Polypropylene resin foamed particle and method for producing the same

Abstract

To provide a polypropylene resin foamed particle which enables formation of a foamed particle molded body which has less steaky patterns and good appearance, and a method for producing the polypropylene resin foamed particle.SOLUTION: A polypropylene resin foamed particle 1 has a cylindrical shape having a through-hole 11 axially penetrating through the particle 1. An average pore diameter d of the through-hole 11 in the foamed particle 1 is less than 1 mm, and a ratio d / D of the average pore diameter d of the through-hole 11 to the average outer diameter D of the foamed particle 1 is 0.4 or less. The foamed particle 1 is composed of a foamed core layer 2 composed of a polypropylene resin, and a coating layer 3 which is composed of a polyolefin resin and coats the side peripheral surface of the foamed core layer 2. Each of the foamed core layer 2 and the coating layer 3 includes colored pigment. A melt mass flow rate MFRS of the polyolefin-based resin composing the coating layer 3 which is measured based on JIS K7210-1:2014 under the conditions of a temperature of 230°C and a load of 2.16 kg is higher than 15 g / 10 min.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an expanded polypropylene resin particle and a method for producing the same. [Background technology]

[0002] Polypropylene resin foamed bead moldings are used in various applications because they are lightweight and have excellent shock-absorbing properties and rigidity. Polypropylene resin foamed bead moldings are produced, for example, by a method called in-mold molding, in which polypropylene resin foamed beads are filled into a mold, and then steam is supplied into the mold to heat it. In the in-mold molding method, when steam is supplied into the mold, the foamed beads undergo secondary foaming and their surfaces melt. This causes the foamed beads in the mold to fuse together, and a molded product having a shape corresponding to the shape of the mold cavity can be obtained. The molded product immediately after molding is prone to expansion due to secondary foaming, so it is cooled in the mold with water, air, etc., and then released from the mold.

[0003] In recent years, the use of expanded bead moldings has expanded, and there is a demand for expanded bead moldings containing a coloring pigment and having a colored appearance. 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. The 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 suppressed color unevenness. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2023-57790 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, when the expanded bead moldings molded using the expanded polypropylene resin beads obtained by the method of Patent Document 1 were observed more carefully, streaky patterns were 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-mentioned 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. [Means for solving the problem]

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

[0009] [2] The expanded polypropylene resin beads according to [1], wherein the average circularity Cb of the outer peripheral edge of the expanded bead is 0.95 or more 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 of ​​the expanded bead is maximum. [3] The expanded polypropylene resin particles according to [1] or [2], wherein the content of the coloring pigment in the expanded core layer is from 0.1% by mass to 5% by mass, and the content of the coloring pigment in the coating layer is from 0.1% by mass to 5% by mass. [4] The expanded polypropylene resin particles according to any one of [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.

[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 The expanded polypropylene resin particles according to any one of [1] to [4], wherein the melt 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 MFR ratio S / MFR C The expanded polypropylene resin particles according to any one of [1] to [5], wherein the number of carbon atoms 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 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 [1] to [7] below: [9] The expanded polypropylene resin beads according to any one of [1] to [8], wherein the through holes have an average circularity Cp of 0.90 or more in a cross section obtained by cutting the expanded bead along a plane perpendicular to the axial direction at a position where the cross-sectional area of ​​the expanded bead is maximum.

[0012]

[10] The expanded polypropylene resin beads according to any one of [1] to [9], wherein a ratio Cb / Cp of an average circularity Cb of the outer peripheral edge of the expanded bead to an 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 a position where the cross-sectional area of ​​the expanded bead 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

[11] .

[0014]

[10] A method for producing expanded polypropylene resin beads, which produces cylindrical expanded polypropylene resin beads having through holes axially penetrating the inside, comprising the steps of: a granulation step for producing multilayer resin particles including 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 to obtain the expanded particles after the granulation step, the through holes in the multilayered resin particles have an average pore diameter dr of less than 0.25 mm; a ratio dr / Dr of an average pore diameter dr of the through holes to an 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; The melt mass flow rate (MFR) of the polyolefin resin constituting the coating layer is measured under conditions of a temperature of 230° C. and a load of 2.16 kg according to JIS K7210-1:2014. S A method for producing expanded polypropylene-based resin particles, wherein the elastic modulus is higher than 15 g / 10 min. Effect of the Invention

[0015] According to the above-mentioned embodiment, it is possible to provide expanded polypropylene resin beads capable of forming an expanded bead molding having few streaks and good appearance, and a production method thereof. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a perspective view of an expanded bead. [Diagram 2] FIG. 2 is a cross-sectional view of the expanded bead of FIG. 1 taken along a cross section passing through the central axis of the expanded bead. [Diagram 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 4 is a perspective view showing an example of an expanded bead having a groove on its circumferential side surface. [Diagram 5] FIG. 5 is a cross-sectional view taken along line VV in FIG. [Figure 6] FIG. 6 is an explanatory diagram showing a method for calculating the area of ​​the high-temperature peak. [Figure 7] FIG. 7 is a plan view that illustrates a schematic view of the surface of the molded body. [Figure 8] FIG. 8 is a photograph showing the appearance of the expanded bead molding of Example 1. [Figure 9] FIG. 9 is a photograph showing the appearance of the expanded bead molding of Comparative Example 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] (Polypropylene resin foam particles) As shown in FIG. 1, the expanded polypropylene resin bead 1 (hereinafter referred to as "expanded bead") has a cylindrical shape with a through hole 11 passing through the inside 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 a shape generally identical to that of the bottom surface 12, and a side peripheral surface 14 connecting the edge of the bottom surface 12 and the edge of the top surface 13. As shown in FIG. 2, the through hole 11 passes through the inside of the expanded bead 1 in the axial direction and opens to the bottom surface 12 and the top surface 13. The aforementioned cylindrical shape includes, for example, a cylinder or a square tube as shown in FIG. 1. The number of the through hole 11 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 S In this way, the side peripheral surface of the foamed core layer of the foamed beads is heated to a melt mass-flow rate MFR S By covering the polypropylene resin foamed bead molded article (hereinafter referred to as "molded article") with a coating layer made of a polyolefin resin having a high melt mass-flow rate MFR, the formation of stripes on the surface of the molded article can be suppressed. S By using a polyolefin resin having the above structure, it is possible to suppress the formation of streaky patterns on the surface of the molded body without impairing the rigidity of the foamed core layer. Therefore, by using the foamed particles, it is possible to suppress the formation of streaky patterns while maintaining the rigidity of the molded body. The detailed configurations of the foamed core layer 2 and the covering layer 3 will be described later.

[0019] [Average hole diameter d of through holes] The average diameter d of the through holes in the expanded beads is less than 1 mm. By making the average diameter d of the through holes less than 1 mm, the moldability of the expanded beads can be improved and a molded polypropylene resin expanded bead having excellent surface properties and rigidity can be obtained. In addition, by making the average diameter d of the through holes less than 1 mm, even if the curing step is omitted, a molded body having a desired shape, excellent surface properties and rigidity, and color unevenness is less noticeable can be obtained. From the viewpoint of obtaining such an effect more reliably, 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. The above-mentioned "curing step" refers to a step of leaving the molded body released from the mold in a high-temperature atmosphere adjusted to a temperature of about 60°C to 80°C for a predetermined time during the manufacturing process of the molded body.

[0020] If the average pore diameter d of the through holes is too large, the molded article may take an excessively long time to dry, and the surface of the molded article may be easily formed with unevenness due to gaps between the expanded particles or 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 effect 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 the preferred range of the average diameter d of the through holes, the upper and lower limits of the average diameter d of the through holes described above can be combined in any way. 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 considered to be, for example, as follows. When the expanded beads are molded in a mold, an open cell structure consisting of open cells, that is, minute spaces communicating with the outside of the molded body, are formed in the molded body. Specifically, the open cell structure is formed by a complex connection of voids formed by the mutual communication of the through holes of a plurality of expanded beads, voids formed by the communication of the through holes of the expanded beads with the gaps between the expanded beads, voids formed by the communication of the gaps between the expanded beads, and open cell portions of the expanded beads constituting the molded body.

[0024] Since the open cell structure is connected to the outside of the molded body, it is believed that when a molded body having a suitable open cell ratio is released from a mold, outside air quickly flows into the bubbles inside the molded body through the open cell structure. Then, the outside air flows into the bubbles inside the molded body, and the internal pressure of the entire molded body quickly becomes balanced with the atmospheric pressure outside the molded body. As a result of the above, it is believed that the dimensions of the molded body are easily stabilized early, and significant shrinkage and deformation of the molded body can be suppressed even if a curing process is not performed.

[0025] In addition, since the expanded beads have through holes, it is believed that when steam is supplied into the mold, the steam can pass through the through holes. This makes it easier for the steam to reach the inside of the mold, and it is believed that the entire expanded beads in the mold can be easily heated. Therefore, even under low molding temperatures during in-mold molding, a molded body having excellent fusion properties and a good appearance can be obtained. As a result, the amount of heat that the expanded beads receive from the steam during in-mold molding can be kept low. In addition, the internal temperature of the molded body after release from the mold is prevented from becoming excessively high. As a result of these, 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 the open cell structure may not be sufficiently formed in the obtained molded body. Therefore, in this case, it is difficult to suppress significant shrinkage, deformation, etc. of the molded body when the curing step is omitted. In addition, in this case, the secondary foaming property of the expanded beads may become excessively high, and the cooling time in the mold may become long.

[0027] The average pore diameter d of the through holes of the expanded beads is obtained as follows. First, the expanded beads are cut at the position where the cross-sectional area is maximum along a plane perpendicular to the axial direction, and the cut surface is exposed 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 regarded 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 operation is carried out for 50 or more expanded beads 1, and the arithmetic mean value of the diameters of the through holes 11 in these expanded beads 1 is defined as the average diameter d of the through holes. 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 where the cross-sectional area of ​​the expanded beads 1 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. In addition, 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 of through holes Cp] The average circularity Cp of the through holes in a cross section obtained by cutting the expanded beads at a position where the cross-sectional area is maximum along a plane perpendicular to the axial direction is preferably 0.90 or more, more preferably 0.92 or more, and even more preferably 0.95 or more. In this case, the effect of improving the moldability of the expanded beads and the effect of shortening the cooling time during molding can be more easily obtained. In addition, by increasing the average circularity Cp of the through holes, the through holes become less likely to be crushed by 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, and the moldability during in-mold molding can be improved. Furthermore, by performing in-mold molding using the expanded beads, a molded body having excellent rigidity can be more easily obtained.

[0031] The average circularity Cp of the through holes of the expanded beads is obtained as follows. First, the expanded beads 1 are cut in a plane perpendicular to the axial direction at the position where the cross-sectional area is maximum, and the cut surface as shown in Fig. 3 is exposed. Next, a photograph of the cut surface of the expanded beads 1 is taken, and the cross-sectional area S p (i.e., the opening area of ​​the through hole 11 on the cut surface) and the perimeter L of the through hole 11 p (That is, the length of the periphery of the through hole 11 on the cut surface) is measured. Then, the circularity of the through hole 11 in each expanded bead 1 is calculated based on the following formula (1). In the following formula (1), π is the circularity factor. Circularity of through hole = 4πS p / (L p ×L p ) · · · (1)

[0032] The above operation is carried out for 50 or more randomly selected expanded beads 1, and the arithmetic mean value of the circularities of the through holes 11 in these expanded beads 1 is defined as the average circularity Cp of the through holes. 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 beads 1 is maximum, as described above.

[0033] The average circularity Cp of the through holes can be adjusted to 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 water temperature (for example, 15° C. or lower).

[0034] [Ratio of average pore diameter d of through holes to average outer diameter D of expanded beads d / D] The 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. 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 a molded product having excellent surface properties and rigidity can be obtained in a wider range of molding pressures from low pressure to high pressure. From the viewpoint of more reliably obtaining such effects, the ratio d / D of the average hole diameter 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 decrease, leading to a decrease in the surface properties and rigidity of the molded product.

[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.

[0036] From the viewpoint of increasing the thickness of the expanded beads to improve 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 more improving the filling property 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] In determining the preferred range of the average outer diameter D of the expanded beads, any combination of the upper and lower limits of the average outer diameter D of the expanded beads can be used. The preferred range of the average outer diameter D of the expanded beads may be, for example, 2 mm or more and 8 mm or less, 2.5 mm or more and 5 mm or less, or 3 mm or more and 4.5 mm or less.

[0038] The method for calculating the average outer diameter D of the expanded beads described above 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, to expose 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 perfect circle having an area equal to the cross-sectional area of ​​the expanded beads 1 is determined 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 cut surface where the cross-sectional area of ​​the expanded beads 1 is maximum, as described above.

[0040] [Average circularity of the outer periphery of the expanded beads Cb] The average circularity Cb of the outer peripheral edge of the expanded beads in a cross section obtained by cutting the expanded beads at a 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 streaky patterns on the surface of the molded product can be more effectively suppressed, and a molded product having a good appearance can be more easily obtained.

[0041] The average circularity Cb of the outer peripheral edge of an expanded bead is obtained as follows. First, the expanded bead is cut in a plane perpendicular to the axial direction at the position where the cross-sectional area is maximum, and the cut surface as shown in Fig. 3 is exposed. 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 cut surface) and the periphery L of the outer periphery of the expanded beads 1 b (That is, the length of the outer periphery of the expanded beads 1 on the cut surface) is measured. Then, the circularity of the outer periphery of each expanded bead 1 is calculated based on the following formula (2). In the following formula (2), π is the circularity factor. Circularity of the outer edge of the foamed particle = 4πS b / (L b ×L b ) · · · (2)

[0042] The above operation is carried out for 50 or more randomly selected expanded beads 1, and the arithmetic mean value of the circularity of these expanded beads 1 is taken as the average circularity Cb of the peripheral edge of the expanded beads. Even if the shape of the cross section of each expanded bead 1 is not uniform in the axial direction, the average circularity Cb of the peripheral edge 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.

[0043] The ratio Cb / Cp of the average circularity Cb of the peripheral edge of the expanded beads to the average circularity Cp of the through holes of the expanded beads is preferably 0.96 to 1.05, more preferably 0.98 to 1.02. In this case, a molded product having excellent rigidity can be obtained while the moldability improving effect due to the through holes is more reliably exhibited, and the formation of streaky patterns on the surface of the molded product can be more effectively suppressed.

[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 foam particles] From the viewpoint of 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 the 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 maximum is preferably 3% or less, more preferably 1.5% 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 surface.

[0046] The aforementioned "groove" refers to a recess formed on the circumferential side surface of the 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 circumferential side surface 14, and has a through hole 11 passing through its interior in the axial direction. Three grooves 15 are formed on the circumferential side 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 portions of the contour of the expanded bead 102 on the cut surface that are recessed inward.

[0047] The method for calculating the area ratio of the total cross-sectional area of ​​the grooves in an expanded bead is as follows. First, the expanded bead 102 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 Fig. 5. Next, a photograph of the cut surface of the expanded bead 102 is taken, and the cross-sectional area of ​​the expanded bead 102 excluding the through-holes 11 (i.e., the area of ​​the region surrounded by the outer peripheral edge and the inner peripheral edge of the expanded bead 102 on the cut surface) is measured.

[0048] Next, a tangent line L1 is drawn outside each groove 15 in the photograph of the cut surface of the expanded bead 102, tangent to the contour of the expanded bead 102 at two points P1 and P2, and does not pass through the interior of the expanded bead 102. Then, the area of ​​the region surrounded by the contour of the expanded bead 102 and the tangent line L1 is calculated, and this area is regarded as the cross-sectional area of ​​each groove 15. In other words, the cross-sectional area of ​​each groove 15 is the area of ​​the region 151 shown by the diagonal lines in Figure 5. The above operation is performed for all grooves 15, and these are summed up to calculate the total cross-sectional area of ​​the grooves 15 in each expanded bead 102.

[0049] The ratio of the total cross-sectional area of ​​the grooves 15 to the cross-sectional area of ​​the expanded beads 102 in each expanded bead 102 is calculated by dividing the total cross-sectional area of ​​the grooves 15 thus obtained by the cross-sectional area of ​​the expanded beads 102 excluding the through holes 11. The above operation is carried out for 100 or more expanded beads 102, and the arithmetic average value of the ratios in 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 specific range. S By using a polyolefin resin having the above structure, the formation of grooves on the peripheral side surface 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 peripheral side surface of the expanded beads obtained.

[0051] [Average thickness of expanded beads t] 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. In addition, the expanded beads having an average wall thickness t within the specific range are less likely to be crushed by external forces, and therefore the rigidity of the molded body obtained by in-mold molding of the expanded beads can be further improved. From the viewpoint of more reliably obtaining 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 further preferably 1.5 mm or more and 2 mm or less.

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

[0053] [Apparent density of expanded beads] The apparent density of the expanded particles is 10 kg / m 3 More than 100kg / m 3 It is preferable that the thickness is less than 20 kg / m 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 3 It 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 having excellent rigidity can be easily obtained. Furthermore, in the past, when a molded body having a particularly low density was produced, the molded body was prone to significant deformation after demolding, and it was difficult to omit the curing step. In contrast, the expanded beads allow the curing step to be omitted even when the apparent density is low, so that 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, the expanded beads are left to stand for one day in an environment of 50% relative humidity, 23°C temperature, and 1 atm pressure to adjust the state of the expanded beads. After measuring the mass of the expanded beads (unit: g), they are submerged in a measuring cylinder containing alcohol (e.g., ethanol) at 23°C using a wire net or the like, and the volume of the expanded beads (unit: L) is calculated from the rise in the liquid level. The apparent density of the expanded beads (unit: kg / m) is then calculated by converting the value obtained by dividing the mass of the expanded beads by the volume of the expanded beads. 3 ) can be calculated.

[0055] [Foam core layer] The foamed core layer of the foamed beads is composed of a polypropylene-based resin. In this specification, the polypropylene-based resin refers to a homopolymer of a propylene monomer and a propylene-based copolymer containing 50% by mass or more of a structural unit 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 foamed 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 an ethylene-propylene copolymer, a butene-propylene copolymer, a hexene-propylene copolymer, and an 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 foamed core layer is preferably 155°C or lower. 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 foamed core layer is preferably 152°C or lower, and more preferably 148°C or lower. 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 foamed core layer is preferably 135°C or higher, more preferably 138°C or higher, and even more preferably 140°C or higher.

[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 performing differential scanning calorimetry (i.e., DSC) based on JIS K7121-1987. Specifically, a test piece made of a polypropylene-based resin is prepared, and the condition of the test piece is adjusted according to "(2) When the melting temperature is measured after a certain heat treatment". The heating rate and cooling rate in the condition adjustment are 10°C / min, and the temperature range is from 30°C to 200°C. The test piece thus adjusted 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 that appears in the DSC curve is the melting point Tmc of the polypropylene-based resin. When multiple melting peaks appear in the DSC curve, the apex temperature of the melting peak with the largest area is the melting point Tmc.

[0059] Melt mass flow rate (MFR) of polypropylene resin that constitutes the foamed core layer C The melt mass-flow rate MFR of the polypropylene-based resin constituting the foamed core layer is preferably 5 g / 10 min or more, more preferably 6 g / 10 min or more, 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 C It is preferable that the elongation rate is 12 g / 10 min or less, and more preferably 10 g / 10 min or less.

[0060] Melt mass flow rate (MFR) of polypropylene resin that constitutes the foamed core layer C In determining the preferred range of the melt mass-flow rate MFR C The upper and lower limits can be arbitrarily combined. For example, the melt mass flow rate MFR 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 are formed such that the side peripheral surface of the foamed core layer is S By covering the molded article with a coating layer made of a polyolefin resin with a high melt mass-flow rate (MFR), the formation of stripes on the surface of the molded article can be suppressed. S The polyolefin resin used has a melt mass flow rate MFR of 1.0 to 1.0 M. C can be set within the above range, for example. This makes it easier to obtain the effect of suppressing the formation of streaky patterns while maintaining the expandability and rigidity of the expanded beads.

[0062] The foamed core layer may contain a polymer other than the polypropylene-based resin, provided that the above-mentioned action and effect is not impaired. Examples of such a polymer include thermoplastic resins other than the polypropylene-based resin, such as polyethylene-based resins and polystyrene-based resins, and thermoplastic elastomers. The content of the polymer other than the polypropylene-based resin 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, and is particularly preferably 0, that is, the foamed core layer contains substantially only the polypropylene-based resin as the polymer.

[0063] The foamed core layer may 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., within a range that does not impair the above-mentioned action and effect. 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 relative to 100 parts by mass of the polypropylene-based resin.

[0064] [Coating layer] The side 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 above-mentioned "non-foamed state" includes a state in which the coating layer is not foamed and does not contain bubbles, and a state in which 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 (expanded core layer:coating layer) 99.5:0.5 to 85:15, more preferably 99:1 to 92:8, and even more preferably 97:3 to 90:10. In this case, the effect of suppressing the formation of streaky patterns can be more easily obtained while maintaining the properties of the expanded beads, such as expandability and rigidity.

[0066] As the polyolefin resin constituting the coating layer, for example, a polyethylene resin, a polypropylene resin, a polybutene resin, etc. can be used. From the viewpoint of adhesion with the foamed core layer, the polyolefin resin constituting the coating layer is preferably a polyethylene resin or a polypropylene resin, and more preferably a polypropylene resin. As the polypropylene resin constituting the coating layer, for example, an ethylene-propylene copolymer, a butene-propylene copolymer, an ethylene-propylene-butene copolymer, a propylene homopolymer, etc. can be used. Among these, the polypropylene resin constituting the coating layer is preferably an ethylene-propylene copolymer or an ethylene-propylene-butene copolymer.

[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 15 g / 10 min or more. S By setting the melt mass-flow rate MFR in the above-mentioned specific range, the formation of streaks on the surface of the molded article can be suppressed, and a molded article having a good appearance can be easily obtained. S By using a polyolefin resin having a relatively low melt mass flow rate MFR as the polypropylene resin constituting the foamed core layer, C As a result, it is possible to impart sufficient rigidity to the expanded beads.

[0068] The reason why such an effect is 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 foamed core layer with the covering layer.

[0069] In order to obtain the above-mentioned effects more reliably, 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 that constitutes the foamed core layerC Difference between 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 MFR ratio S / MFR C is preferably 2 or more, and more preferably 2.5 or more.

[0070] On the other hand, from the viewpoint of 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 S and the melt mass flow rate (MFR) of the polypropylene resin that constitutes the foamed core layer C Difference between 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 MFR ratio S / MFR C is preferably 5 or less, and more preferably 4.0 or less.

[0071] The melt mass flow rate MFR of the polypropylene resin constituting the foamed core layer C The melt mass flow rate MFR of the polyolefin resin constituting the coating layer S MFR ratio S / MFR C In determining the preferred range of the above, the ratio MFR S / MFR C The upper and lower limits of the ratio MFR can be arbitrarily combined. S / MFR CThe preferred range of may be 2 or more and 5 or less, or 2.5 or more and 4.0 or less.

[0072] The MFR of the polyolefin resin that constitutes the coating layer is S is a value measured under the conditions of a test temperature of 230°C and a load of 2.16 kg based on JIS K7210-1:2014. Even if the coating layer is made of a polyethylene 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 covering 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., low 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. From the viewpoint of more reliably obtaining such an effect, 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 more than 0°C and not more than 40°C, more preferably 3°C or more and not more than 35°C, and even more preferably 5°C or more and not more than 30°C.

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

[0075] The coating layer may contain additives such as 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., within a range that does not impair the above-mentioned action and effect. 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 relative to 100 parts by mass of the polyolefin resin.

[0076] [Coloring pigments] The foam core layer and the coating layer contain a coloring pigment. The coloring pigment may be any color other than white, and may be any color having various tones, such as black, red, blue, yellow, and green pigments. The coloring pigment may be 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 color pigment, or may contain two or more types of color pigments. Similarly, the coating layer may contain one type of color pigment, or may contain two or more types of color pigments. The color tone of the color pigment in the foamed core layer and the color tone of the color pigment in the coating layer may be the same or different. From the viewpoint of imparting a high-class feel to the molded article, the color pigment in the foamed core layer and the coating layer is preferably a black pigment, more preferably carbon particles, and even more preferably carbon black.

[0079] The content of the color 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 color pigment in the coating layer is preferably 0.1% by mass or more and 5% by mass or less. In this case, the foamed beads can be given a desired color tone while sufficiently ensuring the secondary foaming property and fusion property of the foamed beads during molding in the mold. This makes it easier to obtain a molded product having a desired color tone and good appearance.

[0080] The streaky pattern formed on the surface of the molded body is considered to be caused, for example, by the grooves formed on the side peripheral surface of the expanded beads being exposed on the surface of the molded body. Such streaky patterns on the surface of the molded body are particularly noticeable in molded bodies obtained by molding expanded beads containing a coloring pigment. This is considered to be because the streaky pattern often appears lighter in color than the surrounding area and is less noticeable in white molded bodies that do not contain a coloring pigment, and because the average circularity of the outer peripheral edge of expanded beads that contain a coloring pigment has traditionally been particularly prone to be low. The expanded beads suppress deterioration of the appearance due to streaky patterns even in colored molded bodies obtained from expanded beads containing a coloring pigment.

[0081] [Closed bubble ratio] From the viewpoint of improving the moldability of the expanded beads and increasing the rigidity of the molded article, 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.

[0082] The closed cell ratio of expanded beads can be measured using an air comparison type specific gravity meter based on ASTM-D2856-70 Procedure C. Specifically, it is measured as follows. First, the expanded beads are left to stand for 24 hours or more in an environment of 50% relative humidity, 23°C temperature, and 1 atm pressure to condition the expanded beads. After the condition is conditioned, the expanded beads are allowed to naturally pile up in a measuring cylinder until the value of the mark is approximately 20 cm. 3 A measurement sample is taken so that the volume becomes 100%. This measurement sample is submerged in a measuring cylinder containing ethanol at 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)×100 / (Va-W / ρ) (4)

[0084] However, Vx in the above formula (4) (unit: cm 3 ) is the true volume of the foamed bead (i.e., the sum of the volume of the resin that composes the foamed bead and the total volume of the air bubbles in the closed cell part within the foamed bead), and Va (unit: cm 3 ) is the apparent volume of the foamed beads (i.e., the volume measured from the rise in the liquid level when the foamed beads are submerged in a graduated 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 regarded as the closed cell ratio of the expanded beads.

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

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

[0088] Specifically, in the DSC curve obtained when heating from 23°C to 200°C at a heating rate of 10°C / min (i.e., the first heating) is performed as described above, both a high-temperature peak and a resin peak specific to the polypropylene resin constituting the foamed core layer appear. In contrast, in the DSC curve obtained when the first heating is performed, the material is cooled from 200°C to 23°C at a cooling rate of 10°C / min, and then heated again from 23°C to 200°C at a heating rate of 10°C / min (i.e., the second heating), only the resin peak specific to the polypropylene resin constituting the foamed core layer appears. 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 temperature of the apex of this resin peak may differ slightly between the first heating and the second heating, but the difference is usually within 5°C.

[0089] From the viewpoint of further improving the moldability of the expanded beads and obtaining a molded body having 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 obtained as follows. First, the expanded beads are left to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm to adjust the state of the expanded beads. 1 to 3 mg of the expanded beads after the adjustment is used as a sample, and a DSC curve is obtained by performing differential scanning calorimetry under conditions of heating from 23°C to 200°C at a heating rate of 10°C / min. An example of a DSC curve is shown in FIG. 6. When the expanded beads have a high-temperature peak, the DSC curve will have a resin-specific peak ΔH1 and a high-temperature peak ΔH2 having a peak at a higher temperature side than the peak of the resin-specific peak ΔH1, 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, that is, 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 intrinsic peak ΔH1 and the high-temperature peak ΔH2 and is parallel to the vertical axis of the graph. This line L3 divides the resin intrinsic peak ΔH1 and the high-temperature peak ΔH2. The endothermic amount of the high-temperature peak ΔH2 can be calculated based on the area of ​​the part of the DSC curve that constitutes the high-temperature peak ΔH2 and the part surrounded by the lines L2 and L3.

[0093] (Method of manufacturing expanded beads) The method for producing the expanded beads includes a granulation step for producing multilayer resin beads having a cylindrical core layer made of a polypropylene resin and having a through hole, and a coating layer made of a polyolefin resin and coating 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 the multilayer resin particles in the granulation step is not particularly limited, but for example, the multilayer resin particles can be produced by the strand cut method. In the strand cut method, the multilayer resin particles are produced using a co-extrusion device equipped with an extruder for forming a core layer, an extruder for forming a coating layer, and a co-extrusion die connected to these two extruders. Specifically, first, a polypropylene-based resin that will be the foamed core layer of the expanded beads, a coloring pigment, and additives that are added as necessary are supplied to the extruder for forming the core layer, and these are melt-kneaded in the extruder for forming the core layer to produce a resin melt-kneaded product for forming the core layer. In addition, a polyolefin-based resin that will be the coating layer of the expanded beads, a coloring pigment, and additives that are added as necessary are supplied to the extruder for forming the coating layer, and these are melt-kneaded in the extruder for forming the coating layer to produce a resin melt-kneaded product for forming the coating layer.

[0095] Then, the molten mixture is discharged from each extruder and merged in a co-extrusion die to form a multilayered composite consisting of a non-foamed core layer and a non-foamed coating layer that covers the lateral surface of the core layer. The composite is then extruded through a small hole in a die equipped with a mechanism for forming through holes in the core layer to form a strand-shaped extrudate having through holes in the core layer. The extrudate is collected and cooled, and then cut to a desired length to obtain multilayered resin particles having through holes in the core layer. The method for producing the multilayered resin particles is not limited to the above-mentioned method, and a hot-cut method, an underwater cut method, or the like may also be used.

[0096] In producing the multilayer resin particles, it is preferable to adopt a strand-cut method in which the strand-shaped extrudate is cooled in water and then cut, which can improve the accuracy of the shape of the multilayer resin particles and more easily make the shape of the multiple through holes in the finally obtained expanded beads into the desired shape.

[0097] In addition, when a strand cutting method is adopted 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 / outer 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 multilayered 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 the mass of 200 randomly selected multilayer resin particles divided by the number of multilayer resin particles. The mass of each multilayer resin particle obtained by the above-mentioned method is sometimes referred to as the "average mass of multilayer resin particles."

[0100] In the above-mentioned production method, expanded beads having the above-mentioned 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 for forming the through holes (i.e., the inner diameter of the die).

[0101] The method for calculating the average outer diameter Dr of the multilayer resin particles and the average pore diameter dr of the through holes is the same as the method 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] [Foaming process] In the foaming step, the multilayer resin particles are foamed. The method for foaming the multilayer resin particles is not particularly limited, but it is preferable to employ, for example, a method called a "direct foaming method" in which the multilayer resin particles containing a foaming agent dispersed in an aqueous medium in a container are released together with the dispersion medium into an atmosphere having a pressure lower than that inside the container.

[0103] In 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 to disperse the multilayer resin particles in the dispersion medium in the container, if necessary.

[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. By adding a dispersant to the dispersion medium, it is possible to suppress fusion between the multilayer resin particles heated in the container in the foaming process. The amount of the dispersant added is preferably 0.001 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the multilayer resin particles. As the dispersant, an organic dispersant or an inorganic dispersant can be used, but it is preferable to use a fine inorganic material as the dispersant because of ease of handling. More specifically, as the dispersant, for example, clay minerals such as amsnite, kaolin, mica, and clay, aluminum oxide, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, iron oxide, etc. can be used. These dispersants may be used alone, or two or more 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 dispersion aid in combination. The amount of the dispersion 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 the physical blowing agent 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, cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane, and halogenated hydrocarbons such as 1,3,3,3-tetrafluoropropene, 1-chloro-3,3,3-trifluoropropene, 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. In addition, the inorganic physical blowing agent and the organic physical blowing agent may be mixed and used. From the viewpoints of environmental load and ease of handling, the physical foaming agent is preferably an inorganic physical foaming agent, and more preferably carbon dioxide.

[0108] The amount of the foaming agent added per 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] As a method for impregnating the multilayer resin particles with a foaming agent, for example, a method of supplying the foaming agent into a container and impregnating the multilayer resin particles in the dispersion medium with the foaming agent can be adopted. In this case, the impregnation of the multilayer resin particles with the foaming 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 terms of gauge pressure. On the other hand, the pressure in the container is preferably 4.0 MPa (G) or less in terms of gauge pressure. If it is within the above range, the expanded beads can be safely produced without risk of damage or explosion of the container.

[0111] In addition, when the dispersion medium is heated, the temperature during foaming can be kept within an appropriate range by increasing the temperature of the dispersion medium at a rate of 1 to 5° C. / min.

[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, whereby the core layer of the multilayer resin particles is foamed to form a cell structure, and the cell structure is stabilized by cooling with the outside air, thereby obtaining the foamed particles.

[0113] When the core layer is made of a polypropylene-based resin, it is preferable to heat and foam in the following manner when impregnating the foaming agent. That is, first, a first-stage holding step is performed in which a temperature is held at a temperature of (melting point of polypropylene-based resin -20°C) or more and less than (end of melting temperature of polypropylene-based resin) for a sufficient time, preferably about 10 to 60 minutes, and then the temperature is adjusted to a temperature of (melting point of polypropylene-based resin -15°C) to less than (end of melting temperature of polypropylene-based resin +10°C). Then, if necessary, a second-stage holding step is performed in which the temperature is held for a further sufficient time, preferably about 10 to 60 minutes. Then, it is preferable to release the contents of the container to the outside while the temperature inside the container is set to (melting point of polypropylene-based resin -10°C) or more, and foam the multilayer resin particles. It is more preferable that the temperature inside the container during foaming is (melting point of polypropylene-based resin) or more and (melting point of polypropylene-based resin +20°C) or less. By heating and expanding the multilayer resin particles in this manner, secondary crystals are formed in the polypropylene-based resin constituting the foamed core layer, and expanded beads having excellent mechanical strength and moldability can be easily obtained.

[0114] In the expansion step, 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 two-stage expansion step are sometimes called second-stage expanded particles.

[0115] A specific method for expanding the multilayer resin particles in two stages is, for example, as follows. First, in the first-stage expansion step, the multilayer resin particles are expanded by, for example, the above-mentioned direct expansion method 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 container, and the pressure container is pressurized with air or an inorganic gas such as carbon dioxide to impregnate the first-stage expanded particles with the inorganic gas. This makes the pressure in the bubbles of the first-stage expanded particles equal to or higher than atmospheric pressure. Then, the first-stage expanded particles are taken out of the pressure-resistant container and heated using a heating medium such as steam or heated air in an environment with a pressure lower than the pressure in the bubbles, thereby causing second-stage expansion of the first-stage expanded particles.

[0116] (Foamed bead molding) 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 complex connections of voids formed by mutual communication between through holes of a plurality of expanded beads, voids formed by communication between through holes of the expanded beads and voids formed between the expanded beads, voids formed by communication between 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, and the like in various fields such as the field of automobiles and other vehicles, and the field of construction.

[0118] In producing the foamed bead molded article, for example, the foamed beads are filled into a mold, and then steam as a heating medium is supplied 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 the filling of 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 foaming. This allows the foamed beads in the mold to be integrated and form a molded article.

[0119] After the heating of the foamed beads is completed, the molded body in the mold is cooled to stabilize the shape. Then, the molded body is removed from the mold to complete the in-mold molding. In the above-mentioned manufacturing method, if necessary, a curing step may be performed in which the molded body after demolding is left for a predetermined 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 demolding 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, for example, the molded body after demolding can be left for 12 hours in an environment of 23°C to stabilize the shape of the molded body. EXAMPLES

[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] [Table 1]

[0123] [Melting Point] The melting point of the resin was determined based on JIS K7121:1987. Specifically, first, the condition of the test piece made of the resin was adjusted based on "(2) When the melting temperature is measured after a certain heat treatment" described in JIS K7121:1987. The heating temperature and cooling temperature in the condition adjustment were 10°C / min, and the temperature range was 30°C to 200°C. The test piece after the condition adjustment was 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 that appeared on the DSC curve was taken as the melting point. A heat flux differential scanning calorimeter (manufactured by SII Nanotechnology Co., Ltd., model number: DSC7020) was used as the measuring device.

[0124] [Resin Melt Mass Flow Rate] The melt mass flow rates of polypropylene-based resins and polyethylene-based resins were measured in accordance with JIS K7210-1:2014 under conditions of a temperature of 230°C and a load of 2.16 kg.

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

[0126] Example 1 1 to 3, the expanded beads 1 of Example 1 have a cylindrical shape and have through holes 11 passing through the inside in the axial direction. The expanded beads 1 of Example 1 have a multilayer structure including an expanded core layer 2 made of PP1 and a non-expanded 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 the granulation step of this example, multi-layer resin particles were produced by the strand cut method. Specifically, a co-extrusion device equipped with a core layer forming extruder, a coating layer forming extruder, and a co-extrusion die connected to these two extruders was used, and the extrudate extruded from the co-extrusion device was cut to an appropriate length to produce multi-layer resin particles. Specifically, in the core layer forming extruder, PP1, carbon black as a coloring pigment, and zinc borate as a bubble regulator were melt-kneaded to obtain a resin melt-kneaded product for forming the core layer. The amount of zinc borate was 500 ppm by mass relative to the mass of PP1, and the amount of carbon black in the resin melt for forming the core layer was 2.7% by mass in the resin melt for forming the core layer. In parallel with this, in the coating layer forming extruder, PP3 and carbon black as a coloring pigment were melt-kneaded to obtain a resin melt-kneaded product for forming the coating layer. The amount of carbon black blended into the molten kneaded resin for forming a coating layer was 2.7% by mass in the molten kneaded resin for forming a coating layer.

[0129] These resin melt kneaded products were merged in the co-extrusion die to form a composite consisting of a non-foamed core layer and a non-foamed coating layer covering the side surface of the core layer. After extruding this composite from the small hole of the co-extrusion die, the extrudate was collected and cooled in water adjusted to 10°C, and cut to an appropriate length using a pelletizer to obtain multilayer resin particles consisting of a core layer and a coating layer covering the side 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 core layer:coating layer=95:5 (i.e., the mass ratio of the coating layer was 5%). The mass of each multilayer resin particle was about 1.5 mg.

[0130] [Foaming process] In the expansion process of this example, the multilayer resin particles were expanded in two stages to produce expanded particles. In the first-stage expansion process, the multilayer resin particles were expanded by a direct expansion method to produce first-stage expanded particles. Specifically, first, 1 kg of the multilayer resin particles was put into 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. In addition, a surfactant (specifically, sodium alkylbenzene sulfonate) was used as the dispersion aid.

[0131] Thereafter, the container was sealed, and carbon dioxide was added to the container as a foaming agent. The container was heated to the temperature shown in the "Expansion temperature" column in Table 2 while stirring. The pressure inside the container (also called the impregnation pressure or carbon dioxide pressure) at this time was the value shown in the "Container pressure" column in 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 a non-expanded coating layer that covers the expanded core layer.

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

[0133] Example 2 Except for the apparent density, the expanded beads of this example have a structure generally similar to that of Example 1. 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 structure 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 similar to the method for producing the expanded beads of Example 1, except that the resin fed into the extruder for forming the core layer in the granulation step is changed from PP1 to PP2, and the conditions of the expansion step are changed as shown in Table 2.

[0135] Example 4 The expanded beads of this example have a structure 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 similar to that 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 structure similar to that of the expanded beads of Example 1, except that the amount of carbon black mixed in 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 similar to the method for producing the expanded beads of Example 1, except that the amount of carbon black added to the extruder for forming the core layer and the extruder for forming the coating layer in the granulation step was changed to 1.0% by mass in each of the melt-kneaded mixtures.

[0137] Comparative Example 1 The expanded beads of this 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 the method for producing the expanded beads of Example 1, except that no coating layer is formed on the lateral surface of the core layer in the granulation step.

[0138] (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.

[0139] (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 is changed to the value shown in Table 4. The method for producing the expanded beads of this example is generally similar to the method for producing the expanded beads of Comparative Example 3, except that the die used in the granulation step is changed to a die having a shape capable of forming a core layer having a larger pore size of the through holes.

[0140] (Reference example 2) Except for the lack of through holes, the expanded beads of this example have a structure generally similar to that of the expanded beads of Comparative Example 3. The method for producing the expanded beads of this example is generally 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.

[0141] (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 of Comparative Example 3, except that no color pigment is added in the granulation step.

[0142] 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.

[0143] (Evaluation of expanded particles) 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 for conditioning.

[0144] [Bulk density] The foamed particles after conditioning were filled into a measuring cylinder so that they would naturally accumulate, and the bulk volume (unit: L) of the foamed particles was read from the graduations on the measuring cylinder. The mass (unit: g) of the foamed particles in the measuring cylinder was then divided by the bulk volume described above, and the unit was converted to obtain the bulk density (unit: kg / m 3 ) was calculated.

[0145] [Apparent density] After measuring the mass of the expanded beads after conditioning, a wire net was used to submerge the expanded beads in a measuring cylinder containing ethanol at 23°C. Taking into account the volume of the wire net, the volume of the expanded beads was measured based on 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 apparent density (unit: kg / m) of the expanded beads was calculated by converting the units. 3 In addition, the "apparent density / bulk density" column in Tables 2 to 4 lists the value obtained by dividing the apparent density of the expanded beads by the bulk density.

[0146] [Closed bubble ratio] The method for measuring the closed cell ratio of the expanded beads is as described above.

[0147] [Average hole diameter d of through holes] 100 expanded beads were randomly selected from the group of expanded beads after the condition adjustment, 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 of ​​the through holes (i.e., the opening area) on the cut surface was measured by performing image analysis. Then, 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 taken as the hole diameter of the through holes in each expanded bead. The above operation was performed for 100 expanded beads, and the arithmetic average value of the obtained hole diameters of the through holes was taken as the average hole diameter d of the through holes of the expanded beads.

[0148] [Average outer diameter D of expanded particles] 100 expanded beads were randomly selected from the group of expanded beads after the condition adjustment, and these expanded beads were cut at the position where the cross-sectional area was maximum along a plane perpendicular to the axial direction 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 expanded beads including the through holes (i.e., the area of ​​the region surrounded by the outer peripheral edge of the expanded beads on the cut surface) was measured by performing image analysis. Then, the diameter of a virtual perfect circle having the same area as the cross-sectional area of ​​the expanded beads was calculated, and this value was taken as the outer diameter of each expanded bead. The above operation was performed for 100 expanded beads, and the arithmetic average value of the obtained outer diameters of the expanded beads was taken as the average outer diameter D of the expanded beads. In addition, the value obtained by dividing the average hole diameter d of the through holes by the average outer diameter D of the expanded beads is shown in the "d / D" column in Tables 2 to 4.

[0149] [Average thickness of expanded beads t] Using the average pore diameter d of the through holes and the average outer diameter D of the expanded beads obtained by the above method, the average wall thickness t of the expanded beads was calculated according to the following formula (3). t = (Dd) / 2 (3)

[0150] [Aspect ratio L / D of expanded particles] For 100 randomly selected expanded beads, the axial length of the expanded beads was 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 obtained by the average outer diameter D of the expanded beads.

[0151] [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 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 resulting cross-sectional photograph was observed. If the contour of the expanded beads in the observed photograph had a portion that was depressed inward from the surrounding area, it was determined that there was a groove, and if there was no depressed portion, it was determined that there was no groove. Furthermore, for the expanded beads that had grooves on the circumferential side surface, the area ratio of the grooves was calculated by the following method.

[0152] First, 100 expanded beads were randomly selected from the group of expanded beads after conditioning, and these expanded beads were cut at the position where the cross-sectional area was maximum along a plane perpendicular to the axial direction 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 expanded beads excluding the through-holes and the total cross-sectional area of ​​the grooves were measured by performing image analysis. 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.

[0153] The above procedure was carried out for 100 expanded beads, and the arithmetic mean value of the ratio of the total cross-sectional area of ​​the grooves obtained was taken as the groove area ratio.

[0154] [Average circularity of through holes Cp] 100 expanded beads were randomly selected from the group of expanded beads after conditioning, and these expanded beads were cut in 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 obtained by performing image analysis. p and the perimeter of the through hole L p These values ​​were then 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)

[0155] 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.

[0156] [Average circularity of the outer periphery of the expanded beads Cb] 100 expanded beads were randomly selected from the group of expanded beads after conditioning, and these expanded beads were cut in 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 image analysis was performed to determine the cross-sectional area S of the expanded beads including the through holes. b and the perimeter L of the outer edge of the foamed beads b These values ​​were then used to calculate the circularity of the outer periphery of each expanded bead based on the following formula (2). Circularity of the outer edge of the foamed particle = 4πS b / (L b ×L b ) · · · (2)

[0157] The arithmetic mean value of the circularity of the outer periphery of 100 expanded beads thus obtained was taken as the average circularity Cb of the outer periphery of the expanded beads. In addition, the "Circularity ratio Cb / Cp" column in Tables 2 to 4 lists the value obtained by dividing the average circularity Cb of the outer periphery of the expanded beads by the average circularity Cp of the through holes, and the "Absolute value of circularity difference |Cp-Cb|" column lists the absolute value of the difference between the average circularity Cb of the outer periphery of the expanded beads and the average circularity Cp of the through holes.

[0158] (Evaluation of Molded Product) Molded bodies were produced by the following method using the expanded beads of the Examples, Comparative Examples, and Reference Examples. 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 set to 10%, and after filling was completed, the mold was clamped in the thickness direction to mechanically compress the expanded beads.

[0159] Next, steam was supplied into the mold to perform in-mold molding. In the in-mold molding, first, steam was 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, to perform a first one-sided heating. Next, steam was supplied from the other side of the mold until a pressure 0.04 MPa (G) lower than the molding pressure during main heating was reached, to perform a second one-sided heating. After that, steam was supplied from both sides of the mold until the molding pressure during main heating shown in Tables 2 to 4 was reached, to perform main heating. After the main heating was completed, the pressure in 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 became 0.04 MPa (G). The time required for cooling in the mold was as shown in the "cooling time" column in Tables 2 to 4. The molding pressure during the main heating was set at a density of approximately 30 kg / m except for Example 2. 3 It was set to be.

[0160] Thereafter, the expanded bead molding was removed from the mold and left to stand in an oven at 80° C. for 12 hours for a curing step. After the curing step, the expanded bead molding was left to stand for 24 hours under conditions of a relative humidity of 50%, 23° C., and 1 atm, and then its yield was evaluated.

[0161] [Density of Molded Body] The mass of the compact (unit: g) is divided by the volume (unit: L) calculated from the external dimensions of the compact, and then the density of the compact (unit: kg / m) is calculated by converting the unit. 3 ) was calculated.

[0162] [50% compressive stress σ 50 〕 A test piece was cut out from the center of the molded body, in the shape of a rectangular prism measuring 50 mm long x 50 mm wide x 25 mm thick, and not including the skin surface, i.e., the surface that was in contact with the inner surface of the mold during molding in the mold. Based on JIS K6767:1999, a compression test was performed at a compression speed of 10 mm / min to measure the 50% compressive stress σ 50(Unit: kPa) was calculated.

[0163] 〔exterior〕 (superficial) 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 no noticeable irregularities caused by through holes, etc. B: Some irregularities due to gaps between particles and / or through holes, etc. are observed on the surface of the molded body. C: The surface of the molded article has significant irregularities due to gaps between particles and / or through holes.

[0164] (Number of streaks and their evaluation) A square observation area with sides of 5 cm was set in the center of a surface of 300 mm long x 250 mm wide of the surface of the molded body. The expanded beads present in this observation area were visually observed, and the number of streak patterns formed on the expanded beads was counted. FIG. 7 shows an example of the observation area. FIG. 8 shows a photograph of the surface of the molded body E1 obtained using the expanded beads of Example 1, and FIG. 9 shows a photograph of the surface of the molded body C3 obtained using the expanded beads of Comparative Example 3. As shown in FIGS. 7 to 9, the surface of the expanded beads 1a without the streak pattern 16 is generally smooth. On the other hand, as shown in FIGS. 7 and 9, the streak pattern 16 passes through the surface of the expanded beads 1b and is observed as a linear pattern that is lighter in color than the surroundings. Note that through holes 11 may appear on the surface of the expanded beads 1 in the observation area.

[0165] Number of streaks that appeared in the observation area (unit: pieces / 25cm 2 ) was as shown in the "Number of streaks" column in Tables 2 to 4. Based on the number of streaks measured, evaluation was performed according to the following criteria. A: 10 stripes / 25cm 2 less than B: 10 stripes / 25cm 2 More than 60 pieces / 25cm 2 less than C: 60 streaks / 25cm 2 End

[0166] [Table 2]

[0167] [Table 3]

[0168] [Table 4]

[0169] As shown in Table 2, the expanded beads of Examples 1 to 5 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. In addition, the expanded beads of these Examples have an expanded core layer and a coating layer that coats the expanded core layer, and the melt mass-flow rate MFR S Therefore, by performing in-mold molding using the expanded beads of these Examples, it was possible to suppress the formation of streaky patterns on the surface of the molded article and obtain a molded article having a good appearance.

[0170] On the other hand, as shown in Table 3, the expanded beads of Comparative Example 1 did not have a coating layer, and therefore streaky patterns were easily formed on the surface of the molded article.

[0171] The coating layer of the expanded beads of Comparative Examples 2 to 4 has a melt mass flow rate MFR S Therefore, streaks tend to form on the surface of the molded article.

[0172] As shown in Table 4, the average pore size d of the through holes in the expanded beads of Reference Example 1 is larger than the specific range. The expanded beads of Reference Example 2 do not have through holes. As shown in Table 4, the expanded beads of these Reference Examples have a melt flow rate of the polyolefin resin constituting the coating layer lower than the specific range, but the streaky pattern is hardly formed on the surface of the molded body made of the expanded beads. Therefore, from the comparison of Examples 1 to 5 and Comparative Examples 1 to 4 with Reference Examples 1 to 2, it can be understood that the formation of the streaky pattern on the surface of the molded body is a problem specific to expanded beads having through holes and having an average pore size d of the through holes within the specific range. The expanded beads of Reference Example 1 have an excessively large average pore size of the through holes, and the surface properties of the molded body are poor. The expanded beads of Reference Example 2 do not have through holes, and the molding pressure is high.

[0173] Moreover, the expanded beads of Reference Example 3 do not contain a color pigment. As shown in Table 4, in the expanded beads of these Reference Examples, although the melt flow rate of the polyolefin resin constituting the coating layer is lower than the specific range, no streaky pattern was observed on the surface of the molded article made of the expanded beads. This is considered to be because the area ratio of the grooves in the expanded beads is relatively small, and the streaky pattern was not observed because the expanded beads are white. Therefore, from a comparison of Examples 1 to 5 and Comparative Examples 1 to 4 with Reference Example 3, it can be understood that the formation of streaky patterns on the surface of a molded article is a problem that is more likely to occur when carbon black is contained as a colorant.

[0174] The specific embodiments of the expanded polypropylene resin particles 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 particles and the method for producing the same according to the present invention are not limited to those in the examples, and the configurations can be appropriately changed within the scope that does not depart from the spirit of the present invention. [Explanation of symbols]

[0175] 1. Foam particles 11 Through hole 2. Foam core layer 3 Covering layer

Claims

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

2. 2. The expanded polypropylene resin beads according to claim 1, wherein the average circularity Cb of the outer edge of the expanded beads 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 maximum is 0.95 or more.

3. 3. The expanded polypropylene resin beads according to claim 1, 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. 3. The expanded polypropylene resin particles according to claim 1, wherein the coloring pigment contained in said foamed core layer is carbon black, and the coloring pigment contained in said coating layer is carbon black.

5. The melt mass-flow rate (MFR) of the polypropylene resin constituting the foamed core layer is 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 claim 1 or 2, wherein the modulus of elasticity is 5 g / 10 min or more and 12 g / 10 min or less.

6. The melt mass-flow rate (MFR) of the polypropylene resin constituting the foamed core layer is 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 / MFR C The expanded polypropylene resin particles according to claim 1 or 2, wherein the number of carbon atoms is 2 or more and 5 or less.

7. The expanded polypropylene resin beads according to claim 1 or 2, 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 beads is 10 kg / m 3 More than 100kg / m 3 The expanded polypropylene resin particles according to claim 1 or 2, wherein:

9. 3. The expanded polypropylene resin beads according to claim 1, wherein the through holes have an average circularity Cp of 0.90 or more 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 is maximum.

10. 3. The expanded polypropylene resin beads according to claim 1, wherein the ratio Cb / Cp of the average circularity Cb of the outer peripheral edge of the expanded beads to the average circularity Cp of the through holes of the expanded beads 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 maximum is 0.96 or more and 1.05 or less.

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

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