Polyolefin-based resin foamed particle and method for producing same

Abstract

Provided are polyolefin-based resin foam particles in which, when A1 represents the cross-sectional area of a cross section perpendicular to a thickness direction of each of the polyolefin-based resin foam particles and A2 represents the area of a region surrounded by an envelope connecting respective ends in each of the polyolefin-based resin foam particles in the cross section, A2 / A1≥1.20 is satisfied, LL / D1 is 0.70-1.20, and LS / LL is 0.50-0.80. As a result, provided are polyolefin-based resin foam particles having excellent sound-absorbing properties and excellent filling properties with respect to a mold.

Description

Expanded polyolefin resin particles and method for producing the same

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

[0002] Polyolefin resin foam molded articles are used in a variety of applications, including automotive interior components, core materials for automotive bumpers, heat insulation materials, cushioning packaging materials, returnable containers, etc. In particular, for automotive interior materials, molded articles with sound-absorbing properties are required.

[0003] Molded articles having sound-absorbing properties are described, for example, in Patent Document 1 and Patent Document 2. Patent Document 1 describes a molded article made of columnar expanded polyolefin resin beads having a predetermined average L / D, etc. Patent Document 2 also describes a molded article made of expanded beads having through holes, the ratio of the maximum length of the through holes in the penetration direction to the maximum diameter of the cross section perpendicular to the length being a predetermined value.

[0004] Japanese Unexamined Patent Publication No. 2007-45979 Japanese Unexamined Patent Publication No. 10-329220

[0005] However, even if the expanded beads described in the above-mentioned prior art have no problems with the sound absorption properties of the molded articles, there is room for improvement in terms of their fillability into a mold for producing the molded articles. That is, the expanded beads described in the above-mentioned prior art are not sufficient from the viewpoint of providing expanded beads that can provide molded articles with excellent sound absorption properties and that also have excellent fillability into a mold for producing the molded articles, and there is room for further improvement.

[0006] One embodiment of the present invention has been made in consideration of the above-mentioned problems, and its object is to provide novel expanded polyolefin resin beads that can provide polyolefin resin foamed molded articles with excellent sound absorption properties and that also have excellent mold fillability.

[0007] As a result of intensive research into solving the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by controlling the shape of expanded polyolefin resin beads to a predetermined shape, and have thus completed the present invention.

[0008] That is, the expanded polyolefin resin beads according to one embodiment of the present invention are polyolefin resin beads, wherein, when A1 is the cross-sectional area of ​​a cross section perpendicular to the thickness direction of the expanded polyolefin resin beads and A2 is the area of ​​a region enclosed by an envelope line connecting each end of the expanded polyolefin resin beads in the cross section, A2 / A1≧1.20, and the expanded polyolefin resin beads have an LL / D1 ratio of 0.70 to 1.20 and an LS / LL ratio of 0.50 to 0.80, where D1 is the diameter of the smallest circle among circles that contain the entire cross-sectional shape of the cross section and inscribe at least two of the end portions in the circle, LL is the length of the longest part in the thickness direction of the expanded polyolefin resin beads in the thickness direction, and LS is the length of the shortest part in the thickness direction of the expanded polyolefin resin beads in the thickness direction.

[0009] a cooling step of solidifying the extruded polyolefin resin composition by cooling it using a cooling device equipped with a cooling medium; a taking-up step of taking up the solidified polyolefin resin composition into a gas phase using a taking-up machine; a chopping step of chopping the taken-up polyolefin resin composition to obtain polyolefin resin particles having an L2 / D2 ratio of 1.8 to 5.0; a dispersion preparation step of mixing the polyolefin resin particles with a dispersion medium and a blowing agent in a container to prepare a dispersion; and a discharging step of discharging the dispersion into a pressure region lower than the internal pressure of the container to obtain expanded polyolefin resin beads, wherein A4 / A3≧1.2 is satisfied, where A3 is the area of ​​a cross section perpendicular to the extrusion direction of the one or more discharge holes and A4 is the area of ​​a region enclosed by an envelope connecting the ends of the discharge holes in the cross section, the distance between the surface of the one or more discharge holes and the surface of the cooling medium facing the one or more discharge holes is 20 mm to 200 mm, and the stretching ratio calculated from the following formula (1) is 1.2 to 4.0; the stretching ratio = take-up speed / resin linear speed formula (1), where the take-up speed is the take-up speed of the polyolefin resin composition by the take-up machine per unit time in the take-up step, and the resin linear speed is a value calculated from the following formula (2); the resin linear speed = discharge rate / (density of the polyolefin resin composition x total area of ​​the discharge holes) formula (2), where the discharge rate is the amount of the polyolefin resin composition extruded from the extruder per unit time in the extrusion step, and the total area of ​​the discharge holes is the sum of the areas of the one or more discharge holes provided in the die provided in the extruder;In the L2 / D2 ratio, L2 is the length in the thickness direction of the surface of the polyolefin resin particle in the thickness direction, and D2 is the diameter of the smallest circle among circles that contain the entire cross-sectional shape of the polyolefin resin particle in a cross section perpendicular to the thickness direction and in which at least two of the ends of the polyolefin resin particle are inscribed.

[0010] According to one aspect of the present invention, there is provided an effect of providing novel expanded polyolefin resin beads that can provide polyolefin resin foamed molded articles with excellent sound absorption properties and that also have excellent mold fillability.

[0011] 1 is a view from one direction of an expanded polyolefin resin bead according to one embodiment of the present invention. FIG. 2 is a view from one direction of an expanded polyolefin resin bead according to one embodiment of the present invention. FIG. 3 is a view from one direction of an expanded polyolefin resin bead according to one embodiment of the present invention. FIG. 4 is a view from one direction of an expanded polyolefin resin bead according to one embodiment of the present invention. FIG. 5 is a view from one direction of an expanded polyolefin resin bead according to one embodiment of the present invention. FIG. 6 is a view from one direction of an expanded polyolefin resin bead according to one embodiment of the present invention. FIG. 7 is a view from one direction of an expanded polyolefin resin bead according to one embodiment of the present invention. FIG. 8 is a view from one direction of an expanded polyolefin resin bead according to one embodiment of the present invention. 8 is a diagram showing an example of polypropylene-based resin particles produced using an extrusion hole having a cross section perpendicular to the extrusion direction in the shape shown in FIG. 7. It is a diagram showing the appearance of an article storage box having a plurality of partition plates. It is a diagram showing an extrusion hole having a cross section called a 1C shape as seen from one direction.

[0012] An embodiment of the present invention will be described below, but the present invention is not limited thereto. The present invention is not limited to the respective configurations described below, and various modifications are possible within the scope of the claims. Furthermore, embodiments or examples obtained by appropriately combining the technical means disclosed in different embodiments or examples are also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment. All academic and patent literature described in this specification is incorporated herein by reference.

[0013] Unless otherwise specified in this specification, "A to B" representing a numerical range means "A or more (including A and greater than A) and B or less (including B and less than B)."

[0014] In this specification, "polyolefin resin particles" may be simply referred to as "resin particles," and "expanded polyolefin resin particles" may be simply referred to as "expanded particles." Furthermore, "expanded polyolefin resin particles according to one embodiment of the present invention" may be simply referred to as "expanded particles," and "expanded polyolefin resin molded article" may be simply referred to as "molded article." Furthermore, "expanded polyolefin resin particles according to one embodiment of the present invention" may be simply referred to as "molded article."

[0015] [1. Technical Concept of One Embodiment of the Invention] Patent Document 1 describes a molded article made of columnar expanded polyolefin resin beads having a predetermined average L / D, etc. Patent Document 2 describes a molded article made of expanded beads having through holes, with a predetermined ratio of the maximum length of the through holes in the penetration direction to the maximum diameter of the cross section perpendicular to the length. These molded articles have sound-absorbing properties, but the expanded beads have difficulty filling a mold during molding. The present inventors have conducted extensive research into the cause of this problem and have found that the expanded beads described in Patent Documents 1 and 2 have insufficient filling properties due to their shape.

[0016] Therefore, the present inventors have conducted extensive research into the shape of expanded polyolefin resin beads that can provide molded articles with excellent sound-absorbing performance and also have excellent filling properties, and have found that the above-mentioned problems can be solved by forming the expanded polyolefin resin beads so that the cross-sectional shape and the longest and shortest parts in the thickness direction of the expanded polyolefin resin beads satisfy predetermined requirements.

[0017] 2. Expanded Polyolefin Resin Beads In an expanded polyolefin resin bead according to one embodiment of the present invention, when A1 is the cross-sectional area of ​​a cross section perpendicular to the thickness direction of the expanded polyolefin resin bead and A2 is the area of ​​a region enclosed by an envelope connecting each end of the expanded polyolefin resin bead in the cross section, A2 / A1≧1.20, and the expanded polyolefin resin bead has an LL / D1 ratio of 0.70 to 1.20 and an LS / LL ratio of 0.50 to 0.80, where D1 is the diameter of the smallest circle among circles that contain the entire cross-sectional shape of the cross section and inscribe at least two of the end portions in the circle, LL is the length of the longest part in the thickness direction of the expanded polyolefin resin bead in the thickness direction, and LS is the length of the shortest part in the thickness direction of the expanded polyolefin resin bead in the thickness direction.

[0018] The present expanded beads have the above-described structure, and therefore have the advantage of being able to provide a polyolefin resin foam molded article having excellent sound absorbing properties and also having excellent mold fillability. Therefore, according to one embodiment of the present invention, a polyolefin resin foam molded article having excellent sound absorbing properties can be provided with high productivity.

[0019] (2-1) Polyolefin Resin Particles The expanded polyolefin resin particles are preferably expanded particles obtained by expanding polyolefin resin particles. The polyolefin resin particles contain a polyolefin resin as a base resin. The "base resin" refers to a resin that accounts for 50% by weight or more of 100% by weight of the resin contained in the resin particles.

[0020] In this specification, the term "resin contained in the resin particles" refers to the polyolefin resin contained in the resin particles, as well as thermoplastic resins and elastomers other than the polyolefin resin that may be contained in the resin particles.

[0021] In this specification, the term "polyolefin resin particles" refers to a resin having the highest content of olefin units among all the structural units constituting the resin. The polyolefin resin contains, for example, 50 mol% or more of olefin units out of 100 mol% of all structural units. The polyolefin resin preferably contains 60 mol% or more of olefin units out of 100 mol% of all structural units, more preferably 70 mol% or more, even more preferably 80 mol% or more, and particularly preferably 90 mol% or more.

[0022] The polyolefin resin particles contain 50% by weight or more of polyolefin resin, preferably 60% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more, of 100% by weight of resin contained in the resin particles.

[0023] The polyolefin resin is not particularly limited as long as it is a polymer compound synthesized using an olefin as a monomer, and examples thereof include polyethylene resins, polypropylene resins, etc. These polyolefin resins may be used alone or in combination of two or more.

[0024] In this specification, the term "polyethylene resin" refers to a resin having the highest content of ethylene units among all structural units constituting the resin. For example, the polyethylene resin contains 50 mol % or more of ethylene units out of 100 mol % of all structural units.

[0025] Examples of polyethylene resins include high-density polyethylene, medium-density polyethylene, low-density polyethylene, linear low-density polyethylene, styrene-modified polyethylene resin, ethylene / vinyl acetate copolymer, ethylene / propylene copolymer, ethylene / 1-butene copolymer, ethylene / 1-butene / propylene copolymer, ethylene / hexene copolymer, ethylene / 4-methyl-1-pentene copolymer, etc. These may be used alone or in combination of two or more.

[0026] In this specification, the term "polypropylene resin" refers to a resin having the highest content of propylene units among all the structural units constituting the resin. For example, the polypropylene resin contains 50 mol % or more of propylene units out of 100 mol % of all structural units.

[0027] Examples of polypropylene resins include polypropylene homopolymers, ethylene / propylene random copolymers, 1-butene / propylene random copolymers, ethylene / 1-butene / propylene random copolymers, ethylene / propylene block copolymers, 1-butene / propylene block copolymers, propylene / chlorinated vinyl copolymers, propylene / maleic anhydride copolymers, etc. These may be used alone or in combination of two or more.

[0028] The following describes a copolymer, such as an ethylene / propylene copolymer, that contains, as structural units, structural units derived from ethylene monomers (also referred to as ethylene units) and structural units derived from propylene monomers (also referred to as propylene units). In this case, a copolymer containing more ethylene units than propylene units is called a polyethylene-based resin, and a copolymer containing more propylene units than ethylene units is called a polypropylene-based resin.

[0029] The polyolefin resin may contain a thermoplastic resin component other than the polyolefin resin and / or an elastomer component, as long as the effects of the present invention are not impaired.

[0030] Examples of thermoplastic resins other than polyolefin resins include vinyl acetate resins, thermoplastic polyester resins, acrylic ester resins, styrene resins, polyamide resins, etc. These thermoplastic resins may be used alone or in combination of two or more.

[0031] Examples of elastomers other than polyolefin-based resins include ethylene / propylene rubber and ethylene / propylene / butadiene rubber. These elastomers may be used alone or in combination of two or more. In this specification, "thermoplastic resin components and / or elastomer components other than polyolefin-based resins" may also be referred to as "other resins."

[0032] The polyolefin resin particles may contain various additives in addition to the polyolefin resin, if necessary.

[0033] In this specification, the additives contained in the polyolefin-based resin particles, in other words, the additives kneaded into the polyolefin-based resin particles, are also referred to as "resin particle additives." The "resin particle additives" can also be said to be components in the resin particles other than the resin (polyolefin-based resin, thermoplastic resin other than polyolefin-based resin, and elastomer).

[0034] Examples of resin particle additives include hydrophilic compounds, antioxidants, heat stabilizers, ultraviolet absorbers, flame retardants, antistatic agents, metal deactivators, carbon black, pigments, dyes, nucleating agents, and cell regulators.

[0035] The amount of these resin particle additives, in total, of all the resin particle additives is preferably 25 parts by weight or less, more preferably 10 parts by weight or less, and even more preferably 5 parts by weight or less, per 100 parts by weight of the resin contained in the resin particles. When the total amount of the resin particle additives is 25 parts by weight or less per 100 parts by weight of the resin contained in the resin particles, the following advantages (a) to (d) are obtained: (a) there is no risk of the dispersibility of the polyolefin resin particles in the dispersion being deteriorated in the process for producing expanded beads using the obtained resin particles; (b) the average cell diameter of the obtained expanded beads does not become too small; (c) the expanded beads can provide a good molded article in the molding process; and (d) the mechanical strength of the obtained molded article is not reduced.

[0036] (2-2. Shape of Expanded Polyolefin Resin Beads) In the expanded beads, when the cross-sectional area of ​​a cross section perpendicular to the thickness direction of the expanded beads is A1 and the area of ​​a region enclosed by an envelope line connecting each end of the expanded polyolefin resin bead in the cross section is A2, A2 / A1≧1.20. A1 and A2 will be described with reference to FIG.

[0037] Fig. 1 is a view of an expanded polyolefin resin bead according to one embodiment of the present invention as seen from one direction. Fig. 2 is a cross-sectional view taken along line A-A' in Fig. 1. That is, Fig. 2 is a view showing a cross section perpendicular to the thickness direction of the expanded polyolefin resin bead shown in Fig. 1. Line A-A' passes through the center in the thickness direction. Fig. 3 is a view showing the region surrounded by an envelope line connecting each end of the expanded polyolefin resin bead in the cross section shown in Fig. 2.

[0038] In this case, the cross-sectional area A1 of the cross section shown in Figure 2 and the area A2 of the region shown in Figure 3 satisfy the relationship A2 / A1 ≥ 1.20. An example of a shape that satisfies this relationship is a shape in which the cross section perpendicular to the thickness direction of the expanded beads has two or more end portions. The end portions refer to the tips of the convex portions that constitute the cross-sectional shape. The convex portions refer to protruding portions in the cross section. In the case of the shape shown in Figure 2, the convex portions are five protruding portions that form a star shape.

[0039] In this specification, a shape having a convex portion and its end portion is also referred to as a “baseline shape.” For example, a star-shaped shape has five convex portions and their end portions, and is therefore also referred to as a “five-baseline shape.”

[0040] The shape having two or more ends can form a recess between a convex portion and another adjacent convex portion. Examples of such shapes include a U-shape, an L-shape, and a V-shape, which have two convex portions and their ends; a Y-shape, which has three convex portions and their ends; an X-shape, which has four convex portions and their ends; and a star shape and a W-shape, which have five convex portions and their ends.

[0041] The end portion is a tip of the expanded particle and therefore has a curved shape, i.e., the end portion is not an intersection of two straight lines but has a rounded shape.

[0042] Here, when the cross-sectional shape is a circle or an ellipse, A2 / A1 = 1. Also, when the cross-sectional shape is a polygon such as a rectangle, which has vertices but does not have recesses formed between adjacent protrusions, A2 / A1 = 1, and the relationship A2 / A1 ≥ 1.20 is not satisfied.

[0043] The larger the A2 / A1 ratio, the higher the porosity and sound absorption coefficient of the molded article obtained from the expanded beads, so A2 / A1 is preferably 1.50 or more. While the upper limit of A2 / A1 is not particularly limited, it is preferably 1.8 or less, since the end portions of the expanded beads are less likely to have a narrow shape, and the compressive strength of the molded article is likely to be high. The above A1 and A2 can be measured by imaging a cross section perpendicular to the thickness direction of the expanded beads with a digital microscope or the like, and measuring the area using the image obtained.

[0044] In the present expanded beads, the LL / D1 of the expanded polyolefin resin beads is 0.70 to 1.20, and the LS / LL is 0.50 to 0.80.

[0045] D1 is the diameter of the smallest circle among the circles that have all of the cross-sectional shapes of the polyolefin resin foam beads in a cross section perpendicular to the thickness direction and in which at least two of the above-mentioned ends are inscribed.

[0046] 4 is a diagram showing an example of a circle having all of the above-described exemplary cross-sectional shapes inside, where two of the end portions in the cross section are inscribed in the circle (points 1 and 2 in the diagram), and the other three end portions are not inscribed in the circle, but are inside the circle, and where the circle has the smallest diameter. It is also possible to use a circle with a larger diameter than the circle shown in FIG. 4 as the circle having two of the end portions inscribed in the circle and the other three end portions not inscribed in the circle, but are inside the circle. However, in this specification, D1, which will be described later, is determined based on the circle with the smallest diameter among the circles.

[0047] In FIG. 4 , there are two end points inscribed in the circle (points 1 and 2), but the number of end points inscribed in the circle is not limited to this. For example, in the case of a star-shaped cross-sectional shape as shown in FIG. 4 , the number of end points inscribed in the circle may be 2, 3, 4, or 5. In a star-shaped cross-sectional shape, if five end points are inscribed in the circle, there will be no end points not inscribed in the circle. In other words, if all end points of a cross-section having two or more end points can form a circle inscribed in the circle, there will be no end points not inscribed in the circle. If all of the end points cannot form a circle inscribed in the circle, when at least two of the end points are inscribed in the circle, at least one end point will be inside the circle that is not inscribed in the circle. In this case, D1, described below, is determined based on the circle that has the shortest possible distance between at least one of the end points that is inside the circle but not inscribed in the circle and the circumference of the circle. This distance is the length of the shortest straight line drawn from the vertex of the end point inscribed in the circle to the circumference. For example, in the case of Fig. 4, it is the length of the shortest straight line drawn from point 3 that is not inscribed in the circle to the circumference. The diameter of the circle set for the expanded polyolefin resin particles in this way is defined as D1.

[0048] FIG. 5 is a view of the thickness direction of the expanded polyolefin resin beads from one direction. The thickness direction is the LL (or LS) direction shown in FIGS. 1 and 5. In the figure, LL is the length of the longest part in the thickness direction of the expanded polyolefin resin beads, and LS is the length of the shortest part in the thickness direction of the expanded polyolefin resin beads. As shown in FIG. 5, the central part of the expanded polyolefin resin beads, measured perpendicular to the thickness direction of the expanded polyolefin resin beads, has the longest thickness direction length (LL), and the upper end part of the expanded polyolefin resin beads, measured perpendicular to the thickness direction of the expanded polyolefin resin beads, has the shortest thickness direction length (LS). As a result, as shown in FIG. 5, the expanded polyolefin resin beads have a curved central portion. During extensive research, the present inventors expanded polyolefin resin beads prepared under specific conditions using an extruder, as described below. The present inventors independently and surprisingly discovered that, in this case, expanded polyolefin resin beads having a curved central portion, as shown in FIG. 5, can be obtained. In addition, when the expanded polyolefin resin beads are expanded beads obtained by expanding polyolefin resin beads obtained using an extruder, the thickness direction of the expanded polyolefin resin beads can also be said to be the extrusion direction of the polyolefin resin beads.

[0049] LL and LS in Fig. 5 correspond to LL and LS shown in Fig. 1. The length of LS2 shown in Fig. 5 is shorter than LL and longer than LS, but in this specification, it is sufficient that LS / LL is 0.50 to 0.80. It is more preferable that the requirement that LS2 / LL is greater than 0.50 and not greater than 0.80 (preferably 0.60 to 0.70) is satisfied, since this results in a shape that is more suitable for filling the expanded beads into a mold.

[0050] Since a molded article having excellent sound absorption coefficient in the low frequency range of around 1000 Hz can be obtained, D1 is preferably 0.3 mm to 3.0 mm, and more preferably 0.6 mm to 2.0 mm, and from another viewpoint, it is preferably 1.0 mm to 10.0 mm, and more preferably 2.0 mm to 7.0 mm.

[0051] LL is preferably 1.0 mm to 10.0 mm, since a molded article having excellent sound absorption coefficient in the low frequency range of around 1000 Hz can be obtained. LL is more preferably 1.5 mm to 5.0 mm, and from another viewpoint, more preferably 2.0 mm to 7.0 mm.

[0052] The LL / D1 ratio is preferably 0.80 to 1.10, since a molded article having an excellent sound absorption coefficient in the low frequency range of around 1000 Hz can be obtained. The LS / LL ratio is preferably 0.60 to 0.70, since this improves the filling property during molding and eliminates or significantly reduces the loss of foamed beads in the fine parts of the molded article.

[0053] The D1 can be determined by measuring the diameter of the circle set on the prepared expanded beads. The D1 is preferably the average value of the D1 values ​​of a plurality of particles. For example, 10 expanded beads are randomly selected, and the average value calculated based on the D1 values ​​of the 10 beads can be used as the D1 value.

[0054] The LL and LS can be determined by measuring the lengths of the longest and shortest parts in the thickness direction of the prepared expanded beads. The LS / LL is preferably the average value of the LS / LL values ​​of a plurality of beads. For example, 10 expanded beads are randomly selected, and the average value calculated based on the LS / LL values ​​of the 10 beads can be used as the LS / LL.

[0055] The present expanded beads satisfy all of the following requirements: A2 / A1≧1.20, LL / D1 between 0.70 and 1.20, and LS / LL between 0.50 and 0.80. When expanded beads have a typical shape, such as a spherical shape, they are excellent at filling into a mold, but irregularly shaped beads, such as those with two or more edges, have better sound-absorbing properties. However, irregularly shaped beads have difficulty filling into a mold. The present expanded beads, despite being irregularly shaped, satisfy all of the above requirements. Therefore, even when molding using a mold with a complex shape, steam passes (almost) uniformly throughout the mold (entire molded article), resulting in the advantageous result of obtaining good molded articles even when using unimpregnated (no internal pressure applied) or lightly impregnated (low internal pressure applied) expanded beads. Therefore, the present expanded beads are excellent not only in sound-absorbing properties but also in mold-filling properties.

[0056] Therefore, the present expanded beads can provide molded articles with excellent sound-absorbing performance with high productivity. The term "cylindrical" expanded beads refers to expanded beads whose cross-sectional shape in a cross section perpendicular to the thickness direction of the expanded beads is circular, and whose central portion in the thickness direction of the expanded beads in the direction perpendicular to the thickness direction is not curved but is straight or nearly straight. In other words, "cylindrical" expanded beads can also be referred to as expanded beads whose LS / LL ratio in this specification is around 1.00 (e.g., 0.90 to 1.10).

[0057] As a method for making the present expanded beads satisfy the above requirements, there is a method for producing expanded polyolefin resin beads, which will be described later. In this method, the present expanded beads satisfying the above requirements can be produced by controlling (i) the distance between the surface of the die discharge hole and the surface of the cooling medium facing said surface, and (ii) the stretching ratio, as will be described later. This point will be described in detail later.

[0058] The expanded beads preferably have 4 to 8 end portions in a cross section perpendicular to the thickness direction of the expanded polyolefin resin beads. This configuration allows the expanded beads to have a number of end portions that is favorable for improving sound absorption performance. This allows for the production of expanded beads that have superior sound absorption performance and excellent mold fillability.

[0059] The number of ends of the cross section is not limited to this, and the cross section may have 9 or more ends. On the other hand, from the viewpoint of (i) obtaining a molded article excellent in both sound absorption performance and compressive strength, and (ii) facilitating the preparation of expanded beads, it is preferable that the cross section have 4 to 8 ends.

[0060] The expanded beads having a cross section with 4 to 8 ends can be obtained, for example, by forming a cross section perpendicular to the extrusion direction of a discharge hole in a die in a shape having 4 to 8 ends in a method for producing expanded polyolefin resin beads described below.

[0061] (2-3. DSC Curve) The present expanded beads have two melting peaks in a DSC curve obtained by differential scanning calorimetry, and when the heat of fusion of the low-temperature peak is Ql (J / g) and the heat of fusion of the high-temperature peak is Qh (J / g), it is preferable that the DSC ratio = {Qh / (Ql+Qh)} x 100 is 20.0% to 35.0%.

[0062] The DSC curve can be obtained by heating 5 to 6 mg of expanded polyolefin resin beads from 40° C. to 220° C. at a heating rate of 10° C. / min using a differential scanning calorimeter.

[0063] Figure 6 shows an example of a DSC curve obtained by subjecting the expanded beads to DSC, raising the temperature from 40°C to 220°C at a heating rate of 10°C / min. In Figure 6, the horizontal axis represents temperature and the vertical axis represents heat quantity. The upward direction of the vertical axis represents heat generation, and the downward direction represents endothermic reaction. The DSC curve shown in Figure 6 can also be considered as the DSC curve during the first heating. The DSC curve shown in Figure 6 has two peaks. These two peaks are a melting peak and an endothermic peak.

[0064] FIG. 6 shows two melting peaks. The lower-temperature peak is referred to as the lower-temperature melting peak, and the peak temperature of this peak is referred to as the lower-temperature peak temperature. The higher-temperature peak is referred to as the higher-temperature melting peak, and the peak temperature of this peak is referred to as the higher-temperature peak temperature. Each point on the DSC curve of the expanded beads shown in FIG. 6 during the first heating cycle will be described. Point A represents the endotherm at a temperature of 100°C. Point B represents the endotherm at the temperature at which the higher-temperature melting ends. Point B can also be considered the intersection of the line extending from the higher-temperature melting peak toward the higher temperature and the higher-temperature baseline (which is also the melting end baseline). Point C represents the point at which the endotherm between the lower-temperature melting peak and the higher-temperature melting peak is smallest. Point D represents the point at which a line parallel to the Y-axis drawn from point C toward line segment AB intersects with this line. The heat quantity based on the range enclosed by the low-temperature melting peak and line segments AD and CD is referred to as the low-temperature melting heat (Ql). The heat quantity based on the range enclosed by the high-temperature melting peak and line segments BD and CD is referred to as the high-temperature melting heat (Qh). The sum of the low-temperature melting heat (Ql) and the high-temperature melting heat (Qh) is the heat quantity of the entire melting peak.

[0065] (2-4. DSC Ratio) The ratio of the high temperature heat of fusion (Qh) to the total heat of the melting peak is called the DSC ratio or high temperature heat ratio. The DSC ratio is expressed by the following formula (3): DSC ratio (%) = (Qh / (Ql+Qh)) x 100 Formula (3).

[0066] The DSC ratio of the present expanded beads is preferably 20.0% to 35.0%, more preferably 21% to 33%, even more preferably 22% to 31%, and particularly preferably 23% to 30%. When the DSC ratio is within this range, a wide range of molding conditions can be selected when using the present expanded beads (e.g., in-mold foam molding). In a molded article produced using the present expanded beads, the structure of the polyolefin resin expanded beads changes, but the composition does not. Therefore, the DSC ratio of a polyolefin resin expanded molded article can be considered to be the same as the DSC ratio of the polyolefin resin expanded beads that are the raw material for the molded article. The DSC ratio of a polyolefin resin expanded molded article can be determined from a DSC curve (specifically, the DSC curve at the first heating time) obtained by the same DSC as that of the polyolefin resin expanded beads.

[0067] (2-5. Bulk Density) The expanded beads preferably have a bulk density of 10.0 g / L to 300.0 g / L. This configuration has the advantage of providing expanded beads with little variation in expansion ratio and good moldability. In view of this advantage, the bulk density is more preferably 12.0 g / L to 100.0 g / L, even more preferably 14.0 g / L to 60.0 g / L, even more preferably 16.0 g / L to 50.0 g / L, and particularly preferably 20.0 g / L to 40.0 g / L. The method for measuring the bulk density of expanded beads will be described in detail in the Examples below.

[0068] (2-6. Average Cell Diameter) The present expanded beads preferably have an average cell diameter of 100 μm to 350 μm. Cell diameter is synonymous with cell diameter. This configuration has the advantage that, when preparing a molded article, it is easy to firmly fuse the expanded beads together, and the porosity of the molded article can be maintained high. In view of these advantages, the average cell diameter is more preferably 150 μm to 300 μm.

[0069] The average bubble diameter can be adjusted to a desired value by using a nucleating agent (inorganic nucleating agent) during the production of polyolefin resin particles. Examples of nucleating agents include talc, titanium oxide, silica (silicon dioxide), silicates, alumina, diatomaceous earth, calcium carbonate, magnesium oxide, magnesium carbonate, magnesium sulfate, calcium phosphate, feldspar apatite, barium sulfate, and zinc borate. Examples of silicates include talc, magnesium silicate, kaolin, halloysite, dickite, aluminum silicate, and zeolite. Only one nucleating agent may be used, or two or more nucleating agents may be used in combination.

[0070] The amount of nucleating agent to be added varies depending on the type of polyolefin resin and the type of cell nucleating agent used and cannot be generally determined, but is generally 0.001 to 2 parts by weight per 100 parts by weight of polypropylene resin.

[0071] The method for measuring the average cell diameter will be described in detail in the Examples below.

[0072] 3. Method for producing expanded polyolefin resin beads A method for producing expanded polyolefin resin beads according to one embodiment of the present invention (hereinafter also referred to as "the present production method") comprises: an extrusion step of extruding a polyolefin resin composition containing a polyolefin resin through one or more discharge holes provided in a die of an extruder; a cooling step of solidifying the extruded polyolefin resin composition by cooling it using a cooling device equipped with a cooling medium; a take-up step of taking up the solidified polyolefin resin composition into a gas phase using a take-up machine; a chopping step of chopping the taken-up polyolefin resin composition to obtain polyolefin resin particles having an L2 / D2 ratio of 1.8 to 5.0; a dispersion preparation step of mixing the polyolefin resin particles with a dispersion medium and a blowing agent in a container to prepare a dispersion; and a release step of releasing the dispersion into a pressure region lower than the internal pressure of the container to obtain expanded polyolefin resin beads. where A3 is the area of ​​a cross section perpendicular to the extrusion direction of the one or more discharge holes and A4 is the area of ​​a region enclosed by an envelope connecting each end of the discharge holes in the cross section, A4 / A3≧1.2, the distance between the surface of the one or more discharge holes and the surface of the cooling medium facing the surface is 20 mm to 200 mm, and the stretching ratio calculated from the following formula (1) is 1.2 to 4.0; the stretching ratio=take-off speed / resin linear speed Formula (1), where the take-off speed is the take-off speed per unit time of the polyolefin resin composition by the take-off machine in the take-off step, and the resin linear speed is a value calculated from the following formula (2): the resin linear speed=discharge rate / (density of the polyolefin resin composition×total area of ​​the discharge holes) Formula (2), where the discharge rate is the amount of the polyolefin resin composition extruded from the extruder per unit time in the extrusion step, the total area of ​​the discharge holes is the sum of the areas of one or more discharge holes provided in the die provided in the extruder;In the L2 / D2 ratio, L2 is the length in the thickness direction of the surface of the polyolefin resin particle in the thickness direction, and D2 is the diameter of the smallest circle among circles that contain the entire cross-sectional shape of the polyolefin resin particle in a cross section perpendicular to the thickness direction and in which at least two of the ends of the polyolefin resin particle are inscribed.

[0073] The present production method has the above-described features and can produce the present expanded beads, which can provide a molded article having excellent sound-absorbing properties and excellent mold-filling properties.

[0074] The "polyolefin resin composition containing a polyolefin resin" is a blend containing a polyolefin resin and, if necessary, the other resins and resin particle additives described above. The polyolefin resin composition can be produced by melt-kneading the blend. This method may also include a resin composition preparation step in which the blend is heated in an extruder to melt and knead to prepare a polyolefin resin composition.

[0075] The apparatus for melt-kneading the blend (melt-kneading apparatus) may be, for example, an extruder as described below, with a twin-screw extruder being more preferred.

[0076] The temperature at which the blend is melt-kneaded (hereinafter, sometimes referred to as "resin temperature") can also be said to be the temperature of the molten kneaded product (polyolefin resin composition) immediately before the molten kneaded product is extruded from a discharge hole provided in a die of an extruder.

[0077] The temperature is not particularly limited, but may be the melting point of the polyolefin resin [Tm １ (°C)], Tm １ +40℃ or more Tm １It may be +110°C or lower. The melting point of the polyolefin resin referred to here is the value obtained by differential scanning calorimetry (DSC) using a differential scanning calorimeter. The specific operating procedure is as follows: (1) 4 to 6 mg of polyolefin resin is heated from 40°C to 220°C at a heating rate of 10°C / min to melt it; (2) the temperature is lowered from 220°C to 40°C at a heating rate of 10°C / min to crystallize it; and (3) the temperature is further increased from 40°C to 220°C at a heating rate of 10°C / min. The temperature of the peak (melting peak) of the DSC curve obtained during the second heating (i.e., during (3)) can be determined as the melting point of the polyolefin resin.

[0078] In expanded polyolefin resin beads produced using polyolefin resin particles containing a polyolefin resin, the structure of the polyolefin resin particles changes, but the composition of the polyolefin resin particles does not change.

[0079] Furthermore, in a polyolefin resin foam molded article produced using expanded polyolefin resin beads produced using polyolefin resin particles, the structure of the expanded polyolefin resin beads changes, but the composition of the expanded polyolefin resin beads does not change.

[0080] Therefore, the melting point value obtained by analyzing the polyolefin resin foam particles or the polyolefin resin foam molded article can be considered to be the melting point value of the polyolefin resin contained in the polyolefin resin particles that are their raw material.

[0081] In this specification, the melting point of the expanded polyolefin resin particles or the expanded polyolefin resin molded article is a value obtained by measuring in the same manner (DSC) as the melting point of the polyolefin resin, except that expanded polyolefin resin particles or expanded polyolefin resin molded article, respectively, are used instead of the polyolefin resin.

[0082] (3-1. Extrusion Step) In this step, the polyolefin resin composition is extruded from one or more discharge holes provided in a die equipped in an extruder. The polyolefin resin composition, which is a melt-kneaded product, can be extruded, for example, in the form of strands, from one or more discharge holes provided in a die attached to the tip of an extruder (for example, a melt-kneading device). "Extruding" the polyolefin resin composition from the discharge hole may also be referred to as "discharging."

[0083] The number of discharge holes provided in the die is not particularly limited as long as it is one or more. The cross-sectional area of ​​the discharge hole perpendicular to the extrusion direction can be adjusted by changing the cross-sectional shape and cross-sectional diameter of the discharge hole. The extrusion direction is the direction in which the polyolefin resin composition (melt-kneaded product) is extruded.

[0084] Here, when the area of ​​a cross section perpendicular to the extrusion direction of the one or more discharge holes is A3 and the area of ​​a region in the cross section enclosed by an envelope connecting each end of the discharge holes is A4, it is preferable that A4 / A3 ≥ 1.2. When there are two or more discharge holes, it is preferable that each discharge hole satisfies the requirement of A4 / A3 ≥ 1.2.

[0085] The above A3 is set to 1.0 mm in order to optimize the size of the expanded particles in consideration of the filling property into the mold. ２ ~6.0mm ２ From this viewpoint, it is preferable that A3 is 1.2 mm. ２ ~5.0mm ２ More preferably, it is 1.6 mm ２ ~4.0mm ２ It is more preferable that:

[0086] Here, Fig. 7 is a front view showing an example of the configuration of one discharge hole provided in a die. That is, Fig. 7 shows a cross section perpendicular to the extrusion direction of the discharge hole. The discharge hole is a discharge hole for obtaining star-shaped resin particles that are the raw material for star-shaped expanded beads. The polyolefin resin composition discharged from the discharge hole can be subjected to each step of the present production method after the cooling step to obtain, for example, the present expanded beads shown in Fig. 1.

[0087] Fig. 8 is a diagram showing a region surrounded by an envelope line connecting each end of the discharge hole shown in Fig. 7. In this case, the cross-sectional area A3 of the cross section shown in Fig. 7 and the area A4 of the region shown in Fig. 8 satisfy the relationship A4 / A3 ≥ 1.2. By satisfying this relationship, the expanded beads of the present invention satisfy the aforementioned relationship A2 / A1 ≥ 1.20.

[0088] An example of a shape that satisfies the relationship A4 / A3≧1.2 is a shape in which the cross-sectional shape has two or more ends. The ends refer to the tips of the convex portions that constitute the cross-sectional shape of the discharge hole. The convex portions refer to the portions that protrude in the cross section. For example, in the case of the shape shown in FIG. 7, the convex portions are five protruding portions that form a star shape. Examples of shapes that have two or more ends include the U-shape, L-shape, V-shape, Y-shape, X-shape, star shape, and W-shape described in (2-2. Shape of expanded polyolefin resin beads).

[0089] The larger the A4 / A3 ratio, the higher the porosity and sound absorption coefficient of the molded article obtained from the expanded beads, so A4 / A3 is 1.2 or more. Although the upper limit of A4 / A3 is not particularly limited, it is preferably 4.0 or less because the end of the expanded beads is less likely to become narrow, and the compressive strength of the molded article is likely to be high.

[0090] As described above, A4 and A3 can be adjusted to satisfy the relationship A4 / A3≧1.2 by changing the cross-sectional shape and cross-sectional diameter of the discharge hole.

[0091] The discharge hole preferably has a slit width of 0.05 mm to 0.80 mm. The slit width refers to the width of the convex portion in a cross section perpendicular to the extrusion direction of the discharge hole. For example, in the cross section shown in FIG. 7, this corresponds to the width of the portion indicated as "slit width." In the cross section shown in FIG. 7, the slit width is constant for all convex portions, but if the slit width varies depending on the convex portion, it is preferable that the average slit width of all convex portions is 0.05 mm to 0.80 mm. Furthermore, if the slit width varies depending on the location of one convex portion, the average slit width can be found for each convex portion and used to calculate the average slit width of all convex portions.

[0092] The slit width is more preferably 0.15 mm to 0.6 mm, and even more preferably 0.25 mm to 0.5 mm, from the viewpoint of (i) setting LS / LL in a suitable range and (ii) preventing nozzle clogging when foreign matter is mixed in.

[0093] The discharge hole preferably has 4 to 8 slits. The number of slits refers to the number of convex portions in a cross section perpendicular to the extrusion direction of the discharge hole. For example, in the cross section shown in Figure 7, the number of slits is 5. Having 4 to 8 slits has the advantage that the discharge hole easily satisfies the requirement of A4 / A3 ≥ 1.2 and easily produces polyolefin resin particles having an L2 / D2 ratio of 1.8 to 5.0. From this perspective, the number of slits is more preferably 4 to 7, and even more preferably 5 to 6.

[0094] (3-2. Cooling Step) In this step, the extruded polyolefin resin composition is solidified by cooling it using a cooling device equipped with a cooling medium.

[0095] The cooling medium may be water, air, or the like, and is preferably water from the viewpoint of cooling efficiency. The cooling device may be, for example, a water tank or a water channel containing water as the cooling medium.

[0096] The temperature of the cooling medium may be referred to as the water temperature when a water tank containing water is used as the cooling medium. The temperature of the cooling medium is not particularly limited, but may be Tm１ -150℃ or more Tm １ It may be -90°C or lower, Tm １ -150℃ or more Tm １ A temperature of −110° C. or lower is preferred.

[0097] When the polyolefin resin composition is discharged from the cooling device into the gas phase for the later-described take-off process, the cooling time may affect the surface temperature of the polyolefin resin composition discharged from the cooling device into the gas phase. The surface temperature is not particularly limited, but may be Tm １ It may be -70°C or lower, Tm １ A temperature of −100° C. or lower is preferred.

[0098] The distance between the surface of the one or more discharge holes and the surface of the cooling medium facing the surface is 20 mm to 200 mm. This distance refers to the distance from the end of the discharge hole in the extrusion direction to the cooling medium. When a water tank containing water as the cooling medium is used as the cooling device, the distance between the surface of the discharge hole and the cooling medium can also be said to be the distance from the end of the discharge hole in the extrusion direction provided in the die to the water surface in the water tank.

[0099] The die is preferably arranged so that the surface of the discharge hole is parallel to the surface of the cooling medium, and in this case, all of the surfaces of one or more discharge holes of the die are positioned equidistant from the surface of the cooling medium, making it easy to satisfy the distance.

[0100] However, this is not limiting, and the distance between the surface of each discharge hole and the surface of the cooling medium may be different as long as it is 20 mm to 200 mm. An example of a case where the distances are different is when the die is positioned so that the surface of the discharge hole forms an angle of 70° with the surface (horizontal) of the cooling medium. In this case, the length of the parabola that the discharged polyolefin resin composition draws before coming into contact with the cooling medium differs between the discharge hole positioned close to the cooling medium and the discharge hole positioned far from the cooling medium. In this case, the distance corresponds to the length between the endpoints of the parabola that the polyolefin resin composition draws before coming into contact with the cooling medium, and this distance may be 20 mm to 200 mm. The length between the endpoints is the length from the surface of the discharge hole to the landing point of the polyolefin resin composition in the cooling medium.

[0101] The distance, together with the stretching ratio described below, affects the value of LS / LL described above. As a result of studies by the inventors, it was found that a distance of 20 mm to 200 mm and a stretching ratio within a range described below are preferable for achieving a LS / LL ratio of 0.50 to 0.80. From this perspective, the distance is preferably 20 mm to 150 mm, and more preferably 25 mm to 70 mm.

[0102] (3-3. Take-up step) In this step, the solidified polyolefin resin composition is taken up into the gas phase from the cooling medium (cooling device) using a take-up machine. Examples of the take-up machine include a roller-type take-up machine that performs take-up using rollers, and can be used to take up the polyolefin resin composition extruded from one or more discharge holes provided in the die.

[0103] In this step, the drawing is carried out so that the draw ratio calculated from the following formula (1) is 1.2 to 4.0: Draw ratio = Take-up speed / Resin linear speed Formula (1).

[0104] The stretch ratio, together with the distance between the surface of the one or more discharge holes and the surface of the cooling medium facing the surface, affects the value of LS / LL. As the stretch ratio increases, LS / LL tends to decrease. To achieve an LS / LL ratio of 0.50 to 0.80, it is preferable to perform take-up so that the distance is 20 mm to 200 mm and the stretch ratio is 1.2 to 4.0. From this perspective, the stretch ratio is preferably 1.5 to 3.5, and more preferably 2.0 to 3.0.

[0105] The take-up speed is the take-up speed of the polyolefin resin composition by the take-up machine per unit time, i.e., the take-up speed of the take-up machine when polyolefin resin particles are produced using the take-up machine.

[0106] The resin linear velocity is a value calculated from the following formula (2): Resin linear velocity=discharge amount / (density of the polyolefin resin composition×total area of ​​the discharge holes) (2).

[0107] As can be seen from formula (2), the resin linear velocity refers to the length in the extrusion direction of the polyolefin resin composition extruded per unit time from each discharge hole provided in a die of the extruder when polyolefin resin particles are produced using the extruder.

[0108] Here, the discharge rate refers to the amount (weight) of the polyolefin resin composition extruded from an extruder per unit time when polyolefin resin particles are produced using the extruder. The discharge rate can also be said to be the total amount (total weight) of the polyolefin resin composition extruded per unit time from one or more discharge holes provided in a die of the extruder when polyolefin resin particles are produced using the extruder.

[0109] The total area of ​​the discharge holes is the sum of the cross-sectional areas of the discharge holes provided in the die of the extruder. In this specification, the "cross-sectional area of ​​the discharge holes" may also be referred to as the "area of ​​the discharge holes." When the die has multiple discharge holes and the cross-sectional areas of the holes are the same, the total area of ​​the discharge holes can be calculated by multiplying the area per discharge hole by the number (total number) of discharge holes.

[0110] (3-4. Shredding Step) In this step, the collected polyolefin resin composition is shredded to obtain polyolefin resin particles having an L2 / D2 ratio of 1.8 to 5.0. L2 in the L2 / D2 ratio is the thickness direction length of the polyolefin resin particle on the thickness direction surface. D2 is the diameter of the smallest circle among circles that contain the entire cross-sectional shape of the polyolefin resin particle in a cross section perpendicular to the thickness direction and inscribe at least two ends of the polyolefin resin particle in the circle.

[0111] By appropriately changing the shredding speed of the polyolefin resin composition that has been subjected to the above-mentioned take-up step so as to satisfy the above-mentioned elongation ratio, polyolefin resin particles having an L2 / D2 ratio of 1.8 to 5.0 can be obtained. The shredding speed refers to the speed at which the polyolefin resin composition taken up by a take-up machine or the like is shredded by a shredding device. When a shredding device equipped with blades such as cutters is used, the shredding speed can also be considered as the rotation speed of the blades.

[0112] D2 corresponds to D1 of the expanded polyolefin resin beads already described. D2 can be explained by replacing D1 with D2 in the explanation of D1 and substituting polyolefin resin particles for expanded polyolefin resin beads. D2 can be determined by measuring the diameter of the circle set as explained for D1. Furthermore, D2 can be set to an appropriate value for achieving L2 / D2 of 1.8 to 5.0 by adjusting the shape of the discharge hole.

[0113] Figure 9 is a view of the thickness direction surface of polyolefin resin particles in one embodiment of the present invention, viewed from one direction. In the figure, L2 is the length in the thickness direction of the surface. By adjusting the shredding speed, L2 can be set to an appropriate value that allows L2 / D2 to be 1.8 to 5.0, taking into account the value of D2. Because the polyolefin resin particles are not foamed and are shredded linearly by the shredding device, they do not have a structure with a curved protruding center, as in the expanded polyolefin resin particles shown in Figure 5. Therefore, only L2 is present, rather than LL and LS as shown in Figure 5.

[0114] The average L2 / D2 value of the produced polyolefin resin particles is preferably 1.8 to 5.0. For example, 10 particles are randomly selected from the produced polyolefin resin particles, and the L2 / D2 values ​​determined for each particle are averaged, and this average value can be used as the L2 / D2 value.

[0115] By setting the L2 / D2 ratio to 1.8 to 5.0, the expanded polyolefin beads obtained by subjecting them to the dispersion preparation step and release step described below can have an LL / D1 ratio of 0.70 to 1.20 and an LS / LL ratio of 0.50 to 0.80. The smaller the L2 / D2 ratio, the higher the apparent bulk density of the resin particles, which has the advantages of (i) allowing a larger amount to be put into a pressure-resistant vessel for expansion, and / or (ii) improving the fluidity of the resin particles, thereby improving the efficiency of piping transportation. For these reasons, the L2 / D2 ratio is preferably 1.8 to 4.0, and more preferably 1.8 to 3.0.

[0116] (3-5. Dispersion Preparation Step) This step can be carried out by adding polyolefin resin particles, a dispersion medium, a foaming agent, and optionally a dispersant, a dispersion aid, etc. to a container and mixing them. The order in which these are added to the container is not particularly limited. The manner in which the added raw materials are mixed is not particularly limited, and examples include a method in which the raw materials are stirred using a stirring blade or the like provided in the container.

[0117] The container is preferably one that can withstand the expansion temperature and expansion pressure during the production of expanded beads, and is more preferably a pressure-resistant container. Examples of the container include an autoclave-type pressure-resistant container. The expansion temperature and expansion pressure will be described in detail later.

[0118] The dispersion medium is not particularly limited. The dispersion medium is preferably an aqueous dispersion medium, and it is preferable to use only water as the dispersion medium. In this production method, a dispersion medium obtained by adding methanol, ethanol, ethylene glycol, glycerin, or the like to water can also be used. Only one type of dispersion medium may be used, or two or more types may be used in combination.

[0119] The amount of the dispersion medium used is preferably 200 parts by weight or more, more preferably 300 parts by weight or more, relative to 100 parts by weight of the polyolefin resin particles. When the amount of the dispersion medium used is 200 parts by weight or more relative to 100 parts by weight of the polyolefin resin particles, the dispersibility of the polyolefin resin particles in the dispersion liquid improves as the amount of the dispersion medium used increases.

[0120] In the present production method, when the polyolefin resin particles contain a hydrophilic compound and the dispersion medium contains water, the water in the dispersion medium can act as a blowing agent, thereby improving the expansion ratio of the resulting expanded beads.

[0121] Examples of the blowing agent include aliphatic hydrocarbons, halogenated hydrocarbons, and inorganic gases. Examples of aliphatic hydrocarbons include propane, butane, isobutane, pentane, and isopentane. Examples of halogenated hydrocarbons include monochloromethane and hydrofluoroolefins. Examples of inorganic gases include air, nitrogen, and carbon dioxide. For example, water contained in the dispersion medium can be used as the blowing agent. These blowing agents may be used alone or in combination of two or more. In this production method, inorganic gases and / or water are preferred as the blowing agent because they do not adversely affect the environment. The blowing agent is preferably air and / or carbon dioxide gas, and more preferably carbon dioxide gas.

[0122] The amount of the foaming agent used cannot be generally specified, as it varies depending on the type of polyolefin resin used, the type of foaming agent, the desired expansion ratio, etc. The amount of the foaming agent used is preferably, for example, 2 parts by weight or more and 60 parts by weight or less per 100 parts by weight of the polyolefin resin particles.

[0123] In the present production method, a dispersant may be used. When a dispersant is used in the dispersion preparation step, (a) coalescence of polyolefin resin particles in the dispersion can be prevented, and / or (b) coalescence of polyolefin resin particles after the dispersion is released can be prevented.

[0124] Examples of dispersants include, but are not limited to, inorganic dispersants such as tricalcium phosphate, trimagnesium phosphate, calcium pyrophosphate, basic magnesium carbonate, calcium carbonate, barium sulfate, and silicates. Examples of silicates include clay minerals such as kaolin, talc, and clay. These dispersants may be used alone or in combination of two or more.

[0125] The amount of the dispersant used is not particularly limited and can be appropriately changed depending on the type of dispersant. The amount of the dispersant used is preferably, for example, 0.01 parts by weight or more and 3.00 parts by weight or less per 100 parts by weight of the polyolefin resin particles.

[0126] In the present production method, a dispersing aid may be used. When a dispersing aid is used in the dispersion preparation step, (a) coalescence of polyolefin resin particles in the dispersion can be prevented, and / or (b) coalescence of polyolefin resin particles after the dispersion is released can be prevented. The dispersing aid is preferably used together with the dispersant.

[0127] The dispersing aid may be, but is not limited to, an anionic surfactant. The dispersing aid may be used alone or in combination of two or more.

[0128] The amount of the dispersing aid used is, for example, preferably 0.001 to 0.300 parts by weight, more preferably 0.010 to 0.200 parts by weight, and more preferably 0.020 to 0.150 parts by weight, per 100 parts by weight of the polyolefin resin particles.

[0129] (3-6. Releasing Step) This step is a step of releasing the dispersion obtained in the dispersion preparing step into a pressure region lower than the internal pressure of the container used in the dispersion preparing step to obtain expanded polyolefin resin beads.

[0130] The discharging step may be carried out, for example, by increasing the pressure inside the container to a pressure higher than atmospheric pressure in advance and discharging the dispersion under atmospheric pressure to obtain expanded polyolefin resin beads having an expansion ratio of 10 times or more. The pressure lower than the pressure inside the container is preferably atmospheric pressure. The expansion ratio of the resulting expanded beads is preferably 60 times or less.

[0131] The temperature and pressure inside the container when the dispersion is released into a pressure region lower than the pressure inside the container are also referred to as the foaming temperature and foaming pressure, respectively. The present production method may further include a heating step of increasing the temperature inside the container to the foaming temperature and / or a pressurizing step of increasing the pressure inside the container to the foaming pressure.

[0132] The foaming temperature varies depending on the type of polyolefin resin used, the type and / or amount of resin particle additive kneaded (used) into the polyolefin resin, and the type and / or amount of foaming agent, and cannot be generally defined. The foaming temperature is calculated by dividing the melting point of the polyolefin resin particles by [Tm ２ (°C)], Tm ２ -30 (℃) or more Tm ２ Preferably, it is +10°C or lower.

[0133] In this specification, the melting point Tm of the polyolefin resin particles ２ The melting point Tm of the polyolefin resin is the same as that of the polyolefin resin except that polyolefin resin particles are used instead of the polyolefin resin. １ The value is obtained by measuring by the same method (DSC) as above.

[0134] In the polyolefin-based resin particles produced using the polyolefin-based resin, the structure of the polyolefin-based resin changes, but the composition of the polyolefin-based resin does not change. Therefore, in one embodiment of the present invention, the melting point Tm ２ is the melting point Tm of the polyolefin resin used to produce the polyolefin resin particles １ can be the same as

[0135] The expansion pressure varies depending on the desired expansion ratio of the expanded polyolefin resin beads and cannot be generally defined. The expansion pressure is preferably 0.50 MPa·G or more and 6.0 MPa·G or less, and more preferably 1.0 MPa·G or more and 4.5 MPa·G or less. In this specification, "G" written after the pressure unit indicates that the pressure is a gauge pressure. In other words, "MPa·G" can also be rewritten as "MPa (gauge pressure)."

[0136] When a dispersion liquid containing polyolefin resin particles is released from a container into a pressure region lower than the container internal pressure, the dispersion liquid may be released through an orifice having an opening of 2 mm to 10 mm in diameter for the purposes of adjusting the release amount (flow rate) and reducing the expansion coefficient variation of the resulting expanded beads. A "pressure region lower than the container internal pressure" is also referred to as a low-pressure atmosphere. In the present expanded bead production method, when the dispersion liquid from the container is released into a low-pressure atmosphere, the temperature of the low-pressure atmosphere may be adjusted for the purpose of increasing the expansion coefficient.

[0137] The dispersion prepared in the vessel may be heated to the foaming temperature and pressurized to the foaming pressure under stirring, and then maintained at the foaming temperature and pressure for a certain period of time. The certain period of time is typically 5 to 180 minutes, and preferably 10 to 60 minutes. The dispersion maintained at the foaming temperature and pressure for the certain period of time is then released into a low-pressure atmosphere (usually atmospheric pressure) by opening a valve provided at the bottom of the vessel, thereby producing expanded polyolefin resin beads.

[0138] [4. Polyolefin-based resin foam molded article] The polyolefin-based resin foam molded article according to one embodiment of the present invention is preferably a molded article obtained by molding (e.g., in-mold foam molding) the polyolefin-based resin foam beads described in the above section [2. Polyolefin-based resin foam beads]. As described above, the polyolefin-based resin foam beads according to one embodiment of the present invention can provide a polyolefin-based resin foam molded article having excellent sound absorption performance and excellent mold fillability. Therefore, this molded article has the advantages of excellent sound absorption performance and can be produced with a high yield.

[0139] The present molded article is preferably a molded article obtained by molding (for example, in-mold foam molding) expanded beads produced by the production method described in the above section [3. Production method of expanded polyolefin resin beads].

[0140] The method for producing the present molded article preferably includes a step of molding (e.g., in-mold foam molding) the expanded polyolefin resin beads described in the above section [2. Expanded polyolefin resin beads]. The method for producing the present molded article also preferably includes a step of molding (e.g., in-mold foam molding) the expanded beads produced by the method described in the above section [3. Expanded polyolefin resin beads].

[0141] More specifically, examples of the manufacturing method (molding method) of the present molded article include the following methods (A) to (C): (A) (i) a method in which expanded beads are pressurized with an inorganic gas to impregnate the expanded beads with the inorganic gas, thereby applying a predetermined internal pressure to the expanded beads, (ii) thereafter, the expanded beads are filled into a mold, and (iii) the mold and the expanded beads are heated with steam to heat-fuse the expanded beads together; (B) (i) a method in which expanded beads are compressed with gas pressure and filled into a mold, and (ii) the foamed beads are heat-fuse to each other by utilizing the recovery force of the filled expanded beads and by heating the mold and the expanded beads with steam; (C) (i) a method in which expanded beads are filled into a mold without any particular pretreatment, and (ii) the mold and the expanded beads are heat-fuse to each other by heating the mold and the expanded beads with steam.

[0142] The present expanded beads can exhibit excellent mold filling properties in any of the above methods (A) to (C).

[0143] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0144] That is, one embodiment of the present invention includes the following configuration.

[0145] [1] Expanded polyolefin resin beads, wherein A2 / A1≧1.20, where A1 is the cross-sectional area of ​​a cross section perpendicular to the thickness direction of the expanded polyolefin resin beads and A2 is the area of ​​a region enclosed by an envelope line connecting each end of the expanded polyolefin resin beads in the cross section; and LL / D1 of the expanded polyolefin resin beads is 0.70 to 1.20, and LS / LL is 0.50 to 0.80, where D1 is the diameter of the smallest circle among circles that contain the entire cross-sectional shape of the cross section and inscribe at least two of the end portions in the circle; LL is the length of the longest part in the thickness direction on the surface of the expanded polyolefin resin beads in the thickness direction; and LS is the length of the shortest part in the thickness direction on the surface of the expanded polyolefin resin beads in the thickness direction.

[0146] [2] The expanded polyolefin resin beads according to [1], wherein the cross-sectional shape perpendicular to the thickness direction of the expanded polyolefin resin beads is one or more shapes selected from the group consisting of U-shape, L-shape, V-shape, Y-shape, X-shape, star-shape and W-shape.

[0147] [3] The expanded polyolefin resin beads according to [1] or [2], wherein the cross section perpendicular to the thickness direction of the expanded polyolefin resin beads has 4 to 8 ends.

[0148] [4] The expanded polyolefin resin beads according to any one of [1] to [3], which have two melting peaks in a DSC curve obtained by differential scanning calorimetry, and when the heat of fusion of the low-temperature peak is Ql (J / g) and the heat of fusion of the high-temperature peak is Qh (J / g), {Qh / (Ql+Qh)} x 100 is 20.0% to 35.0%.

[0149] [5] Expanded polyolefin resin particles according to any one of [1] to [4], having a bulk density of 10.0 g / L to 300.0 g / L.

[0150] [6] Expanded polyolefin resin particles according to any one of [1] to [5], having an average cell diameter of 100 μm to 350 μm.

[0151] [7] The expanded polyolefin resin beads according to any one of [1] to [6], wherein LS2 / LL of the expanded polyolefin resin beads is greater than 0.50 and not greater than 0.80. Here, LS2 is the thickness direction length of the expanded polyolefin resin beads on the thickness direction surface, and is shorter than the LL and longer than the LS.

[0152] [8] The expanded polyolefin resin particles according to any one of [1] to [7], wherein the expanded polyolefin resin particles are formed by expanding polyolefin resin particles, and the polyolefin resin particles contain 25 parts by weight or less of a resin particle additive per 100 parts by weight of the polyolefin resin contained in the polyolefin resin particles.

[0153] [9] A polyolefin resin foam molded article obtained by molding the expanded polyolefin resin beads according to any one of [1] to [8].

[0154]

[10] An extrusion process in which a polyolefin resin composition containing a polyolefin resin is extruded through one or more discharge holes provided in a die of an extruder; a cooling process in which the extruded polyolefin resin composition is solidified by cooling using a cooling device equipped with a cooling medium; a taking-up process in which the solidified polyolefin resin composition is taken up into a gas phase using a take-up machine; a chopping process in which the taken-up polyolefin resin composition is chopped to obtain polyolefin resin particles having an L2 / D2 ratio of 1.8 to 5.0; a dispersion preparation process in which the polyolefin resin particles are mixed with a dispersion medium and a foaming agent in a container to prepare a dispersion; and a discharging process in which the dispersion is discharged into a pressure region lower than the internal pressure of the container to obtain expanded polyolefin resin particles, wherein A4 / A3 is greater than or equal to 1.2, where A3 is the area of ​​a cross section perpendicular to the extrusion direction of the one or more discharge holes, and A4 is the area of ​​a region enclosed by an envelope connecting each end of the discharge holes in the cross section. the distance between the surface of the one or more discharge holes and the surface of the cooling medium facing the one or more discharge holes is 20 mm to 200 mm, and the stretching ratio calculated from the following formula (1) is 1.2 to 4.0; the stretching ratio = take-up speed / resin linear speed formula (1), where the take-up speed is the take-up speed of the polyolefin resin composition by the take-up machine per unit time in the take-up step, and the resin linear speed is a value calculated from the following formula (2); the resin linear speed = discharge rate / (density of the polyolefin resin composition x total area of ​​the discharge holes) formula (2), where the discharge rate is the amount of the polyolefin resin composition extruded from the extruder per unit time in the extrusion step, and the total area of ​​the discharge holes is the sum of the areas of the one or more discharge holes provided in the die provided in the extruder; A method for producing expanded polyolefin resin particles, wherein L2 in the L2 / D2 is the thickness-wise length of the polyolefin resin particle on the thickness-wise plane, and D2 is the diameter of the smallest circle among circles that have all of the cross-sectional shapes of the polyolefin resin particle in a cross section perpendicular to the thickness direction within the circle, and in which at least two of the ends of the polyolefin resin particle are inscribed.

[0155]

[11] The above A3 is 1.0 mm ２ ~6.0mm ２

[11] The method for producing expanded polyolefin resin beads according to

[10] ,

[0156]

[12] The method for producing expanded polyolefin resin beads according to

[10] or

[11] , wherein the discharge hole has a slit width of 0.05 mm to 0.80 mm.

[0157]

[13] The method for producing expanded polyolefin resin beads according to any one of

[10] to

[12] , wherein the discharge hole has 4 to 8 slits.

[0158]

[14] The method for producing expanded polyolefin resin beads according to any one of

[10] to

[13] , wherein the surface of the discharge hole and the surface of the cooling medium are arranged parallel to each other.

[0159]

[15] The method for producing expanded polyolefin resin beads according to any one of

[10] to

[14] , wherein the die is arranged so that the surface of the discharge hole forms an angle of 70° with the surface of the cooling medium, and the length between the endpoints of a parabola drawn by the polyolefin resin composition discharged from the discharge hole before it comes into contact with the cooling medium is the distance between the surface of the one or more discharge holes and the surface of the cooling medium opposite to said surface, and said distance is 20 mm to 200 mm.

[0160]

[16] A method for producing a polyolefin resin foamed molded article, comprising a step of molding the expanded polyolefin resin beads produced by the method for producing expanded polyolefin resin beads according to any one of

[10] to

[15] .

[0161]

[17] Expanded polyolefin resin particles according to any one of [1] to [8], wherein D1 is 0.3 mm to 3.0 mm.

[0162]

[18] The expanded polyolefin resin particles according to any one of [1] to [8] and

[17] , wherein the LL is 1.0 mm to 10 mm.

[0163]

[19] The polyolefin resin foam molded article according to [9], having a maximum sound absorption coefficient of 0.85 or more in the sound absorption frequency range of 700 to 1300 Hz when the thickness is 40 mm.

[0164]

[20] The method for producing expanded polyolefin resin beads according to any one of

[10] to

[15] , wherein there are two or more discharge holes, and each discharge hole satisfies the requirement of A4 / A3≧1.2.

[0165]

[21] The method for producing expanded polyolefin resin beads according to any one of

[10] to

[15] and

[20] , wherein the cross-sectional shape has two or more ends.

[0166]

[22] The method for producing expanded polyolefin resin beads according to any one of

[10] to

[15] ,

[20] and

[21] , wherein the cooling medium is water.

[0167]

[23] The method for producing expanded polyolefin resin beads according to any one of

[10] to

[15] and

[20] to

[22] , wherein the amount of the dispersion medium used is 200 parts by weight or more per 100 parts by weight of the polyolefin resin beads.

[0168]

[24] The method for producing expanded polyolefin resin beads according to any one of

[10] to

[15] and

[20] to

[23] , wherein the blowing agent is one or more blowing agents selected from the group consisting of aliphatic hydrocarbons, halogenated hydrocarbons, inorganic gases, and water.

[0169]

[25] The method for producing expanded polyolefin resin beads according to any one of

[10] to

[15] and

[20] to

[24] , wherein a dispersant is used in the dispersion preparation step.

[0170]

[26] The method for producing expanded polyolefin resin beads according to any one of

[10] to

[15] and

[20] to

[25] , wherein in the discharging step, the dispersion is passed through an orifice having a diameter of 2 mm to 10 mm and discharged into a pressure region lower than the internal pressure of the container.

[0171]

[27] The method for producing expanded polyolefin resin beads according to any one of

[10] to

[15] and

[20] to

[26] , wherein the polyolefin resin particles contain 25 parts by weight or less of a resin particle additive per 100 parts by weight of the polyolefin resin contained in the polyolefin resin particles.

[0172] Hereinafter, one embodiment of the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0173] The substances used in the examples and comparative examples are as follows.

[0174] <Polyolefin Resin> A 1-butene / ethylene / propylene random copolymer (melting point 149° C., 1-butene content 3.8% by weight, ethylene content 0.5% by weight, melt index (MI)=10.1 g / 10 min) was used as a polypropylene resin.

[0175] Further, as the polyethylene resin, linear low-density polyethylene (melting point 123°C, 1-methylpentene content 8.2 wt%, melt index (MI) = 2.0 g / 10 min (190°C), true density 0.926 g / cm ３ ) was used.

[0176] <Resin particle additives> Glycerin: Purified glycerin D (manufactured by Lion Corporation) Talc: Talc Powder (registered trademark) PK-S (manufactured by Hayashi Kasei Co., Ltd.) Hindered amine light stabilizer (HALS): Tinuvin (registered trademark) 622 (manufactured by BASF Japan Ltd.).

[0177] The evaluations in the examples and comparative examples were carried out by the following methods.

[0178] <Melting point Tm of polypropylene resin １Measurement> The melting point of the polypropylene-based resin was measured by a DSC method using a differential scanning calorimeter (manufactured by Seiko Instruments Inc., DSC6200 model). The specific operating procedures were as follows (1) to (4): (1) The temperature of 5 mg to 6 mg of polypropylene-based resin was raised from 40.0°C to 220.0°C at a heating rate of 10.0°C / min to melt the polypropylene-based resin; (2) The temperature of the molten polypropylene-based resin was then lowered from 220.0°C to 40.0°C at a heating rate of 10.0°C / min to crystallize the polypropylene-based resin; (3) The temperature of the crystallized polypropylene-based resin was then further raised from 40.0°C to 220.0°C at a heating rate of 10.0°C / min; (4) The temperature of the peak (melting peak) of the DSC curve of the polypropylene-based resin obtained during the second heating (i.e., during (3)) was determined as the melting point of the polypropylene-based resin. In addition, when there are multiple peaks (melting peaks) in the DSC curve of the polypropylene-based resin obtained during the second heating by the above-mentioned method, the temperature of the peak (melting peak) with the maximum heat of fusion was taken as the melting point of the polypropylene-based resin.

[0179] The MI of the polypropylene-based resin was a value obtained by measurement using an MI measuring device described in JIS K7210:1999 under the following conditions: orifice diameter: 2.0959±0.005 mmφ, orifice length: 8.000±0.025 mm, load: 2.16 kgf, and temperature: 230°C (230±0.2°C).

[0180] <Surface temperature of polyolefin resin composition discharged into the gas phase from the cooling device> The surface temperature was measured by shredding the polyolefin resin composition discharged into the gas phase from the cooling device using a pelletizer, collecting an appropriate amount of the polyolefin resin composition discharged from the pelletizer in a container, and inserting a rod-shaped thermometer into the polyolefin resin composition. The surface temperature is the temperature listed as "surface temperature" in Tables 1 and 2.

[0181] <Measurement of Average Cell Diameter of Expanded Polyolefin Resin Beads> The average cell diameter of expanded beads was measured by the following methods (1) to (5): (1) Using a razor (high-stainless double-edged blade manufactured by Feather Corporation), the expanded beads were cut so that the cut surface passed through the center of the expanded beads; (2) The cut surface of the obtained expanded beads was observed at a magnification of 50 times using an optical microscope (VHX-5000 manufactured by Keyence Corporation); (3) In the image obtained by observation, a straight line passing through the center or approximately the center of the cut surface of the expanded beads was drawn; (4) (4-1) the number n of cells present on the line was measured, and (4-2) the length of the line segment cut from the line at the intersection of the line with the surface of the expanded beads was measured and defined as the expanded bead diameter L; (5) The average cell diameter of expanded beads ("average cell diameter" in Tables 1 and 2) was calculated using the following formula: average cell diameter (μm) = L / n

[0182] <Measurement of DSC Ratio> The measurement of DSC ratio = {Qh / (Ql+Qh)} × 100(%) was carried out using a differential scanning calorimeter (DSC6200 model, manufactured by Seiko Instruments Inc.) Specifically, the DSC ratio ("high-temperature DSC ratio" in Tables 1 and 2) was calculated from the DSC curve (see FIG. 6) obtained during the first temperature rise when 5 to 6 mg of expanded polyolefin resin beads were heated from 40°C to 220°C at a heating rate of 10°C / min.

[0183] <L2 / D2 of Polypropylene Resin Particles> The L2 was determined by measuring the thickness direction length of the polypropylene resin particle on a thickness direction surface using an optical microscope (Keyence VHX-8000) at 40x magnification and measuring the distance between two points on a still image using the measurement function of the device. Hereinafter, the measurement function of the same device was used for measuring length and area. The D2 was defined as the diameter of the smallest circle among circles that contain the entire cross-sectional shape of the polypropylene resin particle in a cross section perpendicular to the thickness direction, and in which at least two of the ends of the polypropylene resin particle are inscribed.

[0184] In Examples 1 to 6 and Comparative Examples 1 to 3, a die having discharge holes with a star-shaped cross section was used to produce polypropylene-based resin particles having a star-shaped cross section (five-baseline shape) in a cross section perpendicular to the thickness direction. In Comparative Example 4, a die having discharge holes with a cross section called a 1C shape was used. The shape of the discharge holes is shown in Figure 12. The polypropylene-based resin particles obtained using these discharge holes had a macaroni structure (hollow tubular structure). In Comparative Example 5, a die having discharge holes with a circular cross section was used. The polypropylene-based resin particles obtained using these discharge holes had a circular cross section.

[0185] The diameter of the smallest circle among the circles that have the entire cross-sectional shape of the produced polypropylene resin particle in a cross section perpendicular to the thickness direction and inscribe at least two ends of the polypropylene resin particle in the circle was determined using an optical microscope (Keyence VHX-8000) taken at 50x magnification.

[0186] Fig. 10 is a view from one direction of an example of polypropylene-based resin particles produced using an extrusion hole having a cross section perpendicular to the extrusion direction with the shape shown in Fig. 7. For the polypropylene-based resin particles shown in Fig. 10, a circle can be set in which five ends are inscribed in a circle, and therefore the DP shown in Fig. 10 becomes the above-mentioned D2. When it is not possible to obtain a circle in which all ends are inscribed in a circle, the above-mentioned D2 is set as explained using Fig. 4 in (2-2. Shape of expanded polyolefin resin beads). In this way, L2 / D2 for 10 polypropylene-based resin particles was calculated, and the average value thereof was taken as the L2 / D2 of the polypropylene-based resin particles.

[0187] The polyolefin resin particles produced in Comparative Examples 4 and 5 did not have protrusions, and the cross-sectional shape was a hollow circle in Comparative Example 4 and a circle in Comparative Example 5. In Comparative Examples 4 and 5, the diameter of the circle was defined as D2.

[0188] <A4 / A3 of the ejection holes> In Examples 1 to 6, the shape shown in FIG. 7 has five slits, and the nozzle area per hole is 2.0 mm ２A die having five discharge holes with a slit width of 0.32 mm was used.

[0189] A3 is the area of ​​the cross section perpendicular to the extrusion direction of the discharge hole, and ２ A4 corresponds to the area of ​​the region surrounded by an envelope connecting each end of the discharge hole in the cross section. The envelope was set by connecting the ends with tangent lines. A4 was determined by area measurement using an image obtained by capturing an image using an optical microscope (Keyence VHX-8000).

[0190] The calculated A4 size is 2.0 mm ２ The A4 / A3 values ​​for the above five holes were all the same.

[0191] In Comparative Examples 1 to 3, discharge holes having the shape, A3, slit width and number of slits shown in Table 1 were used. The obtained polypropylene resin particles were star-shaped, and A4 / A3 was calculated in the same manner as in Examples 1 to 5.

[0192] In Comparative Examples 4 and 5, discharge holes having the shape, A3, slit width, and number of slits shown in Table 1 were used. In Comparative Example 5, since the cross-sectional shape of the discharge hole was circular, (i) there was no slit width, and A3 = A4, so A4 / A3 was 1. Furthermore, in Comparative Example 4, a discharge hole having the shape shown in FIG. 12 (1C shape) was used. The outer periphery of the 1C shape is not a perfect circle, but rather has a portion of the circle interrupted as shown in FIG. 12. In the 1C shape of Comparative Example 4, an envelope was set to connect the interrupted portions of the outer periphery to form a circle. Note that in the 1C shape of Comparative Example 4, there are two circles obtained by connecting the interrupted portions of the outer periphery, but the line forming the circle with the larger diameter was used as the envelope, and the area of ​​the circle with the larger diameter was set as A4. Specifically, in Comparative Example 4, the cross section of the discharge hole was photographed using an optical microscope (VHX-8000 manufactured by Keyence Corporation) to obtain an image, and in the image, an envelope was set so as to connect the periphery of the cross section to form a circle with a large diameter, and the area of ​​the circle was measured and designated as A4.

[0193] <LL / D1 and LS / LL of Expanded Polypropylene Resin Beads> The LL was determined by measuring the length of the longest part in the thickness direction on the surface of the expanded polypropylene resin beads in the thickness direction, using an optical microscope (VHX-8000 manufactured by Keyence Corporation) between two points. The LS was determined by measuring the length of the shortest part in the thickness direction on the surface of the expanded polypropylene resin beads in the thickness direction, using an optical microscope (VHX-8000 manufactured by Keyence Corporation) between two points. The LL and LS are the lengths of the portions shown as LL and LS in Figures 1 and 5, for example. The LS / LL of 10 expanded polypropylene resin beads was determined, and the average value was taken as the LS / LL of the expanded polypropylene resin beads.

[0194] The above D1 is the diameter of the smallest circle among circles that have all of the cross-sectional shapes of the polypropylene-based resin expanded beads in a cross section perpendicular to the thickness direction and in which at least two of the ends of the expanded beads are inscribed.

[0195] In the examples and comparative examples, polypropylene-based resin particles were produced having a star-shaped cross section perpendicular to the thickness direction. Therefore, the expanded polypropylene-based resin particles were also star-shaped. For the produced expanded polypropylene-based resin particles, a circle with the smallest diameter was determined among circles that contained all of the cross sections perpendicular to the thickness direction and inscribed at least two of the expanded beads' ends. The diameter of the circle was measured using an optical microscope (Keyence VHX-8000). When it was possible to determine a circle inscribed with all five ends of the expanded polypropylene-based resin beads, the diameter of the circle was designated D1. When it was not possible to obtain a circle inscribed with all ends, D1 was determined as described in (2-2. Shape of Expanded Polyolefin Resin Beads) using Figure 4. The LL / D1 of 10 expanded polypropylene-based resin beads was calculated in this manner, and the average value was designated as the LL / D1 of the expanded polypropylene-based resin beads.

[0196] The expanded polyolefin resin beads produced in Comparative Examples 4 and 5 did not have any protrusions, and the cross-sectional shape was a hollow circle in Comparative Example 4 and a circle in Comparative Example 5. In Comparative Examples 4 and 5, the diameter of the circle was designated as D1.

[0197] <A2 / A1 of Expanded Polypropylene Resin Beads> A1 is the cross-sectional area of ​​a cross section perpendicular to the thickness direction of the expanded polypropylene resin bead, and A2 is the area of ​​the region enclosed by an envelope connecting each end of the expanded polypropylene resin bead in the cross section. As shown in Figures 1 and 5, the area A1 of a cross section (Figure 2) obtained by cutting the expanded polypropylene resin bead along line A-A' passing through the center of the expanded polypropylene resin bead in the thickness direction was determined by area measurement using an image obtained by capturing an image using an optical microscope (VHX-8000, manufactured by Keyence Corporation). In addition, an envelope was defined by connecting each end of the expanded polypropylene resin bead with a tangent (straight line), and the area A2 of the region enclosed by the envelope (Figure 3) was determined by area measurement using an image obtained by capturing an image using an optical microscope (VHX-8000, manufactured by Keyence Corporation), to calculate A2 / A1. In this way, A2 / A1 of 10 expanded polypropylene resin beads was determined, and the average value was taken as A2 / A1 of the expanded polypropylene resin beads.

[0198] The expanded polyolefin resin beads produced in Comparative Examples 4 and 5 did not have any protrusions, and the cross-sectional shape was a hollow circle in Comparative Example 4 and a circle in Comparative Example 5. In these cases, it was not possible to set an envelope, and therefore "no protrusions" was entered in the A2 / A1 column of Table 1 described later.

[0199] <Bulk Density of Expanded Polypropylene Resin Beads> The bulk density of expanded polypropylene resin beads was measured by the following methods (1) to (3): (1) The expanded polypropylene resin beads were conditioned at room temperature and normal pressure for 4 hours or more, and the volume V1 (cm ３ (2) The powder surface (top end) of the container was scraped off, and the weight W1 (g) of the expanded beads in the container was measured. (3) The bulk density of the expanded beads was calculated using the following formula: Bulk density of expanded beads (g / L) = Weight W1 (g) of expanded beads / {Volume of container V1 (cm３ ) / 1000}.

[0200] <Density of Polypropylene-Based Resin Foam Molded Article> First, a polypropylene-based resin foam molded article was submerged in water, and the volume V (cm ) of the polypropylene-based resin foam molded article was calculated from the rise in the water surface. ３ ) was calculated. Next, the weight W (g) of the foam molded article was measured. The obtained V and W were substituted into the following formula to calculate the density of the polypropylene resin foam molded article (molded article density in Tables 1 and 2): Density of foam molded article (g / L) = (W / V) / 1000.

[0201] <Porosity of Polypropylene-Based Resin Foam Molded Article> A Φ29 x 40 mm test piece was cut out from a polypropylene-based resin foam molded article (length 370 mm x width 320 mm x thickness 50 mm) prepared for porosity measurement to measure normal incident sound absorption coefficient. For the test piece, Vc was measured according to the method described in Procedure C of ASTM D2856-87, and the porosity (%) was calculated according to the following formula: Porosity (%) = {(Va - Vc) x 100} / Va.

[0202] Here, the volume Vc (cm ３ ) is the volume of the test piece (Φ29 × 40 mm) obtained by measurement using an air comparison type hydrometer Model 1000 manufactured by Tokyo Science Co., Ltd. Also, the volume Va (cm ３ ) is the apparent volume calculated by measuring the diameter D (cm) and height H (cm) of the test piece (Φ29 × 40 mm) with a vernier caliper after measuring Vc with the air comparison type hydrometer, using the following formula: Volume Va (cm ３ ) = πD ２ H / 4.

[0203] <Sound absorption coefficient of polypropylene-based resin foam molded body> In accordance with JIS A1405, the normal incidence sound absorption coefficient was measured at 500 Hz to 6400 Hz for a sample thickness of 40.0 mm. A sample with a diameter of 29 mm and a thickness of 50.0 mm was cut out from a polypropylene-based resin foam molded body (370 mm length × 320 mm width × 50 mm thickness). Both skin surfaces of this sample were cut to adjust the thickness to 40.0 mm to prepare a normal incidence sound absorption coefficient measurement sample. The normal incidence sound absorption coefficient was measured in a state where the sample was in close contact with a rigid wall that reflects sound waves, i.e., in a state where there was no air behind it. The normal incidence sound absorption coefficient was measured using an SR-4100 normal incidence sound absorption coefficient measuring device manufactured by Ono Sokki Co., Ltd. From the obtained frequency-normal incidence sound absorption coefficient curve, the normal incidence sound absorption coefficient (maximum sound absorption coefficient) at the frequency at which the normal incidence sound absorption coefficient was maximized was read.

[0204] In this specification, "good" sound absorption performance of a molded article means that the maximum sound absorption coefficient of the molded article in the sound absorption frequency range of 700 to 1300 Hz when the molded article is 40 mm thick is 0.85 or more. Furthermore, "poor" sound absorption performance means that the maximum sound absorption coefficient is less than 0.85.

[0205] <Mold Fillability of Polypropylene-Based Resin Foam Molded Articles> Using a storage box 10 with multiple partition plates as shown in FIG. 11 as a mold for evaluating mold fillability, in-mold foam molding was performed five times with the resulting polypropylene-based resin foam beads. The filling of the foam beads in the partition plate areas was visually observed to evaluate mold fillability. Box 10 measured 350 mm long, 320 mm wide, and 180 mm deep, and contained six sets of partition plates parallel to the width direction. Each partition plate had a different shape and size. The thinnest partition plate was partition plate 11, measuring 8 mm thick at the bottom and 5 mm thick at the top. The upper portion of partition plate 11 was considered the most difficult area for foam beads to fill, and the particle filling ability of this area was evaluated as follows. The above dimensions are those of the mold. Six foam bead filling ports were provided at the bottom of box 10. After in-mold foam molding, the presence or absence of defects of 10 mm or more was examined. The filling property was evaluated according to the following criteria: Good: 0 foamed molded articles had a defect of 10 mm or more. Poor: 1 or more foamed molded articles had a defect of 10 mm or more.

[0206] Example 1 (Preparation of Polypropylene-Based Resin Particles) 100 parts by weight of polypropylene-based resin, 0.2 parts by weight of glycerin, and 0.1 parts by weight of talc were weighed out and dry-blended using a small tumbler manufactured by O.N. Machine Co., Ltd. to obtain a mixture. The mixture was then melt-kneaded at a resin temperature of 200°C using a twin-screw extruder (TEM26-SX manufactured by Toshiba Machine Co., Ltd.). Next, a strand was extruded at 3 kg / Hr per hole through a die having six discharge holes with a shape shown in FIG. 7, with five slits attached to the tip of the twin-screw extruder. The area and slit width of the discharge holes were as described above.

[0207] The extruded product (strand) was water-cooled in a 2-m-long water tank. The distance from the surface of the discharge hole to the water surface facing the surface was 30 mm, and the water-cooled strand in a water tank with a water temperature of 30°C was taken up and chopped using a pelletizer (manufactured by Ishinaka Iron Works) (chopping process). The distance between all six discharge holes was 30 mm. The strand was taken up so that the take-up speed / resin linear speed = elongation ratio was 2.5, and cut using a pelletizer to produce polyolefin-based resin particles with an L2 / D2 = 2.2. This operation yielded polypropylene-based resin particles (weight per particle: 2.5 mg) having the shape (star-shaped, five-baseline shape) shown in Figure 10.

[0208] (Preparation of expanded polypropylene resin particles) A ​​10 L pressure vessel was charged with 100 parts by weight of the obtained polypropylene resin particles, 475 parts by weight of water as an aqueous dispersion medium, 0.50 parts by weight of kaolin as a dispersant, 0.18 parts by weight of sodium dodecylbenzenesulfonate as a dispersion aid, 0.10 parts by weight of citric acid as a pH adjuster, and carbon dioxide as a foaming agent in amounts according to the foaming pressure to prepare a dispersion containing a foaming agent.

[0209] The obtained dispersion was heated to a foaming temperature (temperature inside the pressure vessel) of 152.5° C. while being stirred. After the temperature and pressure inside the pressure vessel reached the predetermined foaming temperature (152.5° C.) and foaming pressure (2.8 MPa (gauge pressure)) (heating step and pressurizing step), the temperature and pressure inside the pressure vessel were maintained at the predetermined foaming temperature and foaming pressure for an additional 30 minutes (maintaining step).

[0210] Next, carbon dioxide was introduced, and the pressure inside the pressure vessel was maintained at the expansion pressure of 2.8 MPa (gauge pressure). The valve at the bottom of the pressure vessel was opened, and the dispersion inside the pressure vessel was released to atmospheric pressure through a 4.0 mm diameter orifice to obtain expanded polypropylene resin beads (release step). The expanded polypropylene resin beads were dried at 75°C for 15 hours. The resulting expanded polypropylene resin beads showed two peaks derived from the polypropylene resin in the DSC curve obtained by measurement using the DSC method. The bulk density of the expanded polypropylene resin beads was evaluated. The results are shown in Table 1.

[0211] (Preparation of Polypropylene-Based Resin Foam Molded Article) The obtained polypropylene-based resin foam beads were filled into an article storage box 10 (mold for box-shaped molded article) shown in FIG. 11 using a polyolefin in-mold foam molding machine (filling step). The polypropylene-based resin foam beads were heat-fused with steam at 0.28 MPa (gauge pressure), then water-cooled and demolded to obtain an in-mold foam. The obtained in-mold foam molded article was left at room temperature for 1 hour, then cured and dried in a thermostatic chamber at 75°C for 15 hours (drying step), and again left at room temperature for 4 hours, and the filling property was evaluated.

[0212] Furthermore, a polypropylene resin foam molded article (length 370 mm × width 320 mm × thickness 50 mm) was obtained using a Planck mold in the same manner as above, and the maximum sound absorption coefficient and porosity were measured. The results are shown in Table 1.

[0213] [Examples 2 to 5 and Comparative Examples 1 to 5] Expanded beads and molded articles were obtained in the same manner as in Example 1, except that the discharge holes, the production conditions for polypropylene-based resin beads, and the expansion conditions for expanded polypropylene-based resin beads were changed as shown in Table 1. The physical properties of the obtained expanded beads and molded articles were measured and evaluated. The results are shown in Table 1.

[0214] Example 6 (Preparation of Polyethylene-Based Resin Particles) 100 parts by weight of polyethylene-based resin, 0.25 parts by weight of glycerin, 0.2 parts by weight of talc, and 0.1 parts by weight of Tinuvin 622 were weighed out and dry-blended using a small tumbler manufactured by O-N Machine Co., Ltd. to obtain a mixture. The mixture was melt-kneaded at a resin temperature of 280°C using a twin-screw extruder (TEM26-SX manufactured by Toshiba Machine Co., Ltd.). Next, a strand was extruded at 3 kg / Hr per hole through a die having six outlet holes with five slits, as shown in FIG. 7, attached to the tip of the twin-screw extruder. The area and slit width of the outlet holes were as described above.

[0215] The extruded product (strand) was water-cooled in a 2-m-long water tank. The distance from the surface of the discharge hole to the water surface facing the surface was 25 mm. The water-cooled strand in a water tank with a water temperature of 30°C was taken up and chopped using a pelletizer (manufactured by Ishinaka Iron Works) (chopping process). The distance between all six discharge holes was 25 mm. The strand was taken up so that the take-up speed / resin linear speed = elongation ratio was 2.6, and cut using a pelletizer to produce polyolefin-based resin particles with an L2 / D2 ratio of 1.9. This procedure yielded polyethylene-based resin particles (weight per particle: 2.5 mg) having the shape shown in Figure 10 (star-shaped, five-baseline shape).

[0216] (Preparation of expanded polyethylene resin particles) A ​​10 L pressure vessel was charged with 100 parts by weight of the obtained polyethylene resin particles, 475 parts by weight of water as an aqueous dispersion medium, 0.50 parts by weight of kaolin as a dispersant, 0.18 parts by weight of sodium dodecylbenzenesulfonate as a dispersing aid, 0.10 parts by weight of citric acid as a pH adjuster, and carbon dioxide as a foaming agent in amounts according to the foaming pressure to prepare a dispersion containing a foaming agent.

[0217] The obtained dispersion was heated to a foaming temperature (temperature inside the pressure vessel) of 123.7° C. while being stirred. After the temperature and pressure inside the pressure vessel reached the predetermined foaming temperature (123.7° C.) and foaming pressure (3.4 MPa (gauge pressure)) (heating step and pressurizing step), the temperature and pressure inside the pressure vessel were maintained at the predetermined foaming temperature and foaming pressure for an additional 30 minutes (maintaining step).

[0218] Next, carbon dioxide was introduced, and the pressure inside the pressure vessel was maintained at the foaming pressure of 3.4 MPa (gauge pressure). The valve at the bottom of the pressure vessel was opened, and the dispersion inside the pressure vessel was released to atmospheric pressure through a 4.0 mm diameter orifice to obtain polyethylene-based resin foamed beads (release step). The polyethylene-based resin foamed beads were dried at 75°C for 15 hours. The resulting polyethylene-based resin foamed beads showed two peaks derived from the polypropylene-based resin in the DSC curve obtained by measurement using the DSC method. The bulk density of the resulting polyethylene-based resin foamed beads was evaluated. The results are shown in Table 2.

[0219] The resulting polyethylene-based resin foamed beads were subjected to a second-stage expansion. Specifically, the polyethylene-based resin foamed beads were placed in a pressure-resistant container, and the polyethylene-based resin foamed beads were impregnated with air to an internal pressure of 3.1 atm. Next, the air-impregnated polyethylene-based resin foamed beads were heat-treated in the pressure-resistant container with water vapor at a pressure of 0.04 MPa for 30 seconds, and the pressure was released after 30 seconds, thereby performing a second-stage expansion. The polyethylene-based resin foamed beads that underwent the second-stage expansion are also referred to as "second-stage expanded beads." The bulk density, LL / D1, LS / LL, and A2 / A1 of the second-stage expanded beads are shown in Table 2.

[0220] (Preparation of polyethylene-based resin foam molded article) The second-stage foamed beads were filled into a storage box 10 (a mold for a box-shaped molded article) shown in FIG. 11 using a polyolefin in-mold foam molding machine without air impregnation (filling step). The second-stage foamed beads were heat-fused with steam at 0.07 MPa (gauge pressure), then water-cooled and demolded to obtain an in-mold foam. The obtained in-mold foam molded article was left at room temperature for 1 hour, then cured and dried in a thermostatic chamber at 75°C for 15 hours (drying step), and then left at room temperature for another 4 hours, and the filling property was evaluated.

[0221] Furthermore, a foamed molded article (370 mm long x 320 mm wide x 50 mm thick) was obtained from the second-stage expanded beads using a Planck mold in the same manner as above, and the maximum sound absorption coefficient and porosity were measured. The results are shown in Table 2.

[0222] The expanded polypropylene resin beads obtained in Examples 1 to 5 were expanded beads produced by the present production method and correspond to the present expanded beads. As shown in Table 1, the expanded polypropylene resin beads have good sound absorption properties and good mold fillability ("fillability" in the table).

[0223] The expanded polyethylene resin beads obtained in Example 6 are also expanded beads produced by the present production method and correspond to the present expanded beads. As shown in Table 2, the expanded polyethylene resin beads have good sound absorption properties and good mold filling properties ("filling property" in the table).

[0224] On the other hand, the expanded polypropylene resin beads of Comparative Example 1 were obtained by a method in which the elongation ratio and L2 / D2 did not satisfy the requirements of the present production method, and the LL / D1 and LS / LL of these particles did not satisfy the requirements of the present expanded beads.

[0225] The expanded polypropylene resin beads of Comparative Example 2 were obtained by a method that did not satisfy the requirements of the present manufacturing method in terms of the elongation ratio, L2 / D2, and the distance between the surface of the discharge hole and the surface of the cooling medium facing that surface ("Distance between the surface of the discharge hole and the water surface" in the table). The LL / D1 and LS / LL of these particles did not satisfy the requirements of the expanded beads of the present invention.

[0226] The expanded polypropylene resin beads of Comparative Example 3 were obtained by a method in which the L2 / D2 ratio and the distance between the surface of the nozzle hole and the water surface did not satisfy the requirements of the present manufacturing method. The LS / LL ratio of these particles did not satisfy the requirements of the expanded beads of the present invention.

[0227] The expanded polypropylene resin beads of Comparative Example 4 were obtained by a method in which the stretching ratio and the distance between the surface of the nozzle hole and the water surface did not satisfy the requirements of the present manufacturing method, and the A2 / A1 and LS / LL ratios of the expanded polypropylene resin beads did not satisfy the requirements of the present manufacturing method.

[0228] In Comparative Example 5, L2 / D2, the elongation ratio, and the distance between the surface of the nozzle hole and the water surface satisfy the requirements of the present manufacturing method, but because a circular nozzle hole is used, A2 / A1 does not satisfy the requirements of the present invention, and LL / D1 and LS / LL also do not satisfy the requirements of the present invention.

[0229] The expanded polypropylene resin beads obtained in the comparative examples did not satisfy any of the requirements for the expanded beads, and were therefore "unacceptable" in both sound absorption properties and mold fillability.

[0230] As described above, the results of the Examples and Comparative Examples show that by satisfying the requirements of the present expanded beads, expanded polyolefin resin beads having excellent sound absorption properties and mold fillability can be obtained. Furthermore, it is clear that the above-mentioned expanded polyolefin resin beads can be produced by a production method that satisfies the requirements of the present production method.

[0231] One embodiment of the present invention can provide expanded polyolefin resin particles that have excellent sound absorption properties and mold fillability, and therefore can be suitably used in a variety of applications, such as cushioning packaging materials, logistics materials, heat insulation materials, civil engineering and construction materials, and automotive components.

[0232] 10 ... Item storage box 11 ... Partition plate

Claims

1. Expanded polyolefin resin beads, wherein A2 / A1≧1.20, where A1 is the cross-sectional area of ​​a cross section perpendicular to the thickness direction of the expanded polyolefin resin beads and A2 is the area of ​​a region enclosed by an envelope connecting each end of the expanded polyolefin resin beads in the cross section; and LL / D1 of the expanded polyolefin resin beads is 0.70 to 1.20, and LS / LL is 0.50 to 0.80, where D1 is the diameter of the smallest circle among circles that contain the entire cross-sectional shape of the cross section and inscribe at least two of the end portions in the circle; LL is the length of the longest part in the thickness direction on the surface of the expanded polyolefin resin beads in the thickness direction; and LS is the length of the shortest part in the thickness direction on the surface of the expanded polyolefin resin beads in the thickness direction.

2. The expanded polyolefin resin beads according to claim 1, wherein the cross section perpendicular to the thickness direction of the expanded polyolefin resin beads has 4 to 8 ends.

3. The expanded polyolefin resin beads according to claim 1, which have two melting peaks in a DSC curve obtained by differential scanning calorimetry, and where the heat of fusion of the lower-temperature peak is Ql (J / g) and the heat of fusion of the higher-temperature peak is Qh (J / g), {Qh / (Ql+Qh)} x 100 is 20.0% to 35.0%.

4. The expanded polyolefin resin particles according to claim 1, having a bulk density of 10.0 g / L to 300.0 g / L.

5. The expanded polyolefin resin particles according to claim 1, wherein the average cell diameter is 100 μm to 350 μm.

6. The expanded polyolefin resin beads according to claim 1, wherein the LS2 / LL of the expanded polyolefin resin beads is greater than 0.50 and not greater than 0.

80. Here, LS2 is the thickness direction length of the expanded polyolefin resin beads on the thickness direction surface, and is shorter than the LL and longer than the LS.

7. The expanded polyolefin resin particles according to claim 1, wherein the expanded polyolefin resin particles are formed by expanding polyolefin resin particles, and the polyolefin resin particles contain 25 parts by weight or less of a resin particle additive per 100 parts by weight of the polyolefin resin contained in the polyolefin resin particles.

8. A polyolefin resin foam molded article obtained by molding the polyolefin resin foam beads according to any one of claims 1 to 7.

9. An extrusion process comprising: an extrusion step of extruding a polyolefin resin composition containing a polyolefin resin through one or more discharge holes provided in a die of an extruder; a cooling step of solidifying the extruded polyolefin resin composition by cooling it using a cooling device equipped with a cooling medium; a take-up step of taking up the solidified polyolefin resin composition into a gas phase using a take-up machine; a chopping step of chopping the taken-up polyolefin resin composition to obtain polyolefin resin particles having an L2 / D2 ratio of 1.8 to 5.0; a dispersion preparation step of mixing the polyolefin resin particles with a dispersion medium and a foaming agent in a container to prepare a dispersion; and a release step of releasing the dispersion into a pressure region lower than the internal pressure of the container to obtain expanded polyolefin resin particles, wherein A4 / A3 is greater than or equal to 1.2, where A3 is the area of ​​a cross section perpendicular to the extrusion direction of the one or more discharge holes and A4 is the area of ​​a region enclosed by an envelope connecting the ends of the discharge holes in the cross section. the distance between the surface of the one or more discharge holes and the surface of the cooling medium facing the one or more discharge holes is 20 mm to 200 mm, and the stretching ratio calculated from the following formula (1) is 1.2 to 4.0; the stretching ratio = take-up speed / resin linear speed formula (1), where the take-up speed is the take-up speed of the polyolefin resin composition by the take-up machine per unit time in the take-up step, and the resin linear speed is a value calculated from the following formula (2); the resin linear speed = discharge rate / (density of the polyolefin resin composition x total area of ​​the discharge holes) formula (2), where the discharge rate is the amount of the polyolefin resin composition extruded from the extruder per unit time in the extrusion step, and the total area of ​​the discharge holes is the sum of the areas of the one or more discharge holes provided in the die provided in the extruder; A method for producing expanded polyolefin resin particles, wherein L2 in the L2 / D2 is the thickness-wise length of the polyolefin resin particle on the thickness-wise plane, and D2 is the diameter of the smallest circle among circles that have all of the cross-sectional shapes of the polyolefin resin particle in a cross section perpendicular to the thickness direction within the circle, and in which at least two of the ends of the polyolefin resin particle are inscribed.

10. The above A3 is 1.0 mm ２ ~6.0mm ２ The method for producing expanded polyolefin resin beads according to claim 9, wherein 11. The method for producing expanded polyolefin resin beads according to claim 9, wherein the discharge hole has a slit width of 0.05 mm to 0.80 mm.

12. The method for producing expanded polyolefin resin beads according to claim 9, wherein the discharge hole has 4 to 8 slits.

13. The method for producing expanded polyolefin resin beads according to claim 9, wherein the surface of the discharge hole and the surface of the cooling medium are arranged parallel to each other.

14. A method for producing expanded polyolefin resin beads according to claim 9, wherein the die is positioned so that the surface of the discharge holes forms an angle of 70° with the surface of the cooling medium, and the length between the endpoints of a parabola drawn by the polyolefin resin composition discharged from the discharge holes before it comes into contact with the cooling medium is the distance between the surface of the one or more discharge holes and the surface of the cooling medium opposite to said surface, and said distance is 20 mm to 200 mm.

15. A method for producing a polyolefin resin foam molded article, comprising a step of molding the polyolefin resin foam beads produced by the method for producing polyolefin resin foam beads described in any one of claims 9 to 14.

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