Thermoplastic resin expanded particles and thermoplastic resin expanded particle molded body

Abstract

This invention provides thermoplastic resin foamed particles that can shorten the cooling time in the molding die and obtain a molded body with a desired shape, appearance, and excellent rigidity even without a curing process, as well as a molded body of thermoplastic resin foamed particles composed of the foamed particles. The foamed particle (1) has a foamed layer (2) comprising thermoplastic resin. The foamed particle (1) has a cylindrical shape and has two to eight through holes (11) extending along its axial direction. The ratio Ct of the total cross-sectional area of ​​the through holes in the cut surface obtained by cutting the foamed particle (1) perpendicular to the axial direction at its center, to the cross-sectional area A of the foamed particle (1), is 0.02 to 0.15.

Description

Technical Field

[0001] This invention relates to thermoplastic resin foamed granules and thermoplastic resin foamed granule molded articles. Background Technology

[0002] Polypropylene resin foamed granules are lightweight and possess excellent cushioning and rigidity, making them suitable for various applications. They are manufactured, for example, by an in-mold molding method, where polypropylene resin foamed granules are filled into a mold and then heated by supplying steam to the mold. In this method, the foamed granules undergo secondary foaming when steam is supplied to the mold, and their surfaces melt simultaneously. This causes the foamed granules within the mold to fuse together, resulting in a molded body with a shape corresponding to the shape of the mold cavity. The freshly molded body expands easily due to secondary foaming and is then cooled within the mold using water, air, or other means before being demolded.

[0003] For example, Patent Document 1 discloses a technique for in-mold molding of foamed particles made from polypropylene resin with its melting point, melt flow index, and Z-average molecular weight adjusted to a specific range. Patent Document 2 discloses a technique for in-mold molding of cylindrical foamed particles with internal through-holes.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2000-129028

[0007] Patent Document 2: Japanese Patent Application Publication No. 7-137064 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] In the aforementioned manufacturing process of the molded body, if the foamed granule molded body is stored at room temperature after being demolded, the steam flowing into the bubbles of the foamed granule molded body during molding will condense within the bubbles, creating a negative pressure inside the bubbles. As a result, the foamed granule molded body may sometimes shrink in volume and deform significantly. Therefore, after demolding the foamed granule molded body from the mold, a curing process is performed by placing it in a high-temperature atmosphere adjusted to, for example, around 60°C to 80°C for a predetermined time to allow the shape of the foamed granule molded body to recover.

[0010] However, since the curing process requires equipment investment and labor, it is desirable to omit the curing process to significantly improve the productivity of foamed granule moldings.

[0011] Furthermore, with increasing environmental awareness in recent years, there is a desire to reduce energy consumption in the manufacturing process of molded parts. Based on this perspective, it is desirable to shorten the cooling time within the molding die while eliminating the curing process.

[0012] However, while the foamed granules described in Patent Document 1 can shorten the curing process, a curing process is still required. If the curing process is omitted, the foamed granule molded body will shrink and deform significantly, making it difficult to obtain a foamed granule molded body with the desired shape. Furthermore, there is room for improvement in shortening the cooling time within the molding die using the foamed granules described in Patent Document 1. Moreover, because the foamed granules described in Patent Document 1 require the use of special raw materials, there is room for improvement in raw material procurement.

[0013] Although the foamed particles described in Patent Document 2 can shorten the cooling time in the molding mold, the appearance of the molded body is significantly worse and the rigidity is insufficient because voids originating from the through holes of the foamed particles are formed on the surface of the obtained molded body.

[0014] The present invention was made in view of the above background and is intended to provide thermoplastic resin foam particles that can shorten the cooling time in the molding die and obtain a foam particle molded body with a desired shape, appearance and excellent rigidity even if the curing process is omitted, as well as thermoplastic resin foam particle molded bodies composed of the foam particles.

[0015] means for solving problems

[0016] One aspect of the present invention is the thermoplastic resin foamed particles described below [1] to [8].

[0017] [1] A thermoplastic resin foamed granule having a thermoplastic resin foamed layer, wherein,

[0018] The foamed particles have a cylindrical shape and have two to eight through holes extending along their axial direction.

[0019] The ratio of the total cross-sectional area Ct of the through holes in the cut surface obtained by cutting the foamed particle perpendicularly to its axial center to the cross-sectional area A of the foamed particle, Ct / A, is 0.02 or more and 0.15 or less.

[0020] [2][1] The thermoplastic resin foamed particles, wherein the ratio of the cross-sectional area Ca of each through hole to the cross-sectional area A of the foamed particles in the cut surface obtained by cutting the foamed particles perpendicular to the axial direction at the center is 0.005 or more and 0.05 or less.

[0021] The thermoplastic resin foamed particles described in [3][1] or [2], wherein the foamed particles have 4 or more but no more than 8 through holes.

[0022] The thermoplastic resin foamed particles described in any one of [4][1] to [3], wherein the diameter d of the through hole in the cut surface obtained by cutting the foamed particles perpendicular to the axial direction at the center of the axial direction is 0.1 mm or more and 0.5 mm or less.

[0023] The thermoplastic resin foamed particles described in any one of [5][1] to [4], wherein the ratio R / d of the through hole spacing R of the foamed particles to the hole diameter d of the through holes in the cut surface obtained by cutting the foamed particles perpendicular to the axial direction at the center of the axial direction is 2.0 or more and 4.5 or less.

[0024] The thermoplastic resin foamed granules described in any one of [6][1] to [5], wherein the bulk density of the foamed granules is 10 kg / m³. 3 Above 50kg / m 3 Hereinafter, the ratio of the apparent density to the bulk density of the foamed particles is 1.7 to 1.9.

[0025] The thermoplastic resin foamed particles described in any one of [7][1] to [6], wherein the thermoplastic resin constituting the foamed layer is an ethylene-propylene random copolymer, the ethylene-propylene random copolymer containing 0.5% by mass and more than 3.5% by mass of ethylene.

[0026] The thermoplastic resin foamed particles according to any one of [8][1] to [7], wherein the foamed particles have a thermoplastic resin coating layer covering the foamed layer, wherein the coating layer is composed of a thermoplastic resin having a melting point or a softening point lower than that of the thermoplastic resin constituting the foamed layer.

[0027] Another aspect of the invention is the thermoplastic resin foamed granule molded body described below [9] to

[10] .

[0028] [9] A thermoplastic resin foamed granule molded body, wherein the thermoplastic resin foamed granule molded body is formed by in-mold molding of thermoplastic resin foamed granules as described in any one of [1] to [8].

[0029]

[10] [9] The thermoplastic resin foamed granule molding body, wherein the open bubble rate of the foamed granule molding body is more than 2% and less than 12%.

[0030] Invention Effects

[0031] According to the above method, it is possible to provide thermoplastic resin foamed particles (hereinafter referred to as "foamed particles") that can shorten the cooling time in the molding die and obtain a foamed particle molded body with a desired shape, appearance and excellent rigidity even if the curing process is omitted, as well as thermoplastic resin foamed particle molded bodies (hereinafter referred to as "foamed particle molded bodies" or "molded bodies") composed of the foamed particles. Attached Figure Description

[0032] Figure 1 This is a 3D view of foamed granules.

[0033] Figure 2 yes Figure 1 The top view along line II-II (a top view of the cut surface of the foamed particles).

[0034] Figure 3 This is an explanatory diagram illustrating the calculation method for the area of ​​a high-temperature peak.

[0035] Figure 4 This is a three-dimensional view of the foamed particles from Example 1.

[0036] Figure 5 yes Figure 4 VV-line sectional view (top view of the cut surface of the foamed particles).

[0037] Figure 6 This is a three-dimensional view of the foamed particles from Example 2.

[0038] Figure 7 yes Figure 6 VII-VII sectional view (top view of the cut surface of the foamed particles).

[0039] Figure 8 This is a three-dimensional view of the foamed particles from Example 4.

[0040] Figure 9 yes Figure 8 IX-IX line sectional view (top view of the cut surface of the foamed particles).

[0041] Explanation of reference numerals in the attached figures

[0042] 1: Foamed granules;

[0043] 11: Through hole;

[0044] 2: Foaming layer;

[0045] 3: Covering layer. Detailed Implementation

[0046] (Thermoplastic resin foam granules)

[0047] The thermoplastic resin foamed granules 1 have a thermoplastic resin foamed layer 2 (hereinafter referred to as "foamed layer 2") mainly composed of thermoplastic resin. For example... Figure 1 As shown, the foamed particle 1 has a cylindrical shape and has two to eight through holes 11 extending axially through its interior. The aforementioned cylindrical shape may include, for example, [example of such a shape]. Figure 1 The shape shown is surrounded by a generally circular bottom surface 12, a top surface 13 disposed above the bottom surface 12 and having a shape substantially the same as the bottom surface 12, and a side peripheral surface 14 connecting the end edge of the bottom surface 12 and the end edge of the top surface 13. The cut surface of the foamed particle 1 obtained by cutting it at the center of the axial direction along a plane perpendicular to the axial direction (see reference). Figure 2 The ratio of the total cross-sectional area Ct of the through holes 11 to the cross-sectional area A of the foamed particles 1, Ct / A, is 0.02 or more and 0.15 or less.

[0048] In this way, by providing two to eight through holes on the cylindrical foamed particles and ensuring that the ratio of the total cross-sectional area Ct of the through holes to the cross-sectional area A of the foamed particles, Ct / A, is within the specified range, the cooling time within the molding die is shortened, and significant shrinkage and deformation of the molded body can be suppressed even without the curing process. Furthermore, the molded body obtained by in-mold forming of the foamed particles exhibits excellent appearance and rigidity.

[0049] The reasons for achieving the aforementioned effects using the foamed particles are as follows: When the foamed particles are in-mold molded, an open-bubble structure composed of open bubbles is formed in the molded body; in other words, a tiny spatial portion communicating with the outside of the molded body is formed. Specifically, the open-bubble structure is formed by the complex connection of gaps formed by interconnected through-holes of multiple foamed particles, gaps formed by interconnected gaps between the through-holes of foamed particles and the gaps between foamed particles, gaps formed by interconnected gaps between foamed particles, and continuous bubble portions of the foamed particles constituting the molded body.

[0050] It is considered that, since the open bubble structure is connected to the outside of the molded body, when the molded body with a suitable open bubble ratio is demolded from the mold, external gas flows rapidly into the bubbles inside the molded body through the open bubble structure. Then, through the inflow of external gas into the bubbles inside the molded body, the overall internal pressure of the molded body can easily and quickly reach parity with the pressure of the atmosphere outside the molded body. It is considered that the above results in the early dimensional stabilization of the molded body, and significant shrinkage and deformation of the molded body can be suppressed even without a curing process.

[0051] Furthermore, since the foamed particles have through-holes, steam can pass through these holes when steam is supplied to the molding die. Therefore, steam can easily reach the interior of the molding die, allowing for easy heating of the foamed particles within the die. Consequently, even at lower molding temperatures during in-mold forming, a molded body with excellent weldability and a good appearance can be obtained. As a result, the heat absorbed by the foamed particles due to steam during in-mold forming can be kept low. Additionally, excessive increases in the internal temperature of the molded body after demolding can be prevented. Consequently, the dimensions of the molded body after in-mold forming tend to stabilize earlier.

[0052] Furthermore, since the foamed particles have multiple through-holes, the secondary foaming property of the foamed particles can be moderately reduced. It is considered that this results in the ability to moderately suppress the outward expansion of the foamed particles during secondary foaming during in-mold forming. Furthermore, by providing multiple through-holes in the foamed particles, the aforementioned open bubble structure is formed in the molded body, increasing the surface area of ​​the small spatial portions originating from the through-holes in the foamed particles within the molded body. It is presumed that this results in efficient cooling of the molded body within the mold and a more rapid reduction in the surface pressure applied to the mold, thus shortening the cooling time within the mold.

[0053] Furthermore, the Ct / A ratio in the foamed particles is within the specified range, and the foamed particles are provided with multiple through holes. Therefore, compared to the case where the number of through holes is one, the pore diameter of each through hole in the foamed particles can be further reduced. It is considered that this can result in improved rigidity of the molded body. Additionally, it is considered that by reducing the pore diameter of each through hole, the through holes are less noticeable on the surface of the molded body, thus improving the appearance.

[0054] When the foamed particles lack through-holes, the molding temperature may easily become too high, and it may be difficult to fully form an open-cell structure in the resulting molded body. Therefore, in this case, it is difficult to suppress significant shrinkage and deformation of the molded body when the curing process is omitted. Furthermore, in this case, there is a risk of excessive secondary foaming of the foamed particles, leading to a longer cooling time within the molding die.

[0055] Furthermore, when the Ct / A ratio in the foamed particles is within the specified range and the number of through-holes is one, the diameter of the through-hole tends to increase compared to the case where the number of through-holes is multiple. Therefore, the surface of the molded body with through-holes after molding may become more noticeable, potentially leading to a deterioration in the appearance of the molded body. Additionally, in this case, the reduction in cooling time within the molding mold may become insufficient. This is because, for example, the surface area of ​​the small spatial portions of the through-holes originating from the foamed particles in the molded body tends to decrease. These problems can be easily avoided by setting the number of through-holes to 2 or more but less than 8, preferably 4 or more but less than 8.

[0056] On the other hand, when the number of through-holes in the foamed particles is nine or more, there is a risk of excessive reduction in the secondary foaming properties of the foamed particles. As a result, there is a risk of reduced rigidity and deterioration in appearance of the molded body.

[0057] When the ratio Ct / A, which is the total cross-sectional area Ct of the through holes to the cross-sectional area A of the foamed particles, is too low, it may be difficult to form an open-bubble structure in the molded body, and a curing process may be required to suppress significant shrinkage and deformation of the molded body. Furthermore, in this case, the secondary foaming property of the foamed particles may become excessively high, and the cooling time may not be sufficiently shortened. By setting the ratio Ct / A to 0.02 or higher, these problems are easily avoided, and significant shrinkage and deformation of the molded body can be suppressed even when the cooling time in the molding die is shortened and a curing process is not performed. From the viewpoint of further shortening the cooling time in the molding die, the ratio Ct / A is preferably 0.03 or higher, more preferably 0.04 or higher, and even more preferably 0.05 or higher.

[0058] On the other hand, if the ratio Ct / A, which is the total cross-sectional area Ct of the through holes to the cross-sectional area A of the foamed particles, is too high, there is a risk of reduced rigidity and deteriorated appearance of the molded article. By setting the ratio Ct / A to 0.15 or less, these problems can be easily avoided, and the rigidity and appearance of the molded article can be improved. From the viewpoint of more reliably obtaining the above-mentioned effects, the ratio Ct / A is preferably 0.13 or less, more preferably 0.11 or less, and even more preferably 0.10 or less.

[0059] From the viewpoint of achieving the effect of suppressing significant shrinkage and deformation of the molded body and shortening the cooling time, while making it easier to obtain molded bodies with good rigidity and appearance, the ratio Ct / A is preferably 0.03 or more and 0.13 or less, and more preferably 0.05 or more and 0.10 or less.

[0060] The calculation method for the cross-sectional area A of the aforementioned foamed particles is as follows. First, the cross-sectional area A of the foamed particles is calculated as follows: Figure 1The foamed particle 1 shown is cut at its center along a plane perpendicular to the axial direction, such that... Figure 2 ( Figure 1 The cut surface of the foamed particle (in other words, the surface of the cut created when the foamed particle is cut perpendicular to the axial direction) shown in the sectional view along line II-II is exposed. The cross-sectional area of ​​the foamed particle 1 in this cut surface is measured. In addition, the cross-sectional area of ​​the foamed particle 1 does not include the cross-sectional area of ​​the through hole 11 (in other words, the opening area).

[0061] For example, such as Figure 2 As shown, when the foamed particle 1 consists only of the foamed layer 2, the cross-sectional area of ​​the foamed particle 1 is equal to the cross-sectional area of ​​the foamed layer 2 in the cut surface. Additionally, for example... Figure 5 As shown, when the foamed particle 1 has a foamed layer 2 and a thermoplastic resin coating layer 3 (hereinafter referred to as "coating layer 3") covering the foamed layer 2, the cross-sectional area of ​​the foamed particle 1 is equal to the sum of the cross-sectional areas of the foamed layer 2 and the coating layer 3 in the cut surface.

[0062] The above operation is performed on more than 100 foamed particles, and the arithmetic mean of the cross-sectional areas of the obtained foamed particles is taken as the cross-sectional area A of the foamed particles in the cut surface. Furthermore, the cross-sectional area of ​​the foamed particles in the cut surface can be measured, for example, by taking a photograph of the cut surface of the foamed particles and performing image analysis. Additionally, the cross-sectional area A of the foamed particles obtained by the aforementioned method is sometimes referred to as the "average cross-sectional area A of the foamed particles".

[0063] The method for calculating the total cross-sectional area Ct of the through hole is as follows. First, as... Figure 2 As shown, the foamed particle 1 is cut at its center along a plane perpendicular to the axial direction, so that... Figure 2 The cut surface of the foamed particle is exposed. Measure the total cross-sectional area of ​​all through holes in this cut surface.

[0064] The above operation is performed on more than 50 foamed particles, and the arithmetic mean of the total cross-sectional areas of the resulting through holes is taken as the total cross-sectional area Ct of the through holes. Furthermore, the cross-sectional area of ​​the through holes in the cut surface can be measured, for example, by taking a photograph of the cut surface of the foamed particles and performing image analysis.

[0065] The ratio of the cross-sectional area Ca of each through-hole to the cross-sectional area A of the foamed particle in the cut surface obtained by cutting the foamed particle at its center along a plane perpendicular to the axial direction is preferably 0.005 or more and 0.05 or less, more preferably 0.005 or more and 0.04 or less, and even more preferably 0.005 or more and 0.03 or less. In this case, the rigidity and appearance of the molded article can be improved more easily. Furthermore, the cross-sectional area Ca of each through-hole is obtained by dividing the total cross-sectional area Ct of the through-holes by the number of through-holes.

[0066] The diameter d of the through hole in the cut surface obtained by cutting the foamed particles at the center of the axial direction along a plane perpendicular to the axial direction is preferably 0.1 mm or more and 0.5 mm or less. By making the diameter d of the through hole 0.5 mm or less, it is easier to improve the rigidity and appearance of the molded article. From the above viewpoint, the diameter d of the through hole is more preferably 0.45 mm or less, and even more preferably 0.4 mm or less. In addition, by making the diameter d of the through hole 0.1 mm or more, steam can pass through the through hole more easily when steam is supplied into the molding die. From the above viewpoint, the diameter d of the through hole is more preferably 0.2 mm or more.

[0067] The pore diameter d of the through-hole in the foamed particle is calculated as follows. First, the foamed particle 1 is cut at its center along a plane perpendicular to the axial direction, such that... Figure 2 The cut surface is shown. Next, a photograph of the cut surface is taken, and the cross-sectional area (in other words, the opening area) of each through hole 11 in the cut surface is calculated. Then, the diameter of an imaginary circle having the same area as the cross-sectional area of ​​the through hole 11 is calculated, and this value is used as the aperture of each through hole.

[0068] The above operation is performed on more than 50 foamed particles, and the arithmetic mean of the diameters of the resulting through-holes is taken as the diameter d of the through-holes in the foamed particles. Furthermore, even when the cross-sectional shape and diameter of each through-hole differ along the axial direction of the foamed particle, the diameter d of the through-hole is determined based on the diameter of the through-hole in the cut surface, as described above. Additionally, the diameter d of the through-holes in the foamed particles obtained by the aforementioned method is sometimes referred to as the "average diameter d of the through-holes in the foamed particles."

[0069] The pore diameter d of the through-hole can be adjusted to the specific range described later by adjusting the size of the pore diameter dr of the through-hole in the resin particles, the apparent density of the foamed particles, etc. Furthermore, by making the foamed particles secondary foamed particles manufactured through secondary foaming, it is easier to adjust the pore diameter d to a smaller value.

[0070] There are no particular limitations on the configuration of the through holes in the cut surface of the foamed particles; various methods can be used. For example,Figure 4 as well as Figure 5 As shown, one of the multiple through holes 11 (11a to 11e) can be configured to penetrate the central axis 10 of the foamed particle 1, while the other through holes 11b to 11e can be configured around the central axis 10 of the foamed particle 1. Figure 5 The through holes 11b to 11e in the foamed particle 1 shown are arranged at approximately equal circumferential intervals in the cut surface of the foamed particle 1. Additionally, for example... Figure 7 As shown, the four through holes 11 (11f~11i) can also be arranged around the central axis 10 of the foamed particle 1, with approximately equal circumferential spacing in the cut surface of the foamed particle 1. Similarly, as Figure 9 As shown, the three through holes 11 (11j~11l) can also be arranged around the central axis 10 of the foamed particle 1, with the circumferential spacing in the cut surface of the foamed particle 1 being approximately equal.

[0071] From the viewpoint of further enhancing the effect of providing the aforementioned plurality of through holes, it is preferable that the plurality of through holes are arranged at substantially equal intervals on the cut surface of the foamed particle. From the same viewpoint, it is more preferable that at least a portion of the plurality of through holes are arranged at substantially equal intervals in the circumferential direction around the central axis of the foamed particle, in the cut surface obtained by cutting the foamed particle at its axial center along a plane perpendicular to the axial direction.

[0072] Furthermore, the ratio R / d of the spacing R between the through holes of the foamed particles and the diameter d of the through holes in the cut surface of the foamed particles is preferably 2.0 or more, more preferably 2.5 or more. In this case, the appearance of the molded article can be further improved and the rigidity of the molded article can be further increased. In addition, the ratio R / d is preferably 4.5 or less, more preferably 3.5 or less. In this case, the cooling time of the molded article in the molding die can be further shortened.

[0073] Furthermore, the aforementioned method for calculating the through-hole spacing R is as follows. First, determine the position of the geometric center of the cross-sectional shape of each through-hole in the cut surface of the foamed particle, and take this position as the center point of each through-hole. Next, for all through-holes, calculate the distance between the center point of the through-hole of the test object and the center point of the through-hole with the nearest center point to that through-hole's center point; in other words, calculate the center point spacing. Then, use the arithmetic mean of the center point spacings of all through-holes as the through-hole spacing of each foamed particle. Perform the above operation on more than 100 foamed particles, and use the arithmetic mean of the obtained through-hole spacings as the through-hole spacing R of the foamed particles. In addition, the through-hole spacing R of the foamed particles obtained by the aforementioned method is sometimes referred to as the "average through-hole spacing R of the foamed particles".

[0074] From the perspective that increasing the wall thickness of the foamed particles improves their secondary foaming properties and the rigidity of the molded body, and from the perspective that omitting the curing process can more reliably suppress deformation and shrinkage of the molded body, the outer diameter D of the foamed particles is preferably 2 mm or more, more preferably 2.5 mm or more, and even more preferably 3 mm or more. On the other hand, from the perspective of improving the filling ability of the foamed particles into the molding die, the outer diameter D of the foamed particles is preferably 8 mm or less, more preferably 5 mm or less, and even more preferably 4.5 mm or less.

[0075] The calculation method for the outer diameter D of the aforementioned foamed particles is as follows. First, the foamed particle 1 is cut at its center along a plane perpendicular to the axial direction, such that... Figure 2 The cut surface of the foamed particle 1 is exposed. Calculate the cross-sectional area of ​​the foamed particle 1 and the cross-sectional area of ​​the through hole 11 in the cut surface. Then, calculate the sum of the cross-sectional areas of the foamed particle 1 and the through hole 11, and use the diameter of an imaginary circle with an area equal to the sum of these areas as the outer diameter of each foamed particle.

[0076] Thus, the outer diameter of more than 100 foamed particles 1 is calculated, and the arithmetic mean of the outer diameters of the obtained foamed particles is taken as the outer diameter D of the foamed particle. Furthermore, the measurement of the outer diameter of the foamed particle 1 in the cut surface can be performed, for example, by taking a photograph of the cut surface of the foamed particle and performing image analysis. Additionally, the outer diameter D of the foamed particle obtained by the aforementioned method is sometimes referred to as the "average outer diameter D of the foamed particle".

[0077] The bulk density of the foamed particles is preferably 10 kg / m³. 3 Above 50kg / m 3 The following is more preferably 10 kg / m 3 Above 35kg / m 3 Furthermore, the ratio of the apparent density to the bulk density of the foamed particles is preferably 1.7 to 1.9, more preferably 1.7 to 1.8. By using foamed particles with a bulk density within the above-mentioned range for in-mold molding, lightweight molded articles with excellent rigidity can be easily obtained. Furthermore, by using foamed particles with an apparent density to bulk density ratio within the specific range (excluding bulk density) for in-mold molding, it is easier to achieve an appropriate open bubble ratio in the molded article, more easily improve the rigidity and appearance of the molded article, and more reliably suppress shrinkage and deformation of the molded article without a curing process.

[0078] The method for calculating the bulk density of foamed granules is as follows. First, the foamed granules are left to stand for at least 24 hours in an environment with a relative humidity of 50%, a temperature of 23°C, and an air pressure of 1 atm to adjust their state. Next, the adjusted foamed granules are naturally piled into a graduated cylinder to fill the cylinder, and the volume of the foamed granule pack (unit: L) is read from the graduated cylinder's scale. Then, by dividing the mass of the foamed granule pack in the graduated cylinder (unit: g) by the aforementioned volume, the bulk density of the foamed granules (unit: kg / m³) can be obtained. 3 ).

[0079] The apparent density of foamed granules is calculated as follows. First, the foamed granule group is left to stand for one day in an environment with a relative humidity of 50%, a temperature of 23°C, and an air pressure of 1 atm to adjust the state of the foamed granules. After measuring the mass (in grams) of the foamed granule group, a metal mesh is immersed in a graduated cylinder containing alcohol (e.g., ethanol) at 23°C. The volume (in liters) of the foamed granule group is calculated based on the rise in liquid level. Then, by converting the unit of the value obtained by dividing the mass of the foamed granule group by its volume, the apparent density (in kg / m³) of the foamed granules can be calculated. 3 ).

[0080] From the perspective of balancing the lightweight and rigidity of the molded body, the apparent density of the foamed granules is preferably 10 kg / m³. 3 Above 150kg / m 3 The following is more preferably 15 kg / m 3 Above 100kg / m 3 The following is a further preferred value: 20 kg / m 3 Above 80kg / m 3 The following is particularly preferred: 25 kg / m 3 Above 60kg / m 3 Therefore, by using foamed particles with low apparent density for in-mold molding, it is easy to obtain lighter molded articles. Furthermore, conventionally, especially when manufacturing low-density molded articles, the articles tend to deform significantly after demolding, making it difficult to omit the curing process. In contrast, since the foamed particles eliminate the need for a curing process even with low apparent density, lightweight molded articles with the desired shape can be manufactured without curing.

[0081] The foamed particles have a thermoplastic resin foam layer. As the thermoplastic resin constituting the foam layer, crystalline thermoplastic resins such as polypropylene resins, polyethylene resins, polyamide resins, and crystalline polyester resins are preferably used. Furthermore, in this specification, polypropylene resins refer to homopolymers of propylene monomers and propylene copolymers containing 50% by mass or more of constituent units derived from propylene. Polyethylene resins refer to homopolymers of ethylene monomers and ethylene copolymers containing 50% by mass or more of constituent units derived from ethylene.

[0082] In this specification, "having crystallinity" means that, according to JIS K7122 (1987), "after undergoing a certain heat treatment, the heat of fusion is measured" (both the heating and cooling rates in the condition adjustment of the test piece are set to 10°C / min), using a differential scanning calorimeter, the endothermic peak heat accompanying the melting of the resin in the DSC curve obtained at a heating rate of 10°C / min is 5 J / g or more. The endothermic peak heat is preferably 15 J / g or more, more preferably 30 J / g or more.

[0083] The foamed layer may contain polymers other than elastomers and other plastic resins, within a range that does not impair the aforementioned effects. The content of other polymers in the foamed layer is preferably 20% by mass or less, more preferably 10% by mass or less, further preferably 5% by mass or less, and particularly preferably 0%. In other words, it is particularly preferred that the foamed layer substantially contains only thermoplastic resins as polymers.

[0084] Furthermore, the foamed layer may contain additives such as bubble modifiers, nucleating agents, flame retardants, flame retardant additives, plasticizers, antistatic agents, antioxidants, UV stabilizers, light stabilizers, conductive fillers, antibacterial agents, and colorants, within a range that does not impair the aforementioned effects. The content of additives in the foamed layer is preferably, for example, 0.01 parts by weight to 10 parts by weight relative to 100 parts by weight of the thermoplastic resin.

[0085] The thermoplastic resin constituting the foam layer is preferably a polypropylene-based resin, more preferably a propylene copolymer formed by copolymerizing propylene with other monomers. Examples of propylene copolymers include copolymers of propylene with α-olefins having 4 to 10 carbon atoms, such as ethylene-propylene copolymers, butene-propylene copolymers, hexene-propylene copolymers, and ethylene-propylene-butene copolymers. These copolymers can be, for example, random copolymers or block copolymers, but random copolymers are preferred. Furthermore, the foam layer may contain one or more polypropylene resins.

[0086] The thermoplastic resin constituting the foam layer is particularly preferably an ethylene-propylene random copolymer that also contains 0.5% to 3.5% by mass of ethylene in these polypropylene resins. Foam particles with such a foam layer exhibit excellent secondary foaming properties and formability. Furthermore, by using the aforementioned foam particles for in-mold molding, it is easier to obtain molded articles with excellent rigidity and surface properties, which suppress deformation and shrinkage even without a curing process.

[0087] From the viewpoint of obtaining a molded article with superior rigidity and less deformation and shrinkage while eliminating the need for curing, the content of ethylene in the ethylene-propylene random copolymer is preferably 0.5% by mass or more and less than 2.0% by mass. On the other hand, from the viewpoint of obtaining a molded article with improved formability of foamed particles and excellent energy absorption characteristics, the content of ethylene in the ethylene-propylene random copolymer is preferably 2.0% by mass or more and 3.5% by mass or less.

[0088] Furthermore, the aforementioned "ethylene component" and "propylene component" refer to the constituent units derived from ethylene and propylene, respectively, in the ethylene-propylene copolymer. Additionally, the ethylene component content is the mass ratio of ethylene component when the total ethylene and propylene components are 100% by mass. The content of each component in the ethylene-propylene copolymer can be determined based on the results of IR spectroscopy measurements.

[0089] When the foam layer is composed of a polypropylene resin, the melting point (Tmc) of the polypropylene resin is preferably 155°C or lower. In this case, a molded article with excellent appearance and rigidity can be formed at a lower molding temperature (in other words, a lower molding pressure). From the viewpoint of improving this effect, the melting point (Tmc) of the polypropylene resin constituting the foam layer is preferably 152°C or lower, more preferably 148°C or lower. On the other hand, from the viewpoint of further improving the heat resistance, mechanical strength, etc., of the molded article, the melting point (Tmc) of the polypropylene resin constituting the foam layer is preferably 135°C or higher, more preferably 138°C or higher, and even more preferably 140°C or higher.

[0090] The melting point of the thermoplastic resin constituting the foam layer can be determined by differential scanning calorimetry (DSC) based on JIS K7121-1987 and based on the obtained DSC curve. Specifically, firstly, the state of the test piece is adjusted according to "(2) after a certain heat treatment, the melting temperature is measured". The DSC curve is obtained by heating the state-adjusted test piece from 30°C to 200°C at a heating rate of 10°C / min, and the temperature of the peak of the melting peak appearing in the DSC curve is taken as the melting point Tmc of the thermoplastic resin. In addition, if multiple melting peaks appear in the DSC curve, the temperature of the peak of the melting peak with the largest area is taken as the melting point Tmc.

[0091] When the foaming layer is composed of a polypropylene resin, the melt flow rate (MFR) of the polypropylene resin is preferably 5 g / 10 min or more, more preferably 6 g / 10 min or more, and even more preferably 7 g / 10 min or more. In this case, foamability and formability can be further improved. On the other hand, from the viewpoint of further improving the rigidity of the molded article, the MFR of the polypropylene resin is preferably 12 g / 10 min or less, more preferably 10 g / 10 min or less. Furthermore, the MFR of the polypropylene resin is a value measured based on JIS K7210-1:2014, under test conditions of 230°C and a load of 2.16 kg.

[0092] When the foam layer is composed of a polypropylene resin, the flexural modulus of the polypropylene resin is preferably 800 MPa or more and 1600 MPa or less. From the viewpoint of improving the rigidity of the molded body and more reliably suppressing dimensional changes when the curing process is omitted, the flexural modulus of the polypropylene resin constituting the foam layer is preferably 800 MPa or more, more preferably 850 MPa or more, further preferably 900 MPa or more, and particularly preferably 1200 MPa or more. On the other hand, from the viewpoint of being able to form a molded body with excellent appearance and rigidity at a lower molding temperature (in other words, a lower molding pressure) and improving the energy absorption characteristics of the molded body, the flexural modulus of the polypropylene resin constituting the foam layer is preferably less than 1200 MPa, more preferably 1100 MPa or less, and further preferably 1000 MPa or less. Furthermore, the flexural modulus of the polypropylene resin can be determined based on JIS K7171:2008.

[0093] Previously, especially when in-mold molding of foamed granules made of polypropylene resin with a flexural modulus of less than 1200 MPa, the molded body tended to shrink and deform significantly after demolding if the curing process was omitted. This was considered to be due to factors such as low resistance to shrinkage and deformation after demolding. In contrast, according to the aforementioned method for manufacturing the molded body, the curing process can be omitted when using foamed granules made of, for example, polypropylene resin with a flexural modulus of less than 1200 MPa.

[0094] The foamed granules may also have a multi-layer structure comprising a foamed layer and a thermoplastic resin coating layer covering the foamed layer. In this case, the coating layer may cover the entire surface of the foamed layer or a portion of the foamed layer, but preferably covers the entire side peripheral surface of the foamed layer.

[0095] The aforementioned coating layer improves the weldability of the foamed particles. Whether the coating layer has this effect can be determined by the following method: First, using foamed particles with both a foaming layer and a coating layer, in-mold molding is performed at various vapor pressures. The weldability of the resulting molded body is measured. Then, the lowest vapor pressure among those that yields a weldability of 90% or higher is determined, and this value is taken as the minimum molding pressure P1.

[0096] In contrast, the same evaluation was performed using foamed particles consisting only of a foam layer, and the lowest vapor pressure that yields a molded body with a weldability of 90% or higher was determined, which was then used as the minimum forming pressure P2. Then, if the minimum forming pressure P1 of the foamed particles with the coating layer is lower than the minimum forming pressure P2 of the foamed particles without the coating layer, it can be determined that the coating layer has the effect of improving weldability.

[0097] Furthermore, the method for determining the weld ratio of the molded body is as follows. First, the molded body is bent and broken to expose the fracture surface. The total number of foamed particles in the fracture surface is counted, along with the number of foamed particles that broke inside the foamed particles (in other words, the foamed particles whose material was destroyed). Then, the ratio of the number of foamed particles that broke inside the foamed particles in the fracture surface to the total number of foamed particles is calculated, and this value is taken as the weld ratio (unit: %).

[0098] The coating layer is preferably made of a thermoplastic resin having a lower melting point or a lower softening point than the thermoplastic resin constituting the foam layer. By coating the foam layer with the coating layer made of the aforementioned thermoplastic resin, the foam particles can be fused at a lower molding temperature (in other words, a lower molding pressure) during in-mold molding. As a result, deformation and shrinkage of the molded body can be more reliably suppressed when the curing process is omitted.

[0099] The thermoplastic resin constituting the coating layer can be either a crystalline thermoplastic resin or a non-crystalline thermoplastic resin. For example, the same crystalline thermoplastic resin used for the coating layer can be used as the same resin used for the foaming layer. Examples of non-crystalline thermoplastic resins used for the coating layer include polystyrene-based resins and non-crystalline polyester-based resins.

[0100] When the foaming layer is composed of a polypropylene-based resin, from the viewpoint of adhesion to the foaming layer, the coating layer is preferably composed of a polyolefin-based resin, more preferably of a polyethylene-based resin and / or a polypropylene-based resin, and even more preferably of a polypropylene-based resin. Examples of polypropylene-based resins used for the coating layer include ethylene-propylene copolymers, butene-propylene copolymers, ethylene-propylene-butene copolymers, and propylene homopolymers. Among these, the thermoplastic resin constituting the coating layer is particularly preferably an ethylene-propylene copolymer and / or an ethylene-propylene-butene copolymer.

[0101] The coating layer may contain additives such as crystallizing nucleating agents, flame retardants, flame retardant auxiliaries, plasticizers, antistatic agents, antioxidants, UV stabilizers, light stabilizers, conductive fillers, antibacterial agents, and colorants, within a range that does not impair the aforementioned effects. The content of additives in the coating layer is preferably, for example, 0.01 parts by weight to 10 parts by weight relative to 100 parts by weight of the thermoplastic resin.

[0102] The coating layer of the foamed particles can be in a foamed or non-foamed state, but is substantially preferably non-foamed. "Substantially non-foamed" means that there is almost no air bubble structure. The thickness of the coating layer is, for example, 0.5 μm to 100 μm. Alternatively, an intermediate layer may be further provided between the foamed layer and the coating layer.

[0103] From the viewpoint of maintaining the rigidity of the molded body while improving its formability, the mass ratio (mass %) of the resin constituting the foam layer to the resin constituting the coating layer is preferably 99.5:0.5 to 80:20, more preferably 99:1 to 85:15, and even more preferably 97:3 to 88:12.

[0104] From the viewpoint of improving the formability of the foamed particles and further improving the rigidity of the molded body, the independent bubble rate of the foamed particles is preferably 90% or more, more preferably 92% or more, and even more preferably 95% or more.

[0105] The independent bubble rate of the foamed particles can be determined using an air comparison hydrometer based on ASTM-D2856-70 procedure C. Specifically, the determination is as follows: First, the foamed particles are allowed to stand for at least 24 hours at a relative humidity of 50%, a temperature of 23°C, and an air pressure of 1 atm to adjust their state. A sample is then selected from the adjusted foamed particles so that the value of the mark when naturally accumulated in the vector cylinder is approximately 20 cm. 3 The sample for the determination was submerged in a graduated cylinder containing ethanol at 23°C, and the apparent volume of the sample was determined based on the rise in liquid level.

[0106] After the test sample, whose apparent volume has been measured, is thoroughly dried, the true volume of the test sample, measured using an Accupyc II 1340 manufactured by Shimadzu Corporation, is determined according to step C as described in ASTM-D2856-70. Then, using these volume values, the independent bubble rate (in %) of the test sample is calculated based on the following formula (1).

[0107] Independent bubble rate = (Vx - W / ρ) × 100 / (Va - W / ρ) ···(1)

[0108] Wherein, Vx (unit: cm) in the above formula (1) 3 Va is the actual volume of the foamed granules (in other words, the sum of the volume of the resin constituting the foamed granules and the total volume of the individual air bubbles within the foamed granules), in cm³. 3 ) is the apparent volume of the foamed particles (in other words, the volume determined by the rise in liquid level when the foamed particles are submerged in a graduated cylinder containing ethanol), W (in g) is the mass of the sample used for the measurement, and ρ (in g / cm³) is the mass of the sample used for the measurement. 3 ) is the density of the thermoplastic resin that makes up the foam layer.

[0109] The independent bubble rate of the foamed particles is obtained by performing the operation more than five times using different test samples and taking the arithmetic mean of the independent bubble rate obtained from these five measurements.

[0110] When the thermoplastic resin constituting the foam layer is a crystalline thermoplastic resin, the foamed particles preferably have a crystalline structure in the DSC curve obtained when heated from 23°C to 200°C at a heating rate of 10°C / min, exhibiting an endothermic peak based on the inherent melting of the crystalline thermoplastic resin constituting the foam layer, and one or more melting peaks located on the higher temperature side than the endothermic peak. Foamed particles possessing the above-mentioned crystalline structure exhibit excellent mechanical strength and formability. Furthermore, hereafter, the endothermic peak based on the inherent melting of the crystalline thermoplastic resin appearing in the DSC curve will be referred to as the "resin-inherent peak," and the melting peak appearing on the higher temperature side than the resin-inherent peak will be referred to as the "high temperature peak." The resin-inherent peak is generated due to the endothermic melting of the crystalline thermoplastic resin inherent in the foam layer. On the other hand, it is inferred that the high temperature peak is generated due to the melting of secondary crystals formed in the crystalline thermoplastic resin constituting the foam layer during the manufacturing process of the foamed particles. That is, it is inferred that when a high temperature peak appears in the DSC curve, secondary crystals are formed in the crystalline thermoplastic resin.

[0111] Whether the foamed particles have the aforementioned crystalline structure can be determined according to JIS K7121:1987 based on the DSC curve obtained by differential scanning calorimetry (DSC) under the aforementioned conditions. Furthermore, 1–3 mg of foamed particles can be used as a sample when performing DSC.

[0112] Specifically, as described above, the DSC curve obtained when heating from 23°C to 200°C at a heating rate of 10°C / min (in other words, the first heating) shows both a high-temperature peak and the inherent resin peak of the crystalline thermoplastic resin constituting the foam layer. In contrast, the DSC curve obtained when, after the first heating, the temperature is cooled from 200°C to 23°C at a cooling rate of 10°C / min, and then heated again from 23°C to 200°C at a heating rate of 10°C / min (in other words, the second heating), shows only the inherent resin peak of the crystalline thermoplastic resin constituting the foam layer. Therefore, by comparing the DSC curve obtained during the first heating and the DSC curve obtained during the second heating, the inherent resin peak and the high-temperature peak can be distinguished. The temperature of the peak of this inherent resin peak may sometimes differ slightly between the first and second heating, but the difference is usually within 5°C.

[0113] From the viewpoint of further improving the formability of foamed particles and obtaining molded bodies with superior rigidity, the melting heat of the high-temperature peak of the foamed particles is preferably 5 J / g or more and 40 J / g or less, more preferably 7 J / g or more and 30 J / g or less, and even more preferably 10 J / g or more and 20 J / g or less.

[0114] The heat of fusion of the aforementioned high-temperature peak was calculated as follows. First, the foamed particles were allowed to stand for 24 hours at a relative humidity of 50%, 23°C, and 1 atm to adjust their state. 1–3 mg of the adjusted foamed particles were used as a sample, and differential scanning calorimetry (DSC) was performed to obtain the DSC curve under the condition of heating from 23°C to 200°C at a heating rate of 10°C / min. Figure 3 An example of a DSC curve is shown. In the case where the foamed particles have a high-temperature peak, such as... Figure 3 As shown, the DSC curve shows a resin-specific peak ΔH1 and a high-temperature peak ΔH2 with a peak on the high-temperature side of the peak that is higher than that of the resin-specific peak ΔH1.

[0115] Next, draw the straight line L1 connecting point α on the DSC curve, which corresponds to 80°C, and point β, which corresponds to the melting end temperature T of the foamed particles. Furthermore, the melting end temperature T is the endpoint of the high-temperature side of the high-temperature peak ΔH2; in other words, it is the intersection of the high-temperature peak ΔH2 on the DSC curve with the baseline on the higher-temperature side of ΔH2.

[0116] After drawing straight line L1, draw straight line L2 parallel to the vertical axis of the graph, passing through the maximum point γ between the resin intrinsic peak ΔH1 and the high-temperature peak ΔH2. The resin intrinsic peak ΔH1 and the high-temperature peak ΔH2 are separated by this straight line L2. The heat endothermic effect of the high-temperature peak ΔH2 can be calculated based on the area enclosed by the portion of the high-temperature peak ΔH2 constituting the DSC curve, straight line L1, and straight line L2.

[0117] (Manufacturing method of foamed granules)

[0118] The foamed particles can be manufactured, for example, by dispersing thermoplastic resin particles (hereinafter referred to as "resin particles") containing a thermoplastic resin in a dispersion medium, impregnating the resin particles with a foaming agent, and then releasing the resin particles containing the foaming agent together with the dispersion medium under low pressure. Furthermore, the above-described foaming method is sometimes referred to as the "direct foaming method".

[0119] Resin granules can be produced, for example, by a slitting process. In this process, firstly, thermoplastic resin constituting the foam layer and additives such as bubble nucleating agents, supplied as needed, are fed into an extruder, heated, and mixed to form a resin melt compound. Then, the resin melt compound is extruded through a small orifice of a die attached to the front end of the extruder to form a cylindrical extrudate with multiple through-holes. After cooling, the extrudate is cut to the desired length, thereby obtaining resin granules with a single-layer structure comprising a thermoplastic resin core layer and having multiple through-holes.

[0120] To obtain foamed granules with a multi-layered structure consisting of a foamed layer and a coating layer covering the foamed layer, a co-extrusion apparatus equipped with a core-forming extruder, a coating-forming extruder, and a co-extrusion die connected to both extruders can be used to produce multi-layered resin granules. In this case, in the core-forming extruder, the thermoplastic resin constituting the foamed layer and additives added as needed are melt-blended to produce a core-forming resin melt-blended compound. Similarly, in the coating-forming extruder, the thermoplastic resin constituting the coating layer and additives added as needed are melt-blended to produce a coating-forming resin melt-blended compound.

[0121] By co-extruding these molten compound mixtures and converging them within a die, a multi-layered composite is formed, consisting of a non-foamed cylindrical core layer and a non-foamed coating layer covering the outer surface of the core layer. This composite is then extruded through a small orifice in the die to form a cylindrical extrudate with multiple through-holes in the core layer. After cooling, the extrudate is cut to the desired length, thereby obtaining resin particles with a multi-layered structure and multiple through-holes in the core layer. Furthermore, the method for manufacturing resin particles is not limited to the aforementioned method; thermal cutting, underwater cutting, etc., can also be used.

[0122] When producing resin granules, it is preferable to use a slitting method where the cylindrical extrudate is cooled in water and then cut into strips. This method improves the shape accuracy of the resin granules and makes it easier to achieve the desired shape for the multiple through-holes in the final foamed granules.

[0123] The particle size of the resin particles is preferably 0.1 mm to 3.0 mm, and more preferably 0.3 mm to 1.5 mm.

[0124] Furthermore, the mass of each resin particle is preferably 0.1 mg to 20 mg, more preferably 0.2 mg to 10 mg, even more preferably 0.3 mg to 5 mg, and particularly preferably 0.4 mg to 2 mg. In addition, the mass of each resin particle is a value obtained by dividing the mass of 200 randomly selected resin particles by the number of resin particles. Furthermore, the mass of each resin particle obtained by the aforementioned method is sometimes referred to as the "average mass of each resin particle".

[0125] When the resin particles have a core layer and a coating layer, the mass ratio of the core layer to the coating layer is preferably 99.5:0.5 to 85:15, more preferably 99:1 to 92:8, and even more preferably 97:3 to 90:10.

[0126] When forming foamed particles with through-holes, the pore diameter d of the through-holes in the core layer of the resin particles can be adjusted to the specific range by adjusting the pore diameter dr of the through-holes. More specifically, by making the pore diameter dr of the through-holes in the resin particles 0.03 mm or more and less than 0.15 mm, preferably 0.05 mm or more and less than 0.1 mm, foamed particles with a pore diameter d of 0.1 mm or more and less than 0.5 mm can be easily manufactured. The pore diameter dr of the through-holes in the core layer of the resin particles can be adjusted, for example, by the diameter of the aperture (in other words, the inner diameter of the die) of the die used to form the through-holes.

[0127] The method for calculating the pore diameter dr of the resin particles is the same as the method for calculating the pore diameter d of the foamed particles, except that resin particles are used instead of foamed particles. In addition, the pore diameter dr of the resin particles obtained by the above method is sometimes referred to as the "average pore diameter dr of the resin particles".

[0128] Furthermore, when a strip-cutting method is used in the cutting of the extrudate, that is, when the cylindrical extrudate extruded from the die is pulled and cooled in water while being cut to an appropriate length, the particle size, length / outer diameter ratio and mass of each resin particle can be adjusted by appropriately changing the extrusion speed, pulling speed, cutting speed, etc. when extruding the resin melt compound.

[0129] After the resin particles are prepared as described above, they are dispersed in a dispersion medium. The dispersion of the resin particles in the dispersion medium can be performed in a closed container used in the subsequent foaming process, or in a different container than the closed container used in the foaming process. From the viewpoint of simplifying the manufacturing process, it is preferable to perform the dispersion process in a closed container used in the foaming process.

[0130] As the dispersion medium, an aqueous dispersion medium with water as the main component is used. In addition to water, the aqueous dispersion medium may also contain hydrophilic organic solvents such as ethylene glycol, glycerol, methanol, and ethanol. The proportion of water in the aqueous dispersion medium is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more.

[0131] It is preferable to add a dispersant to the dispersion medium. By adding a dispersant to the dispersion medium, the melting of heated resin particles within the container can be suppressed during the foaming process. The amount of dispersant added is preferably between 0.001 parts by weight and 5 parts by weight per 100 parts by weight of resin particles. Organic dispersants and inorganic dispersants can be used as dispersants; from the perspective of ease of processing, particulate inorganic substances are preferred. More specifically, dispersants such as alumina, kaolin, mica, clay minerals, alumina, titanium dioxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, and iron oxide can be used. These dispersants can be used alone or in combination. Among these, clay minerals are preferred as dispersants. Clay minerals can be natural or synthetic.

[0132] Furthermore, when using dispersants, it is preferable to simultaneously employ anionic surfactants such as sodium dodecylbenzenesulfonate, sodium alkylbenzenesulfonate, sodium dodecyl sulfate, and sodium oleate as dispersing aids. The amount of dispersing aid added is preferably between 0.001 parts by weight and 1 part by weight per 100 parts by weight of resin particles.

[0133] After dispersing resin particles in a dispersion medium, a foaming agent is impregnated into the resin particles in a sealed container. The foaming agent impregnated into the resin particles is preferably a physical foaming agent. Examples of physical foaming agents include inorganic physical foaming agents such as carbon dioxide, air, nitrogen, helium, and argon; aliphatic hydrocarbons such as propane, butane, and hexane; cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane; and halogenated hydrocarbons such as 1,3,3,3-tetrafluoropropene, 1-chloro-3,3,3-trifluoropropene, chlorofluoromethane, trifluoromethane, 1,1-difluoromethane, 1-chloro-1,1-dichloroethane, 1,2,2,2-tetrafluoroethane, chloromethane, chloroethane, and dichloromethane. These physical foaming agents can be used alone or in combination. Furthermore, inorganic and organic physical foaming agents can also be mixed. From the perspectives of environmental impact and operability, inorganic physical foaming agents are preferred, and carbon dioxide is even more preferred.

[0134] Based on 100 parts by weight of resin particles, the amount of foaming agent added is preferably 0.1 parts by weight or more and 30 parts by weight, more preferably 0.5 parts by weight or more and 15 parts by weight.

[0135] In the manufacturing process of foamed granules, one method for impregnating resin granules with a foaming agent is to supply the foaming agent into a sealed container, thereby increasing the pressure inside the container and impregnating the resin granules in the dispersion medium with the foaming agent. In this case, heating the resin granules and the dispersion medium together further promotes the impregnation of the foaming agent into the resin granules.

[0136] The pressure inside the sealed container during foaming is preferably 0.5 MPa(G) or higher in gauge pressure. On the other hand, the pressure inside the sealed container is preferably 4.0 MPa(G) or lower in gauge pressure. As long as it is within the above range, foamed granules can be manufactured safely without the risk of damage to the sealed container or explosion.

[0137] Furthermore, when heating the dispersion medium, the foaming temperature can be kept within an appropriate range by increasing the temperature of the dispersion medium at a rate of 1 to 5 °C / min.

[0138] After the foaming agent is impregnated into the resin particles, the contents of the sealed container are released into an environment with a lower pressure than the sealed container. This causes the core layer of the resin particles to foam, forming a bubble structure, which is then stabilized by cooling with external gas to obtain foamed particles.

[0139] When the core layer is made of polypropylene resin, it is preferable to heat and foam the resin during impregnation with the foaming agent by the following method: First, a primary holding process is performed, where the temperature is maintained at a level above (melting point of polypropylene resin - 20°C) and below (melting completion temperature of polypropylene resin) for a sufficient time, preferably about 10 to 60 minutes. Then, the temperature is adjusted from (melting point of polypropylene resin - 15°C) to below (melting completion temperature of polypropylene resin + 10°C). Next, a secondary holding process is performed, where the temperature is maintained at this level for a further sufficient time, preferably about 10 to 60 minutes, as needed. Afterward, the contents of the sealed container are preferably released to the outside while maintaining a temperature above (melting point of polypropylene resin - 10°C), causing the resin particles to foam. More preferably, the temperature inside the sealed container during foaming is above (melting point of polypropylene resin) and below (melting point of polypropylene resin + 20°C). In this way, by heating the resin particles to make them foam, secondary crystals can be formed in the polypropylene resin that constitutes the foam layer, making it easy to obtain foam particles with excellent mechanical strength and formability.

[0140] In the fabrication of molded articles, the foamed particles obtained above can be used directly. Alternatively, the foamed particles obtained through the aforementioned direct foaming method can be further foamed, and molded articles can be fabricated using these foamed particles with reduced apparent density. Furthermore, in the case of foaming resin particles in two stages, the first stage foaming process is called the primary foaming process, and the foamed particles obtained through the primary foaming process are called primary foamed particles. The second stage foaming process is called the secondary foaming process. The foamed particles obtained through the secondary foaming process are sometimes referred to as secondary foamed particles.

[0141] The method for reducing the apparent density of foamed particles through secondary foaming is as follows. First, as a primary foaming process, resin particles are foamed using the aforementioned direct foaming method to obtain primary foamed particles. Then, internal pressure is applied to the primary foamed particles. More specifically, after placing the primary foamed particles into a pressure-resistant container, the particles are pressurized within the container using inorganic gases such as air or carbon dioxide, causing the inorganic gases to impregnate the foamed particles. This raises the pressure within the bubbles of the primary foamed particles to above atmospheric pressure. Then, the primary foamed particles removed from the pressure-resistant container are heated using a heating medium such as steam or heated air at a lower pressure compared to the pressure within the bubbles, thereby causing the primary foamed particles to undergo secondary foaming.

[0142] (Foamed granule molded body)

[0143] By in-mold forming the foamed particles, a molded body of foamed particles can be obtained. The molded body has an open bubble structure. The open bubble structure is a small spatial portion that communicates with the outside of the molded body. The open bubble structure is formed by a complex connection of gaps formed by interconnected through holes of multiple foamed particles, gaps formed by interconnected through holes of foamed particles and gaps formed between foamed particles, gaps formed by interconnected gaps between foamed particles, and continuous bubble portions of foamed particles constituting the molded body.

[0144] The open bubble ratio of the molded body is preferably between 2% and 12%. By maintaining the open bubble ratio within this specific range, significant shrinkage and deformation of the molded body can be suppressed even without a curing process, and the appearance and rigidity of the molded body can be improved. This is because the molded body has an open bubble structure in the aforementioned specific ratio, so that after demolding, air quickly flows into the bubbles inside the molded body, increasing the overall internal pressure of the molded body. As a result, the dimensions of the molded body can be stabilized more easily and earlier.

[0145] If the open bubble rate of the molded body is too low, omitting the curing process may result in significant shrinkage and deformation of the molded body, preventing the acquisition of the desired shape. From the viewpoint that even omitting the curing process can better suppress significant shrinkage and deformation of the molded body, the open bubble rate of the molded body is preferably 2% or more, more preferably 2.5% or more, further preferably 3% or more, and particularly preferably 4% or more. On the other hand, if the open bubble rate of the molded body is too high, the appearance of the molded body may deteriorate and its rigidity may decrease. Additionally, there is a risk of insufficient water resistance depending on the application. From the viewpoint of further improving the appearance, rigidity, and water resistance of the molded body, the open bubble rate of the molded body is preferably 12% or less, more preferably 10% or less, further preferably 8% or less, particularly preferably 7.5% or less, and most preferably 6% or less.

[0146] The open bubble rate of the molded body was determined according to ASTM 2856-70, step B. Specifically, the molded body was first conditioned by standing at 23°C for 12 hours. Then, a first test piece with a cube shape (2.5cm long × 2.5cm wide × 2.5cm high) was cut from the center of the molded body, and its geometric volume Va (unit: cm³) was calculated. 3 In other words, calculate the product of the length dimension (in cm), the width dimension (in cm), and the height dimension (in cm). Next, measure the true volume V1 (in cm³) of the first test piece using a dry automatic density meter (specifically, a Shimadzu Accupyc II 1340). 3 ).

[0147] Next, the first test piece was divided into eight equal parts to make a second test piece in the shape of a cube, measuring 1.25cm in length, 1.25cm in width, and 1.25cm in height. The true volume V2 of the second test piece was measured using a dry automatic density meter (unit: cm²). 3 Furthermore, the true volume V2 of the second test piece is the sum of the true volumes of each of the eight pieces cut from the first test piece.

[0148] The open bubble rate (in %) of the first test piece is expressed by the geometric volume Va of the first test piece obtained above, the true volume V1 of the first test piece, and the true volume V2 of the second test piece by the following formula (2). In addition, the open bubble rate thus measured is a value after correcting for the influence of the independent bubbles that are destroyed when the second test piece is cut from the first test piece, and is also called the corrected continuous bubble rate.

[0149] Open bubble rate = (Va - 2V1 + V2) × 100 / Va···(2)

[0150] The above operations were performed on five first test pieces, and the open bubble rate of each first test piece was calculated. Then, the arithmetic mean of the open bubble rates of the five first test pieces was taken as the open bubble rate Co of the molded body.

[0151] Furthermore, the open bubble rate Co in this specification is a property value determined according to step B of ASTM 2856-70 as described above, and is a property value with a different concept from the porosity of the molded body. The porosity of the molded body is determined, for example, as follows. Specifically, a rectangular test piece (e.g., 20 mm long × 100 mm wide × 20 mm high) is first cut from the center of the molded body. Then, the test piece is immersed in a graduated cylinder containing ethanol, and the true volume Vc (unit: L) of the test piece is determined based on the rise of the ethanol level. In addition, the apparent volume Vd (unit: L) is determined based on the external dimensions of the test piece. The porosity (unit: %) of the molded body is expressed by the true volume Vc and apparent volume Vd of the test piece obtained above using the following formula (3).

[0152] Porosity = [(Vd-Vc) / Vd]×100···(3)

[0153] In the determination of the porosity of the molded body, individual air bubbles that are broken during the cutting of the test piece are not considered. Furthermore, the method for determining the open bubble rate Co differs from the method described above in that it uses a liquid such as ethanol as the measurement medium. Therefore, it is difficult to infer the value of the open bubble rate Co based on the value of the porosity of the molded body. Moreover, the porosity of the molded body must necessarily be a value larger than the open bubble rate Co.

[0154] From the viewpoint that dimensional changes can be more effectively suppressed even if the curing process is omitted, the porosity of the molded body is preferably 4% or more, more preferably 4.5% or more, and even more preferably 5% or more. On the other hand, from the viewpoint of improving rigidity and appearance, the porosity of the molded body is preferably 12% or less, more preferably 10% or less. The porosity of the molded body can be measured by the above-described measurement method.

[0155] The density of the molded body is preferably 10 kg / m³. 3 Above 60kg / m 3 The following applies. In this case, the balance between lightweight and rigidity of the molded part can be well improved. From the viewpoint of further improving the rigidity of the molded part, the density of the molded part is more preferably 15 kg / m³. 3 The above is further optimized to 20 kg / m 3 The above. From the viewpoint of further improving the lightweight nature of the molded article, the density of the molded article is more preferably 50 kg / m³. 3 The following is a further preferred value: 45 kg / m 3 The density of the molded body is calculated by dividing the mass of the molded body (in grams) by the volume (in liters) obtained from the external dimensions of the molded body, and then performing a unit conversion. Furthermore, in cases where the molded body has a complex shape, at least partially, making it difficult to determine the volume from the external dimensions, the volume of the molded body can be determined using water.

[0156] Previously, when manufacturing molded articles with low density, the articles tend to deform significantly after demolding, making it particularly difficult to omit the curing process. In contrast, the foamed granule molded articles can omit the curing process even with low density, achieving the desired shape and exhibiting excellent appearance and rigidity even without curing. From the viewpoint of effectively utilizing this effect, it is preferable to have the density of the molded article within the aforementioned range.

[0157] Molded materials can also be used in various fields such as motor vehicles, construction, and other fields as sound-absorbing materials, impact-absorbing materials, and cushioning materials.

[0158] (Manufacturing method of foamed granule molded body)

[0159] When manufacturing the foamed particle molded body, for example, after filling the mold with the foamed particles, steam, as a heating medium, is supplied into the mold for in-mold forming. Specifically, first, foamed particles are filled into a mold having a cavity corresponding to the desired shape of the molded body. After the foamed particles are filled, steam is supplied into the mold to heat the foamed particles. The foamed particles in the mold are heated by the steam, undergo secondary foaming, and simultaneously fuse together. This allows the foamed particles in the mold to be integrated, forming a molded body.

[0160] After the foamed particles are heated, the molded body inside the mold is cooled to stabilize its shape. Then, the molded body is removed from the mold, completing the in-mold molding process. In this manufacturing method, a curing process can be performed, where the demolded molded body is placed in a high-temperature atmosphere (e.g., adjusted from 60°C to approximately 80°C) for a predetermined time. However, even without this high-temperature curing process, shrinkage and deformation of the molded body can be suppressed. If the curing process is omitted, for example, the shape of the molded body can be stabilized by placing it in an environment at 23°C for 12 hours.

[0161] In the manufacturing method described above, the open bubble rate of the molded body can be easily adjusted to a range of 2% to 12% by in-mold forming of the specific foamed particles. When the foamed particles do not have through-holes, it is difficult to achieve an open bubble rate of 2% or higher in the molded body.

[0162] Furthermore, there is a trend that the higher the ratio Ct / A (the total cross-sectional area of ​​the through-holes in the foamed particles, Ct, to the cross-sectional area A of the foamed particles), the greater the open bubble rate of the molded article. Therefore, when the ratio Ct / A is too low, the open bubble rate tends to be less than 2%, and when the ratio Ct / A is too high, the open bubble rate tends to be higher than 12%.

[0163]

Example

[0164] Examples of the foamed particles, the foamed particle molded body, and the manufacturing method thereof are described below.

[0165] (Polypropylene resin)

[0166] Table 1 shows the properties of the polypropylene resins used in the manufacture of foamed granules. Furthermore, the ethylene-propylene copolymer and ethylene-propylene-butene copolymer used in this example are both random copolymers. Additionally, the density of PP1 and PP2 shown in Table 1 is 900 kg / m³. 3 .

[0167] Table 1

[0168]

[0169] <Monomer content of polypropylene resins>

[0170] The monomer content of polypropylene resins (specifically, ethylene-propylene copolymers and ethylene-propylene-butene copolymers) is determined by a known method based on IR spectroscopy. Specifically, it is determined quantitatively by the method described in the Handbook of Polymer Analysis (edited by the Polymer Analysis Research Conference of the Japan Society for Analytical Chemistry, published January 1995, Kinokuniya Shoten, page numbers and item names: 615-616"II.2.32.3.4 Propylene / Ethylene Copolymer", 618-619"II.2.32.3.5 Propylene / Butene Copolymer"). In other words, it is determined by the relationship between the absorbance values ​​of ethylene and butene after correction with predetermined coefficients and the thickness of the film-like test piece.

[0171] More specifically, polypropylene resin was first hot-pressed into a film at 180°C, producing multiple test pieces of varying thicknesses. Then, the IR spectra of each test piece were measured, and the 722 cm⁻¹ wavelength originating from ethylene was read. -1 And 733cm -1 absorbance at (A) 722 A 733 ), 766cm derived from butene -1 absorbance at (A) 766 Next, for each test piece, the ethylene content (in mass%) in the polypropylene resin was calculated using the following formulas (4) to (6). The arithmetic mean of the ethylene content obtained for each test piece was taken as the ethylene content (in mass%) in the polypropylene resin.

[0172] (K′ 733 ) c =1 / 0.96{(K′)733 ) a -0.268(K′ 722 ) a}···(4)

[0173] (K′ 722 ) c =1 / 0.96{(K′) 722 ) a -0.150(K′ 733 ) a}···(5)

[0174] Ethylene content = 0.575{(K′) 722 ) c +(K′ 733 ) c}···(6)

[0175] Among them, K′ in equations (4) to (6) a The apparent absorption coefficient (K′) at each frequency. a =A / ρt), K′ c The corrected absorbance coefficient is given by A, where A is the absorbance and ρ is the density of the resin (unit: g / cm³). 3 ), where t is the thickness of the film-like test piece (unit: cm). In addition, the above formulas (4) to (6) can be applied to random copolymers.

[0176] In addition, the butene content in the polypropylene resin was calculated for each test piece using the following formula (7). The arithmetic mean of the butene content obtained for each test piece is taken as the butene content in the polypropylene resin (unit: mass%).

[0177] Butene content = 12.3 (A) 766 / L)···(7)

[0178] In Equation (7), A represents absorbance and L represents the thickness of the film-like test piece (unit: mm).

[0179] <Flexural modulus of polypropylene resins>

[0180] Polypropylene resin was hot-pressed at 230°C to form a 4mm sheet, from which test pieces measuring 80mm long × 10mm wide × 4mm thick were cut. The flexural modulus of elasticity of the test piece was determined according to JIS K7171:2008. Furthermore, the radius of the indenter and the support platform were both 5mm, the distance between the support points was 64mm, and the testing speed was 2mm / min.

[0181] <Melting point of polypropylene resins>

[0182] The melting point of the polypropylene resin was determined based on JIS K7121:1987. Specifically, firstly, the state of the test piece made of polypropylene resin was adjusted based on "(2) after certain heat treatment, the melting temperature was measured" as described in JIS K7121:1987. The DSC curve was obtained by heating the state-adjusted test piece from 30°C to 200°C at a heating rate of 10°C / min. Then, the melting point was determined by the peak temperature of the melting peak that appeared in the DSC curve. In addition, a differential scanning calorimeter (manufactured by SII Nano Technology Co., Ltd., model: DSC7020) was used as the measuring device.

[0183] <Mel flow rate of polypropylene resins>

[0184] The melt flow rate (MFR) of polypropylene resins was determined according to JIS K7210-1:2014, at a temperature of 230°C and a load of 2.16 kg.

[0185] Next, the composition and manufacturing method of the foamed particles used in this example will be explained.

[0186] (Example 1)

[0187] like Figure 4 as well as Figure 5 As shown, the foamed particle 1 of Example 1 has a cylindrical shape and five through holes 11 extending through its axial direction. One of the five through holes 11 (11a to 11e) is configured to penetrate the central axis 10 of the foamed particle 1. The remaining four through holes 11b to 11e are arranged around the central axis 10 at approximately equal circumferential intervals in the cut surface obtained by cutting the foamed particle 1 along a plane perpendicular to the axial direction at its center.

[0188] In addition, such as Figure 5 As shown in Table 2, the foamed particle 1 of Example 1 has a multi-layer structure, which includes a foamed layer 2 made of PP1 and a non-foamed coating layer 3 made of PP2 that covers the foamed layer 2.

[0189] In producing the foamed granules in this example, a co-extrusion apparatus was first used, comprising an extruder for core layer formation, an extruder for coating layer formation, and a co-extrusion die connected to both extruders. The extrudate from the co-extrusion apparatus was cut into strips to produce multilayer resin granules. Specifically, in the core layer forming extruder, PP1 was melt-blended with zinc borate as a bubble modifier to obtain a core layer forming resin melt blend. Furthermore, the maximum set temperature in the core layer forming extruder was set to 245°C, and the amount of zinc borate was 500 ppm by mass relative to the polypropylene resin. Simultaneously, in the coating layer forming extruder, PP2 was melt-blended at the maximum set temperature of 245°C to obtain a coating layer forming resin melt blend.

[0190] By co-extruding these resin melt blends and converging them within a die, a composite is formed consisting of a non-foamed core layer and a non-foamed coating layer covering the side surfaces of the core layer. After extruding this composite from the die, the extrudate is stretched and cooled in water, then cut to appropriate lengths using a granulator, thereby obtaining multilayer resin particles consisting of a core layer and a coating layer covering the side surfaces of the core layer, with five through-holes formed in the core layer. The mass ratio of the core layer to the coating layer in the multilayer resin particles is set to core layer:coating layer = 95:5 (in other words, the mass ratio of the coating layer is 5%). Furthermore, the mass of each multilayer resin particle is set to approximately 1.5 mg.

[0191] Next, foamed granules were obtained by foaming the multilayer resin particles using a direct foaming method. Specifically, 1 kg of multilayer resin particles and 3 L of water as the dispersion medium were added to a 5 L sealed container. Then, 0.3 parts by weight of dispersant and 0.004 parts by weight of dispersant relative to 100 parts by weight of multilayer resin particles were added to the sealed container to disperse the multilayer resin particles in the dispersion medium. Kaolin was used as the dispersant. Additionally, a surfactant (sodium alkylbenzene sulfonate) was used as the dispersant.

[0192] Next, carbon dioxide, used as a foaming agent, was added to a sealed container, which was then sealed. The container was heated to the temperature listed in the "Foaming Temperature" column of Table 1 while being stirred. The pressure inside the container at this time (also called impregnation pressure or carbon dioxide pressure) is the value shown in the "Container Pressure" column of Table 1. After maintaining the aforementioned foaming temperature for 15 minutes, the sealed container was opened to release the contents to atmospheric pressure, resulting in foamed particles 1 having a foamed core layer 2 and a non-foamed coating layer 3 covering the foamed core layer 2.

[0193] Next, a secondary foaming process is performed to reduce the apparent density of the primary foamed particles. Specifically, the primary foamed particles are placed in a pressure vessel (specifically, a metal drum), air is supplied into the pressure vessel to increase the pressure inside, and the air is impregnated within the bubbles. The internal pressure of the bubbles in the primary foamed particles removed from the pressure vessel is shown in Table 2. Then, the primary foamed particles are placed in the metal drum, steam is supplied to heat the particles so that the drum pressure is the value shown in Table 2, thereby obtaining foamed particles 1 with the apparent density shown in Table 2.

[0194] (Example 2)

[0195] The foamed particle 1 in this example has the same structure as the foamed particle 1 in Example 1, except that the number of through holes 11 is changed to four. Specifically, as Figure 6 as well as Figure 7 As shown, in this example, the through holes 11 (11f~11i) of the foamed particle 1 are arranged at approximately equal circumferential intervals around the central axis 10 of the foamed particle 1, in the cut surface obtained by cutting the foamed particle 1 at its center along a plane perpendicular to the axial direction. The manufacturing method of the foamed particle 1 in this example is the same as that of the foamed particle 1 in Example 1, except that the shape of the small holes in the co-extrusion die is changed.

[0196] (Example 3)

[0197] The foamed particle 1 in this example has the same structure as the foamed particle 1 in Example 1, except that the pore diameter d of the through hole 11 is changed to the value shown in Table 2. The manufacturing method of the foamed particle 1 in this example is substantially the same as that of the foamed particle 1 in Example 1, except that the shape of the small hole in the co-extrusion die is changed.

[0198] (Example 4)

[0199] The foamed particle 1 in this example has the same structure as the foamed particle 1 in Example 1, except that the number of through holes 11 is changed to three. Figure 8 as well as Figure 9 As shown, in this example, the through holes 11 (11j~11l) of the foamed particle 1 are arranged at approximately equal circumferential intervals around the central axis 10 of the foamed particle 1, in the cut surface obtained by cutting the foamed particle 1 at its center along a plane perpendicular to the axial direction. The manufacturing method of the foamed particle 1 in this example is the same as that of the foamed particle 1 in Example 1, except that the shape of the small holes in the co-extrusion die is changed.

[0200] (Comparative Example 1)

[0201] The foamed particles in this example are conventional polypropylene resin foamed particles that have a solid spherical shape but no through-holes. The manufacturing method of the foamed particles in this example is largely the same as the manufacturing method of foamed particles 1 in Example 1, except that resin particles with a solid spherical shape are used instead of having through-holes.

[0202] (Comparative Example 2)

[0203] The foamed particles of Comparative Example 2 have the same structure as the foamed particles 1 of Example 1, except that the number of through holes 11 is changed to one. Although not shown, the through holes in the foamed particles of this example are configured to extend through the central axis of the foamed particles. The manufacturing method of the foamed particles of this example is substantially the same as that of the foamed particles 1 of Example 1, except that the shape of the small holes in the co-extrusion die is changed.

[0204] (Comparative Example 3)

[0205] The foamed particles of Comparative Example 3 have the same structure as the foamed particles 1 of Example 1, except that the number of through holes 11 is changed to one and the diameter d of the through hole is changed to the value shown in Table 3. Although not shown, the through holes in the foamed particles of this example are configured to penetrate the central axis of the foamed particles. The manufacturing method of the foamed particles of this example is the same as that of the foamed particles 1 of Example 1, except that the shape of the small holes in the co-extrusion die is changed and secondary foaming is not performed.

[0206] (Comparative Example 4)

[0207] The foamed particles in this example have the same structure as the foamed particles of Example 2, except that the pore diameter d of the through holes is changed to the value shown in Table 3. The manufacturing method of the foamed particles in this example is largely the same as the manufacturing method of foamed particles 1 in Example 1, except that the shape of the small holes in the co-extrusion die is changed.

[0208] The properties of the foamed particles and molded articles in the examples and comparative examples are shown in Tables 2 and 3. The evaluation methods for the properties shown in Tables 2 and 3 are as follows.

[0209] (Evaluation of foamed granules)

[0210] The foamed particles were used in the physical property determination and evaluation of the foamed particles after being conditioned by standing for 24 hours under conditions of 50% relative humidity, 23°C and 1 atm.

[0211] <Bulk density>

[0212] The conditioned foamed granules were filled into a graduated cylinder by natural stacking, and the bulk volume (in L) of the foamed granule group was read from the graduated cylinder scale. Then, the mass (in g) of the foamed granule group in the graduated cylinder was divided by the aforementioned bulk volume, and further unit conversion was performed to calculate the bulk density (in kg / m³). 3 ).

[0213] Apparent density

[0214] After measuring the mass of the foamed granule assembly after conditioning, a metal mesh was immersed in a graduated cylinder containing ethanol at 23°C. Then, taking into account the volume of the metal mesh, the volume of the foamed granule assembly, as read from the water level rise, was measured. The apparent density of the foamed granules (kg / m³) was calculated by dividing the mass (g) of the foamed granule assembly obtained in this way by the volume (L) and then converting the units. 3 ).

[0215] <Independent bubble rate>

[0216] The method for determining the independent bubble rate of foamed particles is as described above.

[0217] <Diameter d of the through hole>

[0218] One hundred foamed particles were randomly selected from the conditioned foamed particle group. These particles were then cut at their central position along a plane perpendicular to the axial direction, exposing the cut surfaces. Next, photographs of the cut surfaces were taken and the images were analyzed to measure the cross-sectional area (in other words, the opening area) of the through-hole in each particle. Then, the diameter of an imaginary circle with the same area as the cross-sectional area of ​​the through-hole was calculated, and this value was used as the aperture diameter of each through-hole. This process was repeated for all 100 foamed particles, and the arithmetic mean of the resulting aperture diameters was taken as the aperture diameter *d* of the foamed particle's through-holes.

[0219] <Cross-sectional area A of the foamed particles>

[0220] One hundred foamed particles were randomly selected from the conditioned foamed particle group. These particles were then cut at their central position along a plane perpendicular to the axial direction, exposing the cut surfaces. Next, photographs of the cut surfaces were taken and the images were analyzed to calculate the cross-sectional areas of the foam layer and the coating layer in each particle. The sum of the cross-sectional areas of the foam layer and the coating layer was then used as the cross-sectional area of ​​each foamed particle. Note that the cross-sectional area of ​​the through-holes is not included in the cross-sectional area of ​​the foamed particles. This process was repeated for all 100 foamed particles, and the arithmetic mean of the resulting cross-sectional areas was taken as the cross-sectional area A of the foamed particles.

[0221] <Total cross-sectional area Ct of the through holes, cross-sectional area Ca of each through hole>

[0222] One hundred foamed particles were randomly selected from the conditioned foamed particle group. These particles were then cut at their central position along a plane perpendicular to the axial direction, exposing the cut surfaces. Next, photographs of the cut surfaces were taken and the images were analyzed to measure the cross-sectional area of ​​the through-holes in each particle. Finally, the total cross-sectional area of ​​the through-holes for each foamed particle was calculated.

[0223] The above operations were performed on 100 foamed particles. The arithmetic mean of the total cross-sectional areas of the resulting through holes was taken as the total cross-sectional area Ct of the through holes. The cross-sectional area Ca of each through hole was obtained by dividing the total cross-sectional area Ct by the number of through holes. Furthermore, Tables 2 and 3 show the ratio Ct / A of the total cross-sectional area Ct of the through holes in the cut surface to the cross-sectional area A of the foamed particle, and the ratio Ca / A of the cross-sectional area Ca of each through hole to the cross-sectional area A of the foamed particle.

[0224] <Outer diameter D of foamed particles>

[0225] One hundred foamed particles were randomly selected from the conditioned foamed particle group. These particles were then cut at their central position along a plane perpendicular to the axial direction, exposing the cut surfaces. Next, photographs of the cut surfaces were taken and the images were analyzed to measure the cross-sectional areas of the foam layer, the coating layer, and the through-hole for each particle. Then, the diameter of an imaginary circle with the same area as the sum of the cross-sectional areas of the foam layer, the coating layer, and the through-hole was calculated, and this value was taken as the outer diameter of each foamed particle. This process was repeated for all 100 foamed particles, and the arithmetic mean of the resulting outer diameters was taken as the outer diameter D of each foamed particle.

[0226] <Through hole spacing R>

[0227] One hundred foamed particles were randomly selected from the conditioned foamed particle group. These particles were then cut at their central position along a plane perpendicular to the axial direction, exposing the cut surfaces. Next, photographs of the cut surfaces were taken, and image analysis was performed to measure the center-to-center distance in each through-hole. In other words, the distance between the center point of each through-hole and the center point of the through-hole with the nearest center point to that center point was measured. This measurement was performed on all through-holes formed in the foamed particles of the test object, and the arithmetic mean of the obtained center-to-center distances was taken as the through-hole distance R of the foamed particles. This operation was repeated on 100 foamed particles, and the arithmetic mean of the obtained through-hole distances was taken as the through-hole distance R of the foamed particles.

[0228] <Minimum forming pressure>

[0229] In the evaluation of the minimum forming pressure, foamed granule molded bodies were produced by in-mold forming with the forming pressure during formal heating varying by 0.02 MPa each time between 0.18 and 0.38 MPa(G). The lowest forming pressure among those that yielded molded bodies with good weldability and resilience was then defined as the minimum forming pressure. The specific method for manufacturing the molded body is as follows.

[0230] First, after drying the foamed particles at 23°C for 24 hours, air was impregnated to increase the internal pressure, or in other words, the pressure inside the bubbles, to the values ​​shown in the "Internal Pressure of Foamed Particles" column of Tables 2 and 3. Furthermore, the internal pressure of the foamed particles was measured as follows: The mass Q (g) of the foamed particle group before filling into the molding die and after the internal pressure increase was measured, as well as the mass U (g) of the foamed particle group after 48 hours. The difference between Q and U was taken as the increase in air volume W (g). The internal pressure (MPa(G)) of the foamed particles was calculated using these values ​​based on the following equation (8).

[0231] P=(W / M)×R×T / V···(8)

[0232] In equation (8), M is the molecular weight of air, R is the gas constant, T is the absolute temperature, and V is the volume (in L) of the apparent volume of the foamed particle group minus the volume of resin occupied by the foamed particle group. In this example, let M = 28.8 (g / mol), R = 0.0083 (MPa·L / (K·mol)), and T = 296 (K).

[0233] Next, foamed particles were filled into a flat mold with dimensions of 300 mm (length) × 250 mm (width) × 60 mm (thickness) using a pyrolysis filling method. The pyrolysis amount during filling (specifically, the ratio of the mold opening amount to the inner dimension in the thickness direction) is set to the values ​​shown in Tables 2 and 3. After filling, the mold was closed along the thickness direction to mechanically compress the foamed particles.

[0234] Next, in-mold forming was performed by supplying steam into the mold. During in-mold forming, preheating was first performed by supplying steam into the mold for 5 seconds with the mold's discharge valve open. Then, the discharge valve was closed, and steam was supplied from one side of the mold to a pressure 0.08 MPa (G) lower than the forming pressure during formal heating to heat that side. Next, steam was supplied from the other side of the mold to a pressure 0.04 MPa (G) lower than the forming pressure during formal heating to heat that side. Afterward, formal heating was performed by supplying steam from both sides of the mold to the forming pressure required for formal heating. After formal heating was completed, the pressure inside the mold was released, and the molded body was cooled within the mold until the surface pressure, based on the foaming force of the molded body, became 0.04 MPa (G).

[0235] Following this, a curing process was performed where the foamed granule molded body removed from the molding die was placed in an oven at 80°C for 12 hours. After the curing process, the foamed granule molded body was subjected to a conditioning process by placing it at 50% relative humidity, 23°C, and 1 atm for 24 hours. The weldability and resilience of the conditioned foamed granule molded body were evaluated, and the lowest molding pressure among those molding pressures that met the evaluation criteria described later (in other words, the molding pressure that yields a qualified product) was defined as the minimum molding pressure. It can be determined that the lower the minimum molding pressure, the better the formability.

[0236] The evaluation methods for weldability and resilience in the evaluation of minimum forming pressure are as follows.

[0237] • Weldability

[0238] The foamed granule molded body is broken in a manner that is approximately equally divided along its length. By visually observing more than 100 randomly selected foamed granules exposed at the fracture surface, it is determined whether the foamed granules broke internally (in other words, the material was destroyed) or at the interfaces between the foamed granules. Then, the ratio of the number of foamed granules that broke internally to the total number of observed foamed granules is calculated as a percentage (in other words, the material failure rate), and this value is used as the weld rate. A weld rate of 90% or higher is then considered acceptable, while a weld rate of less than 90% is considered unacceptable.

[0239] • Restorative

[0240] The thickness of the foamed granules was measured at four locations 10 mm inside each vertex in the center direction, as well as at the central part, when viewed from above in the thickness direction. Then, the ratio (in %) of the thickness at the thinnest location to the thickness at the thickest location was calculated. A thickness ratio of 95% or higher was considered acceptable, while a ratio less than 95% was considered unacceptable.

[0241] <Forming Cycle Evaluation>

[0242] Cooling time within the molding die

[0243] Except for setting the forming pressure during formal heating to any one of the aforementioned minimum forming pressure, a pressure 0.2 MPa (G) higher than the minimum forming pressure, or a pressure 0.4 MPa (G) higher than the minimum forming pressure, in-mold forming was performed using the same method as for evaluating the aforementioned minimum forming pressure. Then, the time from the completion of formal heating until the pressure inside the forming mold was released until the surface pressure based on the foaming force of the molded body became 0.04 MPa (G) was measured; in other words, the cooling time of the molded body inside the forming mold was measured. Furthermore, in the in-mold forming of foamed particles, there is a trend that the higher the forming pressure, the longer the cooling time inside the forming mold.

[0244] Can it be formed without curing?

[0245] The minimum forming pressure during formal heating was used as the aforementioned minimum forming pressure, and in-mold forming was performed using the same method as the evaluation of the aforementioned minimum forming pressure. The molded body was left to stand for 24 hours at 50% relative humidity, 23°C, and 1 atm without undergoing a curing process after demolding, thus achieving state conditioning of the molded body. The thickness of the molded body was measured at four locations 10 mm inward from the vertices in the center direction, as well as the thickness of the central portion of the molded body, when viewed from above in the thickness direction. Then, the ratio (in %) of the thickness of the thinnest location to the thickness of the thickest location among the measured locations was calculated.

[0246] In the "Can it be molded without curing" column of Tables 2 and 3, cases with a thickness ratio of 95% or more are recorded as "Yes", and cases with a thickness ratio of less than 95% are recorded as "No". In addition, in Comparative Examples 3 and 4, since the appearance evaluation was unqualified as described later, the feasibility of molding without curing was not evaluated.

[0247] (Evaluation of the molded part)

[0248] The molded body used in the physical property determination and evaluation of the molded body was a molded body formed in the mold at the minimum molding pressure using the same method as that used in the evaluation of the minimum molding pressure, and after demolding from the mold, it was subjected to condition conditioning by standing at 50% relative humidity, 23°C and 1 atm for 12 hours without performing a curing process.

[0249] <Density of the molded body>

[0250] The density of the molded body (unit: g) was calculated by dividing its mass (in g) by the volume (in L) obtained from its external dimensions, and then converting the units to obtain the density (in kg / m³). 3 ).

[0251] <Open bubble rate>

[0252] The open bubble rate was determined according to ASTM 2856-70, step B. Specifically, a first test piece with a cube shape (2.5 cm long × 2.5 cm wide × 2.5 cm high) was first cut from the center of the molded body, and its geometric volume Va (unit: cm³) was calculated. 3 In other words, the product of the length dimension (in cm), width dimension (in cm), and height dimension (in cm) was calculated. Next, the true volume V1 (in cm³) of the first test piece was measured using a dry automatic density meter (specifically, a Shimadzu Accupyc II 1340). 3 ).

[0253] Next, the first test piece was divided into eight equal parts to make a second test piece in the shape of a cube, measuring 1.25cm in length, 1.25cm in width, and 1.25cm in height. The true volume V2 of the second test piece was measured using a dry automatic density meter (unit: cm³). 3 Furthermore, the true volume V2 of the second test piece is the sum of the true volumes of each of the eight pieces cut from the first test piece.

[0254] Using the geometric volume Va of the first test piece obtained above, the true volume V1 of the first test piece, and the true volume V2 of the second test piece, the open bubble rate (unit: %) of the first test piece was calculated based on the following formula (2).

[0255] Open bubble rate = (Va - 2V1 + V2) × 100 / Va···(2)

[0256] The above operations were performed on five first test pieces, and the open bubble rate of each first test piece was calculated. Then, the arithmetic mean of the open bubble rates of the five first test pieces was taken as the open bubble rate Co of the molded body.

[0257] <Porosity of the molded body>

[0258] A cuboid-shaped test piece with a length of 20 mm × width of 100 mm × height of 20 mm was cut from the center of the molded body. The test piece was submerged in a graduated cylinder containing ethanol, and the true volume Vc (unit: L) of the test piece was determined based on the rise of the ethanol level. In addition, the apparent volume Vd (unit: L) of the test piece was determined from its external dimensions. The porosity (unit: %) of the molded body was calculated based on the following formula (3) using the true volume Vc and apparent volume Vd obtained above.

[0259] Porosity = [(Vd-Vc) / Vd]×100···(3)

[0260] <50% compressive stress σ 50 >

[0261] A prism-shaped piece, 50 mm long × 50 mm wide × 25 mm thick, was cut from the center of the molded body, excluding the surface layer (i.e., the surface that contacts the inner surface of the mold during in-mold forming). Based on JIS K6767:1999, a compression test was conducted at a compression rate of 10 mm / min to determine the 50% compressive stress σ of the molded body. 50 (Unit: kPa). Additionally, Tables 2 and 3 record the 50% compressive stress σ of the molded body. 50 The value obtained by dividing by the density of the shaped body (σ) 50 / density).

[0262] <Appearance>

[0263] The surface of the molded body was visually observed, and its surface properties were evaluated based on the following criteria.

[0264] A+: Indicates a very good surface condition with very few intergranular gaps on the surface of the molded body, and the unevenness caused by the through holes is mostly inconspicuous.

[0265] A: This indicates a good surface condition where there are sufficiently few intergranular gaps on the surface of the molded body, and the unevenness caused by the through holes is not very noticeable.

[0266] B: Slightly confirm the intergranular gaps and / or unevenness caused by through holes on the surface of the molded body.

[0267] C: Particle gaps and / or irregularities caused by through holes are clearly identified on the surface of the molded body.

[0268] Table 2

[0269]

[0270] Table 3

[0271]

[0272] As shown in Table 2, the foamed particles of Examples 1-4 have a cylindrical shape and multiple through holes extending through their axial direction. Furthermore, the ratio Ct / A of the total cross-sectional area Ct of these through holes to the cross-sectional area A of the foamed particle is within the specified range. By using the above-described foamed particles for in-mold forming, the cooling time within the molding die is shortened. Even when the molding pressure of the foamed particles of Examples 1-4 is higher than the minimum molding pressure, the cooling time can be sufficiently shortened, and even without the curing process, a foamed particle molded body with the desired shape, appearance, and excellent rigidity is obtained. In the foamed particles of Examples 1-4, the ratio Ct / A is within the specified range, and compared to the foamed particle molded body obtained from the foamed particles of Comparative Example 2, which has only one through hole, the appearance is further improved.

[0273] On the other hand, the foamed particles of Comparative Example 1 are conventional polypropylene resin foamed particles without through-pores. The foamed particles of Comparative Example 1 exhibit excessively high secondary foaming properties, resulting in a significantly longer cooling time within the molding die, as shown in Table 3. Furthermore, the foamed particles of Comparative Example 1 are difficult to mold to create vapor channels during molding, thus requiring a high molding pressure to obtain the molded article. Moreover, the vast majority of the resulting molded articles do not form open-cell structures. Therefore, the molded articles made using the foamed particles of Comparative Example 1 show significant shrinkage and deformation after demolding without a curing process.

[0274] Compared to the foamed particles of Examples 1-4, the cooling time within the molding die was longer for the foamed particles of Comparative Example 2. This is believed to be because, with only one through-hole, the surface area of ​​the tiny spatial portion originating from the through-hole of the foamed particle in the molded body tends to be small.

[0275] In Comparative Examples 3 and 4, the ratio of the total cross-sectional area Ct of the through holes to the cross-sectional area A of the foamed particles, Ct / A, is too large. As a result, the unevenness of the through holes is clearly formed on the surface of the molded body, and the appearance becomes unqualified.

[0276] The above description illustrates the specific methods of thermoplastic resin foamed particles and thermoplastic resin foamed particle molded articles involved in the present invention based on the embodiments. However, the specific methods of thermoplastic resin foamed particles involved in the present invention are not limited to the embodiments, and the configuration can be appropriately modified within the scope of the present invention without affecting the spirit of the present invention.

Claims

1. A thermoplastic resin foamed granule, which is a thermoplastic resin foamed granule having a thermoplastic resin foam layer, wherein, The foamed particles have a cylindrical shape and have two to eight through holes extending along their axial direction. The outer diameter D of the foamed particles is greater than 2 mm and less than 8 mm. The diameter d of the through hole in the cut surface obtained by cutting the foamed particle perpendicularly to its axial direction at its center is 0.1 mm to 0.5 mm. The ratio of the total cross-sectional area Ct of the through holes in the cut surface obtained by cutting the foamed particle perpendicularly to its axial center to the cross-sectional area A of the foamed particle, Ct / A, is 0.02 or more and 0.15 or less.

2. The thermoplastic resin foamed granules according to claim 1, wherein, The ratio of the cross-sectional area Ca of each through hole to the cross-sectional area A of the foamed particle in the cut surface obtained by cutting the foamed particle perpendicularly to its axial center is 0.005 or more and 0.05 or less.

3. The thermoplastic resin foamed granules according to claim 1 or 2, wherein, The foamed particles have 4 to 8 through holes.

4. The thermoplastic resin foamed granules according to claim 1 or 2, wherein, The ratio R / d of the through hole spacing R of the foamed particle to the hole diameter d of the through hole in the cut surface obtained by cutting the foamed particle perpendicularly to its axial center is 2.0 or more and 4.5 or less.

5. The thermoplastic resin foamed granules according to claim 1 or 2, wherein, The bulk density of the foamed particles is between 10 kg / m³ and 50 kg / m³, and the ratio of the apparent density to the bulk density of the foamed particles is between 1.7 and 1.

9.

6. The thermoplastic resin foamed granules according to claim 1 or 2, wherein, The thermoplastic resin constituting the foam layer is an ethylene-propylene random copolymer, wherein the ethylene-propylene random copolymer contains 0.5% by mass and 3.5% by mass of ethylene.

7. The thermoplastic resin foamed granules according to claim 1 or 2, wherein, The foamed particles have a thermoplastic resin coating layer covering the foamed layer, the coating layer being composed of a thermoplastic resin having a lower melting point or a lower softening point than the thermoplastic resin constituting the foamed layer.

8. A thermoplastic resin foamed granule molded body, wherein, The thermoplastic resin foamed granule molded body is a thermoplastic resin foamed granule molded by in-mold forming of the thermoplastic resin foamed granules as described in claim 1 or 2.

9. The thermoplastic resin foamed granule molded article according to claim 8, wherein, The open bubble rate of the foamed granule molded body is more than 2% and less than 12%.

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