Thermoplastic foam particles and thermoplastic foam particle molded articles

Abstract

To provide a thermoplastic resin foam particle which can shorten a cooling time in a mold, and can obtain a foam particle molding that has a desired shape even if a curing step is omitted and excellent appearance and rigidity, and a thermoplastic resin foam particle molding composed of the foam particle.SOLUTION: A foam particle 1 has a foam layer 2 containing a thermoplastic resin. The foam particle 1 has a columnar shape, and has two or more and eight or less through holes 11 penetrating in its axial direction. In a cross section obtained by cutting the foam particle 1 at a center in the axial direction vertically to the axial direction, a ratio Ct / A of a cross sectional area A of the foam particle 1 to the total Ct of the cross sectional areas of the through holes is 0.02 or more and 0.15 or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to thermoplastic resin foam particles and a thermoplastic resin foam particle molded body.

Background Art

[0002] Polypropylene-based resin foam particle molded bodies are lightweight and excellent in cushioning properties, rigidity, etc., and are therefore used in various applications. A polypropylene-based resin foam particle molded body is manufactured, for example, by a method called an in-mold molding method in which polypropylene-based resin foam particles are filled into a mold and then steam is supplied into the mold for heating. In the in-mold molding method, when steam is supplied into the mold, the foam particles undergo secondary foaming and their surfaces melt. As a result, the foam particles in the mold fuse with each other, and a molded body having a shape corresponding to the shape of the cavity of the mold can be obtained. Since the molded body immediately after molding is likely to expand due to secondary foaming, it is cooled with water, air, etc. in the mold and then removed from the mold.

[0003] For example, Patent Document 1 discloses a technique for in-mold molding of foam particles using a polypropylene-based resin in which the melting point, melt flow index, Z-average molecular weight, etc. are adjusted within a specific range. Further, Patent Document 2 discloses a technique for in-mold molding of cylindrical foam particles having through holes inside.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the aforementioned manufacturing process for molded products, if the foamed particle molded product is stored at room temperature after being released from the mold, the steam that flowed into the bubbles of the foamed particle molded product during in-mold molding condenses within the bubbles, creating negative pressure inside the bubbles. As a result, volume contraction occurs in the foamed particle molded product, which can cause significant deformation. Therefore, after releasing the foamed particle molded product from the mold, a curing process is performed in which the product is left to stand for a predetermined time in a high-temperature atmosphere adjusted to a temperature of, for example, 60°C to 80°C, to restore the shape of the foamed particle molded product.

[0006] However, the curing process requires capital investment and is labor-intensive, so there is a strong desire to eliminate the curing process and significantly improve the productivity of foam particle molded products.

[0007] Furthermore, with the growing environmental awareness in recent years, there is a desire to reduce energy consumption in the manufacturing process of molded products. From this perspective, in addition to eliminating the curing process, there is a desire to shorten the cooling time within the mold.

[0008] However, although the foamed particles described in Patent Document 1 can shorten the curing process, a curing process is still necessary. If the curing process is omitted, the foamed particle molded body shrinks and deforms significantly, making it difficult to obtain a foamed particle molded body with the desired shape. Furthermore, there was room for improvement in shortening the cooling time within the mold for the foamed particles described in Patent Document 1. Moreover, since the foamed particles described in Patent Document 1 require the use of special raw materials, there was also room for improvement in raw material procurement.

[0009] Although the foamed particles described in Patent Document 2 can shorten the cooling time in the mold, they have the problem that the appearance of the molded body is significantly inferior and the rigidity is insufficient because voids are formed on the surface of the molded body due to through-holes in the foamed particles.

[0010] The present invention has been made in view of the above background, and aims to provide thermoplastic resin foam particles that can shorten the cooling time in the mold and, even without a curing process, can produce a foam particle molded article having a desired shape and excellent appearance and rigidity, and a thermoplastic resin foam particle molded article made of these foam particles. [Means for solving the problem]

[0011] One aspect of the present invention relates to thermoplastic resin foam particles according to the following [1] to [8]. [1] Thermoplastic foam particles having a thermoplastic foam layer, The foamed particles have a cylindrical shape and have two to eight through holes that penetrate in the axial direction. Thermoplastic resin foam particles, wherein the ratio Ct / A of the total cross-sectional area of ​​the through holes to the cross-sectional area A of the foam particles at the cross-section obtained by cutting the foam particle perpendicular to the axial direction at its axial center is 0.02 or more and 0.15 or less.

[0012] [2] The thermoplastic resin foam particle according to [1], wherein the ratio Ca / A of the cross-sectional area of ​​one through hole to the cross-sectional area A of the foam particle at the cross-sectional surface obtained by cutting the foam particle perpendicular to the axial direction at its axial center is 0.005 or more and 0.05 or less. [3] The foamed particle has four to eight through holes as described in [1] or [2]. [4] The thermoplastic resin foam particle according to any one of [1] to [3], wherein the diameter d of the through hole in the cross-section obtained by cutting the foam particle perpendicular to the axial direction at its axial center is 0.1 mm or more and 0.5 mm or less.

[0013] [5] A thermoplastic resin foam particle according to any one of [1] to [4], wherein the ratio R / d of the distance between through holes of the foam particle to the diameter d of the through hole in the cross-section obtained by cutting the foam particle perpendicular to the axial direction at its axial center is 2.0 or more and 4.5 or less. [6] The bulk density of the foamed particles is 10 kg / m³ 3 More than 50kg / m 3 The thermoplastic resin foamed particle according to any one of [1] to [5], wherein the ratio of the apparent density to the bulk density of the foamed particle is 1.7 or more and 1.9 or less.

[0014] [7] The thermoplastic resin foam particle according to any one of [1] to [6], wherein the thermoplastic resin constituting the foam layer is an ethylene-propylene random copolymer, and the ethylene-propylene random copolymer contains 0.5% by mass or more and 3.5% by mass or less of ethylene components. [8] The foamed particle has a thermoplastic resin coating layer covering the foamed layer, wherein the coating layer is made of a thermoplastic resin having a melting point or a lower softening point than the thermoplastic resin constituting the foamed layer. The thermoplastic resin foamed particle according to any one of [1] to [7].

[0015] Another aspect of the present invention relates to a thermoplastic foam particle molded article according to the following [9] to

[10] .

[0016] A molded article of thermoplastic resin foam particles obtained by in-mold molding thermoplastic resin foam particles described in any one of [9][1] to [8].

[10] The thermoplastic resin foam particle molded article according to [9], wherein the open bubble rate of the foam particle molded article is 2% or more and 12% or less. [Effects of the Invention]

[0017] According to the above embodiment, it is possible to provide thermoplastic resin foam particles (hereinafter referred to as "foam particles") that can shorten the cooling time in the mold and, even without a curing process, can be used to obtain a foam particle molded article having a desired shape and excellent appearance and rigidity, and a thermoplastic resin foam particle molded article (hereinafter referred to as "foam particle molded article" or "molded article") made from these foam particles. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 is a perspective view of foamed particles. [Figure 2] Figure 2 is a cross-sectional view taken along line II-II in Figure 1 (a plan view of the cross-section of the foamed particle). [Figure 3] Figure 3 is an explanatory diagram showing the method for calculating the area of ​​the high-temperature peak. [Figure 4] Figure 4 is a perspective view of the foamed particles of Example 1. [Figure 5] Figure 5 is a cross-sectional view taken along the VV line in Figure 4 (a plan view of the cross-section of the foamed particle). [Figure 6] Figure 6 is a perspective view of the foamed particles of Example 2. [Figure 7] Figure 7 is a cross-sectional view taken along the line VII-VII in Figure 6 (a plan view of the cross-section of the foamed particle). [Figure 8] Figure 8 is a perspective view of the foamed particles of Example 4. [Figure 9] Figure 9 is a cross-sectional view taken along the line IX-IX in Figure 8 (a plan view of the cross-section of the foamed particle). [Modes for carrying out the invention]

[0019] (Thermoplastic resin foam particles) The thermoplastic resin foam particle 1 has a thermoplastic resin foam layer 2 (hereinafter referred to as "foam layer 2") mainly composed of thermoplastic resin. As shown in Figure 1, the foam particle 1 has a cylindrical shape and has two to eight through holes 11 that penetrate through its interior in the axial direction. The aforementioned cylindrical shape includes, for example, a shape enclosed by a substantially circular base surface 12, a top surface 13 positioned above the base surface 12 and having a shape generally identical to the base surface 12, and a side surface 14 connecting the edge of the base surface 12 and the edge of the top surface 13, as shown in Figure 1. In the cross-section of the foam particle 1 obtained by cutting the foam particle 1 at its axial center with a plane perpendicular to the axial direction (see Figure 2), the ratio Ct / A of the total cross-sectional area of ​​the through holes 11 to the cross-sectional area A of the foam particle 1 is 0.02 or more and 0.15 or less.

[0020] In this way, by providing two to eight through-holes in cylindrical foam particles, and setting the ratio Ct / A of the total cross-sectional area of ​​the through-holes to the cross-sectional area A of the foam particles to the specified range, the cooling time in the mold can be shortened, and significant shrinkage and deformation of the molded body can be suppressed even when the curing process is omitted. Furthermore, the molded body obtained by in-mold molding the foam particles has excellent appearance and rigidity.

[0021] The reason why such effects can be obtained with the aforementioned foamed particles is thought to be as follows, for example. That is, when the foamed particles are molded in a mold, an open-cell structure consisting of open bubbles is formed in the molded body, that is, minute spaces that communicate with the outside of the molded body. Specifically, the open-cell structure is formed by a complex interconnection of voids formed by the interconnection of through-holes of multiple foamed particles, voids formed by the interconnection of through-holes of foamed particles with the gaps between foamed particles, voids formed by the interconnection of gaps between foamed particles, and continuous cell portions of the foamed particles that make up the molded body.

[0022] Because the open-cell structure communicates with the outside of the molded body, when a molded body with an appropriate open-cell ratio is released from the mold, outside air is thought to quickly flow into the cells inside the molded body through the open-cell structure. As a result of outside air flowing into the cells inside the molded body, the internal pressure of the entire molded body can quickly equalize with the pressure of the atmosphere outside the molded body. As a result, the dimensions of the molded body can be stabilized more quickly, and it is thought that significant shrinkage and deformation of the molded body can be suppressed even without a curing process.

[0023] Furthermore, since the foam particles have through-holes, it is believed that when steam is supplied into the mold, the steam can pass through these holes. This makes it easier for the steam to reach the inside of the mold, and thus easier to heat the entire foam particle inside the mold. As a result, even under low molding temperatures during in-mold molding, a molded body with excellent fusion properties and a good appearance can be obtained. Consequently, the amount of heat absorbed by the foam particles by the steam during in-mold molding can be kept low. In addition, the internal temperature of the molded body after demolding is prevented from becoming excessively high. As a result, it is believed that the dimensions of the molded body after in-mold molding can be stabilized more quickly.

[0024] Furthermore, since the foam particles have multiple through-holes, the secondary foaming properties of the foam particles can be moderately reduced. As a result, it is believed that the outward expansion of the foam particles during secondary foaming in mold molding can be moderately suppressed. Moreover, by providing multiple through-holes in the foam particles, the aforementioned open-cell structure is formed in the molded body, and the surface area of ​​the minute spaces originating from the through-holes of the foam particles in the molded body can be increased. As a result, it is presumed that the molded body can be cooled efficiently in the mold, and the surface pressure applied to the mold can be reduced more quickly, thus shortening the cooling time in the mold.

[0025] Furthermore, the ratio Ct / A value in the foamed particles is within the specified range, and the foamed particles are provided with multiple through-holes. This allows for smaller diameters of individual through-holes in the foamed particles compared to the case where there is only one through-hole. As a result, it is believed that the rigidity of the molded body can be increased. In addition, because the diameters of the individual through-holes are smaller, the through-holes become less noticeable on the surface of the molded body, which is believed to improve its appearance.

[0026] If the foam particles do not have through-holes, the molding temperature tends to be high, and there is a risk that the resulting molded body will not adequately form an open-cell structure. 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, the secondary foaming properties of the foam particles may become excessively high, potentially leading to a longer cooling time within the mold.

[0027] Furthermore, when the ratio Ct / A value in the foamed particles is within the specified range and there is only one through-hole, the diameter of the through-hole tends to be larger compared to when there are multiple through-holes. As a result, the through-holes become more noticeable on the surface of the molded body after in-mold molding, which may lead to a deterioration in the appearance of the molded body. In this case, the reduction in cooling time within the mold may be insufficient. Possible causes of this include, for example, the fact that the surface area of ​​the minute spaces in the molded body resulting from the through-holes of the foamed particles tends to be smaller. These problems can be easily avoided by setting the number of through-holes to two or more and preferably four or more and eight or fewer.

[0028] On the other hand, if the number of through-holes in the foamed particles is nine or more, the secondary foaming properties of the foamed particles may be excessively reduced. As a result, this may lead to a decrease in the rigidity of the molded product and a deterioration in its appearance.

[0029] If the ratio Ct / A of the total cross-sectional area of ​​through holes to the cross-sectional area A of the foamed particles is too low, it becomes difficult to form an open-cell structure in the molded body, and a curing process may be required to suppress significant shrinkage and deformation of the molded body. In this case, the secondary foaming properties of the foamed particles may become excessively high, and the reduction in cooling time may be insufficient. By setting the ratio Ct / A to 0.02 or higher, these problems can be easily avoided, the cooling time in the mold can be shortened, and significant shrinkage and deformation of the molded body can be suppressed even without a curing process. From the viewpoint of further shortening the cooling time in the mold, the ratio Ct / A is preferably 0.03 or higher, more preferably 0.04 or higher, and even more preferably 0.05 or higher.

[0030] On the other hand, if the ratio Ct / A of the total cross-sectional area of ​​the through holes to the cross-sectional area A of the foamed particles is too high, it may lead to a decrease in the rigidity of the molded article and deterioration of its appearance. 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 these 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.

[0031] From the viewpoint of more easily obtaining a molded body with good rigidity and appearance while suppressing significant shrinkage and deformation of the molded body and shortening the cooling time, 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.

[0032] The method for calculating the cross-sectional area A of the foamed particle described above is as follows. First, the foamed particle 1 shown in Figure 1 is cut at its center in the axial direction by a plane perpendicular to the axial direction, exposing the cut surface of the foamed particle (i.e., the surface of the cut when the foamed particle is cut perpendicular to the axial direction) as shown in Figure 2 (cross-sectional view taken along line II-II in Figure 1). The cross-sectional area of ​​the foamed particle 1 at this cut surface is measured. Note that the cross-sectional area of ​​the through hole 11 (i.e., the opening area) is not included in the cross-sectional area of ​​the foamed particle 1.

[0033] For example, as shown in Figure 2, if 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 at the cut surface. Also, as shown in Figure 5, for example, if 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 area of ​​the foamed layer 2 and the cross-sectional area of ​​the coating layer 3 at the cut surface.

[0034] The above procedure is performed on 100 or more foam particles, and the arithmetic mean of the resulting cross-sectional areas of the foam particles is defined as the cross-sectional area A of the foam particles at the cut surface. The cross-sectional area of ​​the foam particles at the cut surface can be measured, for example, by taking a photograph of the cut surface of the foam particles and performing image analysis. Furthermore, the cross-sectional area A of the foam particles obtained by the method described above is sometimes referred to as the "average cross-sectional area A of the foam particles."

[0035] The method for calculating the total cross-sectional area Ct of the through-holes is as follows: First, as shown in Figure 2, the foam particle 1 is cut at its center in the axial direction by a plane perpendicular to the axial direction, exposing the cut surface of the foam particle as shown in Figure 2. The sum of the cross-sectional areas of all the through-holes at this cut surface is measured.

[0036] The above procedure is performed for 50 or more foamed particles, and the arithmetic mean of the sum of the cross-sectional areas of the through-holes obtained is defined as the total cross-sectional area of ​​the through-holes, Ct. The cross-sectional area of ​​the through-holes at the cross-section can be measured, for example, by taking a photograph of the cross-section of the foamed particle and performing image analysis.

[0037] The ratio Ca / A of the cross-sectional area of ​​each through-hole to the cross-sectional area A of the foamed particle at the axial center with 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 more easily improved. 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.

[0038] In the cross-section obtained by cutting the foamed particle at its axial center with a plane perpendicular to the axial direction, the diameter d of the through-hole is preferably 0.1 mm or more and 0.5 mm or less. By setting the diameter d of the through-hole to 0.5 mm or less, the rigidity and appearance of the molded article can be more easily improved. From this 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. Furthermore, by setting the diameter d of the through-hole to 0.1 mm or more, steam can pass through the through-hole more easily when steam is supplied into the mold. From this viewpoint, the diameter d of the through-hole is more preferably 0.2 mm or more.

[0039] The hole diameter d of the through-holes in the foamed particle is determined as follows: First, the foamed particle 1 is cut at its axial center with a plane perpendicular to the axial direction, exposing the cut surface as shown in Figure 2. Next, a photograph of the cut surface is taken, and the cross-sectional area (i.e., opening area) of each through-hole 11 in the cut surface is calculated. Then, the diameter of a virtual perfect circle having the same area as the cross-sectional area of ​​the through-hole 11 is calculated, and this value is taken as the hole diameter of each through-hole.

[0040] Perform the above procedure on 50 or more foam particles, and the arithmetic mean of the resulting through-hole diameters is defined as the through-hole diameter d of the foam particles. Even if the cross-sectional shape and diameter of individual through-holes are not uniform in the axial direction of the foam particle, the through-hole diameter d is determined based on the diameter of the through-hole at the cross-section as described above. Furthermore, the through-hole diameter d of the foam particles obtained by the method described above is sometimes referred to as the "average through-hole diameter d of the foam particles."

[0041] The pore diameter d of the through-holes can be adjusted to the aforementioned specific range by adjusting the size of the pore diameter dr in the resin particles described later, the apparent density of the foamed particles, etc. Furthermore, by using two-stage foamed particles manufactured by two-stage foaming, the pore diameter d can be adjusted to a smaller value more easily.

[0042] The arrangement of through-holes in the cross-section of the foam particle is not particularly limited and can take various forms. For example, as shown in Figures 4 and 5, one of the multiple through-holes 11 (11a to 11e) is arranged so as to penetrate the central axis 10 of the foam particle 1, and the other through-holes 11b to 11e are arranged around the central axis 10 of the foam particle 1. In the foam particle 1 shown in Figure 5, the through-holes 11b to 11e are arranged at positions where the circumferential spacing on the cross-section of the foam particle 1 is approximately equal. Alternatively, as shown in Figure 7, for example, four through-holes 11 (11f to 11i) may be arranged around the central axis 10 of the foam particle 1, at positions where the circumferential spacing on the cross-section of the foam particle 1 is approximately equal. Similarly, as shown in Figure 9, three through-holes 11 (11j to 11l) may be arranged around the central axis 10 of the foam particle 1, at positions where the circumferential spacing on the cross-section of the foam particle 1 is approximately equal.

[0043] From the viewpoint of further enhancing the effects of providing the aforementioned multiple through holes, it is preferable that the multiple through holes are arranged at positions that are approximately equally spaced on the cross-section of the foamed particle. From a similar viewpoint, it is more preferable that at least some of the multiple through holes are arranged around the central axis of the foamed particle, at positions where the circumferential spacing on the cross-section obtained by cutting the foamed particle at its axial center with a plane perpendicular to the axial direction is approximately equal.

[0044] Furthermore, the ratio R / d of the distance R between through-holes of the foam particles to the diameter d of the through-holes in the cross-section of the foam particles is preferably 2.0 or greater, and more preferably 2.5 or greater. In this case, the appearance of the molded body can be further improved, and the rigidity of the molded body can be further increased. Furthermore, the ratio R / d is preferably 4.5 or less, and more preferably 3.5 or less. In this case, the cooling time of the molded body in the mold can be further shortened.

[0045] The method for calculating the through-hole distance R described above is as follows: First, the position of the geometric center in the cross-sectional shape of each through-hole is determined on the cross-section of the foamed particle, and this position is taken as the center point of each through-hole. Next, for all through-holes, the distance between the center point of the through-hole to be measured and the center point of the through-hole having the closest center point to the center point of the through-hole in question is calculated, i.e., the distance between the center points. Then, the arithmetic mean of the distances between the center points for all through-holes is taken as the through-hole distance of each individual foamed particle. The above operations are performed for 100 or more foamed particles, and the arithmetic mean of the obtained through-hole distances is taken as the through-hole distance R of the foamed particle. The through-hole distance R of the foamed particle obtained by the above method is sometimes called the "average through-hole distance R of the foamed particle".

[0046] From the viewpoint of increasing the thickness of the foam particles and improving the secondary foaming properties of the foam particles and the rigidity of the molded article, and from the viewpoint of more reliably suppressing deformation and shrinkage of the molded article when the curing process is omitted, the outer diameter D of the foam 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 viewpoint of improving the filling of foam particles into the mold, the outer diameter D of the foam particles is preferably 8 mm or less, more preferably 5 mm or less, and even more preferably 4.5 mm or less.

[0047] The method for calculating the outer diameter D of the foamed particle is as follows: First, the foamed particle 1 is cut at its center in the axial direction by a plane perpendicular to the axial direction, exposing the cut surface of the foamed particle 1 as shown in Figure 2. The cross-sectional area of ​​the foamed particle 1 and the cross-sectional area of ​​the through hole 11 at this cut surface are calculated. Then, the sum of the cross-sectional area of ​​the foamed particle 1 and the cross-sectional area of ​​the through hole 11 is calculated, and the diameter of a virtual perfect circle with an area equal to this sum is taken as the outer diameter of each foamed particle.

[0048] In this manner, the outer diameter is calculated for 100 or more foam particles 1, and the arithmetic mean of the obtained outer diameters of the foam particles is defined as the outer diameter D of the foam particles. The outer diameter of the foam particles 1 at the cross-section can be measured, for example, by taking a photograph of the cross-section of the foam particles and performing image analysis. Furthermore, the outer diameter D of the foam particles obtained by the method described above is sometimes referred to as the "average outer diameter D of the foam particles."

[0049] The bulk density of the foamed particles is 10 kg / m³. 3 More than 50kg / m 3 Preferably, it is 10 kg / m 3 More than 35kg / m 3It is more preferable that the following conditions are met. Further, the ratio of the apparent density to the bulk density of the foamed particles is preferably 1.7 or more and 1.9 or less, and more preferably 1.7 or more and 1.8 or less. By performing in-mold molding using foamed particles having a bulk density within the above range, a molded body that is lightweight and has excellent rigidity can be easily obtained. Further, by performing in-mold molding using foamed particles in which, in addition to the bulk density, the ratio of the apparent density to the bulk density is within the specific range, the open cell ratio of the molded body can be more easily made appropriate, the rigidity and appearance of the molded body can be more easily improved, and the effect of suppressing shrinkage and deformation of the molded body when the curing process is not performed can be more surely obtained.

[0050] The method for calculating the bulk density of the foamed particles is as follows. First, the foamed particles are left standing for 24 hours or more in an environment of 50% relative humidity, 23°C temperature, and 1 atm pressure to adjust the state of the foamed particles. Next, the state-adjusted foamed particles are filled into a graduated cylinder so as to naturally accumulate, and the bulk volume (unit: L) of the foamed particle group is read from the scale of the graduated cylinder. Then, the value obtained by dividing the mass (unit: g) of the foamed particle group in the graduated cylinder by the above-mentioned bulk volume is converted to the unit to obtain the bulk density of the foamed particles (unit: kg / m<� 3 )

[0051] The method for calculating the apparent density of the foamed particles is as follows. First, the foamed particle group is left standing for 1 day in an environment of 50% relative humidity, 23°C temperature, and 1 atm pressure to adjust the state of the foamed particles. After measuring the mass (unit: g) of this foamed particle group, it is submerged in a graduated cylinder containing alcohol (for example, ethanol) at 23°C using a wire mesh or the like, and the volume (unit: L) of the foamed particle group is determined from the rise in the liquid level. Then, the value obtained by dividing the mass of the foamed particle group by the volume of the foamed particle group is converted to the unit to calculate the apparent density of the foamed particles (unit: kg / m 3 )

[0052] From the viewpoint of the balance between the lightweight property and rigidity of the molded body, the apparent density of the foamed particles is 10 kg / m 3 or more and 150 kg / m 3Preferably, it is 15 kg / m 3 More than 100kg / m 3 It is more preferable that the following is 20 kg / m 3 More than 80kg / m 3 It is even more preferable that the following conditions apply: 25 kg / m 3 More than 60kg / m 3 The following is particularly preferable. By performing in-mold molding using foamed particles with a low apparent density, a lighter molded body can be easily obtained. Furthermore, conventionally, when manufacturing molded bodies with particularly low density, the molded body tends to deform significantly after demolding, making it difficult to omit the curing process. In contrast, since the foamed particles allow for the omission of the curing process even when the apparent density is low, a lightweight molded body with a desired shape can be manufactured without curing.

[0053] The foamed particles have a thermoplastic resin foam layer. Suitable thermoplastic resins for the foam layer include, for example, polypropylene resins, polyethylene resins, polyamide resins, and crystalline polyester resins. In this specification, polypropylene resin refers to a propylene copolymer containing 50% by mass or more of a propylene monomer homopolymer and propylene-derived structural units. Polyethylene resin refers to an ethylene copolymer containing 50% by mass or more of an ethylene monomer homopolymer and ethylene-derived structural units.

[0054] In this specification, "crystalline" means that the endothermic peak heat amount associated with the melting of the resin is 5 J / g or more, based on the DSC curve obtained using a differential scanning calorimetry device with a heating rate of 10°C / min, employing the method described in JIS K7122 (1987) for measuring the heat of fusion after a certain heat treatment (the heating rate and cooling rate in conditioning the test specimen are both 10°C / min). Preferably, the endothermic peak heat amount is 15 J / g or more, and more preferably 30 J / g or more.

[0055] The foamed layer may contain polymers other than plastic resins such as elastomers, to the extent that they do not impair the effects described above. The content of other polymers in the foamed layer is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and especially preferably 0, that is, the foamed layer contains substantially only thermoplastic resins as polymers.

[0056] Furthermore, the foamed layer may contain additives such as foam regulators, nucleating agents, flame retardants, flame retardant aids, plasticizers, antistatic agents, antioxidants, UV inhibitors, light stabilizers, conductive fillers, antibacterial agents, and colorants, to the extent that they do not impair the effects described above. The amount of additives in the foamed layer is preferably, for example, 0.01 parts by mass or more and 10 parts by mass or less per 100 parts by mass of thermoplastic resin.

[0057] The thermoplastic resin constituting the foam layer is preferably a polypropylene resin, and more preferably a propylene copolymer obtained by copolymerizing propylene with another monomer. Examples of propylene copolymers include copolymers of propylene and α-olefins having 4 to 10 carbon atoms, such as ethylene-propylene copolymer, butene-propylene copolymer, hexene-propylene copolymer, and ethylene-propylene-butene copolymer. These copolymers may be random copolymers, block copolymers, etc., but random copolymers are preferred. Furthermore, the foam layer may contain one type of polypropylene resin, or two or more types of polypropylene resins.

[0058] The thermoplastic resin constituting the foam layer is particularly preferably an ethylene-propylene random copolymer containing 0.5% to 3.5% by mass of ethylene components among these polypropylene resins. Foamed particles with such a foam layer exhibit excellent secondary foaming properties and moldability. Furthermore, by performing in-mold molding using such foamed particles, it is possible to more easily obtain a molded article with excellent rigidity and surface properties, and which can suppress deformation and shrinkage even when the curing process is omitted.

[0059] From the viewpoint of obtaining a molded article with superior rigidity and less deformation and shrinkage when the curing process is omitted, it is preferable that the ethylene content in the ethylene-propylene random copolymer is 0.5% by mass or more and less than 2.0% by mass. On the other hand, from the viewpoint of improving the moldability of the foamed particles and obtaining a molded article with excellent energy absorption characteristics, it is preferable that the ethylene content in the ethylene-propylene random copolymer is 2.0% by mass or more and 3.5% by mass or less.

[0060] The aforementioned "ethylene component" and "propylene component" refer to the ethylene-derived and propylene-derived constituent units, respectively, in the ethylene-propylene copolymer. Furthermore, the ethylene component content is the mass ratio of the ethylene component when the total of the ethylene and propylene components is considered to be 100% by mass. The content of each component in the ethylene-propylene copolymer can be determined based on the results of IR spectral measurements.

[0061] When the foamed 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 (i.e., lower molding pressure). From the viewpoint of improving this effect, the melting point Tmc of the polypropylene resin constituting the foamed layer is preferably 152°C or lower, and more preferably 148°C or lower. On the other hand, from the viewpoint of further improving the heat resistance and mechanical strength of the molded article, the melting point Tmc of the polypropylene resin constituting the foamed layer is preferably 135°C or higher, more preferably 138°C or higher, and even more preferably 140°C or higher.

[0062] The melting point of the thermoplastic resin constituting the foam layer can be determined by differential scanning calorimetry (i.e., DSC) based on JIS K7121-1987 and based on the acquired DSC curve. Specifically, first, the test specimen is conditioned according to "(2) When measuring the melting temperature after performing a certain heat treatment". The conditioned test specimen is heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and the peak temperature of the melting peak appearing in the DSC curve is taken as the melting point Tmc of the thermoplastic resin. If multiple melting peaks appear in the DSC curve, the peak temperature of the melting peak with the largest area is taken as the melting point Tmc.

[0063] When the foamed layer is composed of a polypropylene resin, the melt flow rate (i.e., 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 moldability can be further improved. On the other hand, from the viewpoint of further increasing the rigidity of the molded article, the MFR of the polypropylene resin is preferably 12 g / 10 min or less, and more preferably 10 g / 10 min or less. The MFR of the polypropylene resin is a value measured under the conditions of a test temperature of 230°C and a load of 2.16 kg, based on JIS K7210-1:2014.

[0064] 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 increasing the rigidity of the molded article 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, even more preferably 900 MPa or more, and particularly preferably 1200 MPa or more. On the other hand, from the viewpoint of being able to mold an article with excellent appearance and rigidity at a lower molding temperature (i.e., lower molding pressure) and improving the energy absorption characteristics of the molded article, the flexural modulus of the polypropylene resin constituting the foam layer is preferably less than 1200 MPa, more preferably 1100 MPa or less, and even more preferably 1000 MPa or less. The flexural modulus of the polypropylene resin can be determined based on JIS K7171:2008.

[0065] Conventionally, when foamed particles made from polypropylene resins with a flexural modulus of less than 1200 MPa were molded in a mold, omitting the curing process tended to cause significant shrinkage and deformation of the molded product after demolding. This is thought to be due, for example, to low resistance to shrinkage and deformation after demolding. In contrast, the molded product manufacturing method described above allows for the omission of the curing process, even when using foamed particles made from polypropylene resins having a flexural modulus of less than 1200 MPa.

[0066] The foamed particles may have a multilayer 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 part of the foamed layer, but it is preferable that the coating layer covers the entire side surface of the foamed layer.

[0067] The aforementioned coating layer has the effect of improving the fusion properties of the foamed particles. Whether or not the coating layer has such an effect can be determined by the following method. First, foamed particles equipped with a foamed layer and a coating layer are used to perform in-mold molding at various steam pressures. The fusion rate of the molded body obtained in this way is measured. Then, the lowest steam pressure at which a molded body with a fusion rate of 90% or more is obtained is identified, and this value is defined as the minimum molding pressure P1.

[0068] Separately, a similar evaluation is performed using foamed particles consisting only of a foamed layer, and the lowest steam pressure at which a molded product with a fusion rate of 90% or more is obtained is identified, and this value is defined as the minimum molding pressure P2. Then, if the minimum molding pressure P1 of foamed particles with a coating layer is less than the minimum molding pressure P2 of foamed particles without a coating layer, it can be determined that the coating layer has the effect of improving fusion properties.

[0069] The method for measuring the fusion rate of a molded body is as follows: First, the molded body is bent and broken to expose the fracture surface. The total number of foam particles and the number of foam particles that have fractured inside the foam particles (i.e., foam particles that have undergone material destruction) are counted on this fracture surface. Then, the ratio of the total number of foam particles to the number of foam particles that have fractured inside the foam particles present on the fracture surface is calculated, and this value is defined as the fusion rate (unit: %).

[0070] Preferably, the coating layer is made of a thermoplastic resin having a melting point or softening point lower than that of the thermoplastic resin constituting the foam layer. By coating the foam layer with such a thermoplastic resin coating layer, the foam particles can be fused at a lower molding temperature (i.e., lower molding pressure) during in-mold molding. As a result, deformation and shrinkage of the molded article can be more reliably suppressed when the curing process is omitted.

[0071] The thermoplastic resin constituting the coating layer may be a crystalline thermoplastic resin or an amorphous thermoplastic resin. As the crystalline thermoplastic resin used in the coating layer, for example, the same crystalline thermoplastic resin used in the foam layer can be used. Examples of amorphous thermoplastic resins used in the coating layer include polystyrene resins and amorphous polyester resins.

[0072] When the foamed layer is composed of a polypropylene resin, the coating layer is preferably composed of a polyolefin resin, more preferably of a polyethylene resin and / or a polypropylene resin, and even more preferably of a polypropylene resin, from the viewpoint of adhesion to the foamed layer. Examples of polypropylene resins used in the coating layer include ethylene-propylene copolymer, ethylene-butene copolymer, ethylene-propylene-butene copolymer, and propylene homopolymer. Among these, the thermoplastic resin constituting the coating layer is particularly preferably ethylene-propylene copolymer and / or ethylene-propylene-butene copolymer.

[0073] The coating layer may contain additives such as nucleating agents, flame retardants, flame retardant aids, plasticizers, antistatic agents, antioxidants, UV inhibitors, light stabilizers, conductive fillers, antibacterial agents, and colorants, to the extent that they do not impair the effects described above. The amount of additives in the coating layer is preferably, for example, 0.01 parts by mass or more and 10 parts by mass or less per 100 parts by mass of thermoplastic resin.

[0074] The coating layer of the foamed particles may be foamed or non-foamed, but it is preferable that it be substantially non-foamed. "Substantially non-foamed" means that there is almost no cellular structure. The thickness of the coating layer is, for example, 0.5 μm to 100 μm. An intermediate layer may also be provided between the foamed layer and the coating layer.

[0075] From the viewpoint of improving moldability while maintaining the rigidity of the molded article, the mass ratio (mass %) of the resin constituting the foam layer and 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.

[0076] From the viewpoint of improving the moldability of the foam particles and further increasing the rigidity of the molded article, the closed-cell ratio of the foam particles is preferably 90% or more, more preferably 92% or more, and even more preferably 95% or more.

[0077] The closed-cell ratio of foamed particles can be measured using an air-comparison hydrometer based on ASTM-D2856-70 Procedure C. Specifically, it is measured as follows: First, the foamed particles are left to stand for 24 hours or more in an environment of 50% relative humidity, 23°C, and 1 atm pressure to adjust the state of the foamed particles. After adjusting the state, the value of the mark when the foamed particles are naturally deposited in a graduated cylinder is approximately 20 cm³. 3 A sample is taken for measurement in the following manner. This sample is then submerged in a graduated cylinder containing ethanol at 23°C, and the apparent volume of the sample is measured based on the rise in the liquid level.

[0078] After thoroughly drying the sample from which the apparent volume has been measured, the true volume of the sample is measured using a Shimadzu AccuPic II 1340 according to procedure C described in ASTM-D2856-70. Then, using these volume values, the closed-cell percentage (in %) of the sample is calculated based on the following formula (1). Closed cell ratio = (Vx-W / ρ)×100 / (Va-W / ρ) (1)

[0079] However, Vx in the above formula (1) (unit: cm) 3 ) is the true volume of the foamed particle (i.e., the sum of the volume of the resin constituting the foamed particle and the total volume of the closed-cell portion of the foamed particle), and Va (unit: cm) 3) is the apparent volume of the foaming particles (i.e., the volume measured from the rise in the liquid level when the foaming particles are submerged in a graduated cylinder containing ethanol), W (unit: g) is the mass of the sample used for measurement, and ρ (unit: g / cm³) is the apparent volume of the foaming particles (i.e., the volume measured from the rise in the liquid level when the foaming particles are submerged in a graduated cylinder containing ethanol), W (unit: g / cm³) is the mass of the sample used for measurement, and ρ (unit: g / cm³) is the apparent volume of the foaming particles (i.e., the volume measured from the rise in the liquid level when the foaming particles are submerged in a graduated cylinder containing ethanol), W (unit: g) is the mass of the sample used for 3 ) is the density of the thermoplastic resin that makes up the foamed layer.

[0080] The above procedure is performed five times using different measurement samples, and the arithmetic mean of the closed-cell ratios obtained from these five measurements is taken as the closed-cell ratio of the foamed particles.

[0081] When the thermoplastic resin constituting the foam layer is a crystalline thermoplastic resin, it is preferable that the foam particles have a crystalline structure such that the DSC curve obtained when heated from 23°C to 200°C at a heating rate of 10°C / min shows an endothermic peak due to the melting inherent to the crystalline thermoplastic resin constituting the foam layer, and one or more melting peaks located at a higher temperature than this endothermic peak. Foam particles having such a crystalline structure have excellent mechanical strength and moldability. In the following, the endothermic peak due to the melting inherent to the crystalline thermoplastic resin that appears in the DSC curve is referred to as the "resin-specific peak," and the melting peak that appears at a higher temperature than the resin-specific peak is referred to as the "high-temperature peak." The resin-specific peak is caused by the endothermic reaction when the crystals inherent in the crystalline thermoplastic resin constituting the foam layer melt. On the other hand, the high-temperature peak is presumed to be caused by the melting of secondary crystals formed in the crystalline thermoplastic resin constituting the foam layer during the manufacturing process of the foam particles. That is, if a high-temperature peak appears in the DSC curve, it is presumed that secondary crystals have been formed in the crystalline thermoplastic resin.

[0082] Whether or not the foamed particles possess the aforementioned crystalline structure can be determined based on the DSC curve obtained by performing differential scanning calorimetry (DSC) under the conditions described above, in accordance with JIS K7121:1987. Furthermore, 1 to 3 mg of foamed particles should be used as a sample for the DSC.

[0083] Specifically, the DSC curve obtained when heating from 23°C to 200°C at a heating rate of 10°C / min (i.e., the first heating) shows both a high-temperature peak and a resin-specific peak of the crystalline thermoplastic resin constituting the foam layer. In contrast, the DSC curve obtained when cooling from 200°C to 23°C at a cooling rate of 10°C / min after the first heating, and then heating again from 23°C to 200°C at a heating rate of 10°C / min (i.e., the second heating), shows only the resin-specific 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, it is possible to distinguish between the resin-specific peak and the high-temperature peak. The temperature at the peak of this resin-specific peak may differ slightly between the first and second heating, but the difference is usually within 5°C.

[0084] The heat of fusion 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, from the viewpoint of further improving the moldability of the foamed particles and obtaining a molded article with superior rigidity.

[0085] The heat of fusion of the aforementioned high-temperature peak is determined as follows. First, the foamed particles are allowed to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm to adjust their state. Using 1 to 3 mg of the adjusted foamed particles as a sample, a differential scanning calorimetry (DSC) curve is obtained by heating them from 23°C to 200°C at a heating rate of 10°C / min. An example of a DSC curve is shown in Figure 3. When the foamed particles have a high-temperature peak, the DSC curve shows a resin-specific peak ΔH1 and a high-temperature peak ΔH2 whose peak is at a higher temperature than the peak of the resin-specific peak ΔH1, as shown in Figure 3.

[0086] Next, draw a straight line L1 connecting point α, which corresponds to 80°C on the DSC curve, and point β, which corresponds to the melting termination temperature T of the foamed particles. Note that the melting termination temperature T is the high-temperature endpoint of the high-temperature peak ΔH2, that is, the intersection point of the high-temperature peak ΔH2 on the DSC curve and the baseline on the side of the high-temperature peak ΔH2 that is higher than ΔH2.

[0087] After drawing the straight line L1, a straight line L2 is drawn parallel to the vertical axis of the graph, passing through the maximum point γ located between the resin intrinsic peak ΔH1 and the high-temperature peak ΔH2. This straight line L2 separates the resin intrinsic peak ΔH1 and the high-temperature peak ΔH2. The amount of heat absorbed by the high-temperature peak ΔH2 can be calculated based on the area enclosed by the portion of the DSC curve that constitutes the high-temperature peak ΔH2, and the straight lines L1 and L2.

[0088] (Method for manufacturing foamed particles) The foamed particles can be manufactured, for example, by dispersing thermoplastic resin particles containing a thermoplastic resin (hereinafter referred to as "resin particles") 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. This type of foaming method is sometimes called the "direct foaming method."

[0089] Resin particles can be produced, for example, by the strand-cutting method. In the strand-cutting method, first, a thermoplastic resin constituting the foam layer and additives such as bubble nucleating agents, which are supplied as needed, are placed in an extruder, heated, and kneaded to form a molten resin mixture. Then, the molten resin mixture is extruded through small holes in a die attached to the tip of the extruder to form a columnar extruder having multiple through holes. After cooling this extruder, it is cut to the desired length to obtain resin particles with a single-layer structure consisting of a core layer containing thermoplastic resin and having multiple through holes.

[0090] To obtain foamed particles with a multilayer structure comprising a foamed layer and a coating layer covering the foamed layer, the multilayer resin particles can be produced using a co-extrusion apparatus equipped with a core layer forming extruder, a coating layer forming extruder, and a co-extrusion die connected to these two extruders. In this case, the core layer forming extruder melts and kneads the thermoplastic resin constituting the foamed layer with additives added as needed to produce a molten resin mixture for core layer formation. Similarly, the coating layer forming extruder melts and kneads the thermoplastic resin constituting the coating layer with additives added as needed to produce a molten resin mixture for coating layer formation.

[0091] These molten mixtures are co-extruded and merged in a die to form a multilayer composite consisting of a non-foamed columnar core layer and a non-foamed coating layer covering the outer surface of the core layer. This composite is extruded through a small hole in the die to form a columnar extruder with multiple through-holes in the core layer. After cooling this extruder, it is cut to the desired length to obtain multilayer resin particles with multiple through-holes in the core layer. The method for producing the resin particles is not limited to the method described above, and methods such as hot cutting or underwater cutting may also be used.

[0092] When producing resin particles, it is preferable to employ a strand-cutting method, in which a columnar extruded material is cooled in water and then cut. In this case, the precision of the resin particle shape can be further improved, and the shape of the multiple through-holes in the final foamed particles can be more easily determined to the desired shape.

[0093] The particle size of the resin particles is preferably 0.1 mm or more and 3.0 mm or less, and more preferably 0.3 mm or more and 1.5 mm or less.

[0094] Furthermore, the mass per resin particle is preferably 0.1 mg to 20 mg, more preferably 0.2 mg to 10 mg, even more preferably 0.3 mg to 5 mg, and particularly preferably 0.4 mg to 2 mg. The mass per resin particle is the value obtained by dividing the mass of 200 randomly selected resin particles by the number of resin particles. The mass per resin particle obtained by the method described above is sometimes referred to as the "average mass per resin particle."

[0095] 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 core layer:coating layer = 99.5:0.5 to 85:15, more preferably 99:1 to 92:8, and even more preferably 97:3 to 90:10.

[0096] When forming foamed particles having through holes, the diameter d of the through holes in the core layer of the resin particles can be adjusted to the specified range by adjusting the diameter dr of the through holes in the core layer of the resin particles. More specifically, by setting the diameter dr of the through holes in the resin particles to 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 diameter d of 0.1 mm or more and less than 0.5 mm can be easily manufactured. The diameter dr of the through holes in the core layer of the resin particles can be adjusted, for example, by the diameter of the small holes in the die used to form the through holes (i.e., the inner diameter of the die).

[0097] The method for calculating the pore diameter dr of through-holes in resin particles is the same as the method for calculating the pore diameter d of through-holes in foamed particles described above, except that resin particles are used instead of foamed particles. The pore diameter dr of through-holes in resin particles obtained by this method is sometimes called the "average pore diameter dr of through-holes in resin particles."

[0098] Furthermore, when employing a strand-cutting method for cutting the extruded material, that is, a method in which the cylindrical extruded material is taken out of the die, cooled in water while being taken up, and then cut to an appropriate length, the particle size, length / outer diameter ratio, and mass per piece of resin can be adjusted by appropriately changing the extrusion speed, take-up speed, cutter speed, etc., during the extrusion of the molten resin mixture.

[0099] After producing resin particles as described above, the resin particles are dispersed in a dispersion medium. The dispersion of resin particles may be carried out in a sealed container used in the subsequent foaming process, or in a container separate from the sealed container used in the foaming process. From the viewpoint of simplifying the manufacturing process, it is preferable to carry out the dispersion process in a sealed container used in the foaming process.

[0100] As the dispersion medium, an aqueous dispersion medium mainly composed of water is used. In addition to water, the aqueous dispersion medium may also contain hydrophilic organic solvents such as ethylene glycol, glycerin, 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.

[0101] It is preferable to add a dispersant to the dispersion medium. By adding a dispersant to the dispersion medium, the fusion of heated resin particles in the container during the foaming process can be suppressed. The amount of dispersant added is preferably 0.001 parts by mass or more and 5 parts by mass or less per 100 parts by mass of resin particles. Organic dispersants and inorganic dispersants can be used as dispersants, but due to their ease of handling, it is preferable to use fine particulate inorganic materials as dispersants. More specifically, as dispersants, for example, clay minerals such as amsnite, kaolin, mica, and clay, or aluminum oxide, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, iron oxide, etc., can be used. These dispersants may be used alone, or two or more dispersants may be used in combination. Among these, it is preferable to use clay minerals as dispersants. Clay minerals may be natural or synthesized.

[0102] When using a dispersant, it is preferable to use an anionic surfactant such as sodium dodecylbenzenesulfonate, sodium alkylbenzenesulfonate, sodium lauryl sulfate, or sodium oleate as a dispersing aid. The amount of dispersing aid added is preferably 0.001 parts by mass or more and 1 part by mass or less per 100 parts by mass of resin particles.

[0103] After dispersing resin particles in a dispersion medium, the resin particles are impregnated with a blowing agent in a sealed container. The blowing agent used to impregnate the resin particles is preferably a physical blowing agent. Examples of physical blowing agents include inorganic physical blowing agents such as carbon dioxide, air, nitrogen, helium, and argon; and organic physical blowing agents such as aliphatic hydrocarbons like propane, butane, and hexane; cyclic aliphatic hydrocarbons like cyclopentane and cyclohexane; and halogenated hydrocarbons such as 1,3,3,3-tetrafluoropropene, 1-chloro-3,3,3-trifluoropropene, chlorofluoromethane, trifluoromethane, 1,1-difluoromethane, 1-chloro-1,1-dichloroethane, 1,2,2,2-tetrafluoroethane, methyl chloride, ethyl chloride, and methylene chloride. These physical blowing agents may be used individually or in combination of two or more. Furthermore, inorganic and organic physical blowing agents can be mixed and used. From the viewpoint of environmental impact and ease of handling, inorganic physical blowing agents are preferred, and carbon dioxide is more preferred.

[0104] The amount of foaming agent added per 100 parts by mass of resin particles is preferably 0.1 parts by mass or more and 30 parts by mass or less, and more preferably 0.5 parts by mass or more and 15 parts by mass or less.

[0105] In the manufacturing process of foamed particles, one method for impregnating resin particles with a foaming agent is to supply the foaming agent into a sealed container and increase the pressure inside the container to impregnate the resin particles in the dispersion medium with the foaming agent. In this case, heating the resin particles together with the dispersion medium can further promote the impregnation of the foaming agent into the resin particles.

[0106] 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. Within these ranges, foamed particles can be manufactured safely without the risk of damage or explosion of the sealed container.

[0107] Furthermore, when heating the dispersion medium, increasing its temperature at a rate of 1-5°C / min allows the foaming temperature to be kept within an appropriate range.

[0108] After the foaming agent has impregnated the resin particles, the contents of the sealed container are released into an environment with lower pressure than the sealed container. This causes the core layer of the resin particles to foam, forming a cellular structure, which is then cooled by the outside air, stabilizing the cellular structure and resulting in foamed particles.

[0109] When the core layer is made of polypropylene resin, it is preferable to heat and foam in the following manner when impregnating with a foaming agent. Specifically, first, a one-stage holding step is performed in which the temperature is held for a sufficient time, preferably about 10 to 60 minutes, at a temperature of (melting point of polypropylene resin - 20°C) or higher and below the (melting end temperature of polypropylene resin). Then, the temperature is adjusted from (melting point of polypropylene resin - 15°C) to below (melting end temperature of polypropylene resin + 10°C). Then, if necessary, a second-stage holding step is performed in which the temperature is held for a further sufficient time, preferably about 10 to 60 minutes. After that, it is preferable to release the contents of the sealed container to the outside while the temperature inside the sealed container is at or above the (melting point of polypropylene resin - 10°C), thereby foaming the resin particles. It is more preferable that the temperature inside the sealed container during foaming is at or above the (melting point of polypropylene resin) and below the (melting point of polypropylene resin + 20°C). By heating and foaming the resin particles in this manner, secondary crystals are formed in the polypropylene resin constituting the foamed layer, making it easy to obtain foamed particles that have excellent mechanical strength and moldability.

[0110] The foamed particles obtained as described above may be used directly to produce the molded article. Alternatively, the foamed particles obtained by the direct foaming method described above can be further foamed to reduce their apparent density, and the resulting foamed particles can be used to produce the molded article. When foaming resin particles in two stages in this way, the first foaming stage is called the one-stage foaming stage, and the foamed particles obtained in the one-stage foaming stage are called one-stage foamed particles. The second foaming stage is called the two-stage foaming stage. The foamed particles obtained in the two-stage foaming stage are sometimes called two-stage foamed particles.

[0111] A method for reducing the apparent density of foamed particles through two-stage foaming is as follows: First, as a one-stage foaming process, resin particles are foamed using the direct foaming method described above to obtain one-stage foamed particles. Then, internal pressure is applied to the one-stage foamed particles. More specifically, after placing the one-stage foamed particles in a pressure vessel, the inside of the pressure vessel is pressurized with an inorganic gas such as air or carbon dioxide to impregnate the foamed particles with the inorganic gas. This makes the pressure inside the bubbles of the one-stage foamed particles equal to or greater than atmospheric pressure. Then, the one-stage foamed particles removed from the pressure vessel are heated using a heating medium such as steam or heated air in an environment with a pressure lower than the pressure inside the bubbles, thereby causing the one-stage foamed particles to undergo two-stage foaming.

[0112] (Foam particle molded body) A molded body of foamed particles can be obtained by in-mold molding the foamed particles. The molded body has an open-cell structure. The open-cell structure is a minute space that communicates with the outside of the molded body. The open-cell structure is formed by a complex network of interconnected voids, such as voids formed by the interconnection of through-holes of multiple foamed particles, voids formed by the interconnection of through-holes of foamed particles with voids formed between foamed particles, voids formed by the interconnection of gaps between foamed particles, and continuous cell portions of the foamed particles constituting the molded body.

[0113] The open air bubble ratio of the molded body is preferably 2% to 12%. By setting the open air bubble ratio of the molded body within the specified 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, when the molded body has an open air bubble structure at the specified ratio, air flows quickly into the air bubbles inside the molded body after demolding, increasing the internal pressure of the entire molded body, which in turn makes it easier for the dimensions of the molded body to stabilize quickly.

[0114] If the open air bubble ratio of the molded article is excessively low, omitting the curing process may cause the molded article to shrink and deform significantly, potentially preventing the acquisition of a molded article with the desired shape. From the viewpoint of further suppressing significant shrinkage and deformation of the molded article even when the curing process is omitted, the open air bubble ratio of the molded article is preferably 2% or more, more preferably 2.5% or more, even more preferably 3% or more, and particularly preferably 4% or more. On the other hand, if the open air bubble ratio of the molded article is excessively high, the appearance of the molded article may deteriorate, and its rigidity may decrease. Furthermore, depending on the application, the water leakage prevention performance may be insufficient. From the viewpoint of further improving the appearance, rigidity, and water leakage prevention performance of the molded article, the open air bubble ratio of the molded article is preferably 12% or less, more preferably 10% or less, even more preferably 8% or less, particularly preferably 7.5% or less, and most preferably 6% or less.

[0115] The open air bubble ratio of the molded body is measured according to ASTM 2856-70 Procedure B. Specifically, first, the molded body is allowed to stand at 23°C for 12 hours to allow it to settle. Then, a first test specimen in the shape of a cube measuring 2.5 cm x 2.5 cm x 2.5 cm is cut from the center of the molded body, and its geometric volume Va (unit: cm³) is measured. 3 Next, calculate the product of the length (unit: cm), width (unit: cm), and height (unit: cm). Then, use a dry automatic densimeter (specifically, Shimadzu Corporation's AccuPic II 1340) to determine the true volume V1 (unit: cm) of the first test specimen. 3 ) Measure.

[0116] Subsequently, the first test specimen was divided into eight equal parts to create a second test specimen in the shape of a cube measuring 1.25 cm x 1.25 cm x 1.25 cm. The true volume V2 (unit: cm³) of the second test specimen was then measured using a dry automatic densimeter. 3 The volume of the second specimen is measured. Note that the true volume V2 of the second specimen is the sum of the true volumes of the eight individual pieces cut from the first specimen.

[0117] The open-cell ratio (in %) of the first specimen is expressed by the following formula (2), using the geometric volume Va of the first specimen, the true volume V1 of the first specimen, and the true volume V2 of the second specimen obtained above. The open-cell ratio measured in this way is a value corrected for the effect of closed cells that are destroyed when the second specimen is cut from the first specimen, and is also called the corrected open-cell ratio. Open cell rate=(Va-2V1+V2)×100 / Va...(2)

[0118] The above procedure is performed on five first test specimens, and the open bubble ratio for each first test specimen is calculated. The arithmetic mean of the open bubble ratios for the five first test specimens is then defined as the open bubble ratio Co of the molded body.

[0119] In this specification, the open bubble ratio Co is a physical property value measured in accordance with ASTM 2856-70 Procedure B, as described above, and is a physical property value with a different concept from the porosity of the molded body. The porosity of the molded body is measured, for example, as follows. Specifically, first, a rectangular parallelepiped-shaped test piece (for example, 20 mm long × 100 mm wide × 20 mm high) is cut from the center of the molded body. Next, this test piece is submerged in a graduated cylinder containing ethanol, and the true volume Vc (unit: L) of the test piece is determined from the rise in the liquid level of the ethanol. The apparent volume Vd (unit: L) is also determined from the external dimensions of the test piece. The porosity (unit: %) of the molded body is expressed by the following formula (3), using the true volume Vc and apparent volume Vd of the test piece obtained above. Porosity=[(Vd-Vc) / Vd]×100...(3)

[0120] In measuring the porosity of a molded body, closed cells that are destroyed when the test specimen is cut are not taken into consideration. Furthermore, it differs from the method for measuring the open cell ratio Co described above in that it uses a liquid such as ethanol as the measurement medium. Therefore, it is difficult to estimate the value of the open cell ratio Co based on the porosity value of the molded body. Note that the porosity of a molded body is always greater than the open cell ratio Co.

[0121] From the viewpoint of being able to more sufficiently suppress dimensional changes even when the curing process is omitted, the porosity of the molded article is preferably 4% or more, more preferably 4.5% or more, and even more preferably 5% or more. On the other hand, from the viewpoint of further improving rigidity and appearance, the porosity of the molded article is preferably 12% or less, and more preferably 10% or less. The porosity of the molded article can be measured by the measurement method described above.

[0122] The density of the molded body is 10 kg / m³ 3 More than 60kg / m 3 The following is preferable. In this case, the lightness and rigidity of the molded body can be improved in a balanced manner. From the viewpoint of further improving the rigidity of the molded body, the density of the molded body should be 15 kg / m³. 3 It is more preferable that the amount be greater than or equal to 20 kg / m 3 It is even more preferable that the above conditions are met. From the viewpoint of further improving the lightweight properties of the molded body, the density of the molded body should be 50 kg / m³. 3 It is more preferable that the following conditions are met: 45 kg / m 3 The following is even more preferable: The density of the molded body is calculated by dividing the mass of the molded body (in g) by the volume (in L) obtained from the external dimensions of the molded body and converting the units. If, for example, the molded body has a complex shape at least partially and it is not easy to determine the volume from the external dimensions of the molded body, the volume of the molded body can be determined by the immersion method.

[0123] Conventionally, when manufacturing molded articles with low density, it has been particularly difficult to omit the curing process because the molded article is prone to significant deformation after demolding. In contrast, the foamed particle molded article described above allows for the omission of the curing process even when the density is low, and it exhibits excellent appearance and rigidity in the desired shape even without curing. From the viewpoint of effectively demonstrating this effect, it is preferable to set the density of the molded article within the above range.

[0124] Molded products are used as sound-absorbing materials, shock-absorbing materials, and cushioning materials in various fields, such as the automotive sector (including automobiles) and the construction sector.

[0125] (Method for manufacturing foamed particle molded products) In producing the aforementioned foam particle molded body, for example, the foam particles can be filled into a mold, and then steam can be supplied into the mold as a heating medium to perform in-mold molding. Specifically, first, foam particles are filled into a mold having a cavity corresponding to the shape of the desired molded body. After the filling of the foam particles is complete, steam is supplied into the mold to heat the foam particles. The foam particles in the mold are heated by the steam and undergo secondary foaming while fusing together. This integrates the foam particles in the mold, forming a molded body.

[0126] After the heating of the foam particles is complete, the molded body in the mold is cooled to stabilize its shape. Then, the molded body is removed from the mold to complete the in-mold molding process. In the above manufacturing method, if necessary, a curing process may be performed in which the molded body after demolding is left to stand for a predetermined time in a high-temperature atmosphere adjusted to a temperature of, for example, 60°C to 80°C. However, even if the molded body after demolding is not cured in a high-temperature atmosphere, 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 leaving the molded body after demolding in an environment of, for example, 23°C for 12 hours.

[0127] In the above manufacturing method, the open air bubble ratio of the molded body can be easily adjusted to a range of 2% to 12% by in-mold molding the specific foamed particles. If the foamed particles do not have through holes, it becomes difficult to make the open air bubble ratio of the molded body 2% or more.

[0128] Furthermore, the open bubble ratio of a molded product tends to increase as the ratio Ct / A (the ratio of the total cross-sectional area of ​​through holes to the cross-sectional area A of the foamed particles) increases. Therefore, if the ratio Ct / A is too low, the open bubble ratio tends to be less than 2%, and if the ratio Ct / A is too high, the open bubble ratio tends to be higher than 12%. [Examples]

[0129] Examples of the foamed particles, the foamed particle molded body, and the method for producing the same will be described below.

[0130] (Polypropylene resin) Table 1 shows the properties of the polypropylene resin used in the production of the foamed particles. Note that the ethylene-propylene copolymer and ethylene-propylene-butene copolymer used in this example are both random copolymers. The densities of PP1 and PP2 shown in Table 1 are 900 kg / m³. 3 That is the case.

[0131] [Table 1]

[0132] <Monomer component content of polypropylene resin> The monomer content of polypropylene resins (specifically, ethylene-propylene copolymer and ethylene-propylene-butene copolymer) was determined by a known method using IR spectroscopy. Specifically, it was determined by the method described in the Polymer Analysis Handbook (edited by the Polymer Analysis Research Group of the Japan Society for Analytical Chemistry, published January 1995, published by Kinokuniya Shoten, page numbers and item names: 615-616 "II.2.3 2.3.4 Propylene / Ethylene Copolymer", 618-619 "II.2.3 2.3.5 Propylene / Butene Copolymer"), that is, by quantitative analysis based on the relationship between the absorbance of ethylene and butene corrected by a predetermined coefficient and the thickness of a film-like test piece.

[0133] More specifically, first, polypropylene resin was hot-pressed at 180°C to form a film, and multiple test pieces of different thicknesses were prepared. Then, by measuring the IR spectrum of each test piece, the ethylene-derived 722 cm⁻¹ spectrum was identified. -1 and 733cm -1 Absorbance at (A 722 , A 733 ) and 766cm derived from butene -1 Absorbance at (A 766 ) was read. Next, for each test piece, the ethylene component content (unit: mass%) in the polypropylene resin was calculated using the following formulas (4) to (6). The arithmetic mean of the ethylene component content obtained for each test piece was taken as the ethylene component content (unit: mass%) in the polypropylene resin.

[0134] (K' 733 ) c =1 / 0.96{(K' 733 ) a -0.268(K') 722 ) a}···(4) (K' 722 ) c =1 / 0.96{(K' 722 ) a -0.268(K') 722 ) a}···(5) Ethylene content = 0.575{(K' 722 ) c +(K' 733 ) c}···(6)

[0135] However, K' in equations (4) to (6) a This is the apparent absorption coefficient (K') at each wavenumber. a =A / ρt) and K' c ρ is the corrected absorption coefficient, 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). Note that the above formulas (4) to (6) can be applied to random copolymers.

[0136] Furthermore, the butene content in the polypropylene resin was calculated for each test specimen using the following formula (7). The arithmetic mean of the butene content obtained for each test specimen was defined as the butene content in the polypropylene resin (unit: mass%). Butene content = 12.3 (A 766 / L)···(7) However, in equation (7), A is the absorbance and L is the thickness of the film-like test piece (in mm).

[0137] <Flexural modulus of polypropylene resins> A 4mm thick sheet was prepared by hot-pressing polypropylene resin at 230°C, and a test specimen measuring 80mm in length, 10mm in width, and 4mm in thickness was cut from this sheet. The flexural modulus of this test specimen was determined in accordance with JIS K7171:2008. The radius of the indenter and the radius of the support base were both 5mm, the distance between the supports was 64mm, and the test speed was 2mm / min.

[0138] <Melting point of polypropylene resins> The melting point of polypropylene resin was determined in accordance with JIS K7121:1987. Specifically, the condition of the test specimen made of polypropylene resin was first adjusted according to "(2) When measuring the melting temperature after performing a certain heat treatment" described in JIS K7121:1987. A DSC curve was obtained by heating the adjusted test specimen from 30°C to 200°C at a heating rate of 10°C / min. The peak temperature of the melting peak that appeared in the DSC curve was defined as the melting point. A differential scanning calorimetry (DSC7020, manufactured by SII Nanotechnology Co., Ltd.) was used as the measuring device.

[0139] <Melt flow rate of polypropylene resin> The melt flow rate (i.e., MFR) of polypropylene resin was measured in accordance with JIS K7210-1:2014 under conditions of 230°C temperature and 2.16 kg load.

[0140] Next, the composition and manufacturing method of the foamed particles used in this example will be explained.

[0141] (Example 1) As shown in Figures 4 and 5, the foamed particle 1 of Example 1 has a cylindrical shape and five through holes 11 that penetrate its axial direction. One of the five through holes 11 (11a to 11e), through hole 11a, is provided so as to penetrate the central axis 10 of the foamed particle 1. The remaining four through holes 11b to 11e are located around the central axis 10, and are positioned so that the circumferential spacing of the cut surface obtained by cutting the foamed particle 1 at its axial center with a plane perpendicular to the axial direction is approximately equal.

[0142] Furthermore, as shown in Figure 5 and Table 2, the foamed particle 1 of Example 1 has a multilayer structure comprising a foamed layer 2 made of PP1 and a non-foamed coating layer 3 made of PP2 that covers the foamed layer 2.

[0143] In preparing the foamed particles in this example, a co-extrusion apparatus was first used, which included a core layer forming extruder, a coating layer forming extruder, and a co-extrusion die connected to these two extruders. The extruded material from the co-extrusion apparatus was then cut using a strand-cutting method to produce multilayer resin particles. Specifically, in the core layer forming extruder, PP1 and zinc borate as a foam regulator were melt-kneaded to obtain a core layer forming resin molten paste. The maximum setting temperature in the core layer forming extruder was 245°C, and the amount of zinc borate added was 500 ppm by mass relative to the mass of the polypropylene resin. In parallel with this, PP2 was melt-kneaded in the coating layer forming extruder at the maximum setting temperature of 245°C to obtain a coating layer forming resin molten paste.

[0144] These molten resin mixtures were co-extruded and merged in a die to form a composite consisting of a non-foamed core layer and a non-foamed coating layer covering the side surface of the core layer. After extruding this composite from the die, the extruded material was cooled in water while being taken up, and then cut to an appropriate length using a pelletizer to obtain multilayer resin particles consisting of a core layer and a coating layer covering the side surface 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 was core layer:coating layer = 95:5 (i.e., the mass ratio of the coating layer was 5%). The mass of each multilayer resin particle was approximately 1.5 mg.

[0145] Next, foamed particles were obtained by foaming multilayer resin particles using a direct foaming method. Specifically, 1 kg of multilayer resin particles was placed in a 5 L sealed container along with 3 L of water as a dispersion medium. Then, 0.3 parts by mass of dispersant and 0.004 parts by mass of dispersion aid were added to the sealed container per 100 parts by mass of multilayer resin particles, and the multilayer resin particles were dispersed in the dispersion medium. Kaolin was used as the dispersant. A surfactant (sodium alkylbenzene sulfonate) was used as the dispersion aid.

[0146] Subsequently, carbon dioxide was added to the sealed container as a foaming agent, and the container was then sealed and heated to the temperature indicated in the "Foaming Temperature" column of Table 1 while stirring the contents of the container. The internal pressure of the container at this time (also called impregnation pressure or carbon dioxide pressure) was the value shown in the "Internal Pressure of Container" column of Table 1. After maintaining the aforementioned foaming temperature for 15 minutes, the sealed container was opened and the contents were released to atmospheric pressure, thereby obtaining foamed particles 1 having a foamed layer 2 with a foamed core and a non-foamed coating layer 3 covering the foamed layer 2.

[0147] Next, a two-stage foaming process was performed to reduce the apparent density of the single-stage foamed particles. Specifically, the single-stage foamed particles were placed in a pressure vessel (specifically, a metal drum), and air was supplied into the pressure vessel to increase the pressure inside the vessel, causing the air to permeate the bubbles. The internal pressure of the bubbles in the single-stage foamed particles removed from the pressure vessel was as shown in Table 2. Subsequently, the single-stage foamed particles were placed in a metal drum, and steam was supplied to heat the particles so that the drum pressure reached the value shown in Table 2, thereby obtaining foamed particles 1 with the apparent density shown in Table 2.

[0148] (Example 2) The foamed particle 1 in this example has the same configuration as the foamed particle 1 in Example 1, except that the number of through holes 11 has been changed to four. Specifically, as shown in Figures 6 and 7, the through holes 11 (11f to 11i) of the foamed particle 1 in this example are located around the central axis 10 of the foamed particle 1, and are positioned so that the circumferential spacing in the cross-section obtained by cutting the foamed particle 1 at its axial center with a plane perpendicular to the axial direction is approximately equal. The method for manufacturing the foamed particle 1 in this example is the same as the method for manufacturing the foamed particle 1 in Example 1, except that the shape of the small holes in the co-extrusion die has been changed.

[0149] (Example 3) The foamed particle 1 in this example has the same configuration 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 method for manufacturing the foamed particle 1 in this example is generally the same as the method for manufacturing the foamed particle 1 in Example 1, except that the shape of the small holes in the co-extrusion die is changed.

[0150] (Example 4) The foamed particle 1 in this example has the same configuration as the foamed particle 1 in Example 1, except that the number of through holes 11 has been changed to three. As shown in Figures 8 and 9, the through holes 11 (11j to 11l) of the foamed particle 1 in this example are located around the central axis 10 of the foamed particle 1, and are positioned so that the circumferential spacing in the cut surface obtained by cutting the foamed particle 1 at its axial center with a plane perpendicular to the axial direction is approximately equal. The method for manufacturing the foamed particle 1 in this example is the same as the method for manufacturing the foamed particle 1 in Example 1, except that the shape of the small holes in the co-extrusion die has been changed.

[0151] (Comparative Example 1) The foamed particles in this example are conventional polypropylene resin foamed particles that do not have through holes and have a solid spherical shape. The method for manufacturing the foamed particles in this example is generally the same as the method for manufacturing foamed particles 1 in Example 1, except that resin particles that do not have through holes and have a solid spherical shape are used.

[0152] (Comparative Example 2) The foamed particles of Comparative Example 2 have the same configuration as foamed particles 1 of Example 1, except that the number of through-holes 11 is changed to one. Although not shown in the figure, the through-holes in the foamed particles of this example are arranged to penetrate the central axis of the foamed particles. The manufacturing method of the foamed particles of this example is generally the same as the manufacturing method of foamed particles 1 of Example 1, except that the shape of the small holes in the co-extrusion die is changed.

[0153] (Comparative Example 3) The foamed particles of Comparative Example 3 have the same configuration as 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-holes is changed to the value shown in Table 3. Although not shown in the figure, the through-holes in the foamed particles of this example are arranged to penetrate the central axis of the foamed particles. The manufacturing method of the foamed particles of this example is the same as the manufacturing method of foamed particles 1 of Example 1, except that the shape of the small holes in the co-extrusion die is changed and two-stage foaming is not performed.

[0154] (Comparative Example 4) The foamed particles in this example have the same configuration as the foamed particles in Example 2, except that the pore diameter d of the through-holes is changed to the value shown in Table 3. The method for manufacturing the foamed particles in this example is generally the same as the method for manufacturing foamed particles 1 in Example 1, except that the shape of the small holes in the co-extrusion die is changed.

[0155] 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.

[0156] (Evaluation of foamed particles) For the measurement and evaluation of the physical properties of the foamed particles, the particles were used after being conditioned by standing for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm.

[0157] <Bulk density> The conditioned foam particles were filled into a graduated cylinder so that they would naturally accumulate, and the bulk volume (in L) of the foam particle group was read from the scale of the graduated cylinder. Then, the mass (in g) of the foam particle group in the graduated cylinder was divided by the aforementioned bulk volume, and the bulk density (in kg / m³) of the foam particles was obtained by further unit conversion. 3 ) was calculated.

[0158] <Apparent Density> After measuring the mass of the conditioned foam particles, they were submerged in a graduated cylinder containing ethanol at 23°C using a wire mesh. The volume of the foam particles was then measured, taking into account the volume of the wire mesh and the resulting rise in water level. The mass (in g) of the foam particles obtained in this way was divided by the volume (in L), and then converted to obtain the apparent density (in kg / m³) of the foam particles. 3 ) was calculated.

[0159] <Closed cell ratio> The method for measuring the percentage of closed cells in foamed particles is as described above.

[0160] <Diameter d of the through hole> 100 foam particles were randomly selected from the conditioned foam particle group. These foam particles were cut at their central position along their axis with a plane perpendicular to the axis to expose the cut surface. Next, photographs of the cut surfaces of the foam particles were taken, and the cross-sectional area (i.e., opening area) of the through-holes in each foam particle was measured by image analysis. The diameter of a virtual circle with the same area as the cross-sectional area of ​​the through-hole was then calculated, and this value was taken as the hole diameter of each through-hole. The above operations were performed for 100 foam particles, and the arithmetic mean of the obtained through-hole diameters was taken as the hole diameter d of the foam particle's through-hole.

[0161] <Cross-sectional area of ​​foamed particles A> 100 foam particles were randomly selected from the conditioned foam particle group. These foam particles were then cut at their central position along their axis using a plane perpendicular to the axis to expose the cross-sectional surface. Next, photographs of the cross-sectional surfaces of the foam particles were taken, and the cross-sectional area of ​​the foam layer and the coating layer for each foam particle were calculated by image analysis. The sum of the cross-sectional areas of the foam layer and the coating layer was then defined as the cross-sectional area of ​​each foam particle. Note that the cross-sectional area of ​​through holes was not included in the cross-sectional area of ​​the foam particles. The above procedure was performed for 100 foam particles, and the arithmetic mean of the obtained cross-sectional areas of the foam particles was defined as the cross-sectional area A of the foam particles.

[0162] <Total cross-sectional area of ​​through holes Ct, cross-sectional area per through hole Ca> One hundred foamed particles were randomly selected from the conditioned foamed particle group. These foamed particles were then cut at their central position along their axis using a plane perpendicular to the axis, exposing the cut surface. Next, photographs of the cut surfaces were taken, and the cross-sectional area of ​​the through-holes in each foamed particle was measured by image analysis. Finally, the sum of the cross-sectional areas of the through-holes in each foamed particle was calculated.

[0163] The above procedure was performed on 100 foamed particles. The arithmetic mean of the sum of the cross-sectional areas of the through-holes was defined as the total cross-sectional area of ​​the through-holes Ct, and the value obtained by dividing the total cross-sectional area of ​​the through-holes Ct by the number of through-holes was defined as the cross-sectional area per through-hole Ca. Tables 2 and 3 show the ratio of the total cross-sectional area of ​​the through-holes Ct to the cross-sectional area of ​​the foamed particle A at the cross-section Ct / A and the ratio of the cross-sectional area per through-hole Ca to the cross-sectional area of ​​the foamed particle A Ca / A.

[0164] <Outer diameter D of foamed particle> One hundred foamed particles were randomly selected from the conditioned foamed particle group. These foamed particles were then cut at their central position along their axis using a plane perpendicular to the axis to expose the cut surface. Next, photographs of the cut surfaces were taken, and image analysis was performed to measure the cross-sectional area of ​​the foamed layer, the coating layer, and the through-holes of each individual foamed particle. The diameter of a virtual perfect circle having the same area as the sum of the cross-sectional areas of the foamed layer, coating layer, and through-holes was calculated, and this value was defined as the outer diameter of each foamed particle. The above procedure was performed for all 100 foamed particles, and the arithmetic mean of the obtained outer diameters was defined as the outer diameter D of the foamed particle.

[0165] <Distance between through holes R> One hundred foamed particles were randomly selected from the conditioned foamed particle group. These foamed particles were then cut at their central position along their axis using a plane perpendicular to the axis to expose the cut surface. Next, photographs of the cut surfaces of the foamed particles were taken, and image analysis was performed to measure the distance between the center points of each through-hole, that is, the distance between the center point of the through-hole being measured and the center point of the through-hole having the closest center point to the center point of the through-hole being measured. This measurement was performed for all through-holes formed in the foamed particles being measured, and the arithmetic mean of the obtained distances between center points was defined as the distance between through-holes of the foamed particles being measured. The above procedure was performed for 100 foamed particles, and the arithmetic mean of the obtained distances between through-holes of the foamed particles was defined as the distance between through-holes R of the foamed particles.

[0166] <Minimum molding pressure> In evaluating the minimum molding pressure, foam particle molded bodies were produced by performing in-mold molding while varying the molding pressure during heating from 0.18 to 0.38 MPa(G) in increments of 0.02 MPa. The lowest molding pressure among those that yielded molded bodies with good fusion properties and recovery properties was defined as the minimum molding pressure. The specific method for producing the molded bodies is as follows.

[0167] First, the foamed particles were dried at 23°C for 24 hours, and then air was impregnated to increase the internal pressure, i.e., the pressure inside the bubbles, to the values ​​shown in the "Internal Pressure of Foamed Particles" column of Tables 2 and 3. The internal pressure of the foamed particles was measured as follows: The mass Q (in g) of the foamed particle group with increased internal pressure immediately before filling the mold and the mass U (in g) of the foamed particle group after 48 hours were measured, and the difference between Q and U was defined as the increased amount of air W (in g). Using these values, the internal pressure of the foamed particles (in MPa (G)) was calculated based on the following formula (8). P = (W / M) × R × T / V ... (8)

[0168] However, in equation (8) above, M is the molecular weight of air, R is the gas constant, T is the absolute temperature, and V is the volume obtained by subtracting the volume of resin in the foamed particle group from the apparent volume of the foamed particle group (unit: L). In this example, M = 28.8 (g / mol), R = 0.0083 (MPa·L / (K·mol)), and T = 296 (K).

[0169] Next, foam particles were filled into a flat mold measuring 300 mm (length) x 250 mm (width) x 60 mm (thickness) using the cracking filling method. The amount of cracking during filling (specifically, the ratio of the mold opening amount to the internal dimensions in the thickness direction) was set to the values ​​shown in Tables 2 and 3. After filling was completed, the mold was clamped in the thickness direction to mechanically compress the foam particles.

[0170] Next, in-mold molding was performed by supplying steam into the mold. In the in-mold molding process, first, preheating was performed by supplying steam into the mold for 5 seconds with the drain valve of the mold open. Then, the drain valve was closed, and one-sided heating was performed by supplying steam from one side of the mold until the pressure reached 0.08 MPa(G) lower than the molding pressure at the time of main heating. Next, one-sided heating was performed by supplying steam from the other side of the mold until the pressure reached 0.04 MPa(G) lower than the molding pressure at the time of main heating. After that, main heating was performed by supplying steam from both sides of the mold until the molding pressure at the time of main heating was reached. After the main heating was completed, the pressure inside the mold was released, and the molded body was cooled inside the mold until the surface pressure due to the foaming force of the molded body reached 0.04 MPa(G).

[0171] Subsequently, the foam particle molded body removed from the mold was subjected to a curing process in which it was left to stand in an 80°C oven for 12 hours. After the curing process, the foam particle molded body was conditioned by leaving it to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm. The fusion properties and recovery properties of the conditioned foam particle molded body were evaluated, and the lowest molding pressure at which a passing grade was obtained (i.e., the molding pressure at which a passing grade could be obtained) was defined as the minimum molding pressure. A lower minimum molding pressure indicates superior moldability.

[0172] The evaluation methods for fusion properties and recovery properties in the evaluation of the minimum molding pressure are as follows:

[0173] • Fusion properties The foam particle molded body was fractured so that it was divided into roughly equal parts along its longitudinal direction. More than 100 foam particles were randomly selected from the foam particles exposed on the fracture surface and visually observed to determine whether they were foam particles that fractured internally (i.e., foam particles that underwent material failure) or foam particles that fractured at the interface between foam particles. The ratio of the number of foam particles that fractured internally to the total number of foam particles observed was then calculated as a percentage (i.e., material failure rate), and this value was defined as the fusion rate. A fusion rate of 90% or higher was judged as passing, and a rate below 90% was judged as failing.

[0174] ·Recovery In a plan view of the foam particle molded body from the thickness direction, the thickness of the foam particle molded body was measured at four locations 10 mm inward from each vertex towards the center, as well as the thickness of the foam particle molded body at the center. Next, the ratio (in %) of the thickness of the thinnest location to the thickness of the thickest location among the measured locations was calculated. A thickness ratio of 95% or more was judged as passing, and a ratio of less than 95% was judged as failing.

[0175] <Molding cycle evaluation> • Cooling time inside the mold Except for setting the molding pressure during this heating phase to one of the following: the minimum molding pressure, a pressure 0.2 MPa(G) higher than the minimum molding pressure, or a pressure 0.4 MPa(G) higher than the minimum molding pressure, in-mold molding was performed in the same manner as the evaluation of the minimum molding pressure described above. Then, after the heating phase was completed, the pressure inside the mold was released, and the time it took for the surface pressure of the molded body due to the foaming force to reach 0.04 MPa(G), i.e., the cooling time of the molded body inside the mold, was measured. It should be noted that in in-mold molding of foamed particles, the cooling time inside the mold tends to increase as the molding pressure increases.

[0176] • Whether or not nutrient-free forms are acceptable. The molding pressure during heating was set to the minimum molding pressure described above, and in-mold molding was performed using the same method as described above for evaluating the minimum molding pressure. After demolding, the molded body was left to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm without any curing process to adjust its condition. In a plan view of the molded body from the thickness direction after adjustment, the thickness of the molded body was measured at four locations 10 mm inward from each vertex toward the center, and the thickness of the molded body at the center was measured. Next, the ratio (in %) of the thickness of the thinnest location to the thickness of the thickest location among the measured locations was calculated.

[0177] In the "Feasibility of Nutrient-Free Forming" column of Tables 2 and 3, "Feasible" was written if the thickness ratio was 95% or more, and "Not acceptable" if it was less than 95%. In Comparative Examples 3 and 4, the feasibility of nurturing-free formation was not evaluated because the appearance evaluation was unsatisfactory, as described later.

[0178] (Evaluation of molded products) For the measurement and evaluation of the physical properties of the molded body, the molded body was prepared by in-mold molding at the minimum molding pressure using the same method as that used for the evaluation of the minimum molding pressure. After demolding from the mold, the molded body was allowed to settle for 12 hours under conditions of 50% relative humidity, 23°C, and 1 atm without any curing process.

[0179] <Density of the molded body> The density of the molded body (in kg / m³) is calculated by dividing the mass (in g) of the molded body by the volume (in L) obtained from the external dimensions of the molded body, and then converting the units. 3 ) was calculated.

[0180] <Open bubble ratio> The open bubble ratio was measured in accordance with ASTM2856-70 Procedure B. Specifically, first, a cubic specimen measuring 2.5 cm x 2.5 cm x 2.5 cm was cut from the center of the molded body, and its geometric volume Va (unit: cm³) was measured. 3 ), that is, the product of the length (unit: cm), width (unit: cm), and height (unit: cm) was calculated. Next, the true volume V1 (unit: cm) of the first test specimen was calculated using a dry automatic densimeter (specifically, Shimadzu Corporation's AccuPic II 1340). 3 ) was measured.

[0181] Subsequently, the first test specimen was divided into eight equal parts to create a second test specimen in the shape of a cube measuring 1.25 cm x 1.25 cm x 1.25 cm. The true volume V2 (unit: cm³) of the second test specimen was then measured using a dry automatic densimeter. 3 The volume was measured. Note that the true volume V2 of the second test specimen is the sum of the true volumes of the eight individual pieces cut from the first test specimen.

[0182] Using the geometric volume Va of the first specimen, the true volume V1 of the first specimen, and the true volume V2 of the second specimen obtained above, the percentage of open bubbles (in %) of the first specimen was calculated based on the following formula (2). Open cell rate=(Va-2V1+V2)×100 / Va...(2)

[0183] The above procedure was performed on five first test specimens, and the open bubble ratio for each first test specimen was calculated. The arithmetic mean of the open bubble ratios for the five first test specimens was then defined as the open bubble ratio Co of the molded product.

[0184] <Porosity of molded body> A rectangular parallelepiped specimen measuring 20 mm (length) x 100 mm (width) x 20 mm (height) was cut from the center of the molded body. The specimen was submerged in a graduated cylinder containing ethanol, and the true volume Vc (in L) of the specimen was determined from the rise in the ethanol level. The apparent volume Vd (in L) of the specimen was determined from its external dimensions. Using the true volume Vc and apparent volume Vd obtained above, the porosity (in %) of the molded body was calculated based on the following formula (3). Porosity=[(Vd-Vc) / Vd]×100...(3)

[0185] <50% compressive stress σ 50 > A test specimen with a rectangular prism shape measuring 50 mm in length, 50 mm in width, and 25 mm in thickness was cut from the center of the molded body, excluding the skin surface, i.e., the surface that was in contact with the inner surface of the mold during in-mold molding. A compression test was performed at a compression speed of 10 mm / min in accordance with JIS K6767:1999 to determine the 50% compressive stress σ of the molded body. 50 (Unit: kPa) was calculated. Furthermore, Tables 2 and 3 show the 50% compressive stress σ of the molded body. 50 The value obtained by dividing by the density of the molded body (σ 50 The density was listed.

[0186] <Exterior> The surface of the molded body was visually inspected, and its surface properties were evaluated based on the following criteria. A+: The molded body exhibits an extremely good surface condition with very few interparticle gaps and almost no noticeable irregularities caused by through-holes. A: The molded body exhibits a good surface condition with sufficiently few interparticle gaps and minimal irregularities caused by through-holes. B: Slight irregularities are observed on the surface of the molded product due to interparticle gaps and / or through-holes. C: Significant irregularities are observed on the surface of the molded body due to interparticle gaps and / or through-holes.

[0187] [Table 2]

[0188] [Table 3]

[0189] As shown in Table 2, the foamed particles of Examples 1 to 4 have a cylindrical shape and multiple through-holes that penetrate along their axial direction. Furthermore, the ratio Ct / A of the total cross-sectional area of ​​the through-holes to the cross-sectional area A of these foamed particles is within the specified range. By performing in-mold molding using such foamed particles, the cooling time within the mold could be shortened. Even when the foamed particles of Examples 1 to 4 were molded under conditions where the molding pressure was higher than the minimum molding pressure, the cooling time could be sufficiently shortened, and a foamed particle molded article with the desired shape, excellent appearance and rigidity could be obtained even without the curing process. The foamed particles of Examples 1 to 4 had an even better appearance than the foamed particle molded article obtained from the foamed particles of Comparative Example 2, which had a ratio Ct / A within the specified range and only one through-hole.

[0190] On the other hand, the foamed particles of Comparative Example 1 are conventional polypropylene resin foamed particles that do not have through holes. The foamed particles of Comparative Example 1 have excessively high secondary foaming properties, resulting in a significantly longer cooling time in the mold, as shown in Table 3. In addition, the foamed particles of Comparative Example 1 do not easily form steam passages during in-mold molding, resulting in a high molding pressure required to obtain a molded product. Furthermore, the resulting molded product hardly forms any open-cell structure. As a result, the molded product made using the foamed particles of Comparative Example 1 shrinks and deforms significantly when no curing process is performed after demolding from the mold.

[0191] The foamed particles of Comparative Example 2 required a longer cooling time in the mold compared to the foamed particles of Examples 1-4. This is thought to be because the number of through-holes was only one, which tends to reduce the surface area of ​​the minute spatial portion of the foamed particle in the molded body that originates from the through-hole.

[0192] In Comparative Examples 3 and 4, the ratio Ct / A of the total cross-sectional area of ​​the through holes to the cross-sectional area A of the foamed particles was too large, resulting in significant unevenness on the surface of the molded article due to the through holes, and thus the appearance was deemed unacceptable.

[0193] Although specific embodiments of thermoplastic resin foam particles and molded articles of thermoplastic resin foam particles according to the present invention have been described above based on the examples, the specific embodiments of thermoplastic resin foam particles, etc. according to the present invention are not limited to the embodiments of the examples, and the configuration can be appropriately changed without impairing the spirit of the present invention. [Explanation of symbols]

[0194] 1. Foaming particles 11 Through hole 2 Foam layer 3 Covering layer

Claims

1. Thermoplastic resin foam particles having a thermoplastic resin foam layer, The foamed particles have a cylindrical shape and have two to eight through holes that penetrate in the axial direction. The diameter d of the through-hole in the cross-section obtained by cutting the foamed particle perpendicular to the axial direction at its axial center is 0.1 mm or more and 0.5 mm or less. Thermoplastic resin foam particles, wherein the ratio Ct / A of the total cross-sectional area of ​​the through holes to the cross-sectional area A of the foam particles at the cross-section obtained by cutting the foam particle perpendicular to the axial direction at its axial center is 0.02 or more and 0.15 or less.

2. A thermoplastic resin foam particle having a thermoplastic resin foam layer, The foamed particles have a cylindrical shape and have two to eight through holes that penetrate in the axial direction. The outer diameter D of the foamed particle is 2 mm or more and 8 mm or less. Thermoplastic resin foam particles, wherein the ratio Ct / A of the total cross-sectional area of ​​the through holes to the cross-sectional area A of the foam particles at the cross-section obtained by cutting the foam particle perpendicular to the axial direction at its axial center is 0.02 or more and 0.15 or less.

3. The thermoplastic resin foam particle according to claim 1 or 2, wherein the ratio Ca / A of the cross-sectional area per through hole to the cross-sectional area A of the foam particle at the cross-sectional surface obtained by cutting the foam particle perpendicular to the axial direction at its axial center is 0.005 or more and 0.05 or less.

4. The foamed particle has four to eight through holes, according to claim 1 or 2.

5. The thermoplastic resin foam particle according to claim 1 or 2, wherein the ratio R / d of the distance between through-holes of the foam particle to the diameter d of the through-hole in the cross-section obtained by cutting the foam particle perpendicular to the axial direction at its axial center is 2.0 or more and 4.5 or less.

6. The bulk density of the foamed particles is 10 kg / m³ 3 More than 50kg / m 3 The thermoplastic resin foamed particle according to claim 1 or 2, wherein the ratio of the apparent density to the bulk density of the foamed particle is 1.7 or more and 1.9 or less.

7. The thermoplastic resin foam particle according to claim 1 or 2, wherein the thermoplastic resin constituting the foam layer is an ethylene-propylene random copolymer, and the ethylene-propylene random copolymer contains 0.5% by mass or more and 3.5% by mass or less of an ethylene component.

8. The thermoplastic resin foam particle according to claim 1 or 2, wherein the foam particle has a thermoplastic resin coating layer covering the foam layer, and the coating layer is made of a thermoplastic resin having a melting point or a lower softening point than the thermoplastic resin constituting the foam layer.

9. A molded article of thermoplastic resin foam particles obtained by in-mold molding of thermoplastic resin foam particles according to claim 1 or 2.

10. The thermoplastic resin foam particle molded article according to claim 9, wherein the open bubble rate of the foam particle molded article is 2% or more and 12% or less.

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