Polypropylene-based foamed particles, polypropylene-based foamed particle molded products, automotive bumper core materials, and automotive toolboxes

Abstract

The present invention provides polypropylene resin foam particles that can improve the dimensional stability of molded articles, polypropylene resin foam particle molded articles made from these foam particles, automotive bumper core materials, and automotive toolboxes. [Solution] The foamed particle 1 has a foamed layer 2 made of a polypropylene resin and has a specific shape with one or more defects 11. The polypropylene resin making up the foamed layer 2 includes a polypropylene resin (A) with a melting point of 135 to 150°C and a flexural modulus of less than 1000 MPa, and a polypropylene resin (B) with a melting point of 145 to 160°C and a flexural modulus of 1000 MPa or more. The mass ratio of resin (A) to resin (B) in the polypropylene resin is resin (A):resin (B) = 65:35 to 35:65.

Description

[Technical Field]

[0001] The present invention relates to polypropylene resin foam particles, polypropylene resin foam particle molded articles, automotive bumper core materials, and automotive toolboxes. [Background technology]

[0002] Polypropylene foam particle molded articles are lightweight and have excellent cushioning and rigidity properties, making them suitable for a variety of applications. Polypropylene foam particle molded articles are manufactured, for example, by a method called in-mold molding, in which polypropylene foam particles are filled into a mold and then heated by supplying steam into the mold. In in-mold molding, 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 together, and a molded article with a shape corresponding to the shape of the molding cavity of the mold can be obtained. Since the molded article is prone to swelling due to secondary foaming immediately after molding, it is cooled with water or air inside the mold before being released from the mold.

[0003] In the aforementioned manufacturing process for molded products, if the molded product is stored at room temperature after being demolded from the mold, the steam that flowed into the air bubbles of the molded product during in-mold molding condenses within the bubbles, creating negative pressure inside the bubbles. As a result, volume shrinkage occurs in the molded product, which can cause significant deformation. Therefore, after demolding the 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, for example, adjusted to a temperature of around 60°C to 80°C, to restore its shape. However, in in-mold molding of polypropylene resin foam particles, the curing process requires capital investment and is time-consuming and labor-intensive. Therefore, it is desirable to eliminate the curing process and significantly improve the productivity of the molded product.

[0004] From this viewpoint, for example, Patent Document 1 proposes a foamed particle molded body made of tubular polypropylene resin foamed particles. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2022 / 270425 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The molded articles described in Patent Document 1, depending on their shape, have room for further improvement in dimensional stability after in-mold molding, and it was desired to reduce the amount of change in the dimensions of the molded article from the time of demolding until the shape stabilizes through curing processes, etc.

[0007] This invention has been made in view of the above background, and aims to provide polypropylene resin foam particles that can improve the dimensional stability of molded articles, polypropylene resin foam particle molded articles made from these foam particles, automotive bumper core material, and automotive toolbox. [Means for solving the problem]

[0008] One aspect of the present invention relates to polypropylene resin foam particles according to the following [1] to [6].

[0009] [1] Polypropylene resin foam particles having a foamed layer made of polypropylene resin, The foamed particle has a columnar shape and has one or more defects selected from the group consisting of through holes penetrating the interior of the foamed particle in the axial direction and grooves extending along the axial direction on the side surface of the foamed particle. In the cross-section obtained by cutting the foamed particle at its axial center with a plane perpendicular to the axial direction, the ratio Ca / A of the average cross-sectional area Ca per defect to the average cross-sectional area A of the foamed particle is 0.01 or more and 0.20 or less, and the ratio Ct / A of the total cross-sectional area Ct of the defects to the average cross-sectional area A of the foamed particle is 0.02 or more and 0.20 or less. The polypropylene resin constituting the foam layer comprises a polypropylene resin (A) having a melting point of 135°C or higher and 150°C or lower, and a flexural modulus of less than 1000 MPa, and a polypropylene resin (B) having a melting point of 145°C or higher and 160°C or lower, and a flexural modulus of 1000 MPa or higher. Polypropylene resin foam particles, wherein the mass ratio of polypropylene resin (A) to polypropylene resin (B) in the polypropylene resin is polypropylene resin (A):polypropylene resin (B) = 65:35 to 35:65.

[0010] [2] The polypropylene resin foam particles according to [1], wherein the absolute value of the difference between the flexural modulus of the polypropylene resin (A) and the flexural modulus of the polypropylene resin (B) is 200 MPa or more and 500 MPa or less. [3] The polypropylene resin foam particles according to [1] or [2], wherein the polypropylene resin (B) is one or more propylene copolymers selected from the group consisting of ethylene-propylene copolymer, butene-propylene copolymer and ethylene-butene-propylene copolymer. [4] The polypropylene resin foam particles according to [3], wherein the sum of the ethylene content and the butene content in the polypropylene resin (B) is 0.5% by mass or more and 2.5% by mass or less.

[0011] [5] The polypropylene resin foam particles according to any one of [1] to [4], wherein the polypropylene resin foam particles have a crystalline structure in which the DSC curve obtained when the foam particles are heated from 23°C to 200°C at a heating rate of 10°C / min shows a resin-specific peak originating from the melting of crystals in the polypropylene resin and a high-temperature peak having a peak temperature higher than the peak temperature of the resin-specific peak, and the heat of fusion of the high-temperature peak is 8 J / g or more and 25 J / g or less. [6] The apparent density of the polypropylene resin foam particles is 15 kg / m³ 3 More than 200kg / m 3Polypropylene resin foam particles as described in any one of the following [1] to [5].

[0012] Another aspect of the present invention is a polypropylene resin foam particle molded article according to [7] below. A polypropylene resin foam particle molded article obtained by in-mold molding polypropylene resin foam particles described in any one of [7], [1] to [6], wherein the polypropylene resin foam particle molded article has a maximum length of 600 mm or more.

[0013] Yet another aspect of the present invention is an automobile bumper core material according to [8] below. A core material for an automobile bumper, obtained by in-mold molding polypropylene resin foam particles as described in any one of [8], [1], to [6].

[0014] Yet another aspect of the present invention is an automotive toolbox according to [9] below. An automotive toolbox made by in-mold molding polypropylene resin foam particles as described in any one of [9], [1], to [6]. [Effects of the Invention]

[0015] According to the above embodiment, it is possible to provide polypropylene resin foam particles that can improve the dimensional stability of a molded article, a polypropylene resin foam particle molded article made from these foam particles, a core material for an automobile bumper, and an automobile toolbox. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 is a schematic diagram of the appearance of foamed particles having through-holes as defects. [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 a schematic diagram of the appearance of foamed particles having grooves as defects. [Figure 4] Figure 4 is a cross-sectional view taken along the line IV-IV in Figure 3 (a plan view of the cross-section of the foamed particle). [Figure 5] Figure 5 is a schematic diagram of the appearance of foamed particles with a coating layer. [Figure 6] Figure 6 is a cross-sectional view taken along the line VI-VI in Figure 5 (a plan view of the cross-section of the foamed particle). [Figure 7] Figure 7 is an explanatory diagram showing the method for calculating the area of ​​the high-temperature peak. [Figure 8] Figure 8 is a schematic diagram of the appearance of foamed particles D in the example. [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). [Figure 10] Figure 10 is a front view of the bumper core material in Example 1-1. [Figure 11] Figure 11 is a cross-sectional view taken along the line XI-XI in Figure 10. [Figure 12] Figure 12 is a perspective view of the toolbox in Example 1-3. [Figure 13] Figure 13 is a top view of the toolbox in Example 1-3. [Modes for carrying out the invention]

[0017] (Polypropylene resin foam particles) Polypropylene resin foam particles (hereinafter referred to as "foam particles") have a columnar shape and have one or more defects of one or two types selected from the group consisting of through holes penetrating the interior of the foam particle in the axial direction and grooves extending along the axial direction on the side surface of the foam particle. In other words, the foam particles have one or more defects consisting of through holes penetrating the interior of the foam particle in the axial direction and / or grooves extending along the axial direction on the side surface of the foam particle. Furthermore, the ratio Ca / A of the average cross-sectional area per defect to the average cross-sectional area A of the foam particles in the cross-section obtained by cutting the foam particle at its axial center with a plane perpendicular to the axial direction is 0.01 or more and 0.20 or less, and the ratio Ct / A of the total cross-sectional area Ct of the defects to the average cross-sectional area A of the foam particles is 0.02 or more and 0.20 or less. Furthermore, the foamed layer of the foamed particles is composed of a polypropylene resin containing the polypropylene resin (A) and the polypropylene resin (B) in the specific mass ratio.

[0018] Conventionally, in in-mold molding of polypropylene resin foam particle molded articles (hereinafter referred to as "molded articles"), molding conditions are adjusted to take into account the dimensional changes of the molded article from the time of release from the mold until the shape stabilizes through curing processes, etc., in order to produce molded articles of the desired dimensions. However, when the curing process is omitted or the curing time is shortened in order to manufacture molded articles, the aforementioned dimensional changes of the molded article tend to become larger. Furthermore, in particular, when manufacturing molded articles with long maximum lengths, molded articles with complex shapes such as those with recesses, or molded articles that require high dimensional accuracy, it may become difficult to adjust the dimensions of the molded article to within the desired range.

[0019] In contrast, the foamed particles easily improve the dimensional stability of the molded body, reducing dimensional changes from the time of demolding until the shape stabilizes. Therefore, even when manufacturing molded bodies that are relatively difficult to mold in-mold, such as those with a relatively long maximum length, complex shapes, or those requiring high dimensional accuracy, by omitting the curing process or shortening the curing time, it is possible to easily obtain molded bodies with the desired dimensions. Furthermore, since the foamed particles allow for the omission of the curing process or a reduction in the curing time, even when molding molded bodies that are relatively difficult to mold in-mold, as described above, the productivity of the molded body can be significantly improved.

[0020] Furthermore, the foamed particles can easily shorten the drying time of the molded article after demolding, thereby further improving the productivity of the molded article. In addition, the foamed particles can shorten the cooling time of the molded article within the mold, and can also improve the surface properties and fusion properties of the molded article.

[0021] The following are some possible reasons why the aforementioned foamed particles produce such an effect.

[0022] As described above, the foamed particles have one or more defects selected from the group consisting of grooves and through holes. When such foamed particles are filled into a mold, it is thought that the defects of the foamed particles, such as through holes and grooves, and the voids between the foamed particles connect in a complex manner within the molding cavity of the mold, forming fine passages through which steam can pass. Therefore, when steam is supplied into the mold, it is thought that the steam can easily reach the inside of the mold via these fine passages, and that the entire foamed particle inside the mold can be easily heated. Furthermore, since the foamed particles with the defects have appropriate secondary foaming properties, it is thought that they expand appropriately when heated by steam. As a result, it is thought that by using these foamed particles, a molded body with excellent fusion properties and a good appearance can be obtained even under low molding temperatures during in-mold molding.

[0023] Furthermore, since the foamed particles with the aforementioned defects have appropriate secondary foaming properties, sufficient secondary foaming can occur within the mold even if no internal pressure is applied beforehand or if the applied internal pressure is relatively low. In addition, because the foamed particles have the aforementioned defects, it is believed that excessive expansion of the molded body at the time of steam heating can be avoided. This shortens the time required from the completion of heating of the foamed particles until the shape of the molded body stabilizes within the mold. As a result, it is believed that the time required for cooling the molded body within the mold can be shortened.

[0024] Furthermore, after in-mold molding is complete, a suitable open-cell structure is formed in the molded body. The open-cell structure is a small space that communicates with the outside of the molded body. The open-cell structure is formed by a complex interconnection of voids formed by the communication of voids between foam particles, continuous cell portions of the foam particles constituting the molded body, and defect portions. The foam particles have a specific Ca / A ratio and Ct / A ratio within the specified range, which can suppress the formation of coarse open-cell structures inside the molded body, thus shortening the drying time required for the molded body after demolding.

[0025] Furthermore, when a molded body having an open-cell structure is removed from the mold, air quickly flows into the cells inside the molded body through the open-cell structure, and as a result, the internal pressure of the molded body is thought to stabilize quickly. In addition, since the foamed layer of the foamed particles is composed of a polypropylene resin containing the polypropylene resin (A) and the polypropylene resin (B) in the specific mass ratio, the dimensional stability of the molded body is enhanced, and the change in the dimensions of the molded body after demolding from the mold can be reduced. Moreover, because the change in the dimensions of the molded body after demolding is small, it is thought that a molded body with the desired dimensions can be easily obtained even when manufacturing a molded body that is relatively difficult to mold in a mold, without performing a curing process or by shortening the curing process time.

[0026] When performing in-mold molding using foamed particles without defects, it becomes difficult to form an open-cell structure in the molded body, which tends to reduce the dimensional stability of the molded body, and the dimensional changes of the molded body from demolding until the shape stabilizes tend to be large. In this case, for example, when manufacturing a molded body that is relatively difficult to mold in-mold, without a curing process or by shortening the curing process time, it may become difficult to adjust the dimensions of the molded body to the desired dimensions. Furthermore, when performing in-mold molding using foamed particles without defects, it may lead to poor fusion between foamed particles and an increase in the cooling time of the molded body in the mold, which may significantly worsen the productivity of the molded body.

[0027] Even if defects are present, if the ratio Ca / A of the average cross-sectional area of ​​each defect to the average cross-sectional area A of the foam particles and / or the ratio Ct / A of the total cross-sectional area Ct of the defects to the average cross-sectional area A of the foam particles are excessively small, the effects of the defects will be difficult to obtain. In this case, there is a risk of reduced dimensional stability of the molded body and poor fusion between foam particles. Furthermore, in this case, the cooling time of the molded body in the mold may become excessively long, potentially reducing productivity.

[0028] By using foamed particles having a Ca / A ratio of 0.01 or higher and a Ct / A ratio of 0.02 or higher, the cooling time of the molded body in the mold can be shortened. From the viewpoint of further enhancing this effect, a Ca / A ratio of 0.02 or higher is preferable. From the same viewpoint, a Ct / A ratio of 0.03 or higher is more preferable.

[0029] On the other hand, if the ratio Ca / A of the average cross-sectional area of ​​each defect to the average cross-sectional area A of the foamed particles and / or the ratio Ct / A of the total cross-sectional area Ct of the defects to the average cross-sectional area A of the foamed particles are excessively large, the secondary foaming properties of the foamed particles may decrease. As a result, relatively large voids are more likely to form in the resulting molded article, which may lead to an increase in moisture content and drying time. On the other hand, if the amount of steam supplied into the mold during in-mold molding is increased or the steam temperature is raised in order to reduce the voids originating from the defects, the dimensional stability of the molded article may decrease.

[0030] By using foamed particles having a Ca / A ratio of 0.20 or less and a Ct / A ratio of 0.20 or less, these problems can be easily avoided and the dimensional stability of the molded article can be improved. Furthermore, such foamed particles can shorten the drying time of the molded article after demolding, and it is possible to easily obtain a molded article with excellent surface properties and rigidity. From the viewpoint of more reliably obtaining these effects, a Ca / A ratio of 0.15 or less is preferable, more preferably 0.10 or less, even more preferably 0.08 or less, and particularly preferable 0.05 or less. From a similar viewpoint, a Ct / A ratio of 0.18 or less is preferable, more preferably 0.15 or less, even more preferably 0.10 or less, and particularly preferable 0.08 or less. The methods for calculating the average cross-sectional area A of the foamed particles, the average cross-sectional area Ca per defect, and the total cross-sectional area Ct of the defects will be described later.

[0031] In constructing a preferred range for the ratio Ca / A, the upper and lower limits of the ratio Ca / A mentioned above can be arbitrarily combined. For example, the preferred range for the ratio Ca / A may be 0.01 to 0.15, 0.01 to 0.10, 0.02 to 0.08, or 0.02 to 0.05. Similarly, in constructing a preferred range for the ratio Ct / A, the upper and lower limits of the ratio Ct / A mentioned above can be arbitrarily combined. For example, the preferred range for the ratio Ct / A may be 0.02 to 0.18, 0.02 to 0.15, 0.03 to 0.10, or 0.03 to 0.08.

[0032] The shape of the foam particles can take the following forms more specifically. For convenience, in the following description, foam particles having through holes and foam particles having grooves will be described separately, but the shape of the foam particles according to the present invention also includes forms having both through holes and grooves.

[0033] [Foam particles with through holes] Figures 1 and 2 show examples of foamed particles 1 (1a) having through holes 111 as defects 11. The foamed particle 1a shown in Figure 1 is composed of a foamed layer 2 and has a cylindrical shape with through holes 111. The overall shape of the foamed particle may be a cylindrical shape as shown in Figure 1, or a prismatic shape. It is preferable that the through holes penetrate in the axial direction of the foamed particle. The number of through holes may be one or more.

[0034] If the foamed particles have through holes as defects, it is preferable that the average hole diameter d of the through holes be 0.1 mm or more and less than 1 mm. In this case, the Ca / A and / or Ct / A of the foamed particles can be easily adjusted to the above range. By setting the average hole diameter d of the through holes to 0.1 mm or more, it is possible to suppress the collapse and blockage of the through holes of the foamed particles during in-mold molding and to more reliably exert the effect of the through holes as defects. From a similar viewpoint, it is more preferable that the average hole diameter d of the through holes be 0.2 mm or more, even more preferable that be 0.3 mm or more, particularly preferable that be 0.4 mm or more, and most preferable that be 0.5 mm or more.

[0035] On the other hand, if the average hole diameter d of the through-holes is too large, the Ca / A and / or Ct / A of the foamed particles may become excessively large. This may result in the molded article taking an excessively long time to dry. Furthermore, there is a risk that irregularities such as gaps between the foamed particles and irregularities caused by the through-holes may be easily formed on the surface of the molded article, and the rigidity of the molded article may decrease.

[0036] Furthermore, increasing the amount of steam supplied to the mold during in-mold molding or raising the steam temperature to avoid the formation of irregularities such as gaps between foam particles or through holes can reduce the dimensional stability of the molded body, potentially leading to larger changes in the dimensions of the molded body from demolding until the shape stabilizes. By setting the average hole diameter d of the through holes to 1 mm or less, these problems can be easily avoided, and the dimensional stability of the molded body can be more easily improved even when manufacturing molded bodies with long maximum lengths or complex shapes. From the viewpoint of more reliably obtaining these effects, the average hole diameter d of the through holes is more preferably 0.95 mm or less, even more preferably 0.9 mm or less, and particularly preferably 0.85 mm or less.

[0037] In determining the preferred range for the average hole diameter d of the through-holes, the upper and lower limits of the average hole diameter d described above can be arbitrarily combined. For example, the preferred range for the average hole diameter d may be 0.2 mm or more and less than 1 mm, 0.3 mm or more and 0.95 mm or less, 0.4 mm or more and 0.9 mm or less, or 0.5 mm or more and 0.85 mm or less.

[0038] The average hole diameter d of the through-holes 111 in the foamed particle 1a is determined as follows. First, the foamed particle 1a is cut at its axial center by 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 (specifically, the opening area) of the through-holes at 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-holes is calculated, and this value is taken as the hole diameter of the through-holes in each foamed particle. The above procedure is performed for 50 or more foamed particles, and the arithmetic mean of the obtained through-hole diameters is taken as the average hole diameter d of the foamed particle's through-holes. Note that even if the hole diameter of the through-holes in each foamed particle is not uniform in the through-hole direction, the hole diameter of the through-holes in each foamed particle is determined by the hole diameter of the through-hole at the cut surface as described above.

[0039] The average pore diameter d of the through-holes can be adjusted to the aforementioned specific range by adjusting the size of the average pore diameter dr of the through-holes in the resin particles, as well as the apparent density of the foamed particles, as described later. Furthermore, by using two-stage foamed particles produced by two-stage foaming, the average pore diameter d can be more easily adjusted to a smaller value.

[0040] 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 suppressing deformation and shrinkage of the molded article when the curing process is omitted, the average 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 average 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.

[0041] In determining the preferred range for the average outer diameter D of the foamed particles, the upper and lower limits of the average outer diameter D mentioned above can be arbitrarily combined. For example, the preferred range for the average outer diameter D may be 2 mm to 8 mm, 2.5 mm to 5 mm, or 3 mm to 4.5 mm. The method for calculating the average outer diameter D of the foamed particles will be described later.

[0042] The ratio d / D of the average hole diameter d of the through-holes to the average outer diameter D of the foamed particles is preferably 0.4 or less, more preferably 0.35 or less, even more preferably 0.3 or less, and particularly preferably 0.25 or less. By setting the ratio d / D to 0.4 or less, the secondary foaming properties of the foamed particles during in-mold molding are moderately improved, making it easier to obtain a molded body with excellent surface properties and rigidity. On the other hand, from the viewpoint of further suppressing the collapse and blockage of the through-holes of the foamed particles during in-mold molding and more reliably demonstrating the effect of the through-holes, the ratio d / D is preferably 0.05 or more, and more preferably 0.1 or more.

[0043] In determining a preferred range for the ratio d / D, the upper and lower limits of the ratio d / D mentioned above can be arbitrarily combined. For example, the preferred range for the ratio d / D may be 0.05 or more and 0.4 or less, 0.05 or more and 0.35 or less, 0.1 or more and 0.3 or less, or 0.1 or more and 0.25 or less.

[0044] The average wall thickness t of the foam particles having through holes is preferably 1.1 mm or more and 2.0 mm or less. If the average wall thickness t is within this range, the thickness of the foam particles is sufficiently thick, which further improves secondary foaming during in-mold molding. In addition, the foam particles become less susceptible to crushing under external force, and the rigidity of the molded article is further improved. From this viewpoint, the average wall thickness t of the foam particles is more preferably 1.2 mm or more.

[0045] The average wall thickness t of the foamed particle is the distance from the surface of the foamed particle (i.e., the outer surface) to the outer edge of the through hole (i.e., the inner surface of the foamed particle), and is a value that can be obtained by the following formula (1). t = (Dd) / 2 ... (1) d: Average diameter of through holes (unit: mm) D: Average outer diameter of foamed particles (unit: mm)

[0046] Furthermore, the ratio of the average wall thickness t to the average outer diameter D of the foamed particles, t / D, is preferably 0.30 or more and 0.50 or less. When t / D is within the above range, the filling of the foamed particles is better during in-mold molding. In addition, secondary foaming can be further enhanced while maintaining a short cooling time. Therefore, molded articles with excellent appearance and rigidity can be manufactured productively at a lower molding pressure (i.e., heating temperature). From this viewpoint, the ratio of the average wall thickness t to the average outer diameter D of the foamed particles, t / D, is more preferably 0.32 or more and 0.50 or less, and even more preferably 0.35 or more and 0.50 or less.

[0047] [Foam particles with grooves] Figures 3 and 4 show examples of foamed particles 1(1b) having grooves 112 as defects 11. The foamed particle 1b shown in Figure 3 is composed of a foamed layer 2. The foamed particle 1b has a columnar shape and has at least one groove 112 on its side surface.

[0048] The aforementioned "groove" refers to a recess provided on the circumferential surface of the foam particle 1b, extending along the axial direction of the foam particle 1b. More specifically, the groove 112 of the foam particle 1b is observed as a portion where the contour of the foam particle is indented inward in a cross-section that appears when the foam particle 1b is cut by a plane perpendicular to the axial direction of the foam particle 1b, as shown in Figure 4.

[0049] The shape of the foam particles 1b can take on various forms. For example, the foam particles 1b may be columnar in shape with a cross-shaped cross section perpendicular to the axial direction, as shown in Figures 3 and 4. Foam particles 1b having such a shape have four grooves 112 on their side surfaces. The cross-sectional shape of the foam particles in the cross section perpendicular to the axial direction is not limited to the shapes shown in Figures 3 and 4, and can take on various forms such as a circular shape, a triangular shape, or a square shape.

[0050] The number of grooves 112 provided in the foamed particle may be one or more. Furthermore, the cross-sectional shape of the grooves 112 in a cross section perpendicular to the axial direction of the foamed particle is not limited to the V-shape shown in Figures 3 and 4, but can take various forms such as semicircular, square, and U-shape. If the foamed particle has two or more grooves 112, all grooves 112 may have the same cross-sectional shape, or they may have different cross-sectional shapes.

[0051] It is preferable that the foamed particles have two to six grooves. In this case, an open-cell structure is more easily formed in the molded body. Therefore, the foamed particles can more easily improve the dimensional stability of the molded body.

[0052] Furthermore, if the foam particles have two or more grooves, it is preferable that these grooves are arranged at equal intervals on the circumferential surface of the foam particles. In other words, if the foam particles have multiple grooves, it is preferable that these grooves are arranged symmetrically with respect to the central axis of the foam particles. When such foam particles are molded in a mold, an open-cell structure is more easily formed in the molded body. Therefore, the foam particles can more easily improve the dimensional stability of the molded body.

[0053] The average cross-sectional area per groove in the cut surface of the foamed particle is 0.05 mm². 2 1.2mm or more 2The following is preferable: By setting the ratio Ca / A of the average cross-sectional area per defect to the average cross-sectional area A of the foamed particles to the aforementioned specific range, and further setting the average cross-sectional area per groove to the aforementioned specific range, the aforementioned effect of the grooves can be obtained more reliably. From a similar viewpoint, the average cross-sectional area per groove is 0.1 mm². 2 1.0mm or more 2 The following is more preferable: 0.2 mm 2 0.8mm or more 2 The following is even more preferable. The method for calculating the average cross-sectional area per groove on the cut surface of the foamed particle is the same as the method for calculating the average cross-sectional area Ca per defect, described later, except that the sum of the cross-sectional areas C of the grooves 112 is used instead of the sum of the cross-sectional areas 11 of the defect parts in each foamed particle 1.

[0054] Preferably, the ratio H / D of the average depth H per groove to the average outer diameter D of the foam particles at the cross-section of the foam particles is 0.20 or less. In this case, an open-cell structure is more easily formed in the molded body, and shrinkage and deformation of the molded body can be more effectively suppressed when the curing process is omitted. Furthermore, in this case, in addition to the effects described above, the rigidity of the molded body can also be increased. From the viewpoint of further enhancing these effects, it is more preferable that the ratio H / D of the average depth H per groove to the average outer diameter D of the foam particles is 0.18 or less, and even more preferable that it is 0.15 or less.

[0055] From a similar viewpoint, the average depth H per groove in the cross-section of the foamed particle is preferably 1.0 mm or less, more preferably 0.8 mm or less, and even more preferably 0.5 mm or less.

[0056] Furthermore, from the viewpoint of suppressing an excessive decrease in the open air bubble ratio of the molded article, the ratio H / D of the average depth H per groove to the average outer diameter D of the foamed particles at the cross-section of the foamed particles is preferably 0.02 or more, and more preferably 0.05 or more. Also, from the same viewpoint, the average depth H per groove at the cross-section of the foamed particles is preferably 0.1 mm or more, and more preferably 0.2 mm or more.

[0057] The method for calculating the average depth H of each groove in the cross-section of the foam particle is as follows. First, the foam particle 1 is cut at its center in the axial direction by a plane perpendicular to the axial direction, exposing the cross-section as shown in Figures 4 and 9. Next, a tangent line L1 is drawn outside each groove 112 in the cross-section, touching the contour of the foam particle 1 at two points Q1 and Q2, and not passing through the interior of the foam particle 1. The maximum value h of the distance from the tangent line L1 to the periphery of the groove 112 (i.e., the contour of the foam particle) in a direction perpendicular to this tangent line L1 is measured. Then, the maximum value h of the aforementioned distance is measured for all grooves 112, and the arithmetic mean of these is taken as the groove depth of each foam particle. Note that the measurement of the groove depth of each foam particle in the cross-section can be performed, for example, by taking a photograph of the cross-section of the foam particle and performing image analysis.

[0058] Then, the arithmetic mean of the groove depth measured for more than 100 foam particles is used for the foam particles Let H be the depth of the groove.

[0059] [Method for calculating the average cross-sectional area Ca per defect, the total cross-sectional area Ct of the defects, and the average cross-sectional area A of the foamed particles] In the cross-section obtained by cutting the foam particles at their axial center with a plane perpendicular to the axial direction, the ratio Ca / A of the average cross-sectional area Ca per defect to the average cross-sectional area A of the foam particles is 0.01 or more and 0.20 or less, and the ratio Ct / A of the total cross-sectional area Ct of the defects to the average cross-sectional area A of the foam particles is 0.02 or more and 0.20 or less. As described above, the foam particles having the specific shape can improve the dimensional stability of the molded article. Furthermore, the foam particles can significantly improve the productivity of the molded article and easily improve the surface properties and fusion properties of the molded article.

[0060] The method for calculating the average cross-sectional area A of the foamed particle is as follows. First, the foamed particle is cut at its center in the axial direction by a plane perpendicular to the axial direction, exposing the cut surface. The cross-sectional area of ​​the foamed particle 1 at this cut surface is measured. For example, as shown in Figures 2 and 4, if the foamed particle 1 consists only of a 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 6 later, if the foamed particle 1 has a foamed layer 2 and a coating layer 3 covering the foamed layer 2, the cross-sectional area of ​​the foamed particle 1 is 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. The measurement of the cross-sectional area of ​​the foamed particle 1 at the cut surface can be done, for example, by taking a photograph of the cut surface of the foamed particle and performing image analysis. Furthermore, the cross-sectional area of ​​defective parts is not included in the cross-sectional area of ​​the foamed particle.

[0061] The above procedure is performed for 100 or more foam particles, and the arithmetic mean of the resulting cross-sectional areas of the foam particles is defined as the average cross-sectional area A of the foam particles at the cross-section.

[0062] The method for calculating the average cross-sectional area Ca per defect and the total cross-sectional area Ct of the defects is as follows. First, the foamed particle is cut at its center in the axial direction by a plane perpendicular to the axial direction, exposing the cut surface. As shown in Figures 2 and 6, if the defect 11 is a through hole 111, the cross-sectional area C of the through hole 111 at this cut surface is measured.

[0063] Furthermore, if the defect 11 is a groove 112, as shown in Figures 4 and 9, a tangent line L1 is drawn outside each groove 112 in the cross-section, touching the contour of the foam particle 1 at two points Q1 and Q2, and not passing through the interior of the foam particle 1. Then, the area of ​​the region enclosed by the contour of the foam particle 1 and the tangent line L1 is calculated, and this area is taken as the cross-sectional area C of each groove 112. In other words, the cross-sectional area C of each groove 112 is the area of ​​the region indicated by the shaded area in Figures 4 and 9.

[0064] The total cross-sectional area of ​​the defects 11 in each foamed particle 1 is calculated by summing the cross-sectional areas C of the through holes 111 and the grooves 112 obtained in this way. Furthermore, the total cross-sectional area of ​​the defects 11 is divided by the number of defects 11 to calculate the cross-sectional area of ​​each defect in each foamed particle 1.

[0065] Perform the above operation for 100 or more foamed particles, and the arithmetic mean of the sum of the cross-sectional areas of the resulting defects is defined as the total cross-sectional area of ​​the defects, Ct. Also, the average value of the cross-sectional area per defect is defined as the average cross-sectional area per defect, Ca.

[0066] [Method for calculating the average outer diameter D of foamed particles] The method for calculating the average outer diameter D of the foam particles is as follows. First, 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 as shown in Figures 2 and 4. Two points Q3 and Q4 are determined on the contour of the foam particle 1 on this cut surface such that the distance between them is longest, and the distance between these two points (i.e., the maximum outer diameter of the foam particle on the cut surface) is taken as the outer diameter r of each individual foam particle 1. Then, the arithmetic mean of the outer diameters r measured for 100 or more foam particles 1 is taken as the average outer diameter D of the foam particles. Note that the outer diameter of the foam particle 1 on the cut surface can be measured, for example, by taking a photograph of the cut surface of the foam particle and performing image analysis. Furthermore, even if the outer diameter of each foam particle is not uniform in the direction of penetration, the outer diameter of each foam particle is determined by the outer diameter on the cut surface as described above.

[0067] [Apparent density and bulk density of foamed particles] The bulk density of the expanded particles is preferably 10 kg / m 3 or more and 100 kg / m 3 or less, more preferably 15 kg / m 3 or more and 75 kg / m 3 or less, still more preferably 20 kg / m 3 or more and 50 kg / m 3 or less. Also, the apparent density of the expanded particles is preferably 15 kg / m 3 or more and 200 kg / m 3 or less, more preferably 20 kg / m 3 or more and 150 kg / m 3 or less, still more preferably 30 kg / m 3 or more and 100 kg / m 3 or less. In these cases, the lightness and rigidity of the molded body can be improved in a well-balanced manner. Conventionally, especially when manufacturing a molded body with a low density (that is, a molded body with a high molding magnification), the molded body was extremely likely to deform significantly after脱模, and it was difficult to omit the curing process. On the other hand, since the expanded particles can omit the curing process even when manufacturing a molded body with a low density, it is possible to manufacture a lightweight and good-looking molded body without curing.

[0068] The ratio of the apparent density of the expanded particles to the bulk density of the expanded particles (that is, apparent density / bulk density) is preferably more than 1.6, and more preferably 1.7 or more. In this case, the cooling time after in-mold molding can be further shortened, and a lightweight and good molded body can be obtained more easily. Also, deformation and shrinkage of the molded body when the curing process is omitted or the time of the curing process is shortened can be more reliably suppressed. Also, the ratio of the apparent density of the expanded particles to the bulk density of the expanded particles (that is, apparent density / bulk density) is preferably less than 2.0, and more preferably 1.9 or less. In this case, the drying time of the molded body can be further shortened. Also, the surface property and rigidity of the obtained molded body can be made better.

[0069] In determining a preferred range for the ratio of the apparent density of foamed particles to the bulk density of the foamed particles, the upper and lower limits of the ratio mentioned above can be arbitrarily combined. For example, the preferred range for the apparent density / bulk density value may be greater than 1.6 and less than 2.0, or it may be between 1.7 and 1.9.

[0070] The method for calculating the bulk density of foamed particles is as follows: First, the foamed particles are left to stand for at least 24 hours in an environment with a relative humidity of 50%, a temperature of 23°C, and a pressure of 1 atm to adjust their state. The adjusted foamed particles are then filled into a graduated cylinder so that they naturally accumulate, and the bulk volume (unit: L) of the foamed particles is read from the scale of the graduated cylinder. Then, the bulk density (unit: kg / m³) of the foamed particles is calculated by dividing the mass (unit: g) of the foamed particles in the graduated cylinder by the aforementioned bulk volume and converting the value to units. 3 This allows us to obtain [the desired result]. Note that the bulk density of the foamed particles is a value measured under a pressure of 1 atm using uncompressed, naturally occurring foamed particles.

[0071] The apparent density of foamed particles is calculated as follows: First, the foamed particle group is left to stand for one day in an environment with a relative humidity of 50%, a temperature of 23°C, and a pressure of 1 atm to allow the foamed particles to settle. After measuring the mass (in g) of the foamed particle group, it is submerged in a graduated cylinder containing alcohol (e.g., ethanol) at 23°C using a wire mesh, and the volume (in L) of the foamed particle group is determined from the rise in the liquid level. Then, the apparent density (in kg / m³) of the foamed particles is calculated by dividing the mass of the foamed particle group by its volume and converting the units. 3 It is possible to calculate ).

[0072] [Foam layer] The foamed particles have a foamed layer made of a polypropylene resin. In this specification, a polypropylene resin refers to a propylene copolymer containing 50% by mass or more of a homopolymer of propylene monomers and structural units derived from propylene. Preferred 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.

[0073] The polypropylene resin constituting the foam layer includes a polypropylene resin (A) having a melting point of 135°C to 150°C and a flexural modulus of less than 1000 MPa, and a polypropylene resin (B) having a melting point of 145°C to 160°C and a flexural modulus of 1000 MPa or more. Furthermore, the mass ratio of the polypropylene resin (A) to the polypropylene resin (B) in the polypropylene resin is polypropylene resin (A):polypropylene resin (B) = 65:35 to 35:65 (where the total amount of both is 100% by mass).

[0074] By using a polypropylene resin containing the polypropylene resin (A) and the polypropylene resin (B) in the specified mass ratio as the polypropylene resin constituting the foamed layer of the foamed particles, the dimensional stability of the molded article can be easily improved, and the change in the dimensions of the molded article from the time of demolding to the time when the shape stabilizes can be reduced. Furthermore, by using such foamed particles, it is possible to easily obtain a molded article with the desired dimensions even when manufacturing a molded article that is relatively difficult to mold in a mold, without performing a curing process or by shortening the curing process time.

[0075] [Polypropylene resin (A)] The polypropylene resin (A) may be a homopolymer of propylene monomers or a propylene copolymer. From the viewpoint of more reliably obtaining the effect of improving the dimensional stability of the molded article, the polypropylene resin (A) is preferably a propylene copolymer, more preferably one or more propylene copolymers selected from the group consisting of ethylene-propylene copolymer, butene-propylene copolymer and ethylene-butene-propylene copolymer, and even more preferably an ethylene-propylene random copolymer. The polypropylene resin (A) may be a virgin resin or a recycled resin. Furthermore, the polypropylene resin (A) may be a resin derived from fossil fuels or a resin containing monomer components derived from biomass.

[0076] The total content of ethylene and butene components in the polypropylene resin (A) is preferably 1.5% by mass or more and 4.5% by mass or less, more preferably 2.0% by mass or more and 3.5% by mass or less, and even more preferably greater than 2.5% by mass and 3.5% by mass or less.

[0077] The aforementioned "ethylene component" refers to the ethylene-derived constituent units in the propylene copolymer. Furthermore, the ethylene component content is the mass ratio of the ethylene component when the total of propylene-derived constituent units and other monomer-derived constituent units in the propylene copolymer is taken as 100% by mass. The ethylene component content in the propylene copolymer can be determined based on the results of IR spectral measurements.

[0078] Furthermore, the aforementioned "butene component" refers to the butene-derived structural units in the propylene copolymer. The butene component content is the mass ratio of the butene component when the total of the propylene-derived structural units and other monomer-derived structural units contained in the propylene copolymer is taken as 100% by mass. The butene component content in the propylene copolymer can be determined based on the results of IR spectral measurement.

[0079] The flexural modulus of the polypropylene resin (A) is preferably 600 MPa or more and less than 1000 MPa, more preferably 700 MPa or more and less than 1000 MPa, even more preferably 800 MPa or more and less than 1000 MPa, and particularly preferably 900 MPa or more and 980 MPa or less. In this case, the moldability of the foamed particles can be more easily improved. The flexural modulus of the polypropylene resin (A) can be determined based on JIS K7171:2008.

[0080] The melting point of the polypropylene resin (A) is more preferably 136°C to 148°C, even more preferably 137°C to 146°C, particularly preferably 138°C to 145°C, and most preferably 140°C to less than 145°C. In this case, the moldability of the foamed particles can be more easily improved.

[0081] The melting point of polypropylene resin (A) can be determined by differential scanning calorimetry (i.e., DSC) based on JIS K7121-1987 and based on the obtained DSC curve. First, the test specimen is conditioned according to "(2) When measuring the melting temperature after performing a certain heat treatment". The heating rate and cooling rate during conditioning are set to 10°C / min. 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 that appears in the DSC curve is taken as the melting point of polypropylene resin (A). 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 of polypropylene resin (A).

[0082] From the viewpoint of further improving the foaming properties and moldability of the foamed particles, the melt mass flow rate (i.e., MFR) of the polypropylene resin (A) is preferably 5 g / 10 min to 15 g / 10 min, more preferably 6 g / 10 min to 12 g / 10 min, and even more preferably 7 g / 10 min to 10 g / 10 min. The MFR of the polypropylene resin (A) 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.

[0083] [Polypropylene resin (B)] The polypropylene resin (B) may be a homopolymer of propylene monomers or a propylene copolymer. From the viewpoint of more reliably obtaining the effect of improving the dimensional stability of the molded article, the polypropylene resin (B) is preferably a propylene copolymer, and more preferably one or more propylene copolymers selected from the group consisting of ethylene-propylene copolymer, butene-propylene copolymer and ethylene-butene-propylene copolymer. In this case, a molded article with excellent appearance and rigidity can be molded at a lower molding temperature (i.e., lower molding pressure). In this case, the shape of the molded article is more easily restored after demolding from the mold, and the curing process time can be more easily shortened. Furthermore, the polypropylene resin (B) may be virgin resin or recycled resin. Furthermore, the polypropylene resin (B) may be a resin derived from fossil fuels or a resin containing monomer components derived from biomass.

[0084] From the viewpoint of further enhancing the effects described above, the total content of ethylene and butene components in the polypropylene resin (B) is preferably 0.5% by mass or more and 2.5% by mass or less, more preferably 0.5% by mass or more and less than 2.5% by mass, even more preferably 0.8% by mass or more and 2.0% by mass or less, and particularly preferably 1.0% by mass or more and 1.8% by mass or less.

[0085] The flexural modulus of polypropylene resin (B) is preferably 1000 MPa or more and 1800 MPa or less, more preferably 1050 MPa or more and 1700 MPa or less, even more preferably 1100 MPa or more and less than 1600 MPa, and particularly preferably 1150 MPa or more and 1500 MPa or less. In this case, the effect of improving the dimensional stability of the molded article can be further enhanced. From a similar viewpoint, the absolute value of the difference between the flexural modulus of polypropylene resin (A) and the flexural modulus of polypropylene resin (B) is preferably 200 MPa or more and 500 MPa or less. The flexural modulus of polypropylene resin (B) can be determined based on JIS K7171:2008.

[0086] The melting point of the polypropylene resin (B) is more preferably 148°C to 158°C, and even more preferably 150°C to 155°C. In this case, the effect of improving the dimensional stability of the molded article can be further enhanced.

[0087] The method for measuring the melting point of polypropylene resin (B) is the same as the method for measuring the melting point of polypropylene resin (A) described above, except that polypropylene resin (B) is used instead of polypropylene resin (A).

[0088] [Mass ratio of polypropylene resin (A) and polypropylene resin (B)] In the polypropylene resin constituting the foamed layer, the mass ratio of polypropylene resin (A) to polypropylene resin (B) is polypropylene resin (A):polypropylene resin (B) = 65:35 to 35:65 (where the total amount of both is 100% by mass). That is, the mass ratio of polypropylene resin (A) to the total of polypropylene resin (A) and polypropylene resin (B) (100% by mass) is 35% by mass or more and 65% by mass or less.

[0089] By setting the mass ratio of polypropylene resin (A) to polypropylene resin (B) within the specified range, the foamed particles improve the dimensional stability of the molded article, reducing dimensional changes from the time of demolding until the shape stabilizes. Therefore, even when manufacturing molded articles with relatively long maximum lengths or complex shapes, which are relatively difficult to mold in-mold, the foamed particles make it easy to obtain molded articles with desired dimensions, even when the curing process is omitted or the curing time is shortened.

[0090] When the mass percentage of polypropylene resin (A) is lower than 35% by mass relative to the total mass of polypropylene resin (A) and polypropylene resin (B) (100% by mass), the molding pressure during in-mold molding tends to become excessively high. In this case, especially when manufacturing molded articles with long maximum lengths or complex shapes, the secondary foaming properties during in-mold molding tend to decrease, which may lead to deterioration of the appearance of the molded article.

[0091] By setting the mass percentage of polypropylene resin (A) to 35% by mass or more, preferably 38% by mass or more, more preferably 40% by mass or more, even more preferably 45% by mass or more, particularly preferably 50% by mass or more, and most preferably 55% by mass or more, based on 100% by mass of the total of polypropylene resin (A) and polypropylene resin (B), these problems can be easily avoided, and a molded article with a good appearance can be easily obtained.

[0092] If the mass percentage of polypropylene resin (A) exceeds 65% by mass relative to the total mass of polypropylene resin (A) and polypropylene resin (B) (100% by mass), it may lead to a decrease in the dimensional stability of the molded article. Furthermore, especially when manufacturing molded articles with relatively high difficulty in in-mold molding, the dimensional change from the time the molded article is demolded to the time when its shape stabilizes may become large, making it difficult to adjust the dimensions of the molded article to a desired range. By setting the mass percentage of polypropylene resin (A) to 65% by mass or less, preferably 64% by mass or less, more preferably 63% by mass or less, even more preferably 62% by mass or less, and particularly preferably 61% by mass or less, relative to the total mass of polypropylene resin (A) and polypropylene resin (B) (100% by mass), these problems can be easily avoided and the dimensional stability of the molded article can be improved.

[0093] In determining a preferred range for the mass ratio of polypropylene resin (A) and polypropylene resin (B), the upper and lower limits of the mass ratio of polypropylene resin (A) and polypropylene resin (B) described above can be arbitrarily combined. For example, polypropylene resin (A):polypropylene resin (B) = 65:35 to 38:62, polypropylene resin (A):polypropylene resin (B) = 64:36 to 40:60, polypropylene resin (A):polypropylene resin (B) = 63:37 to 45:55, polypropylene resin (A):polypropylene resin (B) = 62:38 to 50:50, or polypropylene resin (A):polypropylene resin (B) = 61:39 to 55:45 (provided that the total amount of both is 100% by mass).

[0094] [Other polymers] The polypropylene resin constituting the foamed layer may contain other polymers besides polypropylene resin (A) and polypropylene resin (B) to the extent that they do not hinder the effects described above. Examples of other polymers include thermoplastic resins such as polypropylene resins that do not fall under either polypropylene resin (A) or polypropylene resin (B), polyethylene resins, polyamide resins, and polystyrene resins, as well as elastomers. 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 particularly preferably 0, that is, the foamed layer contains substantially only polypropylene resin (A) and polypropylene resin (B) as polymers.

[0095] Furthermore, the polypropylene resin constituting 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 aforementioned effects. 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 the polypropylene resin.

[0096] The coloring agent contained in the polypropylene resin constituting the foam layer is preferably carbon black. In this case, a high-quality appearance can be given to the molded product. The carbon black content in the foam layer is preferably 0.1% by mass or more and 5% by mass or less, more preferably 0.5% by mass or more and 4% by mass or less, and even more preferably 1% by mass or more and 3.5% by mass or less.

[0097] [Coating layer] The foamed particle may have a foamed layer and a coating layer made of thermoplastic resin that covers the foamed layer. In this case, the coating layer may cover the entire surface of the foamed layer or cover a part of it. For example, as shown in Figures 5 and 6, the coating layer 3 may be provided on the side surface of the foamed particle 1 (1c) and cover the foamed layer 2.

[0098] The coating layer is a layer provided on the surface of foam particles in order to improve the fusion properties between foam particles during in-mold molding. Preferably, the thermoplastic resin constituting the coating layer has a melting point or a lower softening point than the polypropylene resin constituting the foam layer.

[0099] The thermoplastic resin constituting the coating layer may be a crystalline thermoplastic resin or an amorphous thermoplastic resin. Examples of crystalline thermoplastic resins used in the coating layer include polyolefin resins. Examples of amorphous thermoplastic resins used in the coating layer include polystyrene resins. From the viewpoint of adhesion to the foam layer, the thermoplastic resin constituting the coating layer is preferably a polyolefin resin, more preferably a polyethylene resin and / or a polypropylene resin, and even more preferably a polypropylene resin. Examples of polypropylene resins used in the coating layer include ethylene-propylene copolymer, propylene-butene copolymer, ethylene-propylene-butene copolymer, and propylene homopolymer. Among these, it is particularly preferable that the coating layer be composed of ethylene-propylene copolymer and / or ethylene-propylene-butene copolymer.

[0100] When the coating layer is composed of a crystalline polyolefin resin, the melting point of the crystalline polyolefin resin is preferably 110°C to 150°C, more preferably 120°C to 145°C, and even more preferably 125°C to 142°C.

[0101] Furthermore, the difference [Tmc-Tms] between the melting point Tmc of the base resin constituting the foam layer and the melting point Tms of the crystalline polyolefin resin constituting the coating layer is preferably 1°C to 40°C, more preferably 2°C to 35°C, and even more preferably 5°C to 30°C. In this case, the in-moldability of the foam particles can be more easily improved even when the molding pressure is relatively low.

[0102] The thermoplastic resin constituting the coating layer may contain additives such as crystal nucleating agents, flame retardants, flame retardant aids, plasticizers, antistatic agents, antioxidants, ultraviolet 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.

[0103] The coloring agent contained in the thermoplastic resin constituting the coating layer is preferably carbon black. In this case, a high-quality appearance can be given to the molded article. The carbon black content in the coating layer is preferably 0.1% by mass or more and 5% by mass or less, more preferably 0.5% by mass or more and 4% by mass or less, and even more preferably 1% by mass or more and 3.5% by mass or less.

[0104] Furthermore, it is preferable that the melt mass flow rate of the thermoplastic resin constituting the coating layer, measured under the conditions of 230°C and 2.16 kg load according to JIS K7210-1:2014, is higher than 15 g / 10 min. In this case, the formation of streaky patterns on the surface of the molded article can be suppressed without impairing the rigidity of the foam layer. The upper limit of the melt mass flow rate of the thermoplastic resin constituting the coating layer, measured under the conditions of 230°C and 2.16 kg load according to JIS K7210-1:2014, is approximately 35 g / 10 min.

[0105] 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" includes a state in which the coating layer does not foam and contains no bubbles, and a state in which bubbles disappear after foaming, meaning that there is almost no bubble structure within the coating layer. 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.

[0106] From the viewpoint of improving moldability while maintaining the rigidity of the molded article, the mass ratio (mass %) of the polypropylene resin constituting the foam layer and the thermoplastic 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.

[0107] [High temperature peak] Preferably, the foamed particles have a crystalline structure such that the DSC curve obtained when the foamed particles are heated from 23°C to 200°C at a heating rate of 10°C / min shows a resin-specific peak originating from the melting of crystals inherent to the polypropylene resin constituting the foamed layer, and a high-temperature peak having a peak temperature higher than the peak temperature of the resin-specific peak. Foamed particles with such a crystalline structure have excellent mechanical strength and moldability. The resin-specific peak is caused by endothermic reactions when the crystals inherent to the polypropylene resin constituting the foamed layer melt. On the other hand, the high-temperature peak is presumed to be caused by the melting of secondary crystals formed in the polypropylene resin constituting the foamed layer during the manufacturing process of the foamed particles. In other words, if a high-temperature peak appears in the DSC curve, it is presumed that secondary crystals have been formed in the polypropylene resin.

[0108] 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.

[0109] Specifically, when foam particles are heated from 23°C to 200°C at a heating rate of 10°C / min (i.e., the first heating), the resulting DSC curve shows both a high-temperature peak and a resin-specific peak of the polypropylene resin constituting the foam layer. The DSC curve obtained during the first heating shows both a high-temperature peak and a resin-specific peak. In contrast, when the foam particles are cooled from 200°C to 23°C at a cooling rate of 10°C / min after the first heating, and then heated again from 23°C to 200°C at a heating rate of 10°C / min (i.e., the second heating), the resulting DSC curve shows only the resin-specific peak of the polypropylene resin constituting the foam layer. Therefore, by comparing the DSC curve obtained during the first heating and the DSC curve obtained during the second heating, the resin-specific peak and the high-temperature peak can be distinguished. 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.

[0110] The heat of fusion of the high-temperature peak of the foamed particles is preferably 8 J / g or more and 25 J / g or less, more preferably 10 J / g or more and 24 J / g or less, even more preferably 15 J / g or more and 23 J / g or less, and particularly preferably 18 J / g or more and 22 J / g or less, from the viewpoint of further improving the moldability of the foamed particles, obtaining a molded article with superior rigidity, and further improving dimensional stability when trying to obtain a molded article that is relatively difficult to mold in a mold.

[0111] The heat of fusion of the aforementioned high-temperature peak is determined as follows. First, 1 to 3 mg of conditioned foam particles are used as a sample, and a differential scanning calorimetry (DSC) curve is obtained by heating from 23°C to 200°C at a heating rate of 10°C / min. An example of a DSC curve is shown in Figure 7. When foam 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 7.

[0112] Next, draw a straight line L2 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 and the baseline on the high-temperature side of the DSC curve.

[0113] After drawing the straight line L2, a straight line L3 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 L3 separates the resin intrinsic peak ΔH1 and the high-temperature peak ΔH2. The heat of fusion of the resin intrinsic peak ΔH1 can be calculated based on the area enclosed by the portion of the DSC curve that constitutes the resin intrinsic peak ΔH1, and the straight lines L2 and L3. Similarly, the heat of fusion of 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 L2 and L3.

[0114] (Method for manufacturing polypropylene resin foam particles) The foamed particles can be manufactured, for example, by dispersing polypropylene resin particles (hereinafter referred to as "resin particles"), which are composed of polypropylene resin, in a dispersion medium, impregnating the resin particles with a foaming agent, and then releasing the resin particles containing the foaming agent together with the dispersion medium under low pressure. This foaming method is sometimes called the "direct foaming method."

[0115] Resin particles can be produced, for example, by the strand-cutting method. In the strand-cutting method, polypropylene resin (A) and polypropylene resin (B) constituting the foam layer, along with additives such as bubble nucleating agents as needed, are first supplied to 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 an extruder. 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 with polypropylene resin as the base resin.

[0116] To obtain foamed particles with a multilayer structure comprising a foamed layer and a coating 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 polypropylene resin (A) and polypropylene resin (B) constituting the foamed layer, along with additives added as needed, to produce a molten resin mixture for core layer formation. The coating layer forming extruder melts and kneads thermoplastic resin constituting the coating layer, along with additives added as needed, to produce a molten resin mixture for coating layer formation.

[0117] These molten resin mixtures are co-extruded and combined in a die to form a multilayer composite consisting of a non-foamed core layer and a non-foamed coating layer covering the outer surface of the core layer. This composite is extruded through the small holes of the die to form an extruded product. After cooling this extruded product, it is cut to the desired length to obtain multilayer resin particles. The method for producing the resin particles is not limited to the method described above, and methods such as the hot-cut method or the underwater-cut method may also be employed.

[0118] In the direct foaming method, the resin particles foam while generally maintaining their original shape. Therefore, the shape of the foamed particles obtained by the direct foaming method is generally an enlarged version of the original resin particle shape. Consequently, to obtain cylindrical foamed particles with through holes, one should foam cylindrical resin particles with through holes. Such resin particles can be manufactured, for example, in the strand cutting method described above, by using a die equipped with annular holes corresponding to the shape of the cut surface of the desired resin particle.

[0119] Similarly, to obtain foamed particles with a cross-shaped cross-section at the cut surface, columnar resin particles with a cross-shaped cross-section at the cut surface can be foamed. Such resin particles can be manufactured, for example, in the strand-cutting method described above, by using a die equipped with cross-shaped small holes corresponding to the shape of the cut surface of the desired resin particles.

[0120] When producing resin particles, it is preferable to employ a strand-cutting method in which the extruded material is cooled in water before cutting. In this case, the accuracy of the resin particle shape can be further improved, and the shape of any defects in the resulting foamed particles can be more easily determined to the desired shape.

[0121] The particle diameter of the resin particles is preferably 0.1 mm to 3.0 mm, and more preferably 0.3 mm to 1.5 mm. Furthermore, the ratio of the length to the outer diameter of the resin particles is preferably 0.5 to 5.0, and more preferably 1.0 to 3.0.

[0122] Furthermore, the average mass per resin particle is preferably 0.1 mg or more and 20 mg or less, more preferably 0.2 mg or more and 10 mg or less, even more preferably 0.3 mg or more and 5 mg or less, and particularly preferably 0.4 mg or more and 2 mg or less. The average mass per resin particle is the value obtained by dividing the mass of 200 randomly selected resin particles by the number of resin particles.

[0123] 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.

[0124] In foamed particles having through holes, the average pore diameter d of the through holes in the foamed layer can be adjusted to the specified range by adjusting the average pore diameter dr of the through holes in the core layer of the resin particles. More specifically, by setting the average pore diameter dr of the through holes in the resin particles to 0.10 mm or more and less than 0.25 mm, preferably 0.12 mm or more and less than 0.24 mm, and more preferably 0.15 mm or more and less than 0.22 mm, foamed particles with an average pore diameter d of 0.1 mm or more and less than 1 mm can be easily manufactured. The average pore diameter dr of the through holes in the core layer of the resin particles can be adjusted, for example, by the pore diameter of the small holes in the die used to form the through holes (i.e., the inner diameter of the die).

[0125] Furthermore, by adjusting the particle size and average mass of the resin particles, the average outer diameter of the foamed particles can be adjusted to the aforementioned range. More specifically, by setting the ratio dr / Dr of the average pore diameter dr of the through holes to the average outer diameter Dr of the resin particles to 0.4 or less, preferably 0.3 or less, more preferably 0.25 or less, and even more preferably 0.2 or less, foamed particles with a ratio d / D of the average pore diameter d of the through holes to the average outer diameter D of the foamed particles to 0.4 or less can be easily manufactured. From the viewpoint of manufacturing stability of the resin particles, it is preferable that the average pore diameter dr of the through holes of the resin particles is 0.1 mm or more, and that the ratio dr / Dr of the average pore diameter dr of the through holes to the average outer diameter Dr of the resin particles is 0.1 or more.

[0126] The method for calculating the average pore diameter dr of the through-holes of resin particles and the average outer diameter Dr of the resin particles is the same as the method for calculating the average pore diameter d of the through-holes of foamed particles and the average outer diameter D of the foamed particles described above, except that resin particles are used instead of foamed particles.

[0127] Furthermore, when employing a strand-cutting method for cutting the extruded material, that is, a method in which the cylindrical extruded material from the die is taken up, cooled in water, and then cut to an appropriate length, the particle size, length / outer diameter ratio, and average mass of the resin particles can be adjusted by appropriately changing the extrusion speed, take-up speed, cutter speed, etc., during the extrusion of the molten resin mixture.

[0128] 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 foaming process described later, 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.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] 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 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.

[0133] 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.

[0134] 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.

[0135] 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.

[0136] Furthermore, when heating the dispersion medium, the temperature during foaming can be kept within an appropriate range by setting the heating rate of the dispersion medium to a range of 1°C / min to 5°C / min.

[0137] After the foaming agent has been impregnated into the resin particles, the contents of the sealed container are released into an environment with a lower pressure than the sealed container. This causes the core layer of the resin particles to foam, forming a cellular structure, which is then cooled by the outside air, stabilizing the cellular structure and resulting in foamed particles.

[0138] When impregnating the core layer with a foaming agent, it is preferable to perform heating and foaming in the following manner. Specifically, first, a one-stage holding step is performed in which the resin particles are held at a temperature of (melting point of resin particles - 20°C) or higher and below the (melting end temperature of resin particles) for a sufficient time, preferably about 10 to 60 minutes. Then, the temperature is adjusted from (melting point of resin particles - 15°C) to below (melting end temperature of resin particles + 10°C). If necessary, a second-stage holding step is performed in which the resin particles are held at that temperature 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 resin particles - 10°C) to foam the resin particles. It is more preferable that the temperature inside the sealed container during foaming is at or above the (melting point of resin particles) and below the (melting point of resin particles + 20°C). By heating and foaming the resin particles in this way, secondary crystals are formed in the polypropylene resin constituting the foamed layer, and foamed particles with excellent mechanical strength and moldability can be easily obtained.

[0139] The method for measuring the melting point of the resin particles is the same as the method for measuring the melting point of the polypropylene resin (A) described above, except that resin particles are used instead of polypropylene resin (A). The melting termination temperature of the resin particles is the high-temperature endpoint of the DSC curve obtained in the measurement of the melting point of the resin particles, that is, the temperature at the intersection of the melting peak and the baseline on the higher side of the melting peak.

[0140] The foamed particles obtained as described above may be used as is to produce the molded article. Alternatively, the foamed particles obtained by the direct foaming method described above can be further foamed to increase their bulk ratio, 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 first-stage foaming stage, and the foamed particles obtained in the first-stage foaming stage are called first-stage foamed particles. The second foaming stage is called the second-stage foaming stage. The foamed particles obtained in the second-stage foaming stage are sometimes called second-stage foamed particles.

[0141] A method for increasing the bulk ratio 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 further foaming the one-stage foamed particles.

[0142] (Polypropylene resin foam particle molded product) A polypropylene resin foam particle molded body can be obtained by in-mold molding the foam particles. The molded body obtained by in-mold molding the foam particles 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 interconnection of voids formed by the mutual communication of multiple defects in the foam particles, such as through holes and grooves, voids formed by the communication of defects in the foam particles with the voids formed between the foam particles, voids formed by the communication of voids between the foam particles, and continuous cell portions of the foam particles constituting the molded body.

[0143] The density of the molded body is 10 kg / m³ 3 More than 150kg / 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 18 kg / m 3 It is even more preferable that the amount be greater than or equal to 20 kg / m 3 The above is particularly preferable. From the viewpoint of further improving the lightweight properties of the molded body, the density of the molded body should be 100 kg / m³. 3 It is more preferable that the following is 60 kg / m 3 It is even more preferable that the following conditions apply: 50 kg / m 3 The following is particularly preferable: 45 kg / m 3 The following is most 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.

[0144] In determining a preferred range for the density of the molded body, the aforementioned upper and lower limits for the density of the molded body can be arbitrarily combined. For example, a preferred range for the density of the molded body is 10 kg / m³. 3 More than 100kg / m 3 It may also be less than 15 kg / m3 More than 60kg / m 3 It may also be less than 18 kg / m 3 More than 50kg / m 3 It may also be less than 20 kg / m 3 More than 45kg / m 3 The following is also acceptable.

[0145] 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, molded articles produced by in-mold molding of foamed particles can be manufactured without the curing process even when the density is low, and they exhibit 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.

[0146] The shape of the molded body is not particularly limited and can take various shapes depending on its application. Preferably, the maximum length of the molded body, that is, the maximum length when the length of the molded body is measured in various directions, is 600 mm or more. Conventionally, long molded bodies like this tend to shrink after demolding, making it particularly difficult to omit the curing process or shorten the curing time. In contrast, the molded body has a foamed layer made of the specific polypropylene resin and is composed of foamed particles having the specific shape, and as mentioned above, it has excellent dimensional stability, and the change in the dimensions of the molded body from the time of demolding to the time when the shape is stable can be kept small. Therefore, even when the molded body has a relatively long maximum length of, for example, 600 mm or more, the change in the dimensions of the molded body after demolding can be kept small. As a result, even when the curing process is omitted or the curing time is shortened, a molded body with the desired dimensions can be easily obtained. A molded body having such a shape can be used, for example, as a core material for automobile bumpers, an automobile toolbox, or an automobile seat core material.

[0147] The molded articles are used in a wide range of applications, such as sound-absorbing materials, shock-absorbing materials, and cushioning materials, in various fields, including the automotive sector (such as automobiles) and the construction sector. From the viewpoint of more effectively utilizing the excellent dimensional stability provided by the foamed particles, it is preferable that the molded articles be configured as core materials for automobile bumpers or as toolboxes for automobiles.

[0148] Automotive bumper core materials have a shape in which the maximum length is long relative to the cross-sectional area, which has the problem that the dimensional change of the molded body after demolding tends to be large. Similarly, automotive toolboxes have a complex shape with at least one recess for housing tools and a wall portion erected around the recess, which has the problem that the wall portion is prone to distortion due to the dimensional change of the molded body after demolding. In contrast, since the molded body is composed of the foamed particles, dimensional stability can be improved even when manufacturing molded bodies with such long maximum lengths or complex shapes. Therefore, automotive bumper core materials and automotive toolboxes composed of the foamed particles have high dimensional accuracy.

[0149] (Method for manufacturing polypropylene resin foam particle molded articles) In producing the aforementioned 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, the foam particles are filled into a mold having a molding cavity corresponding to the shape of the desired molded body. Methods such as cracking filling and compression filling can be used to fill the foam particles. The cracking filling method is a method in which the foam particles are filled into the mold with gaps called cracks, and then the mold is completely closed to mechanically compress the foam particles inside the mold. The compression filling method is a method in which foam particles that have been pre-compressed with pressurized gas or the like are filled into the mold.

[0150] The foamed particles to be filled into the mold may be the foamed particles obtained by the above manufacturing method as they are. Alternatively, foamed particles to which internal pressure has been applied by impregnating them with a gas such as compressed air in a pressure vessel may be used. When internal pressure is applied to the foamed particles, the internal pressure of the foamed particles to be filled into the mold can be appropriately set from a range of, for example, 0.3 MPa (G) or less in gauge pressure. Preferably, the internal pressure of the foamed particles is 0.20 MPa (G) or less. It is also possible to mold the foamed particles without applying internal pressure. The pressure inside the bubbles (internal pressure) can be measured by, for example, the method described in Japanese Patent Application Publication No. 2003-201361.

[0151] After the foam particles have been filled into the mold, steam is supplied to heat them. The foam particles inside the mold are heated by the steam, causing them to undergo secondary foaming and fuse together. This integrates the foam particles inside the mold, forming a molded body.

[0152] 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, completing 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 in a high-temperature atmosphere for a predetermined time. The temperature of the atmosphere in the curing process can be appropriately set from, for example, within the range of 60°C to 80°C. The standing time in the curing process may be, for example, 12 hours or more.

[0153] Furthermore, in the above manufacturing method, shrinkage and deformation of the molded body can be suppressed even if a curing process is not performed on the molded body after demolding. If the curing process is omitted, for example, the shape of the molded body can be stabilized by leaving it undisturbed for 12 hours in an environment of, for example, 23°C after demolding. Alternatively, if the curing process time is shortened, the shape of the molded body can be stabilized by leaving it undisturbed for 3 hours in a high-temperature atmosphere adjusted to a temperature of about 60°C to 80°C after demolding. [Examples]

[0154] Examples of the foamed particles and molded articles described above will be explained.

[0155] (Polypropylene resin) Table 1 shows the properties of the polypropylene resins used in the production of the foamed particles. The polypropylene resins PP-A, PP-B, and PP-C used in this example are all virgin ethylene-propylene random copolymers derived from fossil fuels.

[0156] [Table 1]

[0157] [Melt mass flow rate of polypropylene resin] The melt mass 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.

[0158] [Melting point of polypropylene resins] The melting point of the polypropylene resin was determined based on 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. The heating and cooling rates during conditioning were set to 10°C / min. A DSC curve was obtained by heating the conditioned 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 on the DSC curve was defined as the melting point. The flow rate of nitrogen gas in the measurement environment was set to 30 mL / min. A differential scanning calorimetry (DSC7020, manufactured by SII Nanotechnology Co., Ltd.) was used as the measuring device. If multiple melting peaks appeared on the DSC curve, the peak temperature of the melting peak with the largest area was defined as the melting point of the polypropylene resin.

[0159] [Crystallization temperature of polypropylene resins] The crystallization temperature of polypropylene resins was measured according to JIS K7121-1987. Specifically, a differential scanning calorimetry (DSC7020) was used to heat a test specimen made of polypropylene resin from 23°C to 200°C at a heating rate of 10°C / min, and then cool it from 200°C to 30°C at a cooling rate of 10°C / min to obtain a DSC curve. The peak temperature of the crystallization peak in this DSC curve was defined as the crystallization temperature. The nitrogen gas flow rate in the measurement environment was set to 30 mL / min. If multiple crystallization peaks appeared in the DSC curve, the peak temperature of the crystallization peak with the largest area was defined as the crystallization temperature of the polypropylene resin.

[0160] [Heat of fusion] The heat of fusion of polypropylene resin was determined from the DSC curve obtained by differential scanning calorimetry in accordance with JIS K7122-1987. Specifically, polypropylene resin was used as a test specimen, and its condition was adjusted according to "(2) When measuring the melting temperature after performing a certain heat treatment" in "3. Conditioning of test specimens" of JIS K7122-1987. In conditioning, the heating rate and cooling rate were set to 10°C / min. Subsequently, the conditioned test specimen was heated again from 23°C to 200°C at a rate of 10°C / min to obtain the DSC curve (DSC curve after the second heating). The nitrogen gas flow rate in the measurement environment was set to 30 mL / min.

[0161] On the DSC curve obtained in this way, a straight line was drawn connecting the point corresponding to 80°C on the DSC curve and the high-temperature endpoint of the melting peak with the highest peak temperature. Then, the heat of fusion of the polypropylene resin was calculated based on the area of ​​the region enclosed by the straight line determined in this way and the melting peak on the DSC curve.

[0162] [Flexural modulus of polypropylene resin] A 4mm thick sheet was prepared by heat-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 R1 of the indenter and the radius R2 of the support base were both 5mm, the distance between the supports was 64mm, and the test speed was 2mm / min.

[0163] [Ethylene content and butene content of polypropylene resins] The ethylene and butene content in polypropylene resins 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 section 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.

[0164] More specifically, first, polypropylene resin was hot-pressed at 180°C to form a film, and multiple test specimens of different thicknesses were prepared. Then, by measuring the IR spectrum of each test specimen, 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 (2) to (4). The arithmetic mean of the ethylene component content obtained for each test piece was defined as the ethylene component content (unit: mass%) in the polypropylene resin.

[0165] (K' 733 ) c=1 / 0.96{(K' 733 ) a -0.268(K') 722 ) a}···(2) (K' 722 ) c =1 / 0.96{(K' 722 ) a -0.150(K') 733 ) a}···(3) Ethylene content = 0.575{(K' 722 ) c +(K' 733 ) c}···(4)

[0166] However, K' in equations (2) to (4) 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 (2) to (4) can be applied to random copolymers.

[0167] Furthermore, the butene content (unit: mass%) in the polypropylene resin was calculated for each test specimen using the following formula (5). The arithmetic mean of the butene content obtained for each test specimen was defined as the butene content (unit: mass%) in the polypropylene resin. Butene content = 12.3 (A 766 / L)···(5) However, in equation (5), A is the absorbance and L is the thickness of the film-like test piece (in mm).

[0168] Next, the composition and manufacturing method of the foamed particles used in this example will be explained.

[0169] (Foaming particle A) As shown in Figures 5 and 6, the shape of the foamed particle A is cylindrical with through holes 111 that serve as defects 11. Furthermore, the foamed particle A has a multilayer structure comprising a foamed layer 2 and a non-foamed coating layer 3 that covers the foamed layer 2. The polypropylene resin constituting the foamed layer 2 contains PP-A (see Table 1) as polypropylene resin (A) and PP-B (see Table 1) as polypropylene resin (B) in a mass ratio of PP-A:PP-B = 60:40, as shown in Table 2. The coating layer 3 is composed of PP-C (see Table 1) as a thermoplastic resin.

[0170] To produce foamed particle A, multilayer resin particles were first prepared by the strand-cut method. A co-extrusion apparatus was used to produce the multilayer resin particles, comprising a core-forming extruder, a coating-forming extruder, and a co-extrusion die connected to these two extruders. Specifically, in the core-forming extruder, PP-A, PP-B, carbon black, and zinc borate as a foam regulator were melt-kneaded to obtain a core-forming resin mixture. The amount of zinc borate was 500 ppm by mass relative to the combined mass of PP-A and PP-B, and the amount of carbon black in the core-forming resin mixture was 2.6% by mass. Simultaneously, in the coating-forming extruder, PP-C and carbon black were melt-kneaded to obtain a coating-forming resin mixture. The amount of carbon black added to the molten resin mixture for forming the coating layer was 2.6% by mass.

[0171] These molten resin mixtures were combined in a co-extrusion die to form a composite consisting of a non-foamed core layer and a non-foamed coating layer covering the side surface of the core layer. After extruding this composite through a small hole in the co-extrusion die, the extruded material was cooled in water adjusted to a temperature of 10°C 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 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.

[0172] Next, foamed particles were produced by foaming multilayer resin particles in two stages. In the first foaming stage, the multilayer resin particles were foamed using a direct foaming method to obtain single-stage foamed particles. Specifically, 1 kg of multilayer resin particles was placed in a 5 L 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 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.

[0173] Next, with the container sealed, carbon dioxide was supplied to the sealed container as a physical blowing agent while stirring the contents, raising the temperature inside the container to the blowing temperature shown in Table 2. The blowing pressure (i.e., the pressure inside the container) at this time was as shown in Table 2. After the temperature inside the container reached the blowing temperature, this temperature was maintained for 15 minutes to impregnate the multilayer resin particles with the physical blowing agent. After the impregnation with the physical blowing agent was complete, the container was opened, and the contents were released to atmospheric pressure while maintaining the blowing pressure inside the container, causing the multilayer resin particles to blow. As a result, a single-stage blown particle was obtained, comprising a blown core layer and a non-blossed coating layer covering the blown layer.

[0174] Next, a two-stage foaming process was performed to further foam the first-stage foamed particles to obtain foamed particles A. In the two-stage foaming process, first, the first-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 first-stage foamed particles removed from the pressure vessel was as shown in Table 2. Subsequently, the first-stage foamed particles were placed in a metal drum, and steam was supplied to heat the first-stage foamed particles so that the drum pressure reached the values ​​shown in Table 2, thereby obtaining foamed particles A.

[0175] (Foaming particle B) Foamed particle B has a composition that is generally similar to foamed particle A, except that the heat of fusion at the high-temperature peak differs. The method for manufacturing foamed particle B is the same as that for manufacturing foamed particle A, except that the conditions for the single-stage foaming process and the double-stage foaming process are changed as shown in Table 2.

[0176] (Foaming particle C) Foamed particle C has a composition that is generally the same as foamed particle B, except that the mass ratio of PP-A to PP-B is changed to the ratio shown in Table 2. The method for manufacturing foamed particle C is the same as the method for manufacturing foamed particle B, except that the mass ratio of PP-A to PP-B and the foaming temperature in the single-stage foaming process are changed as shown in Table 2.

[0177] (Foaming particle D) As shown in Figures 8 and 9, the foamed particle D has a columnar shape with grooves 112 as defects 11. Furthermore, the foamed particle D has a multilayer structure comprising a foamed layer 2 and a non-foamed coating layer 3 covering the foamed layer 2. More specifically, the foamed particle D has a columnar shape with a cross-shaped cross-section perpendicular to its axial direction, i.e., the direction connecting the bottom surface 12 and the top surface 13 (i.e., the shape of the cross-section of the foamed particle). Four grooves 112 are arranged at equal intervals on the side surface 14 of the foamed particle D. The side surface 14 of the foamed particle D is covered by the coating layer 3, and the inside of the coating layer 3 is composed of the foamed layer 2. The grooves 112 of the foamed particle D have a V-shaped cross-section. The average depth H per groove of the foamed particle D was 0.45 mm.

[0178] The method for producing foamed particle D is the same as the method for producing foamed particle A, except that the die shape was changed to produce resin particles with a cross-shaped cross-section, and the conditions for the first-stage foaming process and the second-stage foaming process were changed as shown in Table 3.

[0179] (Foaming particle E) Foamed particle E has a composition that is generally the same as foamed particle A, except that the polypropylene resin does not contain PP-B and is composed only of PP-A. The method for manufacturing foamed particle E is the same as the method for manufacturing foamed particle A, except that a resin molten mixture for core layer formation was prepared without blending PP-B, and the conditions for the first-stage foaming process and the second-stage foaming process were changed as shown in Table 3.

[0180] (Foaming particles F) Foamed particle F has a composition that is generally the same as foamed particle A, except that the mass ratio of PP-A to PP-B is changed to the ratio shown in Table 2. The method for manufacturing foamed particle F is the same as the method for manufacturing foamed particle A, except that the mass ratio of PP-A to PP-B, the conditions for the first foaming process, and the conditions for the second foaming process are changed as shown in Table 3.

[0181] (Foaming particles G) Foamed particle G has a composition that is generally the same as foamed particle A, except that the polypropylene resin does not contain PP-A and is composed only of PP-B. The method for manufacturing foamed particle G is the same as the method for manufacturing foamed particle A, except that a resin molten mixture for core layer formation is prepared without using PP-A, and the conditions of the first foaming step are changed as shown in Table 4.

[0182] (Foaming particle H) Foamed particle H has a configuration that is generally the same as foamed particle A, except that it has a cylindrical shape without defects. The method for manufacturing foamed particle H is the same as the method for manufacturing foamed particle A, except that the shape of the die is changed to produce cylindrical resin particles without through holes, and the conditions for the first-stage foaming process and the second-stage foaming process are changed as shown in Table 4.

[0183] (Foaming Particle I) Foamed particle I has a generally similar structure to foamed particle A, except that the diameter of the through-holes is larger than that of foamed particle A. The manufacturing method for foamed particle I is the same as that for foamed particle A, except that the shape of the die is changed to alter the average diameter of the through-holes in the resin particles as shown in Table 4, and the conditions of the single-stage foaming process are also changed as shown in Table 4.

[0184] (Example 1-1) The molded body 5 in this example is a bumper core material 51 for automobiles. As shown in Figure 10, the bumper core material 51 has a columnar shape. Also, as shown in Figure 11, the cross-sectional shape of the bumper core material 51 in a section perpendicular to the longitudinal direction is approximately C-shaped. The length of the bumper core material 51 in this example is approximately 1300 mm, the width (specifically the outer dimension in the vertical direction in Figure 11) is approximately 150 mm, the depth (specifically the outer dimension in the horizontal direction in Figure 11) is 75 mm, and the wall thickness is approximately 30 mm. The manufacturing method of the molded body 5 in this example is as follows.

[0185] First, foamed particles A were placed in a pressure vessel, and compressed air was impregnated into the vessel to apply an internal pressure of 0.15 MPa (G) in gauge pressure to the foamed particles. Next, foamed particles A were filled into a mold for molding the bumper core material. A cracking filling method was used to fill the mold, with a cracking amount of 10%. Then, steam was supplied into the mold to cause secondary foaming of foamed particles A inside the mold and to fuse them together, thereby forming a molded body. The molding pressure at this time, i.e., the pressure of the steam supplied into the mold, was 0.32 MPa (G) in gauge pressure. Next, the pressure inside the mold was released, and the molded body inside the mold was cooled with water until the surface pressure generated on the inner surface of the mold by the foaming force of the molded body reached 0.04 MPa (G), and the molded body was released from the mold. The molded body after release was left to stand in an atmosphere of 23°C. As a result, the molded body of Example 1-1 (specifically, the bumper core material) was obtained.

[0186] (Examples 1-2) The method for manufacturing the molded article in Example 1-2 is the same as the method for manufacturing the molded article in Example 1-1, except that the molded article, after being released from the mold, was left to cure in an 80°C atmosphere for 3 hours.

[0187] (Examples 1-3) The molded body 5 of Example 1-3 is an automotive toolbox 52. As shown in Figures 12 and 13, the toolbox 52 has a rectangular parallelepiped shape and is provided with two recesses 521 (521a, 521b) for accommodating tools. More specifically, the toolbox 52 has a bottom plate portion 522, four side wall portions 523 (523a to 523d) erected on the outer peripheral edge of the bottom plate portion 522, and a partition wall portion 524 that divides the space enclosed by these side wall portions 523 into a first recess 521a and a second recess 521b. The partition wall portion 524 extends in a direction parallel to the width direction of the toolbox 52.

[0188] The toolbox 52 in this example is approximately 900 mm long, 450 mm wide, and 400 mm high. The first recess 521a has internal dimensions of approximately 550 mm in length, 350 mm in width, and 300 mm in depth, while the second recess 521b has internal dimensions of approximately 100 mm in length, 250 mm in width, and 100 mm in depth. Of the four side walls 523, the first side wall 523a and the second side wall 523b, located at both ends of the toolbox 52 in the longitudinal direction, are each approximately 100 mm thick. The third side wall 523c and the fourth side wall 523d, located at both ends of the toolbox 52 in the width direction, are each approximately 50 mm thick in the portion facing the first recess 521a, and each is approximately 100 mm thick in the portion facing the second recess 521b. The partition wall 524 is approximately 50 mm thick. The manufacturing method for the molded body 5 in this example is as follows.

[0189] First, foamed particles A were placed in a pressure vessel, and compressed air was impregnated into the vessel to apply an internal pressure of 0.15 MPa (G) in gauge pressure to the foamed particles. Next, foamed particles A were filled into a mold for molding a toolbox. A cracking filling method was used to fill the mold, with a cracking amount of 10%. Then, steam was supplied into the mold to cause secondary foaming of foamed particles A inside the mold and to fuse them together, thereby forming a molded body. The molding pressure at this time, i.e., the pressure of the steam supplied into the mold, was 0.34 MPa (G) in gauge pressure. Next, the pressure inside the mold was released, and the molded body inside the mold was cooled with water until the surface pressure generated on the inner surface of the mold by the foaming force of the molded body reached 0.04 MPa (G), and the molded body was released from the mold. After release, the molded body was left to stand in an atmosphere of 23°C. Through the above steps, the molded body of Example 1-3 (specifically, a toolbox) was obtained.

[0190] (Examples 1-4) The method for manufacturing the molded articles in Examples 1-4 is the same as the method for manufacturing the molded articles in Examples 1-3, except that the molded articles were left to cure for 3 hours in an 80°C atmosphere after being demolded from the mold.

[0191] (Examples 2-1, 3-1, 4-1 and Comparative Examples 1-1, 2-1, 3-1, 4-1, 5-1) The molded articles of these examples and comparative examples are automotive bumper core materials having a shape similar to that of the molded article of Example 1-1, and are composed of the foamed particles shown in Tables 6 to 13. The manufacturing method of the molded articles of these examples and comparative examples is generally the same as that of Example 1-1, except that the foamed particles shown in Tables 6 to 13 are used instead of foamed particle A, and the molding pressure is changed as shown in Tables 6 to 13.

[0192] (Examples 2-2, 3-2, 4-2 and Comparative Examples 1-2, 2-2, 3-2, 4-2, 5-2) The molded articles of these examples and comparative examples are automotive bumper core materials having a shape similar to that of the molded articles of Examples 1-2, and are composed of the foamed particles shown in Tables 6 to 13. The manufacturing method of the molded articles of these examples and comparative examples is generally the same as that of Examples 1-2, except that the foamed particles shown in Tables 6 to 13 are used instead of foamed particle A, and the molding pressure is changed as shown in Tables 6 to 13.

[0193] (Examples 2-3, 3-3, 4-3 and Comparative Examples 1-3, 2-3, 3-3, 4-3, 5-3) The molded articles of these examples and comparative examples are automotive toolboxes having a shape similar to that of the molded articles of Examples 1-3, and are composed of the foamed particles shown in Tables 6 to 13. The manufacturing method of the molded articles of these examples and comparative examples is generally the same as that of Examples 1-3, except that the foamed particles shown in Tables 6 to 13 are used instead of foamed particle A, and the molding pressure is changed as shown in Tables 6 to 13.

[0194] (Examples 2-4, 3-4, 4-4 and Comparative Examples 1-4, 2-4, 3-4, 4-4, 5-4) The molded articles of these examples and comparative examples are automotive toolboxes having a shape similar to that of the molded articles of Examples 1-4, and are composed of the foamed particles shown in Tables 6 to 13. The manufacturing method of the molded articles of these examples and comparative examples is generally the same as that of Examples 1-4, except that the foamed particles shown in Tables 6 to 13 are used instead of foamed particle A, and the molding pressure is changed as shown in Tables 6 to 13.

[0195] Tables 2 to 4 show the physical properties of foamed particles A to I. Tables 5 to 13 show the physical properties of the molded articles of the examples and comparative examples. The measurement and evaluation methods for the physical properties shown in Tables 2 to 13 are as follows.

[0196] (Bulk density) First, the foamed particles were allowed to stand for more than 24 hours in an environment of 50% relative humidity, 23°C, and 1 atm pressure to adjust their state. The adjusted foamed particles were then filled into a graduated cylinder so that they would naturally accumulate, and the bulk volume (in L) of the foamed particle group was read from the scale of the graduated cylinder. Subsequently, the mass (in g) of the foamed particle group in the graduated cylinder was divided by the aforementioned bulk volume, and the bulk density (in kg / m³) of the foamed particles was obtained by further unit conversion. 3 ) was calculated.

[0197] (Apparent density) First, the foamed particles were allowed to stand for more than 24 hours in an environment of 50% relative humidity, 23°C, and 1 atm pressure to adjust their state. After measuring the mass of the adjusted foamed particle group, they were submerged in a graduated cylinder containing ethanol at 23°C using a wire mesh. Then, taking into account the volume of the wire mesh, the volume of the foamed particle group was measured from the rise in water level. The mass (in g) of the foamed particle group obtained in this way was divided by the volume (in L), and then the units were converted to obtain the apparent density (in kg / m³) of the foamed particles. 3 ) was calculated.

[0198] (Average hole diameter d of through holes) First, the foamed particle group was left to stand for more than 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm pressure to adjust the state of the foamed particles. From the adjusted foamed particle group, 100 foamed particles were randomly selected and cut at the axial center with a plane perpendicular to the axial direction to expose the cut surface. Photographs were taken of the cut surface of each foamed particle, and the cross-sectional area (opening area) of the through-hole portion in the cross-sectional photograph was determined. The diameter of a virtual perfect circle with the same area as the cross-sectional area was calculated, and the arithmetic mean of these values ​​was taken as the average hole diameter d (unit: mm) of the through-holes in the foamed particles.

[0199] (Average outer diameter D of foamed particles) First, the foam particle group was left to stand for more than 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm pressure to adjust the state of the foam particles. From the adjusted foam particle group, 100 foam particles were randomly selected and cut at the axial center with a plane perpendicular to the axial direction, exposing the cross-sections shown in Figures 6 and 9. Two points Q3 and Q4 were determined on the contour of the foam particle 1 on this cross-section such that the distance between them was longest, and the distance between these two points (i.e., the maximum outer diameter of the foam particle on the cross-section) was defined as the outer diameter r of each individual foam particle 1. The arithmetic mean of the outer diameters r measured for 100 or more foam particles 1 was defined as the average outer diameter D of the foam particles. The outer diameter of the foam particle 1 on the cross-section was measured by taking photographs of the cross-section of the foam particles and performing image analysis.

[0200] (Average cross-sectional area of ​​foamed particles A) First, the foamed particle group was left to stand for more than 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm pressure to adjust the state of the foamed particles. After adjustment, the foamed particles were cut at their center along their axial direction by a plane perpendicular to the axial direction, exposing the cut surface. The cross-sectional area of ​​the foamed particles at this cut surface was measured. Specifically, the cross-sectional area of ​​foamed particles with a multilayer structure is the sum of the cross-sectional area of ​​the foam layer and the cross-sectional area of ​​the coating layer at the cut surface, and the cross-sectional area of ​​defective parts is not included in the cross-sectional area of ​​the foamed particle. The measurement of the cross-sectional area of ​​the foamed particles at the cut surface was performed by taking photographs of the cut surface of the foamed particles and performing image analysis. The above procedure was performed for 100 foamed particles, and the arithmetic mean of the obtained cross-sectional areas of the foamed particles was defined as the average cross-sectional area A of the foamed particles.

[0201] (Average cross-sectional area Ca per defect, total cross-sectional area Ct of defects) First, the foamed particle group was left to stand for more than 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm pressure to adjust the state of the foamed particles. From the adjusted foamed particle group, 100 foamed particles were randomly selected, and these foamed 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 surface of the foamed particles were taken, and the cross-sectional area of ​​the defect in each foamed particle was measured by image analysis.

[0202] Next, the total cross-sectional area of ​​the defects in each foam particle was calculated by summing the cross-sectional areas of the defects obtained in this way. Furthermore, the cross-sectional area per defect in each foam particle was calculated by dividing the total cross-sectional area of ​​the defects by the number of defects.

[0203] The above procedure was performed on 100 foamed particles, and the arithmetic mean of the sum of the cross-sectional areas of the defects obtained for the 100 foamed particles was defined as the total cross-sectional area of ​​the defects, Ct. In addition, the average value of the cross-sectional area per defect obtained for the 100 foamed particles was defined as the average cross-sectional area per defect, Ca. Tables 2 and 3 show the ratio of the total cross-sectional area of ​​defects Ct to the average cross-sectional area A of the foamed particles at the cross-section, Ct / A, and the ratio of the average cross-sectional area per defect Ca to the average cross-sectional area A of the foamed particles, Ca / A.

[0204] (Percentage of closed cells in foamed particles) The percentage of closed cells in the foamed particles was measured using an air-comparative hydrometer based on ASTM-D2856-70 Procedure C. Specifically, the foamed particles were first allowed to stand for more than 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm pressure to adjust their state. The bulk volume after adjustment was approximately 20 cm³. 3 The foamed particles were used as the measurement sample, and the measurement sample was submerged in a graduated cylinder containing ethanol. The apparent volume Va of the measurement sample was measured from the rise in the liquid level at this time. After thoroughly drying the measurement sample whose apparent volume Va was measured, the true volume Vx of the measurement sample was measured using an air comparison hydrometer (Beckman Model 1000 Air Comparison Pycnometer, manufactured by Tokyo Science Co., Ltd.) in accordance with procedure C described in ASTM-D2856-70. Then, using these volume values ​​Va and Vx, the closed-cell ratio (unit: %) of the measurement sample was calculated based on the following equation (6). The above procedure was repeated five times with different measurement samples, and the arithmetic mean (N=5) of the closed-cell ratios of the five measurement samples was taken as the closed-cell ratio (unit: %) of the foamed particles.

[0205] Closed cell ratio = (Vx-W / ρ)×100 / (Va-W / ρ) (6)

[0206] However, the meaning of the symbols in formula (6) above is as follows: Vx: The true volume of the sample measured by the above method, i.e., the sum of the volume of the resin constituting the foam particles and the total volume of the closed-cell portions within the foam particles (unit: cm²). 3 ) Va: The apparent volume of a sample measured by measuring the amount of rise in the liquid level when the sample is submerged in a graduated cylinder containing ethanol (unit: cm³). 3 ) W: Mass of the sample used for measurement (unit: g) ρ: Density of the resin constituting the foam particles (unit: g / cm³) 3 )

[0207] (Heat of fusion at high temperature peak, heat of fusion at resin-specific peak, total heat of fusion) The peak temperature and heat of fusion of the high-temperature peak were determined from the DSC curve obtained by differential scanning calorimetry (DSC) in accordance with JIS K7122:1987. Specifically, first, approximately 3 mg of foamed particles were used as a sample, and the sample was heated from 23°C to 200°C at a heating rate of 10°C / min, and a DSC curve was obtained by differential scanning calorimetry. Next, as shown in Figure 7 as an example, a straight line L2 was drawn connecting point α, which corresponds to 80°C on the DSC curve, and point β, which corresponds to the melting end temperature T of the foamed particles. Furthermore, a straight line L3 was drawn parallel to the vertical axis of the graph, passing through the maximum point γ that exists between the resin-specific peak ΔH1 and the high-temperature peak ΔH2, and the resin-specific peak ΔH1 and the high-temperature peak ΔH2 were separated by the straight line L3.

[0208] Then, the heat of fusion for the high-temperature peak ΔH2 was calculated based on the area of ​​the region enclosed by the portion constituting the high-temperature peak ΔH2 in the DSC curve, and the lines L2 and L3. Similarly, the heat of fusion for the resin-specific peak ΔH1 was calculated based on the area of ​​the region enclosed by the portion constituting the resin-specific peak ΔH1 in the DSC curve, and the lines L2 and L3. In addition, the total heat of fusion was calculated based on the area of ​​the region enclosed by the portion constituting the resin-specific peak ΔH1 in the DSC curve, the portion constituting the high-temperature peak ΔH2 in the DSC curve, and the line L2.

[0209] The above procedure was repeated five times using different samples, and the arithmetic mean of the heat of fusion of the high-temperature peak ΔH2 obtained in each measurement was used to determine the heat of fusion Q of the high-temperature peak ΔH2 of the foamed particles. H2 Similarly, the arithmetic mean of the heat of fusion of the resin-specific peak ΔH1 obtained in each measurement was used as the heat of fusion Q of the resin-specific peak ΔH1 of the foamed particles. H1 Furthermore, the arithmetic mean of the total heat of fusion obtained in each measurement was used as the total heat of fusion Q of the foamed particles. H1 +Q H2 That's what I decided.

[0210] (Moldable range, lower limit molding pressure) In evaluating the moldable range and the lower limit molding pressure, flat molded bodies were produced by performing in-mold molding while varying the molding pressure during heating from 0.24 MPa(G) to 0.44 MPa(G) in 0.02 MPa increments. The moldable range was then determined based on the surface properties, fusion properties, and recovery properties of the resulting molded bodies.

[0211] The manufacturing method for the molded body used to evaluate the moldable range is as follows. First, foamed particles were placed in a pressure vessel, and the vessel was pressurized with air to impregnate the foamed particles with air, thereby applying an internal pressure of 0.12 MPa (G) to the foamed particles. Next, the foamed particles with internal pressure were filled into a mold using a cracking filling method. In this example, a mold with a molding cavity capable of forming a flat molded body measuring 300 mm in length, 250 mm in width, and 60 mm in thickness was used. In the cracking filling method, the foamed particles were filled into the mold with a cracking gap of 6 mm in the thickness direction of the molded body (i.e., a cracking amount of 10%), and then the mold was completely closed to mechanically compress the foamed particles inside the mold.

[0212] 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 steam was supplied to the molding cavity from one side of the mold until the pressure reached 0.08 MPa(G) lower than the molding pressure during main heating, performing the first one-sided heating. Next, steam was supplied to the molding cavity from the other side of the mold until the pressure reached 0.04 MPa(G) lower than the molding pressure during main heating, performing the second one-sided heating. After that, main heating was performed by supplying steam to the molding cavity from both sides of the mold until the molding pressure reached the molding pressure during main heating. 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).

[0213] After that, the molded body taken out from the mold was left standing in an oven at 80°C for 12 hours to perform a curing process. After the curing process, the molded body was left standing for 24 hours under the conditions of a relative humidity of 50%, 23°C, and 1 atm to adjust the state of the molded body. The surface property, fusion property, and recovery property of the molded body after state adjustment were evaluated, and the range of the molding pressure (that is, the molding pressure at which qualified products could be obtained) that passed all items according to the evaluation criteria described below was defined as the moldable range and described in the "Molding Pressure" column of Tables 2 to 4. Also, the number of conditions under which qualified products could be obtained was described in the "Number of Conditions" column of Tables 2 to 4. It can be judged that the wider the moldable range, the better the moldability.

[0214] In addition, among the moldable ranges determined by the above method, the lowest molding pressure was defined as the lower limit molding pressure and described in Tables 2 to 4. The lower the moldable molding pressure, the more possible it is to perform in-mold molding with a small amount of steam, so it can be judged that the productivity is excellent.

[0215] The evaluation methods for surface property, fusion property, and recovery property in the evaluation of the moldable range and the lower limit molding pressure are as follows.

[0216] 〔Surface property〕 A 100 mm × 100 mm square was drawn at the center of one skin surface in the thickness direction of the molded body, and then a diagonal line was drawn from one of the corners of this square. Then, the number of voids existing on the diagonal line, that is, the number of voids having a size of 1 mm × 1 mm or more among the gaps formed between the expanded particles, was counted. When the number of voids was 2 or less, it was judged as qualified, and when it was 3 or more, it was judged as unqualified.

[0217] 〔Fusion property〕 The molded body was fractured so that it was divided into roughly equal parts along its length. 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 bonding rate. A bonding rate of 80% or higher was judged as passing, and a rate below 80% was judged as failing.

[0218] [Recovery] In a plan view of the molded body from the thickness direction, the thickness of the molded body was measured at four locations 10 mm inward from each vertex towards the center, as well as at the center of the molded body. Next, the ratio (in %) of the thickness of the thinnest location to the thickness of the thickest location was calculated. A thickness ratio of 95% or more was judged as passing, and a ratio of less than 95% was judged as failing.

[0219] (Water cooling time) In the in-mold molding of each molded body in the examples and comparative examples, the time required from the completion of steam supply to the mold to the point when the surface pressure of the molded body due to foaming force reaches 0.04 MPa(G) is recorded as the water cooling time in Tables 5 to 13.

[0220] (Density of the molded body) The density of the molded body (in kg / m³) is calculated by dividing the mass (in g) of each molded body in the examples and comparative examples by the volume (in L) of the molded body determined by the immersion method and then converting the units. 3 ) was calculated.

[0221] (exterior) The appearance of each molded product in the examples and comparative examples was evaluated based on visual observation. The meanings of the symbols in the "Appearance" column in Tables 5 to 13 are as follows. A: The molded body has a beautiful surface with no gaps (voids) between foam particles on its surface, and no noticeable irregularities originating from defects. B: The surface of the molded body has slight gaps (voids) between foam particles and / or a surface with somewhat noticeable irregularities originating from defects. C: The surface of the molded body has many gaps (voids) between foam particles, and / or has a surface with noticeable irregularities originating from defects.

[0222] (Dimensional stability) Depending on the shape and curing conditions of the molded product, dimensional stability was evaluated using the following methods.

[0223] [Examples 1-1, 2-1, 3-1, 4-1 and Comparative Examples 1-1, 2-1, 3-1, 4-1, 5-1] In evaluating the dimensional stability of the molded articles of these examples and comparative examples, the molded articles were left to stand in an atmosphere of 23°C after demolding, and the dimensions of a predetermined part of the molded article were measured at 6 hours after demolding and 5 days after demolding. In addition, a reference sample was prepared, which was molded under the same conditions as each example and comparative example, and then cured by standing in an atmosphere of 80°C for 12 hours after demolding. The dimensions of a predetermined part of this reference sample were then measured. Based on these dimensions, the values ​​of index α1 and index β1, described later, were calculated, and the dimensional stability was evaluated.

[0224] The symbols used in the "Dimensional Stability" column of Tables 5 to 13 for the molded articles of these examples and comparative examples have the following meanings. A: α1 is 1% or less, and β1 is 13 mm or less. B: Does not apply to either A or C. C: α1 is greater than 1%, and β1 is greater than 13 mm. Not evaluated: No evaluation was given because the appearance was unacceptable.

[0225] The dimensions of the predetermined part of the molded body mentioned above refer to the longitudinal outer dimension X1 (unit: mm) of the bumper core material 51, that is, the distance from one end to the other in the longitudinal direction of the bumper core material 51.

[0226] The index α1 (unit: %) represents the longitudinal outer dimension X1 of the bumper core material 51 6 hours after it has been removed from the mold. 無養生,6h The longitudinal outer dimension X1 of the bumper core material 51 five days after it has been removed from the mold. 無養生,5d The following formula (7) is used to calculate the value of index α1. A smaller value of index α1 means that the dimensional change from the time of demolding to the time when the shape stabilizes is small. α1=([(X1 無養生,5d -X1 無養生,6h )] / X1 無養生,6h ) × 100 ···(7)

[0227] Furthermore, the index β1 (unit: mm) is the longitudinal outer dimension X1 of the bumper core material 51 6 hours after it has been removed from the mold. 無養生,6h And, the longitudinal outer dimension X1 of the aforementioned reference sample ref The following formula (8) is used to calculate the value of index β1. A smaller value of index β1 means that it is easier to obtain a molded body with the desired dimensions even after a short period of standing after molding. β1 = │X1 ref -X1 無養生,6h │ ···(8)

[0228] [Examples 1-2, 2-2, 3-2, 4-2 and Comparative Examples 1-2, 2-2, 3-2, 4-2, 5-2] In the evaluation of the dimensional stability of the molded bodies of these Examples and Comparative Examples, after the molded body after mold release was left to stand for 3 hours in an atmosphere of 80°C for curing, the molded body was moved to an atmosphere of 23°C and further left to stand. Then, the dimensions of a predetermined portion of the molded body were measured at each of the time points when 6 hours had elapsed since the time of mold release and when 5 days had elapsed since the time of mold release. Further, as a reference sample, a molded body molded under the same conditions as in each of the Examples and Comparative Examples and cured by leaving it to stand in an atmosphere of 80°C for 12 hours after mold release from the mold was prepared. Then, the dimensions of a predetermined portion of this reference sample were measured. Based on these dimensions, the values of Index α2 and Index β2, which will be described later, were calculated, and the dimensional stability was evaluated.

[0229] Regarding the molded bodies of these Examples and Comparative Examples, the meanings of the symbols described in the "Dimensional Stability" columns of Tables 5 to 13 are as follows. A: α2 is 1% or less and β2 is 13 mm or less. B: It does not correspond to either A or C. C: α2 is greater than 1% and β2 is greater than 13 mm. Not evaluated: Not evaluated because the appearance is nonconforming.

[0230] Specifically, the dimensions of the predetermined portion of the molded body described above are the outer dimensions X1 (unit: mm) in the longitudinal direction of the bumper core material 51 shown in FIG. 10.

[0231] Index α2 (unit: %) is the outer dimension X1 in the longitudinal direction of the bumper core material 51 at the time when 6 hours have elapsed since the time of mold release 3h養生,6h and the outer dimension X1 in the longitudinal direction of the bumper core material 51 at the time when 5 days have elapsed since the time of mold release 3h養生,5d are used and calculated based on the following formula (9). The smaller the value of Index α2, the smaller the change in dimensions from the time of mold release to the time when the shape becomes stable. α2 = ([(X1 3h養生,5d - X1 3h養生,6h )] / X1 3h養生,6h ) × 100 ··· (9)

[0232] Furthermore, the index β2 (unit: mm) is the longitudinal outer dimension X1 of the bumper core material 51 6 hours after it has been removed from the mold. 3h養生,6h And, the longitudinal outer dimension X1 of the aforementioned reference sample ref The following formula (10) is used to calculate the value of index β2. A smaller value of index β2 means that it is easier to obtain a molded body with the desired dimensions even after a short period of standing after molding. β2 = │X1 ref -X1 3h養生,6h │ ···(10)

[0233] [Examples 1-3, 2-3, 3-3, 4-3 and Comparative Examples 1-3, 2-3, 3-3, 4-3, 5-3] In evaluating the dimensional stability of the molded articles of these examples and comparative examples, the molded articles were left to stand in an atmosphere of 23°C after demolding, and the dimensions of a predetermined part of the molded article were measured at 6 hours after demolding and 5 days after demolding. In addition, a reference sample was prepared, which was molded under the same conditions as each example and comparative example, and then cured by standing in an atmosphere of 80°C for 12 hours after demolding. The dimensions of a predetermined part of this reference sample were then measured. Based on these dimensions, the values ​​of the indices α3, α4, β3, and β4, described later, were calculated, and the dimensional stability was evaluated.

[0234] The symbols used in the "Dimensional Stability" column of Tables 5 to 13 for the molded articles of these examples and comparative examples have the following meanings. A: All four conditions are met: α3 is 1% or less, α4 is 1% or less, β3 is 9mm or less, and β4 is 5mm or less. B: Does not apply to either A or C. C: All four conditions are met: α3 is greater than 1%, α4 is greater than 1%, β3 is greater than 9 mm, and β4 is greater than 5 mm. Not evaluated: No evaluation was given because the appearance was unacceptable.

[0235] Incidentally, the dimensions of the predetermined part of the molded body described above are specifically the outer dimension X2 (unit: mm) in the longitudinal direction and the outer dimension Y2 (unit: mm) in the width direction of the tool box 52 shown in FIG. 13 on the upper surface of the tool box 52 (that is, the surface where the recess 521 is open). Specifically, the outer dimension X2 in the longitudinal direction of the tool box 52 is the outer dimension from the upper end of the first side wall portion 523a to the upper end of the second side wall portion 523b measured at the center in the width direction of the tool box 52. Further, the outer dimension Y2 in the width direction of the tool box 52 is specifically the outer dimension from the upper end of the third side wall portion 523c to the upper end of the fourth side wall portion 523d measured at the center in the longitudinal direction of the tool box 52.

[0236] The index α3 (unit: %) is the outer dimension X2 in the longitudinal direction of the tool box 52 at the time when 6 hours have elapsed since the time of脱模 from the mold 無養生,6h and the outer dimension X2 in the longitudinal direction of the tool box 52 at the time when 5 days have elapsed since the time of脱模 from the mold 無養生,5d and is calculated based on the following formula (11). The smaller the value of the index α3, the smaller the change in the longitudinal dimension from the time of脱模 to the time when the shape is stable. α3 = ([(X2 無養生,5d - X2 無養生,6h )] / X2 無養生,6h ) × 100 ··· (11)

[0237] The index α4 (unit: %) is the outer dimension Y2 in the width direction of the tool box 52 at the time when 6 hours have elapsed since the time of脱模 from the mold 無養生,6h and the outer dimension Y2 in the width direction of the tool box 52 at the time when 5 days have elapsed since the time of脱模 from the mold 無養生,5d and is calculated based on the following formula (12). The smaller the value of the index α4, the smaller the change in the width dimension from the time of脱模 to the time when the shape is stable. α4 = ([(Y2 無養生,5d - Y2 無養生,6h )] / Y2 無養生,6h ) × 100 ··· (12)

[0238] Indicator β3 (unit: mm) represents the longitudinal outer dimension X2 of the toolbox 52 6 hours after demolding from the mold. 無養生,6h And, the longitudinal outer dimension X2 of the aforementioned reference sample ref The following formula (13) is used to calculate the value of index β3. A smaller value of index β3 means that it is easier to obtain a molded body with the desired longitudinal dimensions even after a short period of standing after molding. β3 = │X² ref -X2 無養生,6h │ ···(13)

[0239] Indicator β4 (unit: mm) represents the external width dimension Y2 of the toolbox 52 6 hours after demolding from the mold. 無養生,6h And, as mentioned above, the external dimension Y2 in the width direction of the reference sample ref The following formula (14) is used to calculate the value of index β4. A smaller value of index β4 means that it is easier to obtain a molded body with the desired width dimensions even after a short period of standing after molding. β4 = │Y2 ref -Y2 無養生,6h │ ···(14)

[0240] [Examples 1-4, 2-4, 3-4, 4-4 and Comparative Examples 1-4, 2-4, 3-4, 4-4, 5-4] In evaluating the dimensional stability of the molded articles of these examples and comparative examples, the molded articles were cured by standing in an 80°C atmosphere for 3 hours after demolding, and then moved to a 23°C atmosphere for further curing. The dimensions of predetermined parts of the molded articles were measured at 6 hours after demolding and at 5 days after demolding. In addition, a reference sample was prepared, which was molded under the same conditions as each example and comparative example, and then cured by standing in an 80°C atmosphere for 12 hours after demolding. The dimensions of predetermined parts of this reference sample were measured. Based on these dimensions, the values ​​of the indices α5, α6, β5, and β6, described later, were calculated, and the dimensional stability was evaluated.

[0241] The symbols used in the "Dimensional Stability" column of Tables 5 to 13 for the molded articles of these examples and comparative examples have the following meanings. A: All four conditions are met: α5 is 1% or less, α6 is 1% or less, β5 is 9mm or less, and β6 is 5mm or less. B: Does not apply to either A or C. C: All four conditions are met: α5 is greater than 1%, α6 is greater than 1%, β5 is greater than 9mm, and β6 is greater than 5mm. Not evaluated: No evaluation was given because the appearance was unacceptable.

[0242] The dimensions of the specified part of the molded body mentioned above refer to the longitudinal outer dimension X2 (unit: mm) and the width outer dimension Y2 (unit: mm) of the toolbox 52 on the upper surface of the toolbox 52, as shown in Figure 13.

[0243] The index α5 (unit: %) represents the longitudinal outer dimension X2 of the toolbox 52 6 hours after demolding from the mold. 3h養生,6h The external dimensions of the toolbox 52 in the longitudinal direction X2 five days after it was removed from the mold. 3h養生,5d It is calculated using the following formula (15). A smaller value of index α5 means that the change in the longitudinal dimension from the time of demolding to the time when the shape stabilizes is small. α5=([(X2 3h養生,5d -X2 3h養生,6h )] / X2 3h養生,6h ) × 100 ···(15)

[0244] The index α6 (unit: %) represents the external width dimension Y2 of the toolbox 52 6 hours after demolding from the mold. 3h養生,6h The external dimensions Y2 in the longitudinal direction of the toolbox 52 five days after it has been removed from the mold. 3h養生,5d It is calculated using the following formula (16). A smaller value of index α6 means that the change in width dimension from the time of demolding to the time when the shape stabilizes is small. α6=([(Y2 3h養生,5d -Y2 3h養生,6h )] / Y2 3h養生,6h ) × 100 ···(16)

[0245] Indicator β5 (unit: mm) represents the longitudinal outer dimension X2 of the toolbox 52 6 hours after demolding from the mold. 3h養生,6h And, the longitudinal outer dimension X2 of the aforementioned reference sample ref The following formula (17) is used to calculate the value of index β5. A smaller value of index β5 means that it is easier to obtain a molded body with the desired longitudinal dimensions even after a short period of standing after molding. β5 = │X² ref -X2 3h養生,6h │ ···(17)

[0246] Indicator β6 (unit: mm) represents the external width dimension Y2 of the toolbox 52 6 hours after demolding from the mold. 3h養生,6h And, as mentioned above, the external dimension Y2 in the width direction of the reference sample ref The following formula (18) is used to calculate the value of index β6. A smaller value of index β6 means that it is easier to obtain a molded body with the desired width dimensions even after a short period of standing after molding. β6 = │Y2 ref -Y2 3h養生,6h │ ···(18)

[0247] [Table 2]

[0248] [Table 3]

[0249] [Table 4]

[0250] [Table 5]

[0251] Table 6

[0252] Table 7

[0253] Table 8

[0254] Table 9

[0255] Table 10

[0256] Table 11

[0257] Table 12

[0258] Table 13

[0259] As shown in Tables 2 and 3, the foamed particles A to D have the specific shape described above. Furthermore, the foamed layer of foamed particles A to D is composed of a polypropylene resin containing the polypropylene resin (A) and the polypropylene resin (B) in the specific mass ratio described above. As a result, these foamed particles were able to improve the dimensional stability of the molded article even when the curing process was omitted or shortened, as shown in Examples 1-1 to 1-4 in Table 5, Examples 2-1 to 2-4 in Table 6, Examples 3-1 to 3-4 in Table 7, and Examples 4-1 to 4-4 in Table 8. In addition, the molded article obtained by in-mold molding of foamed particles A to D did not show any streaky patterns.

[0260] Furthermore, as shown in Tables 5 to 8, the molded articles made from these foamed particles exhibited minimal dimensional changes after demolding. Therefore, these results suggest that by using foamed particles A to D, it is possible to manufacture molded articles with relatively long maximum lengths, complex shapes, and those requiring high dimensional accuracy by omitting the curing process or shortening the curing time.

[0261] In contrast, the foamed layer of foamed particle E shown in Table 3 is composed solely of polypropylene resin (A) and does not contain polypropylene resin (B). Therefore, as shown in Table 9, the molded articles of Comparative Examples 1-1 to 1-4, which were composed of foamed particle E, had inferior dimensional stability compared to the molded articles of the Examples.

[0262] Although the foamed layer of the foamed particle F shown in Table 3 contains polypropylene resin (A) and polypropylene resin (B), the mass proportion of polypropylene resin (B) is less than the specific range mentioned above. Therefore, as shown in Table 10, the molded articles of Comparative Examples 2-1 to 2-4 composed of foamed particle F had inferior dimensional stability compared to the molded articles of the Examples.

[0263] The foamed layer of foamed particle G shown in Table 4 consists solely of polypropylene resin (B) and does not contain polypropylene resin (A). Therefore, as shown in Table 11, the molding pressure was excessively high when manufacturing the molded articles of Comparative Examples 3-1 to 3-4 composed of foamed particle G. In addition, foamed particle G lacked sufficient secondary foaming properties, and the appearance of the molded articles of Comparative Examples 3-1 to 3-4 was unacceptable.

[0264] The foamed particles H shown in Table 4 do not have defects. Therefore, as shown in Table 12, the molded articles of Comparative Examples 4-1 to 4-4, which were composed of foamed particles H, had inferior dimensional stability compared to the molded articles of the Examples. In addition, the molded articles of Comparative Examples 4-1 to 4-4 required a long cooling time during in-mold molding (i.e., water cooling time), resulting in poor productivity.

[0265] The foamed particles of foamed particle I shown in Table 4 have larger through-holes than foamed particles A to D, and the ratio of the average cross-sectional area Ca per defect to the average cross-sectional area A of the foamed particles (Ca / A) and the ratio of the total cross-sectional area Ct of the defects to the average cross-sectional area A of the foamed particles (Ct / A) are greater than the specified range. Therefore, as shown in Table 13, the appearance of the molded articles of Comparative Examples 5-1 to 5-4 composed of foamed particle I was unacceptable.

[0266] The embodiments of the polypropylene-based resin foam particles, polypropylene-based resin foam particle molded articles, automotive bumper core material, and automotive toolbox of the present invention have been described above based on the examples. However, the specific embodiments of the polypropylene-based resin foam particles, polypropylene-based resin foam particle molded articles, automotive bumper core material, and automotive toolbox of the present invention are not limited to the embodiments described in the examples, and the configuration can be appropriately modified without impairing the spirit of the present invention. [Explanation of Symbols]

[0267] 1. Foaming particles 11. Defective parts 111 Through hole 112 Groove 2 Foam layer

Claims

1. Polypropylene resin foam particles having a foamed layer made of polypropylene resin, The foamed particle has a columnar shape and has one or more defects selected from the group consisting of through holes penetrating the interior of the foamed particle in the axial direction and grooves extending axially along the side surface of the foamed particle. In the cross-section obtained by cutting the foamed particle at its axial center with a plane perpendicular to the axial direction, the ratio Ca / A of the average cross-sectional area Ca per defect to the average cross-sectional area A of the foamed particle is 0.01 or more and 0.20 or less, and the ratio Ct / A of the total cross-sectional area Ct of the defects to the average cross-sectional area A of the foamed particle is 0.02 or more and 0.20 or less. The polypropylene resin constituting the foamed layer comprises a polypropylene resin (A) having a melting point of 135°C or higher and 150°C or lower, and a flexural modulus of less than 1000 MPa, and a polypropylene resin (B) having a melting point of 145°C or higher and 160°C or lower, and a flexural modulus of 1000 MPa or higher. Polypropylene resin foam particles, wherein the mass ratio of polypropylene resin (A) to polypropylene resin (B) in the polypropylene resin is polypropylene resin (A): polypropylene resin (B) = 65:35 to 35:

65.

2. The polypropylene resin foam particle according to claim 1, wherein the absolute value of the difference between the flexural modulus of the polypropylene resin (A) and the flexural modulus of the polypropylene resin (B) is 200 MPa or more and 500 MPa or less.

3. The polypropylene resin foam particles according to claim 1 or 2, wherein the polypropylene resin (B) is one or more propylene copolymers selected from the group consisting of ethylene-propylene copolymer, butene-propylene copolymer, and ethylene-butene-propylene copolymer.

4. The polypropylene resin foam particles according to claim 3, wherein the sum of the ethylene content and the butene content in the polypropylene resin (B) is 0.5% by mass or more and 2.5% by mass or less.

5. The polypropylene resin foam particles according to claim 1 or 2, wherein the polypropylene resin foam particles have a crystalline structure in which the DSC curve obtained when the foam particles are heated from 23°C to 200°C at a heating rate of 10°C / min shows a resin-specific peak originating from the melting of crystals in the polypropylene resin and a high-temperature peak having a peak temperature higher than the peak temperature of the resin-specific peak, and the heat of fusion of the high-temperature peak is 8 J / g or more and 25 J / g or less.

6. The apparent density of the polypropylene resin foam particles is 15 kg / m³ 3 More than 200kg / m 3 The polypropylene resin foam particles according to claim 1 or 2, which are as follows:

7. A molded polypropylene resin foam particle article obtained by in-mold molding polypropylene resin foam particles according to claim 1 or 2, wherein the polypropylene resin foam particle article has a maximum length of 600 mm or more.

8. A core material for an automobile bumper, obtained by in-mold molding polypropylene resin foam particles according to claim 1 or 2.

9. An automotive toolbox made by in-mold molding polypropylene resin foam particles according to claim 1 or 2.

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