Polypropylene resin foam particles

Abstract

To provide polypropylene foamed particles which have good moldability and good balance of rigidity with respect to molding pressure.SOLUTION: Each of foamed particles is composed of base material resin containing a polypropylene resin as a main component. The polypropylene resin includes ethylene-propylene rubber. The foamed particle has such a crystal structure that a first endothermic peak Pa having a largest peak area and a second endothermic peak Pb adjacent to the first endothermic peak Pa on a high temperature side of the first endothermic peak Pa appear in a DSC curve obtained when the foamed particle is heated at a heating rate of 10°C / min from 23°C to 200°C. A closed cell ratio of the foamed particle is 85% or more. A temperature Tb of the apex of the second endothermic peak Pb is 158°C or higher, and a difference Tb-Ta between the temperature Tb and a temperature Ta of the apex of the first endothermic peak Pa is 15 to 30°C. A ratio hb / ha of a peak height hb of the second endothermic peak Pb to a peak height ha of the first endothermic peak Pa is 1.2 to 2.8.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] This invention relates to polypropylene resin foam particles. [Background technology]

[0002] Polypropylene foam particle molded articles are lightweight and have excellent cushioning and rigidity, making them suitable for various applications such as packaging materials, containers, and cushioning materials. 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 heated with steam. When steam is supplied into the mold in the in-mold molding method, the foam particles undergo secondary foaming and their surfaces melt. As a result, the foam particles inside the mold fuse together, making it possible to obtain a molded article with a desired shape corresponding to the shape of the mold cavity.

[0003] Incidentally, in recent years, post-consumer materials have been attracting attention from the perspective of reducing environmental impact. Examples of post-consumer materials including polypropylene resins include polypropylene resin components recovered from used home appliances and automobiles, and automobile shredder residue (ASR) generated during the automobile disposal process, as described in Patent Document 1. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2018-122299 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Polypropylene resins used in such post-consumer materials typically contain ethylene-propylene rubber. When polypropylene resins containing ethylene-propylene rubber are used as the base resin for foamed particles, there was room for improvement in the moldability when molding the foamed particles in a mold.

[0006] On the other hand, in order to improve the moldability when using a polypropylene resin containing ethylene-propylene rubber as the base resin for foamed particles, a method can be considered in which a mixed resin is further blended with a polypropylene resin that does not contain rubber components such as ethylene-propylene rubber as the base resin. However, even in this case, the moldability may not be improved, or the balance of the rigidity of the molded product with respect to the molding pressure of the foamed particles may be insufficient.

[0007] This invention was made in view of the above background, and aims to provide polypropylene-based resin foam particles that have good moldability and a good balance of rigidity with respect to molding pressure. [Means for solving the problem]

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

[10] .

[0009] [1] Foamed particles composed of a base resin mainly composed of polypropylene resin, The polypropylene resin contains ethylene-propylene rubber, 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 first endothermic peak Pa with the largest peak area and a second endothermic peak Pb adjacent to the first endothermic peak Pa on the high-temperature side of the first endothermic peak Pa. The closed-cell ratio of the foamed particles is 85% or more. The temperature Tb at the peak of the second endothermic peak Pb is 158°C or higher. The difference Tb-Ta between the temperature Tb at the peak of the second endothermic peak Pb and the temperature Ta at the peak of the first endothermic peak Pa is 15°C or more and 30°C or less. Polypropylene resin foam particles in which the ratio of the peak height hb of the second endothermic peak Pb to the peak height ha of the first endothermic peak Pa is 1.2 or more and 2.8 or less (hb / ha).

[0010] [2] The polypropylene resin foam particles according to [1], wherein the polypropylene resin is a mixed resin of a polypropylene resin (A) containing ethylene-propylene rubber and a polypropylene resin (B). [3] The polypropylene resin foam particle according to [2], wherein the polypropylene resin (A) has a morphology in which homopolypropylene is used as a matrix and rubbery bodies containing the ethylene-propylene rubber are used as domains, and the polypropylene resin (B) is a copolymer of propylene and ethylene and / or butene.

[0011] [4] Polypropylene resin foam particles according to [2] or [3], wherein the mass ratio of the polypropylene resin (A) to the polypropylene resin (B) is (A):(B)=3:97 to 40:60. [5] Polypropylene resin foam particles according to any one of [2] to [4], wherein the polypropylene resin (A) is derived from post-consumer materials.

[0012] [6] Polypropylene resin foam particles according to any one of [2] to [5], wherein the flexural modulus of the polypropylene resin (B) is 900 MPa or more and 1500 MPa or less. [7] Polypropylene resin foam particles according to any one of [1] to [6], wherein the foam particles are heated from 23°C to 200°C at a heating rate of 10°C / min, then cooled to 23°C at a cooling rate of 10°C / min, and then heated again from 23°C to 200°C at a heating rate of 10°C / min, and the melting end temperature in the second DSC curve obtained is 160°C or higher. [8] Polypropylene resin foam particles according to any one of [1] to [7], wherein the full width at half maximum of the first endothermic peak Pa is 15°C or more and less than 25°C.

[0013] [9] Polypropylene resin foam particles according to any one of [1] to [8], wherein the average bubble diameter of the foam particles is 40 μm or more and 100 μm or less.

[10] The apparent density of the foamed particles is 10 kg / m³ 3 More than 80kg / m 3 Polypropylene resin foam particles as described in any one of the following [1] to [9]. [Effects of the Invention]

[0014] According to the above embodiment, even when a polypropylene resin containing ethylene-propylene rubber is used as the base resin for the foamed particles, it is possible to provide polypropylene resin foamed particles that have good moldability and a good balance of rigidity with respect to molding pressure. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 is an explanatory diagram showing an example of a DSC curve for polypropylene resin foam particles. [Figure 2] Figure 2 is an explanatory diagram showing the method for calculating the peak height and full width at half maximum of each endothermic peak in the DSC curve. [Figure 3] Figure 3 is an electron microscope image of a cross-section of polypropylene resin A1 in the example. [Figure 4] Figure 4 is a transmission electron microscope image showing the morphology of the foamed particles in Example 3. [Modes for carrying out the invention]

[0016] (Polypropylene resin foam particles) The base resin of the aforementioned polypropylene resin foam particles (hereinafter referred to as "foam particles") mainly contains polypropylene resin. Specifically, the proportion of polypropylene resin in the foam particles is 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more. The foam particles may contain one or more types of polypropylene resin.

[0017] In this specification, polypropylene resin refers to a propylene copolymer containing 50% by mass or more of a homopolymer of propylene monomers (i.e., homopropylene) 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. Furthermore, the polypropylene resin of the present invention includes ethylene-propylene rubber.

[0018] [DSC characteristics] The foamed particles have a crystalline structure in which, as shown in Figure 1, 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 first endothermic peak Pa with the largest peak area and a second endothermic peak Pb adjacent to the first endothermic peak Pa on the high-temperature side of the first endothermic peak Pa. Note that the DSC curve may show only the two endothermic peaks, the first endothermic peak Pa and the second endothermic peak Pb, or other endothermic peaks may appear in addition to the first endothermic peak Pa and the second endothermic peak Pb.

[0019] The method for measuring the DSC curve and determining the first endothermic peak Pa and the second endothermic peak Pb is more specifically as follows. First, 1 to 3 mg of conditioned foamed particles are used as a sample, and a DSC curve is obtained by differential scanning calorimetry under the condition of heating from 23°C to 230°C at a heating rate of 10°C / min. As mentioned above, at least two endothermic peaks appear in the DSC curve of the foamed particles.

[0020] Next, as shown in Figure 1, a straight line L1 is drawn 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 endothermic peak with the highest temperature at its peak, that is, the intersection point of the endothermic peak with the highest temperature at its peak on the DSC curve and the baseline on the high-temperature side of that endothermic peak.

[0021] After drawing the straight line L1, a straight line L2 is drawn parallel to the vertical axis of the graph, passing through the maximum point γ located between adjacent endothermic peaks. This straight line L2 divides adjacent endothermic peaks into individual endothermic peaks. The area of ​​the region enclosed by the portion constituting each endothermic peak in the DSC curve, the straight line L1, and the straight line L2 is defined as the peak area of ​​that endothermic peak. The endothermic peak with the largest peak area among the multiple endothermic peaks is defined as the first endothermic peak Pa, and the endothermic peak adjacent to the first endothermic peak Pa on the high-temperature side is defined as the second endothermic peak Pb.

[0022] In the DSC curve, the temperature Tb at the peak of the second endothermic peak Pb (see Figure 1) is 158°C or higher. The foamed particles have DSC characteristics such that the temperature Tb at the peak of the second endothermic peak Pb falls within the specified range, thereby increasing the heat resistance of the foamed particles and suppressing damage to the bubbles of the foamed particles during molding heating. In this case, the heat resistance of the polypropylene resin foamed particle molded article (hereinafter referred to as "foamed particle molded article" or "molded article") obtained by in-mold molding the foamed particles can be increased. From the viewpoint of further enhancing these effects, the temperature Tb at the peak of the second endothermic peak Pb is preferably 160°C or higher, and more preferably 162°C or higher. From the viewpoint of further improving the moldability of the foamed particles, the temperature Tb at the peak of the second endothermic peak Pb is preferably 175°C or lower.

[0023] One method for controlling the temperature Tb at the peak of the second endothermic peak Pb to fall within the specified range is to use a polypropylene resin with a relatively high melting point as a raw material. In particular, by using a polypropylene resin containing the ethylene-propylene rubber and having a relatively high melting point as the base resin, the temperature Tb at the peak of the second endothermic peak Pb can be more easily controlled to fall within the specified range.

[0024] Furthermore, the foamed particles have a DSC characteristic in which the difference Tb-Ta between the temperature Tb at the peak of the second endothermic peak Pb and the temperature Ta at the peak of the first endothermic peak Pa (see Figure 1) is between 15°C and 30°C. Conventionally, foamed particles composed of polypropylene resin containing ethylene-propylene rubber tend to have poor moldability because the temperature Tb at the peak of the second endothermic peak Pb is high and the difference Tb-Ta between the peak temperatures tends to be small. In contrast, the foamed particles can improve moldability during in-mold molding by keeping the difference Tb-Ta between the temperature Tb at the peak of the second endothermic peak Pb and the temperature Ta at the peak of the first endothermic peak Pa within the specified range.

[0025] One possible reason for this is that, for example, by setting the difference in peak temperatures Tb-Ta within the aforementioned specific range, even when performing in-mold molding at a relatively low molding pressure, it is possible to maintain the relatively difficult-to-melt crystals (i.e., crystals that produce a second endothermic peak Pb when melted) within the foam particles during in-mold molding, while sufficiently melting the relatively easy-to-melt crystals (i.e., crystals that produce a first endothermic peak Pa when melted), thereby moderately softening the foam particles.

[0026] If the difference in peak temperatures Tb-Ta is too small, it may lead to poor moldability of the foam particles during in-mold molding, and it may not be possible to obtain a good foam particle molded product. From the viewpoint of more reliably improving the moldability of the foam particles during in-mold molding, the difference in peak temperatures Tb-Ta is preferably 18°C ​​or higher, and more preferably 20°C or higher. On the other hand, if the difference in peak temperatures Tb-Ta is too large, the foam particles may shrink easily. From the viewpoint of further suppressing the shrinkage of the foam particles, the difference in peak temperatures Tb-Ta is preferably 28°C or lower, and more preferably 26°C or lower.

[0027] In determining a preferred range for the vertex temperature difference Tb-Ta, the aforementioned upper and lower limits for the vertex temperature difference Tb-Ta can be arbitrarily combined. For example, the vertex temperature difference Tb-Ta may be 18°C ​​or more and 28°C or less, or 20°C or more and 26°C or less.

[0028] One method for controlling the peak temperature difference Tb-Ta within the specified range is to use a mixed resin as the base resin for the foamed particles, which consists of a polypropylene resin with a relatively high melting point containing the ethylene-propylene rubber and a polypropylene resin consisting of a polypropylene random copolymer with a relatively low melting point.

[0029] In the aforementioned DSC curve, the ratio of the peak height hb of the second endothermic peak Pb to the peak height ha of the first endothermic peak Pa, hb / ha, is between 1.2 and 2.8. Generally, polypropylene foam particles tend to have higher rigidity, such as compressive strength, as the molding pressure during in-mold molding, i.e., the molding heating temperature during in-mold molding, increases. Foam particles with a poor balance of rigidity to molding pressure tend to have a molded body with rigidity lower than that expected from the molding pressure. Polypropylene foam particles are usually molded at the lowest molding pressure (i.e., the minimum molding pressure) that yields a good molded body.

[0030] The foamed particles have DSC characteristics such that the peak height ratio hb / ha falls within the specified range, thereby improving moldability and the balance of rigidity with respect to molding pressure, making it easy to obtain a molded body with rigidity appropriate to the molding pressure.

[0031] Conventionally, when ethylene-propylene rubber was included in the polypropylene resin that served as the base resin for the foamed particles, molding sometimes became difficult. Furthermore, even when molding itself was possible, the rubbery material containing ethylene-propylene rubber was dispersed within the cell membrane of the foamed particles, and because the cell membrane was easily damaged to such an extent that it hardly affected the closed-cell ratio of the molded product, the balance of stiffness with respect to molding pressure was easily compromised. In contrast, the foamed particles have DSC characteristics in which the peak height ratio hb / ha falls within the aforementioned specific range, resulting in a good balance of stiffness with respect to molding pressure even when ethylene-propylene rubber is included. The reason why the balance of stiffness with respect to molding pressure is improved by the peak height ratio hb / ha of the foamed particles being within the aforementioned specific range is not entirely clear at present, but the following reasons are possible, for example.

[0032] The peak height ratio hb / ha is thought to be related to the existence and abundance of rigid crystals with a uniform crystal structure (i.e., crystals that produce a second endothermic peak Pb during melting). A high peak height ratio hb / ha indicates that the aforementioned rigid crystals exist in a state with few structural defects. Furthermore, such rigid crystals are thought to affect the thermoworkability of the foamed particles.

[0033] Therefore, by setting the peak height ratio hb / ha within the aforementioned specific range, it is possible to achieve an appropriate balance between the amount of rigid, relatively difficult-to-melt crystals and the amount of relatively easy-to-melt crystals present in the foamed particles, and to moderately broaden the distribution of the melting temperatures of the relatively easy-to-melt crystals. As a result, it is thought that the bubbles in the foamed particles will be less likely to be damaged during in-mold molding.

[0034] If the peak height ratio hb / ha is too small, the shape of the molded body may not recover easily even after a curing process following in-mold molding, potentially leading to a decrease in moldability. From the viewpoint of more easily avoiding a decrease in the recoverability of the molded body, the peak height ratio hb / ha is preferably 1.3 or higher, more preferably 1.4 or higher, and even more preferably 1.5 or higher. On the other hand, if the peak height ratio hb / ha is too large, the balance of rigidity with respect to molding pressure tends to deteriorate, potentially resulting in insufficient rigidity compared to the molding pressure. From the viewpoint of further improving the balance of rigidity with respect to molding pressure, the peak height ratio hb / ha is preferably 2.6 or lower, and more preferably 2.4 or lower.

[0035] In determining a preferred range for the peak height ratio hb / ha, the aforementioned upper and lower limits of the peak height ratio hb / ha can be arbitrarily combined. For example, the peak height ratio hb / ha may be between 1.3 and 2.6, between 1.4 and 2.6, or between 1.5 and 2.4.

[0036] Methods for controlling the peak height ratio hb / ha within the specified range include, for example, adjusting the ratio of the amount of polypropylene resin with a relatively high melting point containing the ethylene-propylene rubber to the amount of polypropylene resin consisting of a polypropylene random copolymer with a relatively low melting point in the base resin of the foamed particles to within the range described later; setting the flexural modulus of the polypropylene resin consisting of the random copolymer to within the range described later; adjusting the holding time in the step of forming the second endothermic peak Pb during the foamed particle manufacturing process; and adjusting the foaming temperature during the foamed particle manufacturing process.

[0037] The peak height of each endothermic peak, as described above, refers to the distance perpendicular to the horizontal axis between the interpolated baseline and the peak apex. Specifically, the peak height of an endothermic peak is determined as follows: First, the DSC curve of the foamed particles is measured using the method described above to determine the first endothermic peak Pa and the second endothermic peak Pb.

[0038] Next, as shown in Figure 2, a straight line L1 is 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. Then, a line segment parallel to the vertical axis of the graph is drawn from the peak of each endothermic peak to line L1, and the length of this line segment is taken as the peak height of each endothermic peak. That is, the peak height ha of the first endothermic peak Pa is the length of line segment L3 drawn parallel to the vertical axis of the graph from the peak of the first endothermic peak Pa to line L1. Similarly, the peak height hb of the second endothermic peak Pb is the length of line segment L4 drawn parallel to the vertical axis of the graph from the peak of the second endothermic peak Pb to line L1.

[0039] The full width at half maximum of the first endothermic peak Pa is preferably 15°C or more and less than 25°C. In this case, the balance of rigidity with respect to molding pressure is further improved, and a molded body with rigidity commensurate with the molding pressure can be obtained more easily. In particular, a molded body with the rigidity expected from the minimum molding pressure can be obtained more easily. Furthermore, the in-moldability of foamed particles can be further improved when in-mold molding is performed at low molding pressures. From the viewpoint of more reliably obtaining these effects, it is more preferable that the full width at half maximum of the first endothermic peak Pa is 18°C ​​or more and less than 25°C.

[0040] The full width at half maximum (FMAX) wa of the first endothermic peak Pa, as described above, is obtained as follows. First, using the method described above, draw a line segment L3 parallel to the vertical axis of the graph from the peak of the first endothermic peak Pa to line L1. Next, as shown in Figure 2, draw a line L5 that passes through the midpoint of line segment L3 and is parallel to line L1. Then, identify the two intersection points of line L5 and the first endothermic peak Pa. The difference between the temperature corresponding to the high-temperature intersection and the temperature corresponding to the low-temperature intersection at the two intersection points obtained in this way is taken as the full width at half maximum (FMAX) wa of the first endothermic peak Pa.

[0041] Note that, depending on the shape of the DSC curve, the line L5 may not intersect the portion of the curve at a higher temperature than the peak of the first endothermic peak Pa. In this case, the intersection point of the line parallel to the vertical axis passing through the melting termination temperature of the first endothermic peak Pa and the line L5 is identified, and the difference between the temperature corresponding to this intersection point and the temperature corresponding to the intersection point on the lower temperature side is taken as the full width at half maximum (FMAX) wa of the first endothermic peak Pa. Furthermore, when determining the full width at half maximum (FMAX) wa of the first endothermic peak Pa, the melting termination temperature of the first endothermic peak Pa refers to the temperature corresponding to the intersection point of the line L1 with the tangent line drawn at the point where the slope of the curve on the high-temperature side of the first endothermic peak Pa is maximum.

[0042] The full width at half maximum of the second endothermic peak Pb is preferably 2.0°C or more and less than 4.0°C. In this case, the balance of rigidity with respect to molding pressure can be further improved, and a molded body with rigidity commensurate with the molding pressure can be obtained more easily. In addition, in this case, it can be more easily suppressed that the molding pressure becomes excessively high.

[0043] The method for calculating the full width at half maximum of the second endothermic peak Pb is the same as the method for calculating the full width at half maximum of the first endothermic peak Pa described above, except that the second endothermic peak Pb is used instead of the first endothermic peak Pa.

[0044] The first endothermic peak Pa is presumed to be a resin-specific melting peak that appears due to the melting of crystals normally present in the resin components constituting the foamed particles. The second endothermic peak Pb is presumed to appear due to the melting of secondary crystals formed in the resin components during the manufacturing process of the foamed particles. As described above, the DSC curve obtained when heating from 23°C to 230°C at a heating rate of 10°C / min (i.e., the first heating) shows both the first endothermic peak Pa and the second endothermic peak Pb. In contrast, the DSC curve obtained when the mixture is cooled from 230°C to 23°C at a cooling rate of 10°C / min after the first heating, and then heated again from 23°C to 230°C at a heating rate of 10°C / min (i.e., the second heating), shows only the first endothermic peak Pa. Therefore, by comparing the DSC curve obtained from the first heating and the DSC curve obtained from the second heating, it is possible to distinguish between the first endothermic peak Pa and the second endothermic peak Pb.

[0045] From the viewpoint of more reliably ensuring the moldability of the foamed particles, the heat of fusion Qa of the first endothermic peak is preferably 30 J / g or more and 80 J / g or less, and more preferably 40 J / g or more and 70 J / g or less. Furthermore, from the viewpoint of ensuring the moldability of the foamed particles and further increasing the mechanical strength of the molded article, the heat of fusion Qb of the second endothermic peak is preferably 10 J / g or more and 30 J / g or less, and more preferably 15 J / g or more and 25 J / g or less. These heats of fusion are calculated based on the area of ​​each peak in the first DSC curve.

[0046] [Percentage of closed cells] The closed-cell ratio of the foamed particles is 85% or higher. If the closed-cell ratio of the foamed particles is too low, it may lead to a decrease in moldability. Furthermore, by having a closed-cell ratio of 85% or higher and also possessing the aforementioned DSC characteristics, it is possible to achieve both high moldability and a balance of rigidity against molding pressure. The closed-cell ratio of the foamed particles is preferably 90% or higher, and more preferably 92% or higher.

[0047] The closed-cell ratio of foamed particles is measured using an air-comparison hydrometer based on ASTM-D2856-70 Procedure C. The specific method for measuring the closed-cell ratio of foamed particles is as follows: A bulk volume of approximately 20 cm³ after conditioning. 3 The foamed particles are used as the measurement sample, and the apparent volume Va of the measurement sample is measured from the rise in the liquid level when the measurement sample is submerged in a graduated cylinder containing ethanol. After thoroughly drying the measurement sample from which the apparent volume Va has been measured, the true volume Vx of the measurement sample is 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 (in %) of the measurement sample is calculated based on the following formula (1). The above procedure is performed for each of the five measurement samples, and the arithmetic mean (N=5) of the closed-cell ratios of the five measurement samples is taken as the closed-cell ratio (in %) of the foamed particles. Closed cell ratio=(Vx-W / ρ)×100 / (Va-W / ρ) ···(1)

[0048] The meanings of the symbols in equation (1) above are 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 )

[0049] The base resin of the foamed particles contains at least ethylene-propylene rubber. In this specification, ethylene-propylene rubber means a rubbery substance composed of an ethylene-propylene copolymer containing an ethylene component and a propylene component. That is, ethylene-propylene rubber may be ethylene-propylene rubber (i.e., EPM). Furthermore, ethylene-propylene rubber may be rubber that further contains components derived from monomers other than ethylene and propylene, such as ethylene-propylene diene rubber (i.e., EPDM). The mass ratio of the ethylene-derived component to the propylene-derived component in ethylene-propylene rubber (ethylene-derived component: propylene-derived component) is usually 30:70 to 80:20. Ethylene-propylene rubber is a different substance from polypropylene resins and polyethylene resins, and those skilled in the art can distinguish between the two. The flexural modulus of ethylene-propylene rubber is usually 50 MPa or less.

[0050] Whether or not ethylene-propylene rubber is contained in the polypropylene resin in the base resin constituting the foamed particles can be determined by observing the morphology of the foamed particles. Specifically, the observation of the morphology of the foamed particles can be performed based on cross-sectional images (i.e., TEM images) of the foamed particles obtained using a transmission electron microscope.

[0051] The content of the ethylene-propylene rubber in the foamed particles is preferably 0.5% by mass or more and 15% by mass or less, and more preferably 1% by mass or more and 10% by mass or less. In this case, the balance between the moldability of the foamed particles and the rigidity of the resulting molded article can be further improved.

[0052] The ethylene-propylene rubber content in the foamed particles can be determined by the following method. First, the foamed particles are placed in a solvent capable of dissolving ethylene-propylene rubber, and the solvent is heated to dissolve the ethylene-propylene rubber in the foamed particles. Then, the liquid phase containing the dissolved ethylene-propylene rubber is separated. A poor solvent for ethylene-propylene rubber is added to the resulting liquid phase to reprecipitate the ethylene-propylene rubber. This reprecipitate is filtered off, dried, and its mass is measured. The ratio of the mass of the reprecipitate to the mass of the foamed particles placed in the solvent, expressed as a percentage, is taken as the ethylene-propylene rubber content in the foamed particles. The ethylene-propylene rubber content in the foamed particles is approximately the same as the ethylene-propylene rubber content in polypropylene resin particles, which will be described later.

[0053] Furthermore, if the base resin of the foamed particles does not contain any polymers other than polypropylene resin and ethylene-propylene rubber, the ethylene-propylene rubber content in the foamed particles is approximately the same as the ethylene-propylene rubber content in the polypropylene resin.

[0054] The ethylene-propylene rubber contained in the polypropylene resin is preferably derived from post-consumer materials. In particular, the base resin of the foamed particles is preferably a recycled polypropylene resin derived from post-consumer materials containing the ethylene-propylene rubber. In this case, post-consumer materials containing polypropylene resin can be effectively utilized as a resource. Furthermore, since recycled polypropylene resin derived from post-consumer materials has a relatively high melting point, the aforementioned DSC properties can be easily achieved by using such a recycled polypropylene resin as the base resin.

[0055] In this specification, "post-consumer materials" refers to "materials discharged from households, or materials generated as products that can no longer be used for their original purpose from commercial facilities, industrial facilities, and various other facilities that are end users of the product," as described in JIS Q14021:2000. Post-consumer materials also include materials returned from distribution channels.

[0056] More specifically, post-consumer materials include, for example, automotive components made of polypropylene resin, such as bumpers and instrument panels, removed from used automobiles; polypropylene resins recovered from ASR (i.e., automotive shredder residue) generated during the automobile disposal process; and polypropylene resins derived from used home appliances.

[0057] In this specification, ASR refers to "automobile shredded residue" as defined in Article 2-5 of the Act on Recycling of End-of-Life Automobiles, Act No. 87 of 2002. More specifically, ASR refers to the material remaining after dismantling an end-of-life automobile, from which recyclable parts such as engines and batteries have been removed, shredding the vehicle, separating metals and other useful materials from the shredded material, and recovering these materials. ASR usually contains polypropylene resin derived from automobile components, as well as other plastics other than polypropylene resin such as polystyrene (PS) and acrylonitrile-butadiene-styrene resin (ABS), rubber, and various metals. However, polypropylene resin can be recovered from ASR by appropriately combining known sorting methods.

[0058] One example of a method for obtaining ASR is a method comprising: a dismantling step of removing reusable and non-reusable parts from a used vehicle; an accessory separation step of removing automotive accessory parts from the dismantled vehicle after the dismantling step; and a crushing and sorting step of crushing the dismantled vehicle and / or automotive accessory parts after the separation step, and separating metals from the resulting crushed material to recover ASR. More specifically, ASR can be obtained by the methods described in Japanese Patent No. 6609877, Japanese Patent No. 6627142, Japanese Patent No. 6762071, etc.

[0059] Furthermore, the method for recovering polypropylene resin from ASR is not particularly limited, and polypropylene resin can be recovered from ASR by appropriately combining known sorting methods. For example, the sorting steps shown in (α) to (δ) below can be carried out individually or in combination of two or more. (α) Magnetic separation process for separating metals from nonmetals using magnetic force. (β) Wind-powered sorting process for separating light and heavy objects using wind power. (γ) Wet specific gravity separation process for separating light and heavy specific gravity substances using a solvent. (δ) Electrostatic separation process for selecting resins based on their susceptibility to electrostatic charge

[0060] For example, in the wet specific gravity separation process (γ), polypropylene resins contained in ASR can be separated by methods such as those described in Japanese Patent No. 3711472, which utilize water flow classification and sedimentation velocity, or by using an apparatus equipped with a pulsating bubbling tank that intermittently discharges bubbles and a sedimentation tank, and a suspended solids separation tank for separating lighter and heavier substances, as described in Japanese Patent Application Publication No. 2004-58032. Furthermore, these separation processes can be combined, as described in Japanese Patent Application Publication No. 2008-178846 and Japanese Patent No. 6762071. For example, by combining the air separation process and the wet specific gravity separation process, polypropylene resins contained in ASR can be separated by separating lighter and heavier substances contained in ASR using air force, and then immersing each separately in a separation liquid for specific gravity separation.

[0061] Other ingredients The base resin constituting the foamed particles may contain other polymers, such as resins other than polypropylene resins or rubbers other than ethylene-propylene rubber, to the extent that the effects described above are not impaired. Examples of resins other than polypropylene resins that may be included in the base resin include polystyrene resins, polyethylene resins, and acrylonitrile-butadiene-styrene resins. Examples of rubbers other than ethylene-propylene rubber that may be included in the base resin include ethylene-butene rubber and ethylene-octene rubber.

[0062] The content of other polymers in the base resin, such as resins other than the polypropylene resin or rubbers other than ethylene-propylene rubber, is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less.

[0063] The base resin may contain inorganic materials such as talc, silica, and glass fibers, to the extent that it does not impair the effects described above. These inorganic materials constitute components of the ash content of the foamed particles.

[0064] Furthermore, the base resin may contain additives such as bubble regulators, crystal nucleating agents, flame retardants, flame retardant aids, plasticizers, antistatic agents, antioxidants, ultraviolet absorbers, light stabilizers, antibacterial agents, colorants, and reinforcing agents such as glass fibers, to the extent that they do not impair the effects described above.

[0065] The polypropylene resin contained in the base resin is preferably composed of a mixed resin of a polypropylene resin (A) containing the ethylene-propylene rubber and a polypropylene resin (B). In this case, the aforementioned DSC characteristics can be more easily achieved, moldability during in-mold molding can be more easily improved, and the balance of rigidity with respect to molding pressure can be more easily improved. From the viewpoint of more reliably obtaining these effects, it is more preferable that the polypropylene resin (A) is a polypropylene resin having a morphology in which a propylene polymer is the matrix (i.e., the continuous phase) and a rubbery substance containing the ethylene-propylene rubber is the domain (i.e., the dispersed phase). The propylene polymer is usually homopropylene. Furthermore, the rubbery substance domains in the polypropylene resin (A) may also contain a polyethylene resin in addition to the ethylene-propylene rubber. Also, from a similar viewpoint, it is more preferable that the polypropylene resin (B) is a propylene copolymer, and even more preferable that it is a copolymer of propylene and ethylene and / or butene.

[0066] In the aforementioned polypropylene resin, the mass ratio of polypropylene resin (A) to polypropylene resin (B) is preferably (A):(B) = 3:97 to 40:60 (provided that the total amount of both is 100% by mass). That is, in the aforementioned polypropylene resin, the mass ratio of polypropylene resin (A) to the total amount of polypropylene resin (B) is preferably 3% by mass or more and 40% by mass or less. In this case, the aforementioned DSC characteristics can be achieved more easily, moldability during in-mold molding can be improved more easily, and the balance of rigidity with respect to molding pressure can be improved more easily.

[0067] Furthermore, by setting the mass ratio of the polypropylene resin (A) to 40% by mass or less, the moldability during in-mold molding can be more reliably improved. From the viewpoint of more easily obtaining foamed particles with excellent in-moldability, the mass ratio of polypropylene resin (A) to the total of polypropylene resin (B) is more preferably 35% by mass or less, even more preferably 30% by mass or less, and particularly preferably 25% by mass or less.

[0068] In determining the preferred range for the mass ratio of polypropylene resin (A), the upper and lower limits of the mass ratio of polypropylene resin (A) described above can be arbitrarily combined. For example, the preferred range for the mass ratio of polypropylene resin (A) to polypropylene resin (B) may be (A):(B) = 3:97 to 35:65, 3:97 to 30:70, or 3:97 to 25:75. However, the total amount of polypropylene resin (A) and polypropylene resin (B) is 100% by mass.

[0069] • Polypropylene resin (A) The polypropylene resin (A) may be a non-recycled polypropylene resin, that is, a polypropylene resin that has not undergone thermal processing such as molding for forming molded products, or it may be a polypropylene resin that has undergone thermal processing (i.e., a recycled polypropylene resin). However, from the viewpoint of effectively utilizing limited resources, it is preferable that the polypropylene resin (A) is a recycled polypropylene resin. As the recycled polypropylene resin, for example, recycled polypropylene resin derived from post-consumer materials can be used, that is, recycled polypropylene resin made from polypropylene resin used in automobiles, automobile components made of polypropylene resin removed from used automobiles, polypropylene resin recovered from ASR, polypropylene resin derived from used home appliances, or polypropylene resin derived from used toys, storage containers, and other daily products. In this case, the effects described above can be exerted particularly effectively.

[0070] The method for observing the morphology of the polypropylene resin (A) is as follows. First, an observation sample is cut from the polypropylene resin (A). There are no particular limitations on how the observation sample is cut, but for example, if the shape of the polypropylene resin (A) is a cylindrical pellet, the pellet can be cut perpendicular to the height direction of the pellet so as to pass through the center of the pellet. In this way, an observation sample can be obtained in which the cross-section of the center of the pellet made of the polypropylene resin (A) is exposed.

[0071] Next, the observation sample is embedded in epoxy resin, electron-stained with ruthenium tetroxide, and then sections are prepared from the sample using an ultramicrotome or the like. These sections are placed on the grid of a transmission electron microscope (e.g., JEOL's "JEM-1040Flash") and observed at a predetermined magnification (e.g., 5000x), and cross-sectional images (i.e., TEM images) of the polypropylene resin (A) are taken. From the cross-sectional images, the morphology of the polypropylene resin phase and the rubbery phase containing ethylene-propylene rubber in the polypropylene resin (A) is visually observed.

[0072] The content of ethylene-propylene rubber in the polypropylene resin (A) is preferably 1% by mass or more and 30% by mass or less, and more preferably 5% by mass or more and 20% by mass or less. The content of ethylene-propylene rubber in the polypropylene resin (A) can be measured in the same manner as the measurement of the content of ethylene-propylene rubber in the base resin, using the raw material of the polypropylene resin (A) as a sample instead of foamed particles.

[0073] The melting point TmA of the polypropylene resin (A) is preferably 155°C to 170°C. In this case, the aforementioned DSC properties can be achieved more easily, and despite using a polypropylene resin (A) with a relatively high melting point, the moldability during in-mold molding can be improved more easily, as can the balance of rigidity with respect to molding pressure. From the viewpoint of more reliably obtaining these effects, the melting point TmA of the polypropylene resin (A) is more preferably 158°C to 168°C, and even more preferably 160°C to 165°C.

[0074] The melting point TmA of polypropylene resin (A) is determined according to JIS K7121:1987. Specifically, first, a test specimen made of polypropylene resin (A) is prepared, and the test specimen is conditioned according to "3. Conditioning of test specimens" "(2) When measuring the melting temperature after performing a certain heat treatment" in JIS K7121:1987. The temperature range for conditioning is 30°C to 230°C, and both the heating rate and cooling rate are 10°C / min. A DSC curve is obtained by heating the conditioned test specimen from 30°C to 230°C at a heating rate of 10°C / min, and the temperature at the peak of the melting peak that appears in the DSC curve is taken as the melting point. If multiple melting peaks appear in the DSC curve, the temperature at the peak of the melting peak with the highest height relative to the baseline is taken as the melting point TmA of polypropylene resin (A).

[0075] The flexural modulus of the polypropylene resin (A) is preferably 800 MPa to 1600 MPa, and more preferably 800 MPa to 1200 MPa. The flexural modulus of the polypropylene resin (A) can be determined based on JIS K7171:2008. In general, in polypropylene resins containing ethylene-propylene rubber, the flexural modulus tends to increase as the melting point increases. On the other hand, when the polypropylene resin (A) is a recycled polypropylene resin derived from post-consumer materials, the ratio of the flexural modulus to the melting point tends to be relatively low. It is believed that by manufacturing resin particles using such a polypropylene resin (A), it is easier to obtain foamed particles that have good foamability and can be molded well even under relatively low molding pressure conditions. From this viewpoint, the ratio (unit: MPa / °C) of the flexural modulus (MPa) of the polypropylene resin (A) to its melting point (unit: °C) is preferably 4 to 8, and more preferably 5 to 7.

[0076] The heat of fusion of the polypropylene resin (A) is preferably 60 J / g or more and 85 J / g or less. In this case, the aforementioned DSC characteristics can be achieved more easily, moldability during in-mold molding can be improved more easily, and the balance of rigidity with respect to molding pressure can be improved more easily.

[0077] The heat of fusion of polypropylene resin (A) is measured by the following method. First, a DSC curve is obtained using the same method as described above for measuring the melting point. On this DSC curve, a straight line is drawn connecting the point corresponding to 80°C and the high-temperature endpoint of the melting peak with the highest peak temperature. The heat of fusion of polypropylene resin (A) can be calculated based on the area of ​​the region enclosed by the straight line determined in this way and the melting peak of the DSC curve.

[0078] Melt flow rate (MFR) of polypropylene resin (A) measured under conditions of temperature 230°C and load 2.16 kg. Ais preferably 10 g / 10 min or more and 40 g / 10 min or less, more preferably 15 g / 10 min or more and 35 g / 10 min or less, and even more preferably 20 g / 10 min or more and 30 g / 10 min or less. In this case, while containing the ethylene-propylene rubber, the in-mold formability of the foamed particles can be further enhanced, and the range of molding pressure in which a good foamed particle molded body can be obtained can be made wider.

[0079] Also, the melt flow rate MFR of the polypropylene resin (A) A and the melt flow rate MFR of the polypropylene resin (B) B The difference MFR A -MFR B is preferably 10 g / 10 min or more and 25 g / 10 min or less, and more preferably 15 g / 10 min or more and 22 g / 10 min or less. In this case, the variation in the in-mold formability of the foamed particles can be further reduced, and foamed particles having excellent in-mold formability can be obtained more easily.

[0080] · Polypropylene resin (B) The polypropylene resin (B) may be a non-recycled polypropylene resin, that is, a polypropylene resin that has not received a heat history due to molding processing or the like for molding a molded product, or a polypropylene resin that has received a heat history (that is, a recycled polypropylene resin). From the viewpoint of more easily realizing the specific DSC characteristics, the polypropylene resin (B) is preferably a non-recycled polypropylene resin. From the same viewpoint, the polypropylene resin (B) preferably does not substantially contain a rubber component. Specifically, the content of the rubber component in the polypropylene resin (B) is preferably less than 0.1% by mass, more preferably less than 0.05% by mass, and even more preferably 0.

[0081] The polypropylene resin (B) is preferably a copolymer of propylene and ethylene and / or butene. Specifically, propylene-ethylene random copolymers and propylene-ethylene-butene random copolymers can be preferably used. From the viewpoint of more reliably obtaining the aforementioned effects and expanding the moldable range, the polypropylene resin (B) is preferably a propylene-ethylene random copolymer.

[0082] The ethylene component content in the propylene-ethylene random copolymer used as the polypropylene resin (B) is preferably 1% by mass or more, and more preferably 2% by mass or more. In this case, the in-moldability of the foamed particles can be further improved when in-mold molding is performed at a low molding pressure.

[0083] Furthermore, the ethylene component content in the propylene-ethylene random copolymer is preferably 6% by mass or less, more preferably 5% by mass or less, even more preferably 4% by mass or less, and particularly preferably 3% by mass or less. In this case, a foamed particle molded article with good compressive strength against molding pressure can be obtained more easily.

[0084] The total content of ethylene and butene components in the propylene-ethylene-butene random copolymer used as the polypropylene resin (B) is preferably 2% by mass or more, more preferably 5% by mass or more, and even more preferably 7% by mass or more. In this case, the in-moldability of the foamed particles can be further improved when in-mold molding is performed at a low molding pressure.

[0085] Furthermore, the total content of ethylene and butene components in the propylene-ethylene-butene random copolymer is preferably 15% by mass or less, and more preferably 12% by mass or less. In this case, a foamed particle molded article with good compressive strength and other properties can be obtained more easily.

[0086] Furthermore, from the viewpoint of being able to mold at relatively low molding pressure and more easily obtain a foamed particle molded body with good compressive strength, the ratio of the butene component content to the ethylene component content in the propylene-ethylene-butene random copolymer is preferably 7 to 25, more preferably 8 to 20, and even more preferably 10 to 18.

[0087] The content of ethylene-derived components and butene-derived components in polypropylene resin (B) can be determined, for example, based on IR spectroscopy. The method for measuring the content of ethylene-derived components and butene-derived components in polypropylene resin (B) will be described in detail in the examples.

[0088] In determining the preferred range for the content of comonomer components in the polypropylene resin (B), the aforementioned upper and lower limits can be arbitrarily combined.

[0089] The flexural modulus of the polypropylene resin (B) is preferably 800 MPa or more and 1500 MPa or less. From the viewpoint of more easily improving moldability during in-mold molding and more easily improving the balance of rigidity with respect to molding pressure, the flexural modulus of the polypropylene resin (B) is more preferably 900 MPa or more, and even more preferably 950 MPa or more. Furthermore, from the viewpoint of more easily improving moldability during in-mold molding and further expanding the moldable range, the flexural modulus of the polypropylene resin (B) is more preferably 1200 MPa or less, and even more preferably 1100 MPa or less. The flexural modulus of the polypropylene resin (B) can be determined based on JIS K7171:2008.

[0090] In determining the preferred range for the flexural modulus of the polypropylene resin (B), the aforementioned upper and lower limits can be arbitrarily combined. For example, the preferred range for the flexural modulus of the polypropylene resin (B) may be 900 MPa to 1500 MPa, 950 MPa to 1500 MPa, 800 MPa to 1200 MPa, or 800 MPa to 1100 MPa.

[0091] The melting point TmB of the polypropylene resin (B) is preferably 130°C or higher and less than 160°C, more preferably 135°C or higher and less than 155°C, even more preferably 138°C or higher and 152°C or lower, and particularly preferably 140°C or higher and 150°C or lower. In this case, the specific DSC characteristics can be more easily achieved, moldability during in-mold molding can be more easily improved, and the balance of rigidity with respect to molding pressure can be more easily improved. The method for measuring the melting point TmB of the polypropylene resin (B) is the same as the method for measuring the melting point TmA of the polypropylene resin (A) described above, except that the polypropylene resin (B) is used instead of the polypropylene resin (A).

[0092] The difference TmA-TmB between the melting point TmA of the polypropylene resin (A) and the melting point TmB of the polypropylene resin (B) is preferably 5°C or more and 35°C or less, more preferably 10°C or more and 30°C or less, and even more preferably 15°C or more and 25°C or less. In this case, the specific DSC characteristics can be achieved more easily, moldability during in-mold molding can be improved more easily, and the balance of rigidity with respect to molding pressure can be improved more easily. Furthermore, shrinkage of foam particles can be suppressed more easily.

[0093] Melt flow rate (MFR) of polypropylene resin (B) measured under conditions of temperature 230°C and load 2.16 kg. BFrom the viewpoint of more reliably ensuring foaming, the amount is preferably 2g / 10 min to 15g / 10 min, and more preferably 4g / 10 min to 10g / 10 min.

[0094] [Melting end temperature of foamed particles] When the foamed particles are heated from 23°C to 200°C at a heating rate of 10°C / min, then cooled to 23°C at a cooling rate of 10°C / min, and then heated again from 23°C to 200°C at a heating rate of 10°C / min, it is preferable that the melting termination temperature in the second DSC curve obtained is 160°C or higher. The melting termination temperature in the second DSC curve is related to the melting point of the polypropylene resin contained in the foamed particles, and the melting termination temperature in the second DSC curve tends to be higher when a polypropylene resin with a high melting point is included. Therefore, foamed particles with a melting termination temperature of 160°C or higher in the second DSC curve can more reliably improve heat resistance. The upper limit of the melting termination temperature in the second DSC curve of the foamed particles is not particularly limited, but it is generally around 175°C.

[0095] The method for measuring the melting termination temperature of foamed particles is as follows: First, using a differential scanning calorimetry (DSC) analyzer, the foamed particles are heated from 23°C to 200°C at a heating rate of 10°C / min. Then, the foamed particles are cooled from 200°C to 23°C at a cooling rate of 10°C / min. After cooling is complete, the foamed particles are heated again from 23°C to 200°C at a heating rate of 10°C / min (i.e., a second heating), and a DSC curve is obtained. The melting termination temperature is defined as the temperature at the high-temperature endpoint of the melting peak that appears in the DSC curve from the second heating, that is, the temperature at the intersection of the melting peak and the baseline on the higher side of the melting peak in the second DSC curve.

[0096] [Apparent density of foamed particles] The apparent density of the foamed particles is 10 kg / m³ 3 More than 150kg / m 3 Preferably, the apparent density of the foamed particles is 10 kg / m³. 3 More than 20 kg / m 3More preferably 30 kg / m 3 By doing so, a molded body with sufficient rigidity can be obtained more easily. Furthermore, the apparent density of the foam particles is preferably 100 kg / m³. 3 More preferably 80 kg / m 3 By doing the following, a lighter molded body can be obtained.

[0097] In determining a preferred range for the apparent density of the foamed particles, the aforementioned upper and lower limits of apparent density can be arbitrarily combined. For example, the apparent density may be 20 kg / m³. 3 More than 100kg / m 3 It may also be less than 30 kg / m 3 More than 80kg / m 3 The following is also acceptable.

[0098] The apparent density of foam particles is calculated as follows: First, the foam 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 foam particles to settle. After measuring the mass (in g) of the foam particle group, it is submerged in a graduated cylinder filled with 23°C water using a wire mesh or similar, and the volume (in L) of the foam particle group is determined from the rise in water level. Then, the apparent density (in kg / m³) of the foam particle group is calculated by dividing the mass of the foam particle group by its volume and converting the units. 3 It is possible to calculate ).

[0099] [Average bubble diameter of foamed particles] The average bubble diameter of the foamed particles is preferably 40 μm to 100 μm, more preferably 45 μm to 90 μm, and even more preferably 50 μm to 80 μm. Conventionally, foamed particles composed of polypropylene resin containing ethylene-propylene rubber, particularly foamed particles composed of polypropylene resin containing ethylene-propylene rubber derived from post-consumer materials, tend to have a small average bubble diameter, possibly due to the high ash content described later, and thus tend to have poor in-moldability. In contrast, the foamed particles of the present invention have the specific DSC characteristics described above, and therefore have good moldability even when the average bubble diameter of the foamed particles is within the range described above, making it easier to obtain good molded articles over a wide range of molding pressures.

[0100] The average bubble diameter of a foam particle is calculated using the following method. First, the foam particle is cut into approximately two equal parts. Next, a magnified photograph is taken so that the entire exposed cut surface is within the field of view. On the resulting magnified photograph, four line segments are drawn from the outermost surface of the foam particle, through the center, to the outermost surface on the opposite side, such that the angles between adjacent line segments are equal (i.e., the angles between adjacent line segments are 45°). The value obtained by dividing the total length of these four line segments by the total number of bubbles that intersect the line segments is taken as the bubble diameter of each foam particle.

[0101] The above procedure is performed on 10 or more randomly selected foamed particles, and the arithmetic mean of the resulting bubble diameters for each foamed particle is defined as the average bubble diameter of the foamed particles.

[0102] [Ash content of foamed particles] The ash content of the foamed particles is preferably 0.02% by mass or more and 4% by mass or less. The ash content of the foamed particles can be easily adjusted by using recycled polypropylene resin derived from post-consumer materials as the base resin.

[0103] From the viewpoint of more effectively utilizing post-consumer materials, the ash content of the foamed particles is more preferably 0.05% by mass or more, even more preferably 0.1% by mass or more, and particularly preferably 0.2% by mass or more. On the other hand, from the viewpoint of more reliably improving moldability during in-mold molding, the ash content of the foamed particles is more preferably 3% by mass or less, and even more preferably 2% by mass or less.

[0104] Furthermore, when determining the preferred range for the ash content of the foamed particles, the upper and lower limits of the ash content of the foamed particles mentioned above can be arbitrarily combined. For example, the preferred range for the ash content of the foamed particles may be 0.02% by mass or more and 3% by mass or less, 0.1% by mass or more and 3% by mass or less, or 0.2% by mass or more and 2% by mass or less.

[0105] The ash content of the foamed particles mentioned above can be calculated from the mass of the residue remaining after the foamed particles are burned.

[0106] (Method for manufacturing foamed particles) The method for producing the foamed particles is not particularly limited; for example, a conventional method for producing polypropylene-based foamed particles can be used as the method for producing the foamed particles.

[0107] [Preparation of resin particles] More specifically, first, polypropylene resin particles containing a rubber component made of ethylene-propylene rubber (hereinafter referred to as "resin particles") are prepared. The method for preparing the resin particles is not particularly limited. For example, when producing resin particles by the strand-cut method, a polypropylene resin that will be the base resin and additives used as needed are supplied to an extruder, and a molten resin mixture is obtained by heating and kneading the polypropylene resin in the extruder. This molten resin mixture is extruded in a strand shape through small holes in a die attached to the downstream side of the extruder. By taking up this strand-shaped extruded material and cutting it to the desired length, resin particles made of the base resin can be obtained.

[0108] Furthermore, when using two or more polypropylene resins as the base resin, the process of producing a molten resin mixture by melting and mixing the raw materials, and the process of producing resin particles from the molten resin mixture, may be carried out in the same extruder as described above. Alternatively, for example, the two processes described above may be carried out using separate extruders or the like.

[0109] [Foaming] The foamed particles are obtained by foaming resin particles using a method called the "direct foaming method." In the direct foaming method, resin particles containing a foaming agent, dispersed in an aqueous medium inside a container, are released together with the aqueous medium into an atmosphere at a pressure lower than the pressure inside the container, thereby causing the resin particles to foam. A preferred embodiment of the foaming method is described below.

[0110] To foam the resin particles, first, the resin particles are placed in a container and dispersed in an aqueous medium. At this time, if necessary, dispersants, dispersion aids, surfactants, etc., may be added to the aqueous medium in the container to disperse the resin particles.

[0111] As dispersants, inorganic fine particles such as aluminum oxide, tricalcium phosphate, magnesium pyrophosphate, zinc oxide, kaolin, and mica can be used. These inorganic fine particles may be used alone or in combination of two or more types. As a dispersing aid, aluminum sulfate can be used, for example. As a surfactant, anionic surfactants such as sodium alkylbenzenesulfonate, sodium dodecylbenzenesulfonate, and sodium alkanesulfonate can be used. These surfactants may be used alone or in combination of two or more types.

[0112] Next, a foaming agent is supplied into the container, and the pressure inside the container is increased to impregnate the resin particles with the foaming agent. This allows for obtaining resin particles containing the foaming agent. At this time, heating the resin particles in the container together with an aqueous medium can accelerate the impregnation of the foaming agent into the resin particles.

[0113] As blowing agents used in the foaming process, for example, inorganic physical blowing agents such as carbon dioxide, air, nitrogen, helium, and argon, or organic physical blowing agents such as hydrocarbons such as propane, n-butane, isobutane, n-pentane, isopentane, and hexane, and halogenated hydrocarbons such as ethyl chloride, 2,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropene, and trans-1-chloro-3,3,3-trifluoropropene can be used. From the viewpoint of environmental impact and ease of handling, carbon dioxide is preferably used as the blowing agent. The amount of blowing agent added is preferably 0.1 parts by mass or more and 30 parts by mass or less, and preferably 0.5 parts by mass or more and 15 parts by mass or less, per 100 parts by mass of resin particles.

[0114] The pressure inside the container immediately before foaming is preferably 0.5 MPa(G) or higher in gauge pressure. On the other hand, the pressure inside the 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 container damage or explosion.

[0115] After impregnating resin particles with a foaming agent, the contents of the container are released into an atmosphere with a lower pressure than the container. This causes the resin particles to foam and form a bubbly structure, which is then cooled by the outside air (i.e., the atmosphere) and stabilized, resulting in foamed particles.

[0116] In the method for producing the resin particles, a step is performed to adjust the crystalline structure of the resin components constituting the resin particles between the time the resin particles are dispersed in an aqueous medium and the time the resin particles are foamed. By foaming the resin particles after adjusting the crystalline structure of the resin components, foamed particles with excellent in-moldability and mechanical strength can be easily obtained.

[0117] The method for adjusting the crystalline structure of the resin component is as follows: First, a holding step is performed in which the mixture is held at a temperature of (melting point of the molten resin mixture - 15°C) or higher and (melting point of the molten resin mixture + 10°C) or lower for a sufficient amount of time, preferably 10 to 60 minutes. By performing this holding step, crystals that produce a second endothermic peak Pb upon melting are more easily formed in the resin constituting the foamed particles, and foamed particles having a crystalline structure in which the second endothermic peak Pb appears can be obtained more easily. The temperature inside the container during foaming is preferably between (melting point of the molten resin mixture) and (melting point of the molten resin mixture + 10°C).

[0118] In the method for producing the foamed particles, the resin particles that have undergone the holding step may be prepared in advance, and the foamed particles may be obtained by foaming these resin particles. From the viewpoint of increasing the productivity of foamed particles, it is preferable to perform the holding step by heating the resin particles dispersed in the dispersion medium in the container in the presence of a foaming agent, and then releasing the contents of the sealed container into an atmosphere with a pressure lower than the pressure inside the container, thereby foaming the resin particles and obtaining foamed particles having a crystalline structure in which a second endothermic peak Pb appears.

[0119] In the method for manufacturing foamed particles, when foaming the resin particles, the resin particles may be foamed in one step as described above, or they may be foamed in two or more steps. When foaming the resin particles in two steps, first, in the first foaming step, the resin particles are foamed by a direct foaming method to obtain one-stage foamed particles. In the second foaming step, for example, the one-stage foamed particles may be pressurized with air or the like to increase the pressure (internal pressure) inside the bubbles of the one-stage foamed particles, and then the one-stage foamed particles may be heated with steam or the like to further foam them. By foaming the resin particles in multiple steps in this way, foamed particles with a higher foaming ratio (i.e., lower bulk density) can be easily obtained.

[0120] (Polypropylene resin foam particle molded product) After filling a mold with the foamed particles, a foamed particle molded body can be obtained by supplying a heating medium such as steam into the mold and performing in-mold molding. The density of the molded body is 10 kg / m³ 3 More than 100kg / m 3 The following is preferable. In this case, the lightweight and rigidity of the molded body can be improved in a balanced manner.

[0121] From the perspective of further increasing the rigidity of the molded body, the density of the molded body should be 12 kg / m³. 3 It is more preferable that the amount be greater than or equal to 15 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 lightness of the molded body, the density of the molded body should be 80 kg / m³. 3 More preferably, the following is preferred: 70 kg / m 3 It is even more preferable that the following conditions apply: 60 kg / m 3 The following is particularly 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 then converting the units. If 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.

[0122] Furthermore, from the viewpoint of further improving the rigidity of the molded article, it is preferable that the closed-cell ratio of the molded article be 75% or more, more preferably 80% or more, and even more preferably 85% or more.

[0123] The closed-cell ratio of the molded body is measured according to ASTM 2865-70 Procedure C. Specifically, first, a test specimen measuring 25 mm (length) x 25 mm (width) x 30 mm (height) is cut from the center of the molded body, and the geometric volume Va (unit: cm³) of the test specimen is measured. 3 ), that is, calculate the product of the length (unit: cm), width (unit: cm), and height (unit: cm). Next, in accordance with procedure C described in ASTM-D2856-70, measure the true volume value Vx of the test specimen using an air comparison hydrometer (specifically, the "Beckman Model 1000 Air Comparison Pycnometer" manufactured by Tokyo Science Co., Ltd.). The true volume value Vx obtained by the air comparison hydrometer is the sum of the volume of the resin constituting the sample and the total volume of the closed-cell portions of the sample (unit: cm). 3 )

[0124] The percentage of closed cells in a test specimen (in %) is calculated using the mass W of the test specimen (in g) and the density ρ of the resin constituting the foam particles (in g / cm³). 3 Using the geometric volume Va and true volume Vx of the test specimen obtained by the method described above, the true volume is expressed by the following equation (2). Closed cell ratio = (Vx-W / ρ)×100 / (Va-W / ρ) (2)

[0125] The above procedure is performed on five test specimens, and the closed-cell ratio of each specimen is calculated. The arithmetic mean of the closed-cell ratios of these five test specimens is then taken as the closed-cell ratio of the molded product. [Examples]

[0126] Examples of the foamed particles described above will now be explained. The polypropylene resin used as the raw material for the foamed particles in these examples is as follows.

[0127] (Polypropylene resin (A)) Table 1 shows the properties of the polypropylene resin (A) used in the production of the foamed particles. The polypropylene resin A1 shown in Table 1 is a recycled polypropylene resin mainly composed of polypropylene resin recovered from ASR (more specifically, polypropylene resin A1 is "PLC-A02" manufactured by Planic Co., Ltd.). The shape of polypropylene resin A1 is cylindrical pellets, with an average length in the height direction of the pellets (average length in the extrusion direction during pellet production) of 3 mm, a pellet diameter of 3 mm, and an average pellet mass of 16 mg. In addition, polypropylene resin A1 contains inorganic substances mainly composed of talc. When the morphology of polypropylene resin A1 used in this example was observed using the method described later, polypropylene resin A1 showed a morphology in which polypropylene was the matrix and the rubbery material containing the ethylene-propylene rubber was the domain.

[0128] [Table 1]

[0129] The method for measuring the physical properties of polypropylene resin (A) shown in Table 1 is as follows.

[0130] [Content of ethylene-derived components in the polypropylene resin (A) mentioned above] The ethylene content in polypropylene resin (A) 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 corrected by a predetermined coefficient and the thickness of a film-like test piece.

[0131] More specifically, first, polypropylene resin (A) was heat-pressed at 180°C to form a film, and several test pieces with different thicknesses of 0.1 to 0.3 mm were prepared. Next, the IR spectrum of each test piece was measured to determine the ethylene-derived 722 cm⁻¹ spectrum. -1 , 733cm -1 Absorbance at (A 722 , A 733 ) and 729 cm² derived from the high ethylene content crystal band. -1 Absorbance at (A 729 ) was read. Next, for each test piece, the ethylene component content (unit: mass%) in the polypropylene resin (A) was calculated using the following formulas (3) to (5). The arithmetic mean of the ethylene component content obtained for each test piece was taken as the ethylene component content (unit: mass%) in the polypropylene resin (A).

[0132] (K' 733 ) c =2.33(K' 733 ) a +0.687(K' 722 ) a -2.02(K' 729 ) a ...(3) (K' 722 ) c =1.05(K' 722 ) a +0.275(K' 733 ) a -0.61(K' 729 ) a ...(4) Ethylene content = 0.575{(K' 722 ) c +(K' 733 ) c}···(5)

[0133] However, K' in equations (3) to (5) 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).

[0134] [Morphology of polypropylene resin (A)] The morphology of polypropylene resin (A) was observed using the following method. First, a cylindrical pellet made of polypropylene resin (A) was cut perpendicular to the height direction of the pellet, passing through its center, to prepare an observation sample with the cross-section of the pellet's center exposed. Next, this observation sample was embedded in epoxy resin and electron-stained with ruthenium tetroxide. Then, sections containing the center of the pellet were prepared from the sample using an ultramicrotome or similar device. These sections were placed on the grid of a transmission electron microscope (e.g., JEOL Ltd. "JEM-1040Flash") and observed at a magnification of 5000x, while cross-sectional images (i.e., TEM images) of the polypropylene resin (A) were taken.

[0135] From cross-sectional photographs, the morphology of the polypropylene phase and the rubbery phase containing ethylene-propylene rubber in polypropylene resin (A) was visually observed. As an example, Figure 3 shows a cross-sectional photograph of polypropylene resin A1. In Figure 3, domain D of the rubbery phase containing ethylene-propylene rubber is shown in a relatively dark tone, while the matrix M consisting of polypropylene is shown in a lighter tone relative to domain D.

[0136] Furthermore, the average diameter of the domains in the rubbery material containing ethylene-propylene rubber was calculated based on the TEM images. More specifically, the longest and shortest diameters of 50 domains randomly selected from the domains appearing in the TEM images were measured. The arithmetic mean of the longest and shortest diameters obtained in this way was taken as the average diameter of the domains in the rubbery material containing ethylene-propylene rubber. As a result, the average diameter of the domains in the rubbery material containing ethylene-propylene rubber in polypropylene resin A1 was 1.2 μm.

[0137] Furthermore, the area ratio of the domains in the morphology was calculated based on the TEM images. More specifically, using image analysis software (WinROOF2013 manufactured by Mitani Corporation), the domain areas in the TEM images were monochrome-processed, making the domain areas black and the non-domain areas white, and these were used as the measurement target. Subsequently, the total area occupied by the domains in the measurement target was calculated using the same image analysis software, and the ratio of the total area occupied by the domains to the total area of ​​the measurement target was also calculated. As a result, the area ratio of the domains in the morphology of the polypropylene resin (A) was 28%.

[0138] [Ash content] Approximately 5 g of polypropylene resin (A) was accurately weighed and placed in a crucible. The polypropylene resin (A) in the crucible was heated for 1 hour in an electric furnace set to an ambient temperature of 600°C to burn it. After heating was complete, the mass of the residue remaining in the crucible was measured. The ratio of the mass of the residue to the mass of the polypropylene resin (A) before heating, expressed as a percentage, was defined as the ash content of the polypropylene resin (A) (unit: mass%).

[0139] [Melting point] The melting point of polypropylene resin (A) was determined in accordance with JIS K7121:1987. Specifically, the condition of the test specimen made of polypropylene resin (A) was first adjusted according to "(2) When measuring the melting temperature after performing a certain heat treatment" in "3. Conditioning of test specimens" described in JIS K7121:1987. A DSC curve was obtained by heating the conditioned test specimen from 30°C to 230°C at a heating rate of 10°C / min. The peak temperature of the melting peak that appeared in the DSC curve was defined as the melting point. A differential scanning calorimetry (DSC7020) manufactured by SII Nanotechnology Co., Ltd. was used as the measuring device.

[0140] [Meltflow Rate] The melt flow rate (i.e., MFR) of polypropylene resin (A) was measured in accordance with JIS K7210-1:2014 under conditions of a temperature of 230°C and a load of 2.16 kg.

[0141] [Bending modulus] A 4mm thick sheet was prepared by heat-pressing a polypropylene resin (A) 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.

[0142] (Polypropylene resin (B)) Table 2 shows the properties of the polypropylene resin (B) used in the production of the foamed particles. Note that all polypropylene resins (B) used in this example are composed of non-recycled polypropylene resins. Furthermore, the polypropylene resins (B) used in this example are either propylene-ethylene random copolymers (B1-B3), propylene-ethylene-butene random copolymers (B4), or propylene homopolymers (B5). Also, the polypropylene resins (B) do not contain rubber components. In Table 2, propylene random copolymers are abbreviated as "rPP," and propylene homopolymers are abbreviated as "hPP."

[0143] The measurement methods for the physical properties shown in Table 2 are the same as those for the polypropylene resin (A) described above. The calculation methods for the "ethylene content" and "butene content" of the polypropylene resin (B) in Table 2 are as follows.

[0144] The ethylene and butene content in polypropylene resin (B) was determined by a known method using IR spectroscopy. Specifically, it was determined by the method described in the Polymer Analysis Handbook (edited by the Polymer Analysis Research Group of the Japan Society for Analytical Chemistry, published January 1995, published by Kinokuniya Shoten, page numbers and item names: 615-616 "II.2.3 2.3.4 Propylene / Ethylene Copolymer", 618-619 "II.2.3 2.3.5 Propylene / Butene Copolymer"), that is, by quantitative analysis based on the relationship between the absorbance of ethylene and butene corrected by a predetermined coefficient and the thickness of a film-like test piece.

[0145] More specifically, first, polypropylene resin (B) was heat-pressed at 180°C to form a film, and several test pieces with different thicknesses of 0.1 to 0.3 mm were prepared. Next, the IR spectrum of each test piece was measured to determine the ethylene-derived 722 cm⁻¹ spectrum. -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 (B) was calculated using the following formulas (6) to (8). The arithmetic mean of the ethylene component content obtained for each test piece was taken as the ethylene component content (unit: mass%) in the polypropylene resin (B).

[0146] (K' 733 ) c =1 / 0.96{(K' 733 ) a -0.268(K') 722 ) a}···(6) (K' 722 ) c =1 / 0.96{(K' 722 ) a -0.150(K') 733 ) a}···(7) Ethylene component content = 0.575{(K´ 722 ) c +(K´ 733 ) c}···(8)

[0147] However, K´ a in formulas (6) to (8) is the apparent absorption coefficient at each frequency (K´ a = A / ρt), K´ c is the absorption coefficient after correction, A is the absorbance, ρ is the density of the resin (unit: g / cm 3 ), and t is the thickness of the film-shaped test piece (unit: cm).

[0148] Also, for each test piece, the content of the butene component (unit: mass%) in the polypropylene-based resin (B) was calculated using the following formula (9). The arithmetic mean value of the butene component content obtained for each test piece was taken as the butene component content (unit: mass%) in the polypropylene-based resin (B). Butene component content = 12.3(A 766 / L)···(9) However, A in formula (9) is the absorbance, and L is the thickness of the film-shaped test piece (unit: mm).

[0149]

Table 2

[0150] Next, the structure and manufacturing method of the foamed particles of this example will be described.

[0151] (Example 1) The polypropylene-based resin (A) shown in Table 3, the polypropylene-based resin (B), and the foam regulator were put into an extruder at the mass ratios shown in Table 3 to form a molten mixture containing the polypropylene-based resin (A) and the polypropylene-based resin (B) in the extruder. Zinc borate was used as the foam regulator. The addition amount of zinc borate was 500 mass ppm based on the total of the polypropylene-based resin (A) and the polypropylene-based resin (B).

[0152] Subsequently, the molten mixture was extruded in a strand-like form through small holes in a die located downstream of the extruder. This strand-like extruded material was taken up, cooled, and then cut to an appropriate length using a pelletizer to obtain resin particles.

[0153] The resin particles obtained in this manner were foamed by a direct foaming method. Specifically, first, 1 kg of resin particles were placed in a 5 L container along with 3 L of water as an aqueous medium. Next, 0.3 parts by mass of dispersant, 0.02 parts by mass of sodium alkylbenzenesulfonate and 0.01 parts by mass of aluminum sulfate were added to the container per 100 parts by mass of resin particles as dispersion aids, and the resin particles were dispersed in the aqueous medium. Kaolin was used as the dispersant.

[0154] Subsequently, carbon dioxide was supplied as a foaming agent into the sealed container while stirring the contents, raising the temperature inside the container to 156.2°C. The pressure inside the container at this time was 2.1 MPa(G). The temperature inside the container was then maintained for 15 minutes to impregnate the resin particles with the foaming agent. The container was then opened, and the contents were released into an atmospheric pressure environment, causing the resin particles to foam. These foamed particles were dried for 24 hours in an atmosphere of 23°C and 50% relative humidity. The foamed particles of Example 1 were obtained as described above.

[0155] (Examples 2-3) The methods for producing foamed particles in Examples 2 and 3 are generally the same as those for producing foamed particles in Example 1, except that the mass ratio of polypropylene resin (A) and polypropylene resin (B) and the foaming conditions were changed as shown in Table 3.

[0156] (Examples 4-5) The methods for producing foamed particles in Examples 4 and 5 are generally the same as those for producing foamed particles in Example 1, except that the type of polypropylene resin (B), the mass ratio of polypropylene resin (A) to polypropylene resin (B), and the foaming conditions are changed as shown in Table 3.

[0157] (Comparative Examples 1-2) As shown in Table 4, the foamed particles of Comparative Example 1 and Comparative Example 2 are composed solely of polypropylene resin (A). The manufacturing method for these comparative examples is generally the same as that of Example 1, except that only polypropylene resin A1 is used as the raw material and the foaming conditions are changed as shown in Table 4.

[0158] (Comparative Example 3) The method for producing foamed particles in Comparative Example 3 is generally the same as the method for producing foamed particles in Example 3, except that the type of polypropylene resin (B) and the foaming conditions are changed as shown in Table 4.

[0159] (Comparative Example 4) The method for producing foamed particles in Comparative Example 4 is generally the same as the method for producing foamed particles in Example 1, except that the mass ratio of polypropylene resin (A) and polypropylene resin (B) and the foaming conditions are changed as shown in Table 4.

[0160] (Comparative Example 5) The method for producing foamed particles in Comparative Example 5 is generally the same as the method for producing foamed particles in Example 3, except that the type of polypropylene resin (B) and the foaming conditions are changed as shown in Table 5.

[0161] (Comparative Example 6, Comparative Example 8) The methods for producing foamed particles in Comparative Examples 6 and 8 are generally the same as those for producing foamed particles in Example 1, except that the foaming conditions were changed as shown in Table 5.

[0162] (Comparative Example 7) The method for producing foamed particles in Comparative Example 7 is generally the same as the method for producing foamed particles in Example 3, except that the foaming conditions were changed as shown in Table 5.

[0163] (Reference example 1) The method for producing foamed particles in Reference Example 1 is generally the same as the method for producing foamed particles in Example 3, except that a polypropylene resin (B6) that does not contain ethylene-propylene rubber was used instead of polypropylene resin (A), and the foaming conditions were changed as shown in Table 5.

[0164] The resin particles, foamed particles, and molded bodies obtained by in-mold molding of the foamed particles in this manner were used to evaluate each of the evaluation items shown in Tables 3 to 5. The evaluation methods for these evaluation items are as follows.

[0165] [Melting point difference of polypropylene resins used in the manufacture of resin particles] The melting point difference was calculated by subtracting the melting point of polypropylene resin B) from the melting point of polypropylene resin A) used in the production of the resin particles, and this value is recorded in the "Melting Point Difference (TmA-TmB)" column of Tables 3 to 5.

[0166] [Difference in MFR of polypropylene resins used in the manufacture of resin particles] Melt flow rate (MFR) of polypropylene resin (A) used in the production of resin particles A From polypropylene resin B) melt flow rate MFR B The value obtained by subtracting is the MFR difference, and Tables 3 to 5 show the "MFR difference (MFR A -MFR B I wrote it in the ")" column.

[0167] [Ash content of resin particles and foamed particles] The method for measuring the ash content of resin particles and foamed particles is the same as the method for measuring the ash content of polypropylene resin (A) described above, except that resin particles or foamed particles are used instead of polypropylene resin (A).

[0168] [Morphology of foamed particles] The morphology of the foamed particles was observed by the following method. First, the foamed particles were cut so as to pass through their central portions, and an observation sample in which the cross-section of the central portion of the foamed particles was exposed was prepared. Next, this observation sample was embedded in an epoxy resin, electron staining was performed with ruthenium tetroxide, and then a section including the central portion of the foamed particles was prepared from the sample using an ultramicrotome or the like. This section was placed on a grid of a transmission electron microscope (for example, "JEM-1040Flash" manufactured by JEOL Ltd.), observed at a magnification of 5000 times, and a cross-sectional photograph of the foamed particles (that is, a TEM photograph) was taken.

[0169] From the cross-sectional photograph, the morphology of the polypropylene-based resin phase and the rubber-like phase containing ethylene-propylene rubber in the cell membrane of the foamed particles was visually observed. As an example, Fig. 4 shows a cross-sectional photograph of the foamed particles of Example 3. As shown in Fig. 4, it was confirmed that the cell membrane of the foamed particles had a morphology in which domains D of a rubber-like body containing ethylene-propylene rubber were dispersed in a matrix M of a polypropylene-based resin.

[0170] 〔Bulk density of foamed particles〕 The foamed particles were allowed to stand for 24 hours or more in an environment of 50% relative humidity, 23 °C temperature, and 1 atm pressure to adjust the state of the foamed particles. The foamed particles thus obtained were filled into a graduated cylinder, and the filling height of the foamed particle group in the graduated cylinder was stabilized by gently tapping the floor surface several times at the bottom surface of the graduated cylinder. Then, the bulk volume (unit: L) of the foamed particle group was read from the scale of the graduated cylinder. Then, the bulk density (unit: kg / m 3 ) of the foamed particles was calculated by unit conversion of the value obtained by dividing the mass (unit: g) of the foamed particle group in the graduated cylinder by the above-mentioned bulk volume.

[0171] 〔Apparent density of foamed particles〕 The foamed particle group was left to stand for one day under conditions of 50% relative humidity, 23°C, and 1 atm pressure to adjust its state. After measuring the mass (in grams) of the foamed particle group, it was submerged in a graduated cylinder filled with 23°C water using a wire mesh, and the volume (in liters) of the foamed particle group was determined from the rise in water level. Subsequently, the apparent density (in kg / m³) of the foamed particle group was calculated by converting the value obtained by dividing the mass of the foamed particle group by its volume. 3 ) was calculated.

[0172] [Average bubble diameter of foamed particles] The foam particles were cut into approximately two equal parts. Next, a magnified photograph was taken so that the entire exposed cut surface was within the field of view. On the resulting magnified photograph, four line segments were drawn from the outermost surface of the foam particle, through the center, to the outermost surface on the opposite side, such that the angles between adjacent line segments were equal (i.e., the angles between adjacent line segments were 45°). The value obtained by dividing the total length of these four line segments by the total number of bubbles intersecting the line segments was defined as the bubble diameter of each foam particle.

[0173] The above procedure was performed on 10 or more randomly selected foamed particles, and the arithmetic mean of the resulting bubble diameters for each foamed particle was defined as the average bubble diameter of the foamed particles.

[0174] [Percentage of closed cells in foamed particles] The percentage of closed cells in the foamed particles was measured using an air-comparison hydrometer based on ASTM-D2856-70 Procedure C. Specifically, first, the bulk volume after conditioning was approximately 20 cm³. 3The 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 formula (1). 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. Closed cell ratio = (Vx-W / ρ)×100 / (Va-W / ρ) (1)

[0175] The meanings of the symbols in equation (1) above are as follows: Vx: The true volume of the foamed particle measured by the above method, i.e., the sum of the volume of the resin constituting the foamed particle and the total volume of the closed-cell portion of the foamed particle (unit: cm) 3 ) Va: The apparent volume of foaming particles, measured from the rise in liquid level when the foaming particles are submerged in a graduated cylinder containing ethanol (unit: cm³). 3 ) W: Mass of foamed particles (sample for measurement) (unit: g) ρ: Density of the resin constituting the foam particles (unit: g / cm³) 3 )

[0176] [Temperature, peak height, and full width at half maximum of the endothermic peak] Using the method described above, the peak temperature Ta, peak height ha, and full width at half maximum (FMAX) wa of the first endothermic peak Pa, and the peak temperature Tb, peak height hb, and FMAX wb of the second endothermic peak Pb were measured. Tables 3-5 show the ratio hb / ha (peak height of the second endothermic peak Pb to peak height ha of the first endothermic peak Pa) instead of the peak heights of each endothermic peak. Tables 3-5 also show the difference in peak temperatures Tb-Ta, calculated based on the peak temperature and FMAX of each endothermic peak.

[0177] [Endothermic peak heat of fusion] First, a DSC curve was obtained by performing differential scanning calorimetry under the same conditions as the measurement of the peak temperatures of each endothermic peak. Next, as shown in Figure 1, a straight line L1 was drawn connecting point α, which corresponds to 80°C, and point β, which corresponds to the melting end temperature T of the foamed particles.

[0178] After drawing the straight line L1, a straight line L2 was drawn parallel to the vertical axis of the graph, passing through the maximum point γ located between the first endothermic peak Pa and the second endothermic peak Pb. This line L2 separated the first endothermic peak Pa and the second endothermic peak Pb. The heat of fusion for each endothermic peak was then calculated based on the area of ​​the region enclosed by the parts constituting each endothermic peak in the DSC curve, and the lines L1 and L2. Tables 3 to 5 show the heat of fusion for each endothermic peak, along with the ratio of heat of fusion Qa / Qb and the sum of heat of fusion Qa+Qb calculated based on these values.

[0179] [Melting point, melting termination temperature, and full width at half maximum of the melting peak of the foamed particles] Using a differential scanning calorimetry (DSC7020) manufactured by SII Nanotechnology Co., Ltd., foam particles were heated from 23°C to 200°C at a heating rate of 10°C / min. Subsequently, the foam particles were cooled from 200°C to 23°C at a cooling rate of 10°C / min. After cooling was complete, the foam particles were heated again from 23°C to 200°C at a heating rate of 10°C / min (i.e., a second heating), and a DSC curve was obtained. The peak temperature of the melting peak that appeared in the DSC curve during the second heating was defined as the melting point of the foam particles, and the temperature at the high-temperature end of the melting peak, that is, the temperature at the intersection of the melting peak and the baseline on the higher temperature side of the melting peak in the second DSC curve, was defined as the melting end temperature.

[0180] Furthermore, the full width at half maximum (FMAX) of the melting peak in the second DSC curve was calculated using the same method as the measurement method for the full width at half maximum (FMAX) of the first endothermic peak Pa.

[0181] [Moldable range, minimum molding pressure] In evaluating the moldable range, foam particle molded bodies were produced by performing in-mold molding while varying the molding pressure during heating in increments of 0.01 MPa between 0.26 and 0.46 MPa (G). The minimum molding pressure and moldable range were determined based on the surface properties, fusion properties, and recovery properties of the obtained molded bodies.

[0182] The method for manufacturing a foamed particle molded body is as follows. First, foamed particles are placed in a pressure vessel, and the inside of the vessel is pressurized with an inorganic gas such as air or carbon dioxide to impregnate the foamed particles with the inorganic gas, thereby applying an internal pressure of 0.10 MPa (G) to the foamed particles. Next, the foamed particles with internal pressure are filled into a mold using a cracking filling method. In this example, a mold with a cavity capable of forming a flat foamed particle molded body measuring 250 mm in length, 200 mm in width, and 50 mm in thickness was used. In the cracking filling process, the foamed particles were filled into the mold with a cracking gap of 5 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.

[0183] Next, in-mold molding was performed by supplying steam into the mold. In the in-mold molding process, first, preheating was performed by supplying steam into the mold for 5 seconds with the drain valve of the mold open. Then, the drain valve was closed, and one-sided heating was performed by supplying steam from one side of the mold until the pressure reached 0.08 MPa(G) lower than the molding pressure at the time of main heating. Next, one-sided heating was performed by supplying steam from the other side of the mold until the pressure reached 0.04 MPa(G) lower than the molding pressure at the time of main heating. After that, main heating was performed by supplying steam from both sides of the mold until the molding pressure at the time of main heating was reached. After the main heating was completed, the pressure inside the mold was released, and the molded body was cooled inside the mold until the surface pressure due to the foaming force of the molded body reached 0.04 MPa(G).

[0184] Subsequently, the foam particle molded body removed from the mold was left to cure in an 80°C oven for 12 hours. After the curing process, the foam particle molded body was conditioned by leaving it to cure for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm. The surface properties, fusion properties, and recovery properties of the conditioned foam particle molded body were evaluated, and the molding pressure range in which all items passed according to the evaluation criteria described later (i.e., the molding pressure during this heating that allowed for the acquisition of a passing product) was defined as the moldable range, and the lowest molding pressure within the moldable range was defined as the minimum molding pressure. A wider moldable range and a lower minimum molding pressure indicate superior moldability.

[0185] Furthermore, for Comparative Examples 1-3 shown in Table 4 and Comparative Examples 6-7 shown in Table 5, it was not possible to obtain acceptable products within the aforementioned molding pressure range. Therefore, "None" is written in the "Moldable Range" column of these tables, and the symbol "-" is written in the "Minimum Molding Pressure" column. In addition, no further evaluation of the molded bodies was performed for these comparative examples. Therefore, the symbol "-" is written in the evaluation items for the molded bodies in Comparative Examples 1-3 and Comparative Examples 6-7.

[0186] The evaluation methods for surface properties, fusion properties, and recovery properties in the assessment of the moldable range are as follows.

[0187] ·Superficiality A 100mm x 100mm square was drawn in the center of one skin surface in the thickness direction of the foam particle molded body, and then a diagonal line was drawn from one of the corners of this square. The number of voids located along the diagonal line, that is, gaps formed between foam particles, that have a size of 1mm x 1mm or larger, was counted. The product was judged to pass if there were two or fewer voids, and to fail if there were three or more voids.

[0188] • Fusion properties The foam particle molded body was fractured so that it was divided into roughly equal parts along its longitudinal direction. More than 100 foam particles were randomly selected from the foam particles exposed on the fracture surface and visually observed to determine whether they were foam particles that fractured internally (i.e., foam particles that underwent material failure) or foam particles that fractured at the interface between foam particles. The ratio of the number of foam particles that fractured internally to the total number of foam particles observed was then calculated as a percentage (i.e., material failure rate), and this value was defined as the fusion rate. A fusion rate of 80% or higher was judged as passing, and a rate below 80% was judged as failing.

[0189] ·Recovery In a plan view of the foam particle molded body from the thickness direction, the thickness of the foam particle molded body was measured at four locations 10 mm inward from each vertex towards the center, as well as the thickness of the foam particle molded body at the center. Next, the ratio (in %) of the thickness of the thinnest location to the thickness of the thickest location among the measured locations was calculated. A thickness ratio of 95% or more was judged as passing, and a ratio of less than 95% was judged as failing.

[0190] [Density of the molded body] A foamed particle molded body was obtained by performing in-mold molding at the lowest molding pressure within the aforementioned moldable range. The mass (in g) of this foamed particle molded body was divided by the volume (in L) obtained from the external dimensions of the molded body, and then the density (in kg / m³) of the molded body was calculated by unit conversion. 3 ) was calculated.

[0191] [Closed cell ratio of molded articles] A foamed particle molded body was obtained by in-mold molding at the lowest molding pressure within the aforementioned moldable range. A test specimen measuring 25 mm (length) x 25 mm (width) x 30 mm (height) was cut from the center of this molded body, and the geometric volume Va (unit: cm) of the test specimen was measured. 3 ), that is, the product of the length (unit: cm), width (unit: cm), and height (unit: cm) was calculated. Next, in accordance with procedure C described in ASTM-D2856-70, the true volume value Vx of the test specimen was measured using an air comparison hydrometer (specifically, the "Beckman Model 1000 Air Comparison Pycnometer" manufactured by Tokyo Science Co., Ltd.). The true volume value Vx obtained by the air comparison hydrometer is the sum of the volume of the resin constituting the sample and the total volume of the closed-cell portions within the sample (unit: cm). 3 )

[0192] The mass W (in g) of the test specimen obtained above, and the density ρ (in g / cm³) of the resin constituting the foam particles are given above. 3 Using the geometric volume Va and true volume Vx of the test specimen obtained by the method described above, the closed-cell ratio (unit: %) of the test specimen was calculated using the following formula (2). Closed cell ratio = (Vx-W / ρ)×100 / (Va-W / ρ) (2)

[0193] The above procedure was performed on five test specimens, and the closed-cell ratio of each specimen was calculated. The arithmetic mean of the closed-cell ratios of these five specimens was then defined as the closed-cell ratio (in %) of the molded product.

[0194] [50% compressive strength of the molded body] A foamed particle molded body was obtained by in-mold molding at the lowest molding pressure within the aforementioned moldable range. A rectangular parallelepiped test specimen measuring 50 mm in length, 50 mm in width, and 25 mm in thickness was taken from the center of this molded body. A compression test was performed on the test specimen at a compression rate of 10 mm / min according to the method specified in JIS K7220:2006, and a stress-strain curve was obtained. The compression test was performed in a laboratory at 23°C. Based on this stress-strain curve, the 50% deformation compressive stress of the test specimen was calculated, and this value was taken as the 50% compressive strength of the molded body. Tables 3 to 5 show the values ​​obtained by dividing the 50% compressive strength by density.

[0195] [Balance between molding pressure and rigidity] The balance between molding pressure and stiffness was evaluated based on the value obtained by dividing the 50% compressive strength by the density. The stiffness of polypropylene foam particle molded articles tends to increase with increasing density. Also, as mentioned above, the stiffness of polypropylene foam particle molded articles tends to increase with increasing molding pressure. Therefore, considering the effect of density, the 50% compressive strength / density value is used as an indicator of the balance, and if the 50% compressive strength / density value is higher than the value expected from the molding pressure, it can be determined that the balance between molding pressure and stiffness is good.

[0196] In this example, the value calculated based on the following formula (10) (unit: kPa·m) 3 The value expected from the molding pressure (kPa) was set as the value obtained from the molding pressure, and the balance between molding pressure and stiffness was evaluated based on the comparison result between this value and the 50% compressive strength / density value. In this example, the molding pressure (unit: kPa) in equation (10) below is the molding pressure at which the molded body for which the 50% compressive strength / density value was measured was molded, i.e., the minimum molding pressure. The minimum molding pressure in equation (10) below is an absolute pressure, and the unit of the minimum molding pressure is "kPa". 0.025 × (molding pressure) - 2.75 ···(10)

[0197] The symbols in the "Balance between molding pressure and rigidity" column in Tables 3 to 5 have the following meanings: Excellent: The 50% compressive strength / density value is greater than or equal to the value of formula (10) + 1.0. Good: The 50% compressive strength / density value is greater than or equal to the value of equation (10) and less than the value of equation (10) + 1.0. Poor: The value of 50% compressive strength / density is less than the value in equation (10).

[0198] In evaluating the balance between molding pressure and rigidity, cases where the 50% compressive strength / density value is greater than or equal to the value in equation (10), resulting in the symbols "Excellent" and "Good," were judged to be acceptable because the rigidity was appropriate for the molding pressure. Cases where the 50% compressive strength / density value is less than the value in equation (10), resulting in the symbol "Poor," were judged to be unacceptable because the rigidity was insufficient for the molding pressure.

[0199] [Table 3]

[0200] [Table 4]

[0201] [Table 5]

[0202] As shown in Table 3, the polypropylene resin foam particles of Examples 1 to 5 contain ethylene-propylene rubber and have the closed-cell ratio within the specified range and the specified DSC characteristics. Therefore, these foam particles have good moldability and an excellent balance of rigidity with respect to molding pressure. Furthermore, from the results of Examples 1 to 5, it can be understood that these exceptional effects can be achieved even when using a high-melting-point polypropylene resin containing ethylene-propylene rubber derived from post-consumer materials as the base resin.

[0203] Among these, the foamed particles of Examples 1 and 2 exhibit a superior balance between molding pressure and molded body rigidity compared to the foamed particles of Examples 3-5. Furthermore, the foamed particles of Examples 1 and 2 have superior moldability compared to the foamed particles of Examples 3-5, resulting in a wider moldable range.

[0204] On the other hand, as shown in Table 4, the foamed particles of Comparative Examples 1 and 3 had too small a temperature difference Tb-Ta at the peak of the endothermic peak in the DSC curve. Furthermore, these foamed particles had too large a ratio hb / ha of peak heights for the first endothermic peak Pa in the DSC curve. As a result, the foamed particles of Comparative Examples 1 and 3 had poor moldability, and good molded articles could not be obtained.

[0205] Although the foamed particles of Comparative Example 2 were able to achieve a peak height ratio hb / ha within the specified range by changing the foaming conditions from Comparative Example 1, the temperature difference Tb-Ta at the peak of the endothermic peak was too small. As a result, the foamed particles of Comparative Example 2 had poor moldability, and a good molded article could not be obtained.

[0206] The foamed particles of Comparative Example 4 (shown in Table 4), Comparative Example 5 (shown in Table 5), and Comparative Example 8 had excessively high peak height ratios (hb / ha) in the DSC curve, resulting in the inability to obtain molded articles with sufficient rigidity for the molding pressure. The ratio of heat of fusion (Qa / Qb) in the foamed particles of Comparative Example 4 was similar to that of Example 3. The ratio of heat of fusion (Qa / Qb) is sometimes used as an indicator of the moldability of foamed particles. Therefore, it can be seen that in foamed particles containing ethylene-propylene rubber, the balance of rigidity with respect to molding pressure is only improved when the peak height ratio (hb / ha) is within a predetermined range.

[0207] The foamed particles of Comparative Examples 6 and 7 had poor recovery properties because the peak height ratio hb / ha in the DSC curve was too small, making it impossible to obtain a good molded product.

[0208] Reference Example 1 consists of foamed particles made from a base resin that is mainly composed of homopolypropylene and does not contain ethylene-propylene rubber. Although the peak height ratio hb / ha of the foamed particles in Reference Example 1 is outside the range of the present invention, the balance of stiffness with respect to molding pressure was good. From this, it is considered that the effect of improving the balance of stiffness with respect to molding pressure by setting the peak height ratio hb / ha within the aforementioned specific range is unique to foamed particles containing ethylene-propylene rubber.

[0209] Although specific embodiments of the foamed particles according to the present invention have been described above based on the examples, the specific embodiments of the foamed particles according to the present invention are not limited to those of the examples, and the configuration can be modified as appropriate without impairing the spirit of the present invention. [Explanation of symbols]

[0210] Pa 1st endothermic peak Pb 2nd endothermic peak

Claims

1. Foamed particles composed of a base resin mainly composed of polypropylene resin, The polypropylene resin contains ethylene-propylene rubber, 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 first endothermic peak Pa with the largest peak area and a second endothermic peak Pb adjacent to the first endothermic peak Pa on the high-temperature side of the first endothermic peak Pa. The closed-cell ratio of the foamed particles is 85% or more. The temperature Tb at the peak of the second endothermic peak Pb is 158°C or higher. The difference Tb-Ta between the temperature Tb at the peak of the second endothermic peak Pb and the temperature Ta at the peak of the first endothermic peak Pa is 15°C or more and 30°C or less. Polypropylene resin foam particles in which the ratio hb / ha of the peak height hb of the second endothermic peak Pb to the peak height ha of the first endothermic peak Pa is 1.2 or more and 2.8 or less.

2. The polypropylene resin foam particles according to claim 1, wherein the polypropylene resin is a mixed resin of a polypropylene resin (A) containing ethylene-propylene rubber and a polypropylene resin (B).

3. The polypropylene resin foam particle according to claim 2, wherein the polypropylene resin (A) has a morphology in which homopolypropylene is used as a matrix and rubbery bodies containing the ethylene-propylene rubber are used as domains, and the polypropylene resin (B) is a copolymer of propylene and ethylene and / or butene.

4. The polypropylene resin foam particles according to claim 2, wherein the mass ratio of the polypropylene resin (A) to the polypropylene resin (B) is (A):(B) = 3:97 to 40:

60.

5. The polypropylene resin foam particles according to claim 2, wherein the polypropylene resin (A) is derived from a post-consumer material.

6. The polypropylene resin foam particles according to claim 2, wherein the flexural modulus of the polypropylene resin (B) is 900 MPa or more and 1500 MPa or less.

7. Polypropylene resin foam particles according to any one of claims 1 to 6, wherein the foam particles are heated from 23°C to 200°C at a heating rate of 10°C / min, then cooled to 23°C at a cooling rate of 10°C / min, and then heated again from 23°C to 200°C at a heating rate of 10°C / min, and the melting end temperature in the second DSC curve obtained is 160°C or higher.

8. Polypropylene resin foam particles according to any one of claims 1 to 6, wherein the full width at half maximum of the first endothermic peak Pa is 15°C or more and less than 25°C.

9. Polypropylene resin foamed particles according to any one of claims 1 to 6, wherein the average bubble diameter of the foamed particles is 40 μm or more and 100 μm or less.

10. The apparent density of the foamed particles is 10 kg / m³ 3 More than 80kg / m 3 The polypropylene resin foam particles according to any one of claims 1 to 6, which are as follows:

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