Multilayer foamed particles

Abstract

To provide polypropylene-based resin multilayer foam particles which enable of in-mold molding of a foam particle molding having small variation in dimensional change due to contraction after molding even when water cooling time is shortened after in-mold molding.SOLUTION: There are provided polypropylene-based resin multilayer foam particles, wherein a foam core layer is composed of an ethylene-propylene random copolymer (A) having a specific ethylene component percentage content, a coating layer is composed of a polyolefin-based resin (B), a DSC curve of the polyolefin-based resin (B) has one or more melting peaks, a melting peak temperature of at least one melting peak in the melting peaks is lower than a melting point (Tm) of the random copolymer (A), a weight average molecular weight (Mw) of the random copolymer (A) is within a specific range, and a ratio (Mw / Mn) of the weight average molecular weight (Mw) to a number average molecular weight (Mn) of the ethylene-propylene random copolymer (A) is within a specific range.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to multilayer foamed particles, and more particularly to multilayer foamed particles for in-mold molding. [Background technology]

[0002] Polypropylene resin foam particles have excellent processability, and the foam particle molded products obtained by in-mold molding can be formed into complex shapes. They offer an excellent balance of mechanical strength, cushioning properties, and price, and are therefore used as cushioning packaging materials, automotive components, and building components.

[0003] Examples of such polypropylene resin foam particles include those disclosed in Patent Document 1. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2004-68016 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, when performing in-mold molding at a low vapor pressure using multilayer foamed particles as described in Patent Document 1, it becomes difficult to distribute the vapor evenly throughout the foamed particles in the mold when trying to obtain molded bodies with complex shapes or large sizes, leading to uneven heating. As a result, the degree of shrinkage that occurs after molding differs from part to part of the foamed particle molded body, and dimensional variations tend to occur, leaving problems such as the burden of strictly controlling the dimensions of the molded body becoming too great.

[0006] On the other hand, in an attempt to resolve this uneven heating issue, if the mold is heated sufficiently by using a higher vapor pressure for in-mold molding or by extending the heating time, dimensional variations are reduced, but the cooling time after in-mold molding tends to be longer, and the overall molding time also tends to be longer.

[0007] The present invention aims to provide multilayer foamed particles that enable in-mold molding of foamed particle molded bodies, even when the cooling time after in-mold molding is shortened, and when the variation in dimensional change due to shrinkage after molding is small. [Means for solving the problem]

[0008] According to the present invention, the following multilayer foamed particles are provided. [1] Multilayer foamed particles having a foamed core layer and a coating layer covering the foamed core layer, The foamed core layer is composed of an ethylene-propylene random copolymer (A) with an ethylene component content of 2.5% by mass or more and 3.5% by mass or less. The coating layer is composed of a polyolefin resin (B), and the DSC curve of the polyolefin resin (B), measured by differential scanning calorimetry (DSC), has one or more melting peaks, and the melting peak temperature of at least one of the melting peaks is lower than the melting point (Tm) of the ethylene-propylene random copolymer (A). Multilayer foamed particles characterized in that the weight-average molecular weight (Mw) of the ethylene-propylene random copolymer (A) is 200,000 or more and 300,000 or less, and the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of the ethylene-propylene random copolymer (A) (Mw / Mn) is 3.5 or more and 5 or less. [2] The multilayer foamed particle according to claim 1, wherein the ratio (Mz / Mn) of the Z-average molecular weight (Mz) of the ethylene-propylene random copolymer (A) to the number-average molecular weight (Mn) of the ethylene-propylene random copolymer (A) is 9.5 or more and less than 13. [3] The multilayer foamed particle according to 1 or 2, wherein the melting point (Tm) of the ethylene-propylene random copolymer (A) is 130°C or higher and 150°C or lower. [4] A multilayer foamed particle according to any one of 1 to 3, wherein the crystallization temperature (Tc) of the ethylene-propylene random copolymer (A) and the melting point (Tmc) of the ethylene-propylene random copolymer (A) satisfy the following formula (1). 40°C <Tm-Tc<50(℃)···(1) (However, the units of Tmc and Tc in the formula are both °C.)

Advantages of the Invention

[0009] According to the present invention, it is possible to shorten the cooling time after in-mold forming, and even if the cooling time is shortened, a multi-layer foamed particle capable of in-mold forming a foamed particle molded body with little variation in dimensional change due to shrinkage or the like after molding can be provided.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 is a drawing showing an example of a DSC curve of the first heating by differential scanning calorimetry of heat flux.

Embodiments for Carrying Out the Invention

[0011] Hereinafter, the multi-layer foamed particles of the present invention will be described in detail. The multi-layer foamed particles of the present invention (hereinafter, also simply referred to as foamed particles) have a foamed core layer and a coating layer covering the foamed core layer. Further, the foamed core layer is composed of an ethylene-propylene random copolymer (A) (hereinafter, also referred to as copolymer (A)), and the coating layer is composed of a polyolefin resin (B).

[0012] The foamed core layer is in a foamed state. The foamed state means a state in which a bubble structure is formed. On the other hand, the coating layer may be in a foamed state or a non-foamed state, but it is preferably in a non-foamed state. The non-foamed state means not only a state in which no bubbles exist at all (including a state in which bubbles once formed when the resin particles are foamed are destroyed and the bubbles disappear), but also a state in which very minute bubbles slightly exist within a range that does not affect the mechanical strength of the resulting molded body.

[0013] The foam core layer may be covered by the coating layer in any way, such as the foam core layer being completely covered by the coating layer or a portion of the foam core layer being exposed. An example of a structure in which the foam core layer is exposed is a structure in which only the circumferential surface of a cylindrical foam core layer is covered by the coating layer, and the foam core layer is exposed on the top or bottom surface of the cylinder.

[0014] Next, the ethylene-propylene random copolymer (A) of the foamed core layer will be described. The ethylene content of the copolymer (A) is 2.5% by mass or more and 3.5% by mass or less. When the content is within this range, it has appropriate mechanical strength and exhibits sufficient secondary foaming properties at low vapor pressure, making it suitable for use as foamed particles for in-mold molding, and the resulting foamed particles have an excellent balance of mechanical strength and moldability. If the content is too low, it may become impossible to perform in-mold molding at low vapor pressure. On the other hand, if the content is too high, it may result in excessively reduced rigidity.

[0015] Furthermore, in the copolymer (A), the propylene component content is preferably 95% by mass or more, and preferably more than 96.5% by mass and less than 97.5% by mass. Since the copolymer (A) has propylene as its main component, it is classified as a propylene-based resin.

[0016] In this specification, the ethylene content of ethylene-propylene random copolymer (A) can be determined by a known method using IR spectroscopy. Specifically, it can be 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, pages 615-616, "II.2.3 2.3.4 Propylene / Ethylene Copolymer"), that is, by a method of quantification based on the relationship between the absorbance of ethylene corrected by a predetermined coefficient and the thickness of a film-like test piece.

[0017] The weight-average molecular weight (Mw) of the ethylene-propylene random copolymer (A) is 200,000 to 300,000, preferably 240,000 to 280,000. Copolymers (A) with a weight-average molecular weight (Mw) within this range can be molded at low vapor pressure, and the resulting foam has excellent strength and a good balance between moldability and mechanical strength.

[0018] Furthermore, the ratio (Mw / Mn) of the weight-average molecular weight (Mw) of the ethylene-propylene random copolymer (A) to the number-average molecular weight (Mn) of the ethylene-propylene random copolymer (A) is between 3.5 and 5. If the ratio (Mw / Mn) is within this range, the variation in dimensional changes of the resulting molded article will be small even if the water cooling time is shortened. The reason for this is that crystallization of the resin is more likely to occur in the early stages of water cooling, making it less likely for dimensional changes to occur even if the water cooling time is shortened. Furthermore, variations in secondary foaming are suppressed, preventing localized excessive secondary foaming that would make it difficult for steam to spread evenly throughout the mold. As a result, variations in dimensional changes of the foamed particle molded article after molding are kept small. From this viewpoint, the ratio (Mw / Mn) is preferably 3.7 or more and 4.8 or less, and more preferably 3.8 or more and 4.5 or less.

[0019] Furthermore, the Z-average molecular weight (Mz) of the ethylene-propylene random copolymer (A) is preferably 500,000 to 1,000,000, more preferably 600,000 to 900,000, and even more preferably 650,000 to 750,000. The ratio of the Z-average molecular weight (Mz) to the number-average molecular weight (Mn) (Mz / Mn) is preferably 9.5 or more and less than 13, and more preferably 10 or more and 12 or less. If the ratio (Mz / Mn) is within this range, a foamed particle molded body can be obtained in which the variation in dimensional changes between parts of the molded body due to shrinkage after molding is small, even with a short cooling time.

[0020] Furthermore, the ratio of the Z-average molecular weight (Mz) to the weight-average molecular weight (Mw) (Mz / Mw) is preferably 2 or more and 5 or less, more preferably 2.5 or more and 3.5 or less, and even more preferably 2.6 or more and 3.2 or less.

[0021] The ethylene-propylene random copolymer (A) used in the present invention can be obtained, for example, by controlling the molecular weight by oxidative decomposition of the ethylene-propylene random copolymer with an organic peroxide such as a peroxide, so that the ratio (Mw / Mn) is 3.5 or more and 5 or less.

[0022] In this specification, the number-average molecular weight (Mn), weight-average molecular weight (Mw), and Z-average molecular weight (Mz) can be measured by gel permeation chromatography. Details of the measurement method will be explained in the examples.

[0023] The melting point (Tm) of the ethylene-propylene random copolymer (A) is preferably 130°C or higher and 150°C or lower. If the melting point (Tm) is within this range, the multilayer foamed particles can be molded at a low vapor pressure, while the resulting molded body will have excellent heat resistance.

[0024] In the ethylene-propylene random copolymer (A) used in the present invention, it is preferable that the crystallization temperature (Tc) and the melting point (Tm) satisfy the following equation (1). 40°C <Tm-Tc<50(℃)···(1) (However, the units of Tm and Tc in the formula are both °C.)

[0025] When a foamed core layer is formed using an ethylene-propylene random copolymer (A) whose crystallization temperature (Tc) and melting point (Tm) satisfy equation (1), a good foamed particle molded body can be obtained even when in-mold molding is performed using low-pressure steam. This is thought to be because, during the cooling process in in-mold molding, ethylene-propylene random copolymer (A) that satisfies equation (1) tends to crystallize early, making it less likely for dimensional changes to occur even if the cooling time during molding is shortened.

[0026] For the reasons stated above, it is preferable that the crystallization temperature (Tc) and the melting point (Tm) satisfy equation (2) below, and more preferably that they satisfy equation (3) below. 41°C ≤ Tm - Tc ≤ 48°C ···(2) 42°C ≤ Tm - Tc ≤ 45°C ... (3)

[0027] In this specification, the melting point of copolymer (A) refers to the maximum melting peak temperature of the DSC curve obtained by differential scanning calorimetry (DSC) based on JIS K7121-1987. For conditioning the test specimen, "(2) When the melting temperature is measured after a certain heat treatment" is adopted, and the cooling rate at this time is set to 10°C per minute. The heating rate when measuring the melting temperature is also set to 10°C per minute. If two or more melting peaks appear, the temperature of the peak of the melting peak with the largest area is taken as the melting point. Furthermore, the melting point (Tmf) of foamed particles and multilayer foamed particles can also be measured using the same method as described above. If the melting peak is not clear, the peak position can be determined by referring to the sign and zero point of the differential curve (DDSC) of the DSC near the relevant temperature.

[0028] In this specification, the crystallization temperature refers to the crystallization peak temperature determined by differential scanning calorimetry based on JIS K7121-1987. For conditioning the test specimen, "(2) When the melting temperature is measured after a certain heat treatment" is adopted, and the cooling rate is set to 10°C per minute. If two or more exothermic crystallization peaks appear in copolymer (A), the crystallization temperature shall be the temperature of the peak of the crystallization peak with the largest area. Furthermore, the crystallization temperature (Tcf) of foamed particles and multilayer foamed particles can also be measured using the same method as described above.

[0029] The flexural modulus of the ethylene-propylene random copolymer (A) is preferably 800 MPa to 1100 MPa, and more preferably 820 MPa to 1000 MPa. If the flexural modulus is within this range, a foamed particle molded article with particularly good rigidity can be obtained.

[0030] In this specification, the flexural modulus is determined in accordance with JIS K7171:2008.

[0031] The melt flow rate (MFR) of the ethylene-propylene random copolymer (A) is preferably 1 g / 10 min to 20 g / 10 min, more preferably 2 g / 10 min to 15 g / 10 min, and even more preferably 3 g / 10 min to 10 g / 10 min. If the MFR is within this range, foamed particles having a multilayer structure, in particular, multilayer resin particles that serve as precursors when obtaining foamed particles, can be easily formed. The MFR of the ethylene-propylene random copolymer (A) is the value measured under test condition M (230°C / 2.16 kg load) of JIS K7210:1999.

[0032] The ethylene-propylene random copolymer (A) constituting the foamed core layer in the present invention may contain polymer components other than the copolymer (A), such as thermoplastic resins and thermoplastic elastomers. However, the resin constituting the foamed core layer preferably contains 80% by weight or more of the ethylene-propylene random copolymer (A), and more preferably 90% by weight or more.

[0033] Furthermore, additives such as antistatic agents, catalyst neutralizers, lubricants, nucleating agents, and colorants such as carbon black can be added to the foamed core layer. The amount of additive added depends on the type of additive and its intended use, but is preferably 15 parts by weight or less, more preferably 10 parts by weight or less, even more preferably 5 parts by weight or less, and particularly preferably 3 parts by weight or less, per 100 parts by weight of copolymer (A) constituting the foamed core layer.

[0034] As described above, the multilayer foamed particles of the present invention have a foamed core layer and a coating layer, and the foamed core layer is covered by the coating layer. The coating layer is composed of a polyolefin resin (B), and the DSC curve of the polyolefin resin (B), measured by differential scanning calorimetry (DSC), must have one or more melting peaks, with the melting peak temperature of at least one of these peaks being lower than the melting point of the ethylene-propylene random copolymer (A). When the coating layer satisfies this configuration, the fusion properties of the foamed particles are improved, making in-mold molding possible at a lower molding temperature than that of single-layer foamed particles consisting only of the foamed core layer.

[0035] Here, "melting point of ethylene-propylene random copolymer (A)" refers to the temperature of the peak of the melting peak with the largest area when two or more melting peaks appear on the DSC curve of the ethylene-propylene random copolymer (A) measured by differential scanning calorimetry (DSC). Furthermore, "the DSC curve of the polyolefin resin (B) has one or more melting peaks, and the melting peak temperature of at least one of these melting peaks is lower than the maximum melting peak temperature of the ethylene-propylene random copolymer (A)" means that when one melting peak appears in the DSC curve of the polyolefin resin (B), the melting peak temperature of that melting peak is lower than the maximum melting peak temperature of the ethylene-propylene random copolymer (A). Furthermore, if multiple melting peaks appear in the DSC curve of the polyolefin resin (B), it means that at least the melting peak temperature of the lowest melting peak in the DSC curve of the polyolefin resin (B) is lower than the maximum melting peak temperature of the ethylene-propylene random copolymer (A).

[0036] In the present invention, the relationship between the melting peak temperature of the polyolefin resin (B) and the maximum melting peak temperature of the ethylene-propylene random copolymer (A) is satisfied. Furthermore, by setting the molecular weight of the resin constituting the foamed core layer within a specific range, the fusion properties of the coating layer are improved, while controlling the secondary foaming properties and crystallinity during water cooling in the foamed core layer. This results in foamed particles that are less prone to dimensional variations even when the water cooling time is shortened.

[0037] Furthermore, "possible to mold at a low molding temperature" means that, using the in-mold molding temperature of single-layer foamed particles made of ethylene-propylene random copolymer (A) as a reference, a molded article having similar properties (e.g., surface appearance, fusion properties, recovery properties) to a molded article formed from said single-layer foamed particles can be molded at a molding temperature lower than the reference in-mold molding temperature of said single-layer foamed particles.

[0038] Next, the polyolefin resin (B) will be described. The polyolefin resin (B) preferably contains 50% by mass or more of the olefin component, more preferably 70% by mass or more, and even more preferably 90% by mass or more. There are no particular restrictions on the composition or synthesis method of the polyolefin resin, as long as it contains 50% by mass or more of olefin components. Specifically, examples of the polyolefin resin include polypropylene resins, polyethylene resins, and mixtures thereof.

[0039] Examples of polypropylene-based resins include propylene homopolymers or copolymers of propylene with other olefins copolymerizable with propylene. Examples of other olefins copolymerizable with propylene include ethylene and α-olefins having 4 or more carbon atoms, such as 1-butene. Examples of copolymers include propylene-ethylene random copolymers, propylene-ethylene block copolymers, and propylene-ethylene-butene random copolymers, and further examples include mixed resins of two or more of these.

[0040] Examples of polyethylene-based resins include high-density polyethylene, medium-density polyethylene, low-density polyethylene, linear low-density polyethylene, ultra-low-density polyethylene, and ethylene-vinyl acetate composites, and further examples include mixed resins of two or more of these.

[0041] Furthermore, the coating layer in the present invention may contain polymers other than polyolefin resin (B), such as thermoplastic resins and thermoplastic elastomers, to the extent that they do not hinder the intended purpose, but it is preferable that the amount of the polymer is 5 parts by mass or less per 100 parts by mass of polyolefin resin (B).

[0042] Additives such as antistatic agents, catalyst neutralizers, lubricants, and colorants such as carbon black can be added to the coating layer. The amount of additives added depends on the type of additive and its intended use, but is preferably 15 parts by weight or less, more preferably 10 parts by weight or less, even more preferably 5 parts by weight or less, and particularly preferably 3 parts by weight or less, per 100 parts by weight of the polyolefin resin (B) forming the coating layer.

[0043] Next, the physical properties of the multilayer foamed particles of the present invention will be described. The bulk density of the multilayer expanded particles is preferably 10 kg / m 3 or more and 300 kg / m 3 or less. When the bulk density is within this range, since the expansion ratio is relatively high, the dimensional change after in-mold forming tends to be large and the cooling time tends to be long. Therefore, the effect of shortening the cooling time specified by the present invention becomes more remarkable. In addition, expanded particle molded articles that can be used for various applications can be obtained, including automotive interior members, core materials for automotive bumpers, and core materials for automotive seats. From such a viewpoint, the bulk density of the multilayer expanded particles is preferably 15 kg / m 3 or more and 200 kg / m 3 or less, more preferably 17 kg / m 3 or more and 100 kg / m 3 or less, and even more preferably 19 kg / m 3 or more and 50 kg / m 3 or less.

[0044] The method for measuring the bulk density of the multilayer expanded particles is as follows. First, prepare a 1 L graduated cylinder. While removing the static electricity of the multilayer expanded particles, fill the multilayer expanded particles up to the 1 L mark of the graduated cylinder so as to be in a natural deposition state. The mass of the multilayer expanded particles in this graduated cylinder, that is, the mass of the multilayer expanded particles per 1 L of volume, is converted to the mass per 1 m 3 per volume (kg / m 3 ), whereby the bulk density (kg / m 3 ) of the multilayer expanded particles can be calculated.

[0045] The average cell diameter of the expanded core layer is preferably 20 to 400 μm, more preferably 40 to 250 μm, even more preferably 80 to 200 μm, and particularly preferably 100 to 160 μm. When the average cell diameter is within this range, the multilayer expanded particles have excellent in-mold formability, and the obtained expanded particle molded article has excellent dimensional recovery after molding.

[0046] The average cell diameter of the expanded core layer is measured as follows. Based on magnified photographs taken under a microscope of a cross-section of a foamed core layer that has been roughly divided in two, the following can be determined. First, in the magnified photograph of the cross-section of the foamed core layer, four line segments are drawn from one surface of the foamed core layer to the other surface, passing through approximately the center of the bubble cross-section. However, these line segments should be drawn in such a way that they form radial lines extending in eight equally spaced directions from approximately the center of the bubble cross-section to the surface of the cut particles. Next, the total number of bubbles N intersecting these four line segments is determined. The sum of the lengths of the four line segments L (μm) is determined, and the value obtained by dividing the sum L by the sum N (L / N) is taken as the average bubble diameter of one foamed core layer. This process is performed for 10 foamed core layers, and the arithmetic mean of the average bubble diameters of each foamed core layer is taken as the average bubble diameter of the foamed core layer.

[0047] In the multilayer foamed particles of the present invention, the closed-cell ratio is preferably 75% or more, more preferably 80% or more, even more preferably 82% or more, and particularly preferably 85% or more. If the closed-cell ratio is within this range, the multilayer foamed particles will have excellent secondary foaming properties, and the resulting foamed particle molded article will have good mechanical properties.

[0048] The percentage of closed cells in the multilayer foamed particles is measured as follows: Multilayer foamed particles are left to cure for 10 days in a constant temperature room under atmospheric pressure, relative humidity of 50%, and temperature of 23°C. Next, in the same constant temperature room, a volume of approximately 20 cm³ is used. 3 The multilayer foamed particles after curing are used as measurement samples, and their apparent volume Va is accurately measured by the immersion method as described below. After thoroughly drying the measurement samples from which the apparent volume Va has been measured, the true volume Vx of the measurement samples is measured using a Toshiba Beckmann Corporation air-comparative hydrometer 930, in accordance with procedure C described in ASTM-D2856-70. Then, based on these volumes Va and Vx, the closed-cell ratio is calculated using equation (4) below, and the average value of N=5 is taken as the closed-cell ratio of the multilayer foamed particles.

[0049] Closed cell ratio (%)=(Vx-W / ρ)×100 / (Va-W / ρ)...(4) however, Vx: The true volume of the multilayer foamed particle measured by the above method, i.e., the sum of the volume of the resin constituting the multilayer foamed particle and the total volume of the closed-cell portion within the multilayer foamed particle (cm³). 3 ) Va: The apparent volume (cm³) of multilayer foamed particles measured by the rise in water level when the particles are submerged in a graduated cylinder filled with water. 3 ) W: Weight (g) of the sample used for measuring multilayer foamed particles ρ: Density of the resin constituting the multilayer foamed particles (g / cm³) 3 )

[0050] In the present invention, it is preferable that the DSC curve of the first heating, obtained by temperature rise measurement using differential scanning calorimetry for multilayer foamed particles, shows an endothermic peak intrinsic to polypropylene resin (hereinafter also simply referred to as "intrinsic peak") and a high-temperature peak caused by secondary crystals formed by the temperature treatment during foaming.

[0051] Specifically, it is preferable that the DSC curve (DSC curve for the first heating) obtained when 2 to 10 mg of the multilayer foamed particles are heated from 23°C to 220°C at a heating rate of 10°C / min using differential scanning calorimetry is as shown in Figure 1, and that it has an endothermic peak a (hereinafter also simply referred to as the "intrinsic peak") intrinsic to the polypropylene resin, and one or more endothermic peaks b (hereinafter also simply referred to as the "high-temperature peak") originating from secondary crystals formed by the temperature treatment during foaming on the high-temperature side of the intrinsic peak.

[0052] Figure 1 shows an example of a DSC curve for the first heating. In Figure 1, a is the intrinsic peak, b is the high-temperature peak, α is the point on the DSC curve corresponding to 80°C, and T E α represents the melting termination temperature, which is the temperature at which the high-temperature side of the endothermic peak b returns to the baseline position. β is the point on the DSC curve corresponding to the melting termination temperature TE of the high-temperature peak. γ is the point at the bottom of the valley between the intrinsic peak a and the high-temperature peak b. δ is the point where a line drawn parallel to the vertical axis of the graph from point γ intersects the line (α-β).

[0053] The peak temperature of the intrinsic peak (intrinsic peak temperature) is preferably 135°C to 145°C, and more preferably 138°C to 142°C. If the intrinsic peak temperature is within this range, highly crystalline crystals that contribute to improved rigidity are easily formed by cooling after molding. The melting termination temperature (γ) of the intrinsic peak is the temperature corresponding to the bottom of the valley between intrinsic peak a and high-temperature peak b, and is preferably 150°C or higher and 160°C or lower, and more preferably 152°C or higher and 155°C or lower.

[0054] The heat of fusion of the intrinsic peak is preferably 50 J / g or more and 70 J / g or less, and more preferably 55 J / g or more and 65 J / g or less. Having the heat of fusion of the intrinsic peak within this range improves dimensional stability after molding.

[0055] The heat of fusion of the intrinsic peak is determined as the area enclosed by the straight line (δ-γ) connecting points δ and γ, the straight line (α-β), and the curve of the high-temperature peak a in Figure 1.

[0056] The heat of fusion of the high-temperature peak (hereinafter also simply referred to as the high-temperature peak heat) is preferably 10 J / g or more and 15 J / g or less. Because the high-temperature peak heat value falls within this range, dimensional changes after molding are less likely to occur, and a foamed particle molded article with excellent mechanical strength can be obtained. Furthermore, the high-temperature peak of the foamed core layer can be adjusted by well-known methods, and specifically, such adjustment methods are disclosed, for example, in Japanese Patent Application Publication No. 2001-151928.

[0057] Furthermore, the high-temperature peak b is observed in the DSC curve obtained during the first heating as described above, but not in the DSC curve obtained after the second heating. In the DSC curve obtained during the second heating, only the endothermic peak characteristic of the propylene resin constituting the foamed core layer is observed.

[0058] The heat of fusion at the high-temperature peak can be determined in Figure 1 as the area enclosed by the straight line (δ-γ), the straight line (α-β), and the curve of the high-temperature peak b.

[0059] Furthermore, when 2 to 10 mg of the multilayer foamed particles are heated from 23°C to 220°C at a heating rate of 10°C / min using differential scanning calorimetry, the total melting peak heat of the DSC curve obtained is preferably 50 J / g or more and 90 J / g or less, and more preferably 55 J / g or more and 80 J / g or more. Note that the total melting peak heat is the sum of the melting heat of the high-temperature peak and the melting heat of the intrinsic peak.

[0060] Next, a multilayer foamed particle molded body (hereinafter simply referred to as a foamed particle molded body or molded body) obtained by in-mold molding the multilayer foamed particles will be described. The molded density of the foamed particle molded body is 10 kg / m³ 3 More than 300kg / m 3 The following is preferable, and more preferably, 11 kg / m³ 3 More than 200kg / m 3 More preferably, 12 kg / m 3 More than 100kg / m 3 The following applies. In particular, when the molded article density of the foamed particle molded article is low, the molding time tends to be longer, so it is useful to use the foamed particles of the present invention.

[0061] The compressive stress of the foamed particle molded body at 50% strain is preferably 100 kPa or more and 500 kPa or less, more preferably 120 kPa or more and 300 kPa or less, and even more preferably 140 kPa or more and 200 kPa or less. The compressive stress of the foamed particle molded body at 50% strain can be measured in accordance with JIS K6767:1999.

[0062] The fusion rate of the foamed particle molded article is preferably 70% or higher. In this case, the molded article can fully exhibit desired mechanical properties, such as rigidity, depending on the resin component composition. From the viewpoint of improving this effect, the fusion rate of the molded article is more preferably 80% or higher, and even more preferably 90% or higher.

[0063] Next, a method for producing multilayer foamed particles according to the present invention will be described. The multilayer foamed particles can be obtained by foaming multilayer resin particles, in which a resin particle core layer made of ethylene-propylene random copolymer (A) is coated with a resin particle coating layer made of polyolefin resin (B).

[0064] The multilayer resin particles can be manufactured, for example, as follows: In this apparatus, two extruders, one for the resin particle core layer and the other for the resin particle coating layer, are connected to a co-extrusion die. The required ethylene-propylene random copolymer (A) and additives such as foam regulators as needed are supplied to the resin particle core layer extruder and melt-kneaded to form a resin molten material for forming the resin particle core layer. Meanwhile, the required polyolefin resin (B) and additives as needed are supplied to the resin particle coating layer extruder and melt-kneaded to form a resin molten material for forming the resin particle coating layer. Next, the resin molten material for forming the resin particle core layer is introduced into the co-extrusion die to form a linear flow, and at the same time, the resin molten material for forming the resin particle coating layer is introduced into the co-extrusion die, so that the resin molten material for forming the resin particle coating layer is layered around the linear flow of the resin molten material for forming the resin particle core layer, forming a multilayer resin molten material. Multilayer resin particles can be produced by extruding this multilayer resin molten material in the form of one or more strands through small holes in a die attached to the outlet of an extruder, cooling the strands by passing them through water and then cutting them to an appropriate length, or by cutting and cooling them simultaneously with extruding them from the die into water. In this specification, the multilayer structure formed in this manner may be referred to as a "sheath core" structure.

[0065] A method for producing multilayer resin particles using the aforementioned co-extrusion die is described in detail in, for example, Japanese Patent Publication No. 41-16125, Japanese Patent Publication No. 43-23858, Japanese Patent Publication No. 44-29522, and Japanese Patent Publication No. 60-185816.

[0066] It is preferable to add a bubble regulator to the resin particle core layer in order to adjust the bubble diameter of the foamed core layer. Examples of the bubble regulator include inorganic substances such as talc, calcium carbonate, borax, zinc borate, aluminum hydroxide, and alum. The amount added is preferably 0.001 to 10 parts by weight, and more preferably 0.01 to 5 parts by weight, per 100 parts by weight of copolymer (A) that forms the resin particle core layer. Furthermore, when adding a foam regulator to copolymer (A), the foam regulator can be added directly, but it is generally preferable to add it as a foam regulator masterbatch, taking into consideration dispersibility and other factors.

[0067] In the present invention, the weight of the multilayer resin particles is preferably 0.02 to 20 mg, and more preferably 0.1 to 6 mg, in order to ensure uniform filling of the multilayer foamed particles into the mold.

[0068] The multilayer foamed particles in the present invention can be manufactured using the multilayer resin particles by, for example, a so-called dispersion medium release foaming method. In this dispersion medium release foaming method, the multilayer resin particles are dispersed in a dispersion medium such as water in a sealed container such as an autoclave together with a physical blowing agent, the dispersion medium is heated to a temperature above the softening temperature of the resin particles, and the blowing agent is impregnated into the resin particles. Next, while maintaining the pressure inside the sealed container at a pressure above the vapor pressure of the blowing agent, one end of the sealed container below the water surface is opened, and the foamable multilayer resin particles containing the blowing agent are released from the sealed container together with the dispersion medium such as water into an atmosphere at a lower pressure than the pressure inside the sealed container, usually under atmospheric pressure, to foam and obtain multilayer foamed particles. Alternatively, the foamable multilayer resin particles containing the physical blowing agent may be removed from the sealed container and heated with a heating medium such as steam to foam them. On the other hand, when producing multilayer resin particles with the extruder, foamed particles can also be obtained by injecting a foaming agent into an extruder for forming a resin particle core layer to form a foamed molten resin composition, laminating the foamed molten resin composition with a molten resin for forming the resin particle coating layer to form a molten resin composition with a sheath core structure, extruding the molten resin composition with a sheath core structure from a die to foam it, and then cutting it into granular pieces.

[0069] In the dispersion medium discharge foaming method, the pressure difference between the high-pressure and low-pressure conditions when discharging from a high-pressure condition where foaming does not occur to a low-pressure condition where foaming occurs is preferably 400 kPa or more, more preferably 500 to 15000 kPa.

[0070] Examples of blowing agents used in the dispersion medium release foaming method include organic physicoblasting agents such as propane, isobutane, n-butane, isopentane, n-pentane, cyclopentane, n-hexane, cyclobutane, cyclohexane, chlorofluoromethane, trifluoromethane, 1,1,1,2-tetrafluoroethane, 1-chloro-1,1-difluoroethane, 1,1-difluoroethane, 1-chloro-1,2,2,2-tetrafluoroethane, and hydrofluoroolefins, as well as inorganic physicoblasting agents such as nitrogen, carbon dioxide, argon, and air. Among these, inorganic physicoblasting agents that do not deplete the ozone layer and are inexpensive are preferred, with nitrogen, air, and carbon dioxide being particularly preferred. Furthermore, two or more of these blowing agents can be used in combination.

[0071] The amount of blowing agent used is appropriately selected according to the relationship between the bulk density of the multilayer foamed particles to be obtained and the foaming temperature. Specifically, in the case of the aforementioned blowing agents, excluding nitrogen and air, the amount of blowing agent used is usually 2 to 50 parts by weight per 100 parts by weight of resin particles. In the case of nitrogen and air, the amount used is such that the pressure inside the sealed container is within the pressure range of 1 to 5 MPa(G), preferably within the pressure range of 1.5 to 3.5 MPa(G).

[0072] In a sealed container, water is preferred as the dispersion medium for dispersing resin particles, but any medium that does not dissolve the resin particles can be used. Examples of such dispersion media include ethylene glycol, glycerin, methanol, and ethanol.

[0073] The average bubble diameter is controlled by the type and amount of blowing agent, the foaming temperature, and the amount of foam regulator added. The bulk density (foaming ratio) is controlled by the amount of blowing agent added, the foaming temperature, and the differential pressure during foaming. Within a reasonable range, generally, the more blowing agent added, the higher the foaming temperature, and the greater the differential pressure, the lower the bulk density of the resulting foamed particles.

[0074] When dispersing resin particles in a dispersion medium in a sealed container and heating to the foaming temperature, an anti-fusing agent can be used to prevent the multilayer resin particles from fusing together. Any anti-fusing agent that does not dissolve in water or other liquids and does not melt when heated can be used, regardless of whether it is inorganic or organic, but inorganic agents are generally preferred.

[0075] Suitable inorganic anti-fusing agents include powders such as kaolin, talc, mica, aluminum oxide, titanium oxide, and aluminum hydroxide. The anti-fusing agent has an average particle size of 0.001 to 100 μm, and is particularly preferably 0.001 to 30 μm. The amount of anti-fusing agent added is usually 0.01 to 10 parts by weight per 100 parts by weight of resin particles.

[0076] Furthermore, anionic surfactants such as sodium dodecylbenzenesulfonate and sodium oleate, as well as aluminum sulfate, are preferably used as dispersing aids. It is generally preferable to add 0.001 to 5 parts by weight of the dispersing aid per 100 parts by weight of resin particles.

[0077] When producing multilayer foamed particles with low bulk density, it is preferable to produce the multilayer foamed particles by the dispersion medium release foaming method, and then perform so-called two-stage foaming, which involves further foaming the obtained multilayer foamed particles. In two-stage foaming, the multilayer foamed particles are filled into a pressurized sealed container, and the internal pressure of the multilayer foamed particles is increased to 0.01 to 0.6 MPa (G) by pressurizing with a gas such as air. After that, the multilayer foamed particles are removed from the container and heated using a heating medium such as steam, thereby easily obtaining multilayer foamed particles with low bulk density.

[0078] A foamed particle molded article can be obtained by conventionally known methods, which involve filling a mold with multilayer foamed particles and then heat-molding it using a heating medium such as steam. Specifically, after filling the mold with multilayer foamed particles, steam is introduced into the mold to heat and foam the multilayer foamed particles, causing them to fuse together and resulting in a molded article with the shape of the molded space already formed. Furthermore, as a method for obtaining a molded body with a low molded body density, if necessary, a pressurization operation similar to the operation in the two-stage foaming described above is performed to increase the pressure within the multilayer foamed particles, adjusting the internal pressure within the multilayer foamed particles to 0.01 to 0.2 MPa (G), and then in-mold molding is performed.

[0079] When performing in-mold molding using the multilayer foamed particles of the present invention, the molding vapor pressure is preferably 0.05 to 0.46 MPa (G), more preferably 0.07 to 0.35 MPa (G), and even more preferably 0.09 to 0.32 MPa (G).

[0080] In the present invention, the desired molded article can also be obtained by employing a method in which multilayer foamed particles are filled into a mold so that the compressibility is 4 to 25 volume%, preferably 5 to 20 volume%, and then molded in the mold using steam.

[0081] The compression ratio is adjusted by cracking the foam particles when filling the mold cavity with them, by using a method called cracking. Cracking is a filling method in which the mold is not completely closed during the filling process to allow air to be exhausted from inside the mold and to efficiently fill the mold with multilayer foam particles. Cracking refers to the open portion of the mold, which is ultimately closed when steam is introduced after the multilayer foam particles have been filled into the mold, resulting in the compression of the filled multilayer foam particles. [Examples]

[0082] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples.

[0083] Table 1 shows the types and properties of ethylene-propylene random copolymers used as raw materials in the examples and comparative examples.

[0084] [Table 1]

[0085] The types and properties of the ethylene-propylene random copolymer used as the polyolefin resin (B) in the examples and comparative examples are shown below. Polyolefin resin (B): Ethylene-propylene random copolymer, ethylene component content (3.0% by mass), melting point 134°C, flexural modulus 770 MPa, MFR 7 g / 10 min (230°C / 2.16 kg load).

[0086] In Table 1, the melting point (Tm), crystallization temperature (Tc), MFR, molecular weight Mn, molecular weight Mw, and molecular weight Mz were measured by the method described above.

[0087] [Measurement of melting point, heat of fusion, and crystallization temperature] The melting point and crystallization temperature of copolymer (A) and resin (B) were determined in accordance with JIS K7121:1987. Specifically, the melting point was determined by the temperature at the peak of the endothermic peak obtained from the DSC curve, which was obtained by heating a 2 mg pellet sample from 30°C to 200°C at a heating rate of 10°C / min, then cooling it down to 30°C at a cooling rate of 10°C / min, and then heating it again from 30°C to 200°C at a heating rate of 10°C / min, based on the differential scanning calorimetry method described in JIS K7121:1987. The heat quantity of the endothermic peak obtained from the DSC curve was defined as the heat of fusion. Furthermore, the crystallization temperature is a value measured in accordance with JIS K7121 (1987). The crystallization temperature (°C) was defined as the peak temperature of the crystallization heat peak in the DSC curve obtained when a sample melted by heating from 30°C to 200°C at a heating rate of 10°C / min and then cooled down to 30°C at a cooling rate of 10°C / min. The measurement device used was a differential scanning calorimetry system (DSC.Q1000, manufactured by T.A. Instruments).

[0088] (MFR (Melt Flow Rate)) The MFR of the ethylene-propylene random copolymer (A) and polyolefin resin (B) was measured under test condition M (230°C / 2.16 kg load) of JIS K7210:1999.

[0089] [Method and conditions for measuring molecular weight] Sample preparation: 30 mg of the sample was added to 20 mL of o-dichlorobenzene and shaken at 145°C to dissolve. The solution was then thermally filtered through a sintered filter with a pore size of 1.0 μm to obtain the analytical sample, which was measured under the following conditions. Measurement device: HLC-8321GPC / HT type high-temperature gel permeation chromatograph (manufactured by Tosoh Corporation) Analysis equipment: Data processing software Empower3 (manufactured by Waters Japan Co., Ltd.) Columns: 2 x TSKgel GMH6-HT, 2 x TSKgel GMH6-HTL (each with an inner diameter of 7.5 mm and a length of 300 mm, manufactured by Tosoh Corporation) Mobile phase: o-dichlorobenzene (containing 0.025% BHT) Column temperature: 140℃ Detector: Differential refractometer (RI) Flow rate: 1.0mL / min Sample concentration: 0.15% (W / V)-o-dichlorobenzene Injection volume: 400μL Sampling time interval: 1 second Column calibration: Monodisperse polystyrene (manufactured by Tosoh Corporation) Molecular weight conversion: Polypropylene (PP) conversion / General calibration method Table 1 shows the molecular weight of the raw materials before the production of foamed particles. Since significant fluctuations in molecular weight are not expected during the foamed particle production process, it is also possible to measure the molecular weight by removing the foamed core layer and performing the above procedure.

[0090] In Table 1, the ethylene content (wt%) of copolymer (A) and copolymer (B) was measured by the method described above.

[0091] (Ethylene content) The ethylene content of the polypropylene resin was determined by a known method using IR spectroscopy. Specifically, first, the polypropylene resin was hot-pressed at 180°C to form a film, and several test pieces of different thicknesses were prepared. Then, by measuring the IR spectrum of each test piece, the ethylene-derived 722 cm⁻¹ was determined. -1 and 733cm -1 The absorbance (A722, A733) was read. Next, the ethylene content in the polypropylene resin was calculated for each test specimen using the following equations (5) to (7). The arithmetic mean of the ethylene content obtained for each test specimen was defined as the ethylene content in the polypropylene resin (unit: mass%). (K ´ 733)c=1 / 0.96{(K ´ 733)a-0.268(K ´ 722)a}··(5) (K´ 722)c=1 / 0.96{(K ´ 722)a-0.268(K ´ 722)a}··(6) Ethylene content (%) = 0.575 {(K ´ 722)c+(K ´ 733)c}···(7) However, in equations (5) to (7), K ´ a: Apparent absorption coefficient at each wavenumber (K'a = A / ρt), K ´ c: corrected absorption coefficient, A: absorbance, ρ: resin density (unit: g / cm3), t: thickness of the film-like test piece (unit: cm).

[0092] The flexural modulus of the resin was measured according to JIS K 7171 (2008). The measurement was performed using a Tensilon universal tester RTF-1350 manufactured by A&D Company, Limited.

[0093] The bubble regulator used was Borax's zinc borate product, "Fire Break ZB".

[0094] Carbon dioxide was used as a physical foaming agent.

[0095] Examples 1-4, Comparative Examples 1-3 [Manufacturing of multilayer resin particles] An apparatus was used in which a die for forming multilayer strands was attached to the outlet side of an extruder for a resin particle core layer with an inner diameter of 50 mm and an extruder for a resin particle coating layer with an inner diameter of 30 mm. The types and amounts of ethylene-propylene random copolymer (A), carbon masterbatch, and foam regulator masterbatch shown in Table 2 were supplied to an extruder for the resin particle core layer with an inner diameter of 50 mm. Simultaneously, the types and amounts of propylene resin (B) as a polyolefin resin, also shown in Table 2, were supplied to an extruder for the resin particle coating layer with an inner diameter of 30 mm. Each was heated, melted, and kneaded at a set temperature of 200-220°C, then supplied to the die, where they were combined in a sheath-core structure (mass ratio of resin particle core layer to resin particle coating layer of 95:5). The mixture was then co-extruded through the pores of a die attached to the tip of the extruder as a multilayer strand in which the coating layer was applied to the sides of the core layer. The co-extruded strand was water-cooled by passing it through water, and then cut in a pelletizer to obtain cylindrical multilayer resin particles (1.2 mg, L / D=2.9) formed in two layers (sheath-core structure) (the coating layer was formed on the circumferential surface of the cylinder relative to the cylindrical core). The weight and L / D ratio of the multilayer resin particles are arithmetic mean values ​​obtained from 100 multilayer resin particles randomly selected from the group of multilayer resin particles.

[0096] [Table 2]

[0097] Examples 1-4, Comparative Examples 1-3 [Manufacturing of multilayer foamed particles] 20 kg of the obtained multilayer resin particles were placed in a 100 L autoclave along with 60 L of water as a dispersion medium. 15 g of kaolin was added to the dispersion medium as a dispersant, 12 g of sodium alkylbenzenesulfonate and 3 g of aluminum sulfate as dispersion aids. Carbon dioxide was injected into a sealed container as a foaming agent to achieve an internal pressure of 2.2 MPa(G). The mixture was heated and stirred to a foaming temperature of 150°C and maintained at that temperature for 15 minutes to adjust the high-temperature peak heat. The contents of the autoclave were then released with water under atmospheric pressure to obtain multilayer foamed particles.

[0098] The multilayer foamed particles obtained in this manner have a foamed core layer made of a polypropylene resin and a coating layer made of a polyolefin resin that covers the foamed core layer.

[0099] In Table 2, the bulk density of the multilayer foamed particles was measured by the method described above.

[0100] In Table 2, the high-temperature peak heat of fusion was measured using the method described above.

[0101] [Manufacturing of foamed particle molded products] A large mold with a rectangular parallelepiped-shaped molding cavity having internal dimensions of 600 mm in the vertical direction, 1250 mm in the horizontal direction, and 50 mm in the thickness direction was used as the molding die. With the mold completely closed, it was opened 5 mm (resulting in a molding cavity thickness of 55 mm), and multilayer foamed particles were filled into the molding cavity. After filling was complete, the mold was completely closed (cracking amount 5 mm, 10%). Then, steam was supplied into the molding cavity to heat the multilayer foamed particles, causing secondary foaming and fusion of the particles to form a foamed particle molded body. The heating method involved preheating (exhaust process) by supplying steam for 5 seconds with the drain valves on both sides of the mold open, followed by heating on one side at a pressure 0.08 MPa (G) lower than the molding vapor pressure listed in Table 2, and then heating on the other side from the opposite direction at a pressure 0.04 MPa (G) lower than the molding vapor pressure listed in Table 2. Finally, both sides were heated (main heating) at the molding vapor pressure shown in Table 2. After heating was complete, the pressure was released, and the mold was water-cooled until the surface pressure of the foamed particle molded body inside the mold reached 0.04 MPa (G). The mold was then opened, and the foamed particle molded body was removed from the mold. The obtained foamed particle molded body was cured for 12 hours under atmospheric pressure and at a temperature of 80°C. The water cooling time was defined as the time it took to cool the foamed particle molded body in the mold until the surface pressure reached 0.04 MPa(G) after the completion of the main heating. The physical properties of the obtained foamed particle molded body are shown in Table 2. The molding vapor pressure shown in Table 2 represents the lowest molding vapor pressure at which a molded body with a good appearance can be obtained. The criteria for a molded body with a good appearance were defined as having a fusion rate of 80% or more and showing almost no gaps or depressions on the surface of the molded body.

[0102] In Table 2, the density of the molded body, shrinkage rate, deviation from the reference dimensions, and compressive stress at 50% strain were measured as follows.

[0103] [Molded density of foam particle molded body] The weight of the foamed particle molded body is divided by the volume obtained from the external dimensions of the foamed particle molded body, and the value is calculated as [kg / m 3 The units were converted to [ ].

[0104] [Fusion rate] The fusion rate was measured using the following method. Specifically, test specimens measuring 150 mm in length, 75 mm in width, and 20 mm in thickness were randomly cut from the foam particle molded body (leaving at least one skin surface intact). A 5 mm deep cut was made across the entire width of this rectangular molded sample in the longitudinal center of one surface (leaving at least one skin surface without the cut intact), and this was designated as the test specimen. Next, the test specimens were placed on two support plates, each 100 mm high, 80 mm or wider, and 10 mm thick, which were erected parallel to each other with a distance of 70 mm between their centers and rounded upper ends to a radius of 5 mm. The test specimens were positioned with the cut side facing downwards and evenly straddling the support plates so that the length of the specimen was perpendicular to the length of the support plates. Next, a pressure plate made of a rigid body with a tip rounded to a radius of 5 mm, measuring 60 mm in height, 80 mm in width, and 10 mm in thickness, was used. The test specimen was set so that the center of the pressure plate in the thickness direction coincided with the notch in the specimen. A three-point bending test was then performed from the opposite side of the notch in the specimen, with the pressure plate applied at a pressing speed of 200 mm / min. The test specimen was pressed until it fractured or until it came off the support plates and was completely inserted between them. Next, the fracture surface of the test specimen was observed, and the number of foam particles that fractured internally and the number of foam particles that detached at the interface were counted visually. Then, the ratio of foam particles that fractured internally to the total number of foam particles that fractured internally and foam particles that detached at the interface was calculated and expressed as a percentage to determine the fusion rate (%). Naturally, the foam particles that were initially present on the 5mm cut were not counted. Furthermore, when focusing on a single foam particle on the fracture surface of the test specimen, if it includes both a fractured portion and a portion separated between foam particles, the area was considered. If the area of ​​the fractured portion was 50% or more, it was counted as a fractured particle; if the area of ​​the fractured portion was less than 50%, it was counted as a portion separated between foam particles. In addition, if the test specimen was not completely fractured as a result of this test, the entire unfractured portion was considered fractured, and the unfractured portion was cut perpendicularly with a knife (in the thickness direction of the test specimen). The number of foam particles present on the cut surface was counted as the number of foam particles that fractured internally, and the fusion rate (%) was calculated as described above. The fusion rate of the obtained foam particle molded bodies was all 90% or higher.

[0105] [Shrinkage rate] The shrinkage rate [%] of the foam particle molded body was calculated using the formula: (Longest side dimension of the molding die [mm] - Longest side length of the molded body [mm]) / Longest side dimension of the molding die [mm] × 100. The "Longest side length of the molded body [mm]" refers to the length of the longest side of the foam particle molded body obtained in the examples and comparative examples after curing in an 80°C atmosphere for 12 hours, slow cooling, and further curing in a 23°C atmosphere for 6 hours (n=10).

[0106] [Deviation rate from standard dimensions] In a mold having a rectangular molding cavity with internal dimensions of 600 mm in the vertical direction, 1250 mm in the horizontal direction, and 50 mm in the thickness direction, four reference points were established at each vertex of the rectangular cavity. Meanwhile, the longitudinal deviation between each point of the foam particle molded body at the corresponding location from these reference points was measured. The value obtained by dividing the longitudinal dimensional difference of each point by the longitudinal length of the foam particle molded body was defined as the deviation rate from the reference dimensions.

[0107] [Compressive stress at 50% strain] A test specimen measuring 50 mm (length) x 50 mm (width) x 25 mm (thickness) was cut out after removing the skin, and a compression test was performed at a compression speed of 10 mm / min according to JIS K6767-1999 to determine the 50% compressive stress of the foam particle molded body. The test was performed on 10 test specimens, and the average value obtained was taken as the compressive stress at 50% strain.

[0108] Examples 1 to 4 are examples of producing multilayer foamed particles using copolymer (A) that satisfies the configuration of the present invention. In-mold molding is possible at low vapor pressure, and the water cooling time in the cooling process is shorter than that of the comparative example, demonstrating the effect of reducing water cooling time. In addition, the shrinkage rate and the deviation from the reference dimensions are smaller compared to the comparative example. Reference Example 1 is an example in which single-layer foamed particles were produced in the same manner as in Example 1, except that the same copolymer (A) as in Example 1 was used and no coating layer was provided. The molding vapor pressure was higher than in Example 1, and the shrinkage rate and the deviation from the reference dimensions were larger compared to Example 1. Reference Example 2 is an example in which single-layer foamed particles were produced in the same manner as Comparative Example 1, except that a coating layer was not provided, using the same ethylene-propylene random copolymer as in Comparative Example 1. The molding vapor pressure was higher in Comparative Example 2 than in Comparative Example 1, and the deviation from the standard dimensions was larger compared to Comparative Example 1. Furthermore, comparing Example 1 with Comparative Example 1 against Reference Example 1 and Reference Example 2, it can be seen that the effect of reducing the water cooling time of Example 1 compared to Comparative Example 1 is significantly higher than the effect of reducing the water cooling time of Reference Example 1 compared to Reference Example 2. Moreover, it can be seen that the multilayer foamed particles are superior in terms of the deviation rate from the standard dimensions. These effects are considered to be unique to multilayer foamed particles. [Explanation of Symbols]

[0109] a. Intrinsic peak b. High temperature peak Point on the α-DSC line corresponding to 80°C T E Melting termination temperature β Melting end temperature T E A point on the DSC curve corresponding to this point γ is the point at the bottom of the valley between the intrinsic peak a and the high-temperature peak b. δ is the point where the line parallel to the vertical axis of the graph intersects with the line (α-β) from point γ.

Claims

1. A multilayer foamed particle having a foamed core layer and a coating layer covering the foamed core layer, The foamed core layer is composed of an ethylene-propylene random copolymer (A) with an ethylene component content of 2.5% by mass or more and 3.5% by mass or less. The melting point (Tm) of the ethylene-propylene random copolymer (A) is 130°C or higher and 150°C or lower. The coating layer is composed of a polyolefin resin (B), and the DSC curve of the polyolefin resin (B), measured by differential scanning calorimetry (DSC), has one or more melting peaks, and the melting peak temperature of at least one of the melting peaks is lower than the melting point (Tm) of the ethylene-propylene random copolymer (A). Multilayer foamed particles characterized in that the weight-average molecular weight (Mw) of the ethylene-propylene random copolymer (A) is 200,000 or more and 300,000 or less, and the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of the ethylene-propylene random copolymer (A) (Mw / Mn) is 3.5 or more and 5 or less.

2. The multilayer foamed particle according to claim 1, wherein the ratio (Mz / Mn) of the Z-average molecular weight (Mz) of the ethylene-propylene random copolymer (A) to the number-average molecular weight (Mn) of the ethylene-propylene random copolymer (A) is 9.5 or more and less than 13.

3. The multilayer foamed particle according to claim 1 or 2, wherein the crystallization temperature (Tc) and the melting point (Tm) of the ethylene-propylene random copolymer (A) satisfy the following formula (1). 40(℃)<Tm-Tc<50(℃) (1) (However, the units of Tmc and Tc in the formula are both °C.)

Images