Method for producing polyolefin resin foam particles

Abstract

To provide a method for producing polyolefin-based resin foam particles that can produce polyolefin-based resin foam particles which are have small variations in particle diameters and have excellent moldability.SOLUTION: A method for producing polyolefin-based resin foam particles includes: a dispersion step of dispersing polyolefin-based resin particles in an aqueous medium; a foaming agent addition step of adding a foaming agent into a sealed container; and a foaming step of discharging the resin particles immersed with the foaming agent in the sealed container from the sealed container together with the aqueous medium, foaming the resin particles and thereby producing foam particles having an apparent density of 50-300 kg / m3. The foaming agent contains hydrofluoroolefin and nitrogen. The addition amount of the hydrofluoroolefin in the foaming agent addition step is 1-12 pts.mass with respect to 100 pts.mass of the resin particles. In the foaming step, the pressure in the sealed container immediately before foaming is adjusted to a range of 1.8-4.5 MPa (G).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a method for producing polyolefin-based resin foam particles. [Background technology]

[0002] Polyolefin resin foam particle molded articles, which are produced by in-mold molding of polyolefin resin foam particles, are used in a variety of applications such as packaging materials, automotive components, and building materials. Polyolefin resin foam particle molded articles are manufactured by a method called in-mold molding, in which polyolefin resin foam particles are filled into a mold, and then heated by supplying a heating medium such as steam into the mold. When a heating medium 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, and a molded article with a shape corresponding to the shape of the mold cavity can be obtained.

[0003] Polyolefin resin foam particles are manufactured, for example, by impregnating polyolefin resin particles dispersed in an aqueous medium with a foaming agent in a sealed container, and then releasing the foamed polyolefin resin particles together with the aqueous medium into an environment with a pressure lower than the pressure inside the sealed container. This foaming method is sometimes called the "direct foaming method."

[0004] For example, Patent Document 1 describes a method for pre-foaming polyolefin resin foam particles using a direct foaming method with a chlorofluoromethane or other CFC as a foaming agent. However, CFCs, when released into the atmosphere, cause ozone layer depletion. Furthermore, because CFCs have a high global warming potential, their release into the atmosphere may accelerate global warming.

[0005] Therefore, in recent years, as a substitute for Freon, hydrofluoroolefin (HFO) has been proposed, which has excellent properties of Freon while reducing the adverse impact on the environment. For example, Patent Document 2 describes that in order to obtain a foam of a thermoplastic resin with excellent heat insulation properties, a foam was produced by foaming a thermoplastic resin using a foaming agent containing 1-chloro-2,3,3,3-tetrafluoropropene.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, when hydrofluoroolefin is used as a foaming agent and polyolefin resin particles are foamed by the direct foaming method, the variation in the particle diameter of the obtained foamed particles tends to be large. Such a problem was particularly prominent when producing polyolefin resin particles with a high apparent density.

[0008] The present invention has been made in view of such a background, and even when producing foamed particles with a high apparent density by the direct foaming method using hydrofluoroolefin as a foaming agent, it is intended to provide a method for producing polyolefin resin foamed particles that can produce polyolefin resin foamed particles with small variation in particle diameter and excellent moldability.

Means for Solving the Problems

[0009] One aspect of the present invention relates to a method for producing polyolefin resin foamed particles according to the following [1] to [6].

[0010] [1] A dispersion step of dispersing polyolefin resin particles in an aqueous medium, A foaming agent addition step in which a foaming agent is added to a sealed container, After impregnating the polyolefin resin particles with the foaming agent in the sealed container, the polyolefin resin particles are released from the sealed container together with the aqueous medium, causing the polyolefin resin particles to foam, resulting in an apparent density of 50 kg / m³. 3 More than 300kg / m 3 The following foaming process for producing polyolefin-based resin foam particles is included: The aforementioned blowing agent comprises hydrofluoroolefin and nitrogen. The amount of hydrofluoroolefin added in the foaming agent addition step is 1 part by mass or more and 12 parts by mass or less per 100 parts by mass of the polyolefin resin particles. A method for producing polyolefin resin foam particles, comprising the foaming step, in which the pressure inside the sealed container immediately before foaming is adjusted to a range of 1.8 MPa(G) to 4.5 MPa(G) and the polyolefin resin particles are released from the sealed container.

[0011] [2] The method for producing polyolefin resin foam particles according to [1], wherein the hydrofluoroolefin has a carbon skeleton with 3 to 5 carbon atoms. [3] A method for producing polyolefin resin foam particles according to [1] or [2], wherein the hydrofluoroolefin has a chlorine atom in its molecular structure. [4] The method for producing polyolefin resin foam particles according to any one of [1] to [3], wherein the hydrofluoroolefin is one or more compounds selected from the group consisting of 1-chloro-3,3,3-trifluoropropene and 1-chloro-2,3,3,3-tetrafluoropropene.

[0012] [5] A method for producing polyolefin resin foam particles according to any one of [1] to [4], wherein the hydrofluoroolefin content in the polyolefin resin foam particles three days after the completion of the foaming process is 1% by mass or less. [6] A method for producing polyolefin resin foam particles according to any one of [1] to [5], wherein the average value of the short diameter of the polyolefin resin foam particles obtained by the foaming step is 1.0 mm or more and 5.0 mm or less, and the coefficient of variation is 10% or less. [Effects of the Invention]

[0013] According to the above embodiment, even when foamed particles with a high apparent density are produced by a direct foaming method using hydrofluoroolefin as a foaming agent, it is possible to provide a method for producing polyolefin-based resin foamed particles that have small particle size variations and excellent moldability. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is an explanatory diagram showing the method for calculating high-temperature peak heat energy. [Figure 2] Figure 2 is an explanatory diagram showing an example of a stress-strain curve for a foamed particle molded body. [Modes for carrying out the invention]

[0015] (Method for producing polyolefin resin foam particles) In the method for producing the polyolefin resin foam particles (hereinafter referred to as "foam particles"), the foam particles can be obtained by foaming the polyolefin resin particles (hereinafter referred to as "resin particles") using a direct foaming method with the specific foaming agent. A more detailed configuration of the method for producing the polyolefin resin foam particles will be described below.

[0016] <Polyolefin resin particles> The polyolefin resin particles used in the production of the foamed particles are unfoamed resin particles with a polyolefin resin as the base resin. A polyolefin resin refers to a homopolymer of olefin monomers and a copolymer containing 50 mol% or more of components derived from olefin monomers. Examples of olefin monomers include ethylene, propylene, butene, and pentene.

[0017] More specifically, as polyolefin resins, for example, polyethylene resins, polypropylene resins, polybutene, polypentene, and copolymers of olefin monomers with other monomers can be used.

[0018] Examples of polyethylene-based resins include polyethylene such as high-density polyethylene (PE-HD), medium-density polyethylene (PE-MD), low-density polyethylene (PE-LD), linear low-density polyethylene (PE-LLD), and ultra-low-density polyethylene; and ethylene-based copolymers containing 50 mol% or more of ethylene-derived components, such as ethylene-vinyl acetate copolymer (EVA), ethylene-ethyl acrylate copolymer (EEAK), and ethylene-methyl methacrylate copolymer (EMMA). Among these, linear low-density polyethylene is preferred as the polyethylene-based resin from the viewpoint of foaming properties and moldability. Linear low-density polyethylene is preferably a copolymer of ethylene and an α-olefin having 4 to 8 carbon atoms, having substantially linear molecular chains and a density of 910 kg / m³. 3 More than 930kg / m 3 It is polyethylene of less than 110°C. The melting point of polyethylene resin is preferably 110°C or higher and 130°C or lower.

[0019] Examples of polypropylene resins include propylene homopolymers such as isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene; and propylene copolymers containing 50 mol% or more of propylene-derived components, such as propylene-ethylene copolymer, propylene-butene copolymer, propylene-ethylene-butene ternary copolymer, propylene-hexene copolymer, propylene-acrylic acid copolymer, and propylene-maleic anhydride copolymer. These copolymers are, for example, random copolymers and block copolymers, with random copolymers being preferred.

[0020] The resin particles may contain one resin selected from these polyolefin resins, or two or more resins. The melting point of the polypropylene resin is preferably 130°C to 160°C, and more preferably 135°C to 155°C.

[0021] From the viewpoint of foaming properties and moldability, it is preferable to use one or more propylene copolymers selected from propylene-ethylene random copolymer, propylene-butene random copolymer, and propylene-ethylene-butene random copolymer as the polypropylene resin.

[0022] The polyolefin resin constituting the resin particles is preferably a polypropylene resin. Polypropylene resin foam particles obtained by foaming such resin particles have excellent cushioning properties, compressive strain recovery properties, and lightweight properties. Therefore, by in-mold molding polypropylene resin foam particles, foam particle molded articles suitable for applications such as packaging materials, automotive components, and building materials can be easily obtained.

[0023] In addition to the polyolefin resin as the base resin, the resin particles may also contain polymers other than polyolefin resins, such as other resins or elastomers, to the extent that they do not impair the objectives and effects of the present invention. Examples of resins other than polyolefin resins include thermoplastic resins such as polystyrene resins, polyamide resins, and polyester resins. Examples of elastomers include olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers. The proportion of polymers other than polyolefin resins in the resin particles is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and particularly preferably 0% by mass, that is, the resin particles contain only polyolefin resin as a polymer.

[0024] Furthermore, the polyolefin resin, which is the base resin of the resin particles, may contain additives such as foam regulators, crystal nucleating agents, flame retardants, flame retardant aids, plasticizers, antistatic agents, antioxidants, ultraviolet inhibitors, light stabilizers, conductive fillers, antibacterial agents, and colorants, to the extent that they do not impair the effects described above. The content of additives in the resin particles is preferably, for example, 0.01 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the polyolefin resin.

[0025] The method for producing the foamed particles is not particularly limited, and various methods such as the strand cutting method, the hot cutting method, and the underwater cutting method can be employed. For example, in the strand cutting method, a polyolefin resin, which will be the base resin, and additives such as bubble nucleating agents, which are supplied as needed, are placed in an extruder, heated, and kneaded to form a molten resin mixture. Then, the molten resin mixture is extruded through small holes in a die attached to the tip of the extruder to form an extruder. After cooling this extruder in water, resin particles with a polyolefin resin as the base resin can be obtained by cutting it to the desired length.

[0026] <Dispersion process> In the dispersion process, polyolefin resin particles are dispersed in an aqueous medium to prepare a dispersion. The dispersion of resin particles in the aqueous medium may be carried out in a sealed container used in the foaming agent addition process or the foaming process, or in a container separate from the sealed container used in those processes.

[0027] As the aqueous medium, a liquid mainly composed of water is used. In addition to water, the aqueous medium may also contain hydrophilic organic solvents such as ethylene glycol, glycerin, methanol, and ethanol. The proportion of water in the aqueous medium is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more.

[0028] A dispersant may be added to the aqueous medium. Adding a dispersant to the aqueous medium can suppress the fusion of heated resin particles in the container during the foaming agent addition step and the subsequent foaming step. The amount of dispersant added is preferably 0.001 parts by mass or more and 5 parts by mass or less per 100 parts by mass of resin particles. Organic dispersants and inorganic dispersants can be used as dispersants, but due to their ease of handling, fine inorganic materials are preferred as dispersants. More specifically, examples of dispersants that can be used include clay minerals such as amsnite, kaolin, mica, and clay, as well as aluminum oxide, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, and iron oxide. These dispersants may be used alone or in combination of two or more dispersants. Among these, clay minerals are preferred as dispersants. Clay minerals may be natural or synthesized.

[0029] When a dispersant is added to an aqueous medium, it is preferable to use anionic surfactants such as sodium dodecylbenzenesulfonate, sodium alkylbenzenesulfonate, sodium lauryl sulfate, and sodium oleate as dispersing aids. The amount of dispersing aid added is preferably 0.001 parts by mass or more and 1 part by mass or less per 100 parts by mass of resin particles.

[0030] <Foaming agent addition process> In the foaming agent addition step, a foaming agent containing hydrofluoroolefin and nitrogen is added to a sealed container. The timing of the foaming agent addition step is not particularly limited as long as it is performed before the foaming step. For example, the foaming agent addition step may be performed before the dispersion step, or in parallel with the dispersion step. Alternatively, the foaming agent addition step may be performed after the dispersion step is completed. In any case, the polyolefin resin particles and the foaming agent come into contact in the sealed container, thereby impregnating the polyolefin resin particles with at least hydrofluoroolefin.

[0031] Furthermore, the foaming agent addition process may be configured to add the entire amount of foaming agent in one step, or it may be configured to add the foaming agent in multiple steps. Preferably, the addition of the foaming agent in the foaming agent addition process is carried out in two steps, a first addition step and a second addition step, as described later.

[0032] The blowing agent used in the blowing agent addition step contains at least hydrofluoroolefin and nitrogen. The nitrogen may be added as elemental nitrogen or as air. The amount of hydrofluoroolefin added in the blowing agent addition step is 1 to 12 parts by mass per 100 parts by mass of polyolefin resin particles. Furthermore, in the blowing agent addition step, it is preferable to adjust the pressure inside the sealed container to a range of 1.8 MPa(G) to 4.5 MPa(G) using the blowing agent. In particular, it is preferable to adjust the pressure inside the sealed container to a range of 1.8 MPa(G) to 4.5 MPa(G) by adding nitrogen as the blowing agent. This allows the pressure inside the sealed container immediately before foaming, as described later, to be within the range of 1.8 MPa(G) to 4.5 MPa(G). By adding a specific amount of hydrofluoroolefin to the sealed container in the blowing agent addition step and adjusting the pressure inside the sealed container to the specific range using nitrogen, the variation in particle size of the foamed particles obtained in the foaming step can be more easily reduced. If nitrogen is not added as a blowing agent in the blowing agent addition process, it becomes difficult to maintain a pressure of 1.8 MPa(G) or higher inside the sealed container.

[0033] The pressure inside the sealed container mentioned above refers to the pressure measured as the sum of the partial pressures of nitrogen (or air), hydrofluoroolefin, and water vapor derived from the aqueous medium added in the blowing agent addition process. Furthermore, the proportion of the total pressure inside the container that is accounted for by the partial pressure of nitrogen added in the blowing agent addition process is preferably 30% or more, and more preferably 50% or more. In this case, variations in the particle size of the resulting foamed particles can be suppressed more reliably. In addition, the upper limit of the proportion of the total pressure inside the container that is accounted for by the partial pressure of nitrogen added in the blowing agent addition process is generally 85% or less.

[0034] Furthermore, by foaming resin particles using a direct foaming method with a foaming agent containing hydrofluoroolefin, foamed particles that can be molded over a wide range of molding pressures, from low to high, can be obtained. The reason for this is not entirely clear, but possible explanations include the good affinity of hydrofluoroolefin to polyolefin resins, which causes the foaming agent to gradually separate from the resin particles during the foaming process, thereby suppressing the miniaturization of bubbles formed in the foamed particles.

[0035] If the amount of hydrofluoroolefin added in the foaming agent addition process is too small, the effect of the hydrofluoroolefin may decrease, potentially leading to a decrease in moldability. This problem can be easily avoided by adding 1 part by mass or more per 100 parts by mass of resin particles. From the viewpoint of more reliably obtaining the effect of hydrofluoroolefin, the amount of hydrofluoroolefin added is preferably 1.5 parts by mass or more, more preferably 2 parts by mass or more, and even more preferably 3 parts by mass or more, per 100 parts by mass of resin particles.

[0036] On the other hand, if the hydrofluoroolefin content is too high, the apparent density of the resulting foamed particles may become excessively low. This problem can be easily avoided by limiting the amount of hydrofluoroolefin added to 12 parts by mass or less per 100 parts by mass of resin particles. From a similar viewpoint, the amount of hydrofluoroolefin added is preferably 10 parts by mass or less, more preferably 8 parts by mass or less, even more preferably 7 parts by mass or less, and particularly preferably 5 parts by mass or less per 100 parts by mass of resin particles.

[0037] From the viewpoint of obtaining the effects of hydrofluoroolefins while more reliably achieving the desired structure of foamed particles, the hydrofluoroolefin content is preferably 1.5 parts by mass or more and 10 parts by mass or less, and more preferably 2 parts by mass or more and 8 parts by mass or less, per 100 parts by mass of resin particles.

[0038] Furthermore, if the pressure inside the sealed container during the foaming agent addition process is too low, the resulting foamed particles tend to exhibit greater variation in particle size. By adding nitrogen in addition to hydrofluoroolefin as a foaming agent, and adjusting the pressure inside the sealed container during the foaming agent addition process to 1.8 MPa(G) or higher in gauge pressure, foamed particles with less variation in particle size can be easily obtained. From the viewpoint of more reliably obtaining these effects, the pressure inside the sealed container during the foaming agent addition process is preferably 2.0 MPa(G) or higher, more preferably 2.5 MPa(G) or higher, and even more preferably 3.0 MPa(G) or higher.

[0039] On the other hand, if the pressure inside the sealed container during the foaming agent addition process is too high, the sealed container may be damaged. Also, the bubbles in the resulting foamed particles may become excessively fine, reducing their moldability. These problems can be easily avoided by adding nitrogen in addition to hydrofluoroolefin as a foaming agent and adjusting the pressure inside the sealed container during the foaming agent addition process to 4.5 MPa(G) or less in gauge pressure. From the viewpoint of more reliably obtaining these effects, it is preferable that the pressure inside the sealed container during the foaming agent addition process be 4.2 MPa(G) or less.

[0040] Hydrofluoroolefins (HFOs) are fluorine-containing hydrocarbons having a structure in which at least one hydrogen atom and one fluorine atom are bonded to a carbon skeleton with unsaturated bonds. For example, a hydrofluoroolefin may have a structure in which one hydrogen atom and one fluorine atom are bonded to a carbon skeleton with unsaturated bonds. In addition to hydrogen and fluorine atoms, a chlorine atom may also be bonded to the carbon skeleton of a hydrofluoroolefin. That is, the term hydrofluoroolefin as used herein is a concept that includes hydrochlorofluoroolefins (HCFOs) having a structure in which one hydrogen atom, one fluorine atom, and one chlorine atom are bonded to a carbon skeleton with unsaturated bonds. Hydrofluoroolefins act as physical blowing agents.

[0041] More specifically, examples of hydrofluoroolefins that can be used as blowing agents include 1,3,3,3-tetrafluoropropene (HFO1234ze), 1,1,1-4,4,4-hexafluoro-2-butene (HFO1336mzz), 2,3,3,3-tetrafluoropropene (HFO1234yf), 1-chloro-3,3,3-trifluoropropene (HCFO1233zd), and 1-chloro-2,3,3,3-tetrafluoropropene (HCFO1224yd). These hydrofluoroolefins may be in trans or cis form. The physical blowing agent may contain one type of hydrofluoroolefin or two or more types of hydrofluoroolefins.

[0042] The hydrofluoroolefin preferably has a carbon skeleton with 3 to 5 carbon atoms. By using such a hydrofluoroolefin as a blowing agent, the shrinkage of foamed particles immediately after foaming can be more effectively suppressed. The reason for this is not clear, but one possible reason is that the hydrofluoroolefin has a relatively large molecular skeleton, which slows down the gas permeation rate in the polyolefin resin. In this case, the moldability of the foamed particles can also be further improved. From a similar viewpoint, the molecular weight of the hydrofluoroolefin is preferably 100 or more, and more preferably 130 or more. On the other hand, the upper limit of the molecular weight of the hydrofluoroolefin is approximately 200.

[0043] The hydrofluoroolefin preferably has a chlorine atom in its molecular structure. That is, in the blowing agent addition step, it is preferable to use a physical blowing agent containing hydrochlorofluoroolefin. Hydrochlorofluoroolefin has a higher affinity for polyolefin resins than hydrofluoroolefin that does not contain a chlorine atom. Therefore, by using hydrochlorofluoroolefin as a blowing agent, the miniaturization of bubbles in the foamed particles can be more effectively suppressed, and the moldability of the foamed particles can be further improved.

[0044] From the viewpoint of further improving the moldability of the foamed particles, it is more preferable that the hydrofluoroolefin is one or more compounds selected from the group consisting of 1-chloro-3,3,3-trifluoropropene and 1-chloro-2,3,3,3-tetrafluoropropene.

[0045] Furthermore, in the blowing agent addition step, other blowing agents may be added along with hydrofluoroolefin and nitrogen as blowing agents. Examples of such blowing agents include inorganic blowing agents such as water, carbon dioxide, and argon. The amount of blowing agents other than hydrofluoroolefin and nitrogen added in the blowing agent addition step is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 0% by mass, i.e., only hydrofluoroolefin and nitrogen are added as blowing agents.

[0046] In the foaming agent addition step, a foaming agent containing hydrofluoroolefin and nitrogen is added to a sealed container, and the foaming agent is brought into contact with the polyolefin resin particles, thereby impregnating the resin particles in the aqueous medium with at least hydrofluoroolefin. In the foaming agent addition step, the entire amount of foaming agent may be added to the sealed container at once, or the foaming agent may be added to the sealed container in multiple portions. Furthermore, in the foaming agent addition step, the impregnation of the resin particles with the foaming agent may be promoted by pressurizing the sealed container or heating the contents of the sealed container as needed.

[0047] For example, the foaming agent addition step preferably comprises a first addition step in which the foaming agent is initially added to the sealed container, and a second addition step in which the foaming agent is further added to the sealed container after the first addition step, and the pressure inside the sealed container is adjusted to a range of 1.8 MPa(G) to 4.5 MPa(G), thereby configuring the process to add the foaming agent in two stages.

[0048] When the blowing agent is added in two stages during the blowing agent addition process, the timing of the first addition step may be before or after the dispersion step. Alternatively, the first addition step may be performed in parallel with the dispersion step. In the first addition step, it is preferable to add a blowing agent containing at least hydrofluoroolefin into a sealed container to impregnate the resin particles, and it is more preferable to add a blowing agent containing hydrofluoroolefin and nitrogen into a sealed container to impregnate the resin particles. In this case, the aforementioned effects can be obtained more reliably.

[0049] The timing of the second addition step is not particularly limited as long as it is performed after the first addition step, but from the viewpoint of more reliably obtaining the effects described above, it is preferable to perform the second addition step immediately before the foaming step. Furthermore, it is preferable that the foaming agent used in the second addition step is nitrogen. By adding nitrogen to the sealed container in the second addition step, the pressure inside the sealed container immediately before the foaming step can be reliably adjusted to the aforementioned specific range. As a result, the variation in particle size of the resulting foamed particles is reduced, and foamed particles with excellent moldability can be obtained more easily.

[0050] Furthermore, the manufacturing method preferably further includes a high-temperature peak formation step in which the temperature inside a sealed container is maintained at a temperature of (the melting point of the polyolefin resin - 30°C) or higher and (the melting end temperature of the polyolefin resin) or lower for 1 to 60 minutes. By performing the high-temperature peak formation step, secondary crystals are formed in the polyolefin resin constituting the foamed particles, making it possible to easily obtain foamed particles that have excellent mechanical strength and moldability.

[0051] While there are no particular limitations on the timing of the high-temperature peak formation process, from the viewpoint of sufficiently impregnating the resin particles with hydrofluoroolefin, it is preferable to perform the high-temperature peak formation process after the impregnation of the resin particles with hydrofluoroolefin is complete. Specifically, it is preferable to perform the high-temperature peak formation process after the foaming agent addition process. For example, if the foaming agent is added to a sealed container in two stages during the foaming agent addition process, it is most preferable to perform the high-temperature peak formation process after the hydrofluoroolefin has been impregnated into the resin particles by the first and second addition steps.

[0052] <Foaming Process> In the foaming agent addition step, the foaming agent added to the sealed container is impregnated into the resin particles, and then the foaming step is performed. In the foaming step, the contents of the sealed container are released into an environment with a lower pressure than the sealed container while the pressure inside the sealed container (i.e., the pressure inside the container immediately before foaming) is adjusted to a range of 1.8 MPa(G) to 4.5 MPa(G). The pressure inside the sealed container immediately before foaming is adjusted by adding the foaming agent in the foaming agent addition step. For example, if the foaming agent addition step has a first addition step and a second addition step, the pressure inside the sealed container is adjusted to the above range by the second addition step, and then the foaming step is performed.

[0053] When resin particles inside a sealed container are released into an environment with lower pressure than the sealed container, the resin particles foam up, forming a cellular structure, and are then cooled by the outside air, stabilizing the cellular structure. As a result, the apparent density becomes 50 kg / m³. 3 More than 300kg / m 3 The following foamed particles can be obtained. In the above manufacturing method, hydrofluoroolefin and nitrogen are used as foaming agents, and the resin particles are released from the sealed container while the pressure inside the sealed container is adjusted to a range of 1.8 MPa(G) to 4.5 MPa(G). This makes it possible to easily obtain foamed particles with high apparent density, small variation in particle size, and excellent moldability.

[0054] The reason why adding nitrogen to hydrofluoroolefin as a blowing agent makes it possible to easily produce foamed particles with high apparent density and suppressed particle size variation is not entirely clear, but possible reasons include the fact that nitrogen has a low plasticizing ability with polyolefin resins, which makes it easier to adjust the pressure inside the sealed container within the above range while suppressing the apparent density of the resulting foamed particles from becoming excessively low.

[0055] From the viewpoint of more reliably obtaining the aforementioned effects, it is preferable to pressurize the inside of the sealed container while the contents of the sealed container are being released during the foaming process to maintain the pressure immediately before foaming (i.e., in the range of 1.8 MPa(G) to 4.5 MPa(G)). In this case, an inorganic gas such as nitrogen can be used to pressurize the inside of the sealed container. Note that the inorganic gas used to maintain the pressure inside the sealed container during the foaming process is not included in the foaming agent described herein.

[0056] (Polyolefin resin foam particles) According to the above manufacturing method, polyolefin resin foam particles can be obtained. The base resin of the polyolefin resin foam particles is a polyolefin resin. The polyolefin resin constituting the foam particles is the same as the polyolefin resin constituting the resin particles described above. Therefore, for the specific composition of the polyolefin resin in the foam particles, refer to the explanation of the polyolefin resin in the resin particles as appropriate.

[0057] The foamed particles are obtained by foaming resin particles using a foaming agent containing hydrofluoroolefin and nitrogen. For specific details of the foaming agent, refer to the description of the foaming agent used in the foaming agent addition step as appropriate.

[0058] <Apparent Density> The apparent density of the foamed particles is 50 kg / m³. 3 More than 300kg / m 3The following is the case. As described above, even when producing foamed particles with an apparent density within the specific range by the direct foaming method using a foaming agent containing nitrogen in addition to hydrofluoroolefin, the variation in the particle diameter of the foamed particles can be reduced. In particular, even when producing foamed particles with a large apparent density where the variation in particle diameter is particularly significant in the conventional direct foaming method using hydrofluoroolefin, a sufficient effect can be exhibited. From the perspective of making such an effect more beneficial, the apparent density of the foamed particles is preferably 60 kg / m 3 or more, more preferably 70 kg / m 3 or more, and even more preferably 80 kg / m 3 or more. Also, the upper limit of the apparent density of the foamed particles is preferably 250 kg / m 3 and more preferably 200 kg / m 3 .

[0059] The method for calculating the apparent density of the foamed particles is as follows. First, the foamed particle group is left standing for 1 day in an environment with a relative humidity of 50%, a temperature of 23°C, and an atmospheric pressure of 1 atm to adjust the state of the foamed particles. After measuring the mass (unit: g) of this foamed particle group, it is submerged in a graduated cylinder containing alcohol (e.g., ethanol) at 23°C using a wire mesh or the like, and the volume (unit: L) of the foamed particle group is obtained from the rise in the liquid level. Then, the apparent density (unit: kg / m 3 ) of the foamed particles can be calculated by unit conversion of the value obtained by dividing the mass of the foamed particle group by the volume of the foamed particle group.

[0060] <Average value and coefficient of variation of the short diameter> The average value of the short diameter of the foamed particles is 1.0 mm or more and 5.0 mm or less, and the coefficient of variation of the short diameter is 12% or less. As mentioned above, by using a blowing agent containing nitrogen in addition to hydrofluoroolefin, foamed particles with small particle size variation can be obtained, such that the average value and coefficient of variation of the short diameter are both within the specified range, even when the apparent density is relatively high. Furthermore, by producing a foamed particle molded article using foamed particles in which the average value and coefficient of variation of the short diameter are both within the specified range, the energy absorption performance of the foamed particle molded article can be more easily improved. From this viewpoint, the coefficient of variation of the short diameter is preferably 11% or less, and more preferably 10% or less. The lower limit of the coefficient of variation of the short diameter is approximately 1%.

[0061] Furthermore, from the viewpoint of further improving the filling properties of the foam particles and the energy absorption performance of the molded body, it is preferable that the average value of the short diameter of the foam particles be 2.0 mm or more and 4.0 mm or less.

[0062] The average value of the minor axis and the coefficient of variation of the minor axis of the foamed particle can be measured using a projection image type particle size distribution analyzer. The minor axis of the foamed particle refers to the length at which the distance between two parallel lines flanking the foamed particle is minimized in the projection image of the foamed particle captured by the projection image type particle size distribution analyzer (i.e., the minimum Ferret diameter). Average value T of the minor axis of the foamed particle av The value (in mm) is calculated based on the following formula (1). Also, the coefficient of variation T of the minor axis of the foamed particles is also calculated. cv (Unit: %) is the average value T of the short diameter of the foamed particles, as shown in formula (2) below. av Standard deviation of the minor axis T sd This value represents the ratio of (unit: mm) as a percentage. Note that the standard deviation T of the minor axis of the foamed particles is also shown. sd This value is calculated based on the following formula (3). T av =Σ(T i ) / n ···(1) T cv =T sd / T av ×100 ···(2) T sd=(Σ(T i -T av ) 2 (n-1) 1 / 2 ...(3)

[0063] Note that T in equations (1) and (3) above i is the value of the short axis (in mm) of the i-th measured foam particle, and n is the total number of foam particles measured. The value of n should be, for example, 1000 or greater.

[0064] <Average bubble diameter> The average bubble diameter of the foamed particles is preferably 80 μm or more. By making the average bubble diameter of the foamed particles 80 μm or more, the moldability of the foamed particles can be further improved. From a similar viewpoint, the average bubble diameter of the foamed particles is more preferably 100 μm or more. On the other hand, the upper limit of the average bubble diameter of the foamed particles is preferably 300 μm, more preferably 250 μm, and even more preferably 200 μm.

[0065] 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.

[0066] 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.

[0067] <Remaining amount of hydrofluoroolefin in foamed particles> The hydrofluoroolefin content in the polyolefin resin foam particles is preferably 1% by mass or less, more preferably 0.5% by mass or less, even more preferably 0.1% by mass or less, and particularly preferably 0% by mass. By performing in-mold molding using such foam particles, a foam particle molded body with excellent rigidity can be easily obtained. The hydrofluoroolefin content in the foam particles can be measured using a headspace gas chromatograph-mass spectrometer.

[0068] Furthermore, the hydrofluoroolefin impregnated into the resin particles as a physical blowing agent rapidly dissipates from the polyolefin-based resin foam particles immediately after foaming. Therefore, in the case of foam particles obtained by the above manufacturing method, for example, the amount of hydrofluoroolefin in the foam particles after standing for 3 days from immediately after manufacturing at normal pressure and room temperature will be 1% by mass or less. Hydrofluoroolefins are sometimes used to improve thermal insulation by taking advantage of their property of easily remaining in foams, such as polystyrene-based resin foams. However, the dissipation behavior of hydrofluoroolefins in polyolefin-based resin foam particles obtained by the above manufacturing method is completely different from the dissipation behavior of hydrofluoroolefins in polystyrene-based resin foams.

[0069] <High temperature peak> Preferably, the foamed particles have a crystalline structure in which the DSC curve obtained when heated from 23°C to 200°C at a heating rate of 10°C / min shows an endothermic peak due to the melting inherent to the polyolefin resin constituting the foamed particles, and one or more melting peaks located at a higher temperature than this endothermic peak. Foamed particles with such a crystalline structure have excellent mechanical strength and moldability. In the following, the endothermic peak due to the melting inherent to the polyolefin resin that appears in the DSC curve is referred to as the "resin-specific peak," and the melting peak that appears at a higher temperature than the resin-specific peak is referred to as the "high-temperature peak." The resin-specific peak is caused by the endothermic reaction when the crystals inherent in the polyolefin resin constituting the foamed particles melt. On the other hand, the high-temperature peak is presumed to be caused by the melting of secondary crystals formed in the polyolefin resin constituting the foamed particles during the manufacturing process. That is, if a high-temperature peak appears in the DSC curve, it is presumed that secondary crystals have been formed in the polyolefin resin.

[0070] Whether or not the foamed particles possess the aforementioned crystalline structure can be determined based on the DSC curve obtained by performing differential scanning calorimetry (DSC) under the conditions described above, in accordance with JIS K7121:1987. Furthermore, 1 to 3 mg of foamed particles should be used as a sample for the DSC.

[0071] Specifically, the DSC curve obtained when heating from 23°C to 200°C at a heating rate of 10°C / min (i.e., the first heating) shows both a high-temperature peak and a resin-specific peak of the polyolefin resin constituting the foamed particles. The DSC curve obtained during the first heating shows both the resin-specific peak and the high-temperature peak. In contrast, the DSC curve obtained when the material is cooled from 200°C to 23°C at a cooling rate of 10°C / min after the first heating, and then heated again from 23°C to 200°C at a heating rate of 10°C / min (i.e., the second heating), shows only the resin-specific peak of the polyolefin resin constituting the foamed particles. Therefore, by comparing the DSC curve obtained during the first heating and the DSC curve obtained during the second heating, the resin-specific peak and the high-temperature peak can be distinguished. The temperature at the peak of this resin-specific peak may differ slightly between the first and second heating, but the difference is usually within 5°C.

[0072] The heat of fusion of the high-temperature peak of the foamed particles is preferably 5 J / g or more and 40 J / g or less, more preferably 7 J / g or more and 30 J / g or less, and even more preferably 10 J / g or more and 20 J / g or less, from the viewpoint of further improving the moldability of the foamed particles and obtaining a molded article with superior rigidity.

[0073] The heat of fusion of the aforementioned high-temperature peak is determined as follows. First, 1 to 3 mg of conditioned foam particles are used as a sample, and a differential scanning calorimetry (DSC) curve is obtained by heating from 23°C to 200°C at a heating rate of 10°C / min. An example of a DSC curve is shown in Figure 1. When foam particles have a high-temperature peak, the DSC curve shows a resin-specific peak ΔH1 and a high-temperature peak ΔH2 whose peak is at a higher temperature than the peak of the resin-specific peak ΔH1, as shown in Figure 1.

[0074] Next, draw a straight line L1 connecting point α, which corresponds to 80°C on the DSC curve, and point β, which corresponds to the melting termination temperature T of the foamed particles. Note that the melting termination temperature T is the high-temperature endpoint of the high-temperature peak ΔH2, that is, the intersection point of the high-temperature peak ΔH2 on the DSC curve and the baseline on the side of the high-temperature peak ΔH2 that is higher than ΔH2.

[0075] After drawing the straight line L1, a straight line L2 is drawn parallel to the vertical axis of the graph, passing through the maximum point γ located between the resin intrinsic peak ΔH1 and the high-temperature peak ΔH2. This straight line L2 separates the resin intrinsic peak ΔH1 and the high-temperature peak ΔH2. The amount of heat absorbed by the high-temperature peak ΔH2 can be calculated based on the area enclosed by the portion of the DSC curve that constitutes the high-temperature peak ΔH2, and the straight lines L1 and L2. [Examples]

[0076] Examples of the foamed particles and their manufacturing method will be described below.

[0077] (Example 1) In this example, polypropylene resin foam particles were produced by foaming polypropylene resin particles using a direct foaming method with hydrochlorofluoroolefin and nitrogen as foaming agents. A more detailed method for producing the foam particles in this example is as follows.

[0078] <Manufacturing of polypropylene resin particles> Polypropylene resin particles were manufactured by the strand-cut method. An extrusion apparatus equipped with an extruder with an inner diameter of 50 mm and a die attached to the end of the extruder was used to manufacture the polypropylene resin particles. First, polypropylene resin and zinc borate as a bubble nucleating agent were introduced into the extruder and melt-kneaded to form a molten resin mixture. Then, the molten resin mixture was extruded from the extruder die to obtain an extruder. This extruder was cooled by passing it through a water bath, and then cut to an appropriate length using a pelletizer to obtain polypropylene resin particles. The mass of each polypropylene resin particle in this example was approximately 1.0 g.

[0079] The polypropylene resin used in this example is an ethylene-propylene random copolymer. The melting point of the ethylene-propylene random copolymer, measured according to JIS K7121:1987, is 143°C. Furthermore, the melt mass flow rate of the ethylene-propylene random copolymer, measured according to JIS K7210-1:2014 at a temperature of 230°C and a load of 2.16 kg, is 8 g / 10 min. The density of this ethylene-propylene random copolymer is 900 kg / m³. 3 In Table 1, the ethylene-propylene random copolymer used in this example is denoted as "PP".

[0080] Furthermore, the bubble nucleating agent used in this example was zinc borate. The amount of zinc borate added was 1000 ppm by mass relative to the mass of the resin particles, as shown in Table 1.

[0081] <Dispersion process> In the dispersion process, 1 kg of polypropylene resin particles were placed in a 5 L sealed container along with 3 L of water as the aqueous medium. Next, 0.3 parts by mass of dispersant, 0.004 parts by mass of surfactant, and 0.01 parts by mass of dispersion aid were added to the sealed container per 100 parts by mass of resin particles to disperse the resin particles in the aqueous medium. Kaolin was used as the dispersant, sodium dodecylbenzenesulfonate ("Neogen®," manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) as the surfactant, and aluminum sulfate as the dispersion aid.

[0082] <Foaming agent addition process> In the foaming agent addition process of this example, the foaming agent was added to the sealed container in two separate steps. First, with the sealed container in place, the amount of hydrochlorofluoroolefin shown in Table 1 was added to 100 parts by mass of polypropylene resin, and nitrogen was added to the sealed container to pressurize the container to 1.0 MPa(G) (first addition step). Specifically, the hydrochlorofluoroolefin used in this example is trans-1-chloro-3,3,3-trifluoropropene (HCFO1233zd). In Table 1, the hydrochlorofluoroolefin used in this example is indicated as "HCFO1".

[0083] After the first addition step was completed, the sealed container was heated while being stirred, and the temperature inside the container was raised to 0.5°C lower than the foaming temperature shown in Table 1. Next, nitrogen was added to the sealed container, and the pressure inside the container was raised to the value shown in Table 1 (second addition step).

[0084] <High-temperature peak formation process> After the second addition step was completed, the sealed container was further heated to raise the temperature inside the container to the foaming temperature shown in Table 1. By maintaining this temperature for 15 minutes, the state of the resin particles was adjusted so that a high-temperature peak, described later, would appear in the DSC curve of the resulting foamed particles.

[0085] <Foaming Process> After the high-temperature peak formation process was completed, the contents of the sealed container were released under atmospheric pressure to foam the resin particles. The pressure inside the container at the time of release was the pressure shown in the "Pressure inside the container immediately before foaming" column of Table 1, which was adjusted in the second addition process. In addition, to suppress the decrease in pressure inside the container during foaming, nitrogen was used as an inorganic gas to pressurize the container while the contents were being released from the sealed container, maintaining the pressure inside the container at the pressure shown in Table 1. Through the above process, foamed polypropylene resin particles were obtained.

[0086] (Example 2) In this example, polypropylene resin foam particles were prepared in the same manner as in Example 1, except that 1-chloro-2,3,3,3-tetrafluoropropene (HCFO1224yd) was used as the foaming agent. In Table 1, the hydrochlorofluoroolefin used in this example is indicated as "HCFO2".

[0087] (Example 3) In this example, instead of hydrochlorofluoroolefin as a foaming agent, a hydrofluoroolefin that does not contain chlorine atoms in its molecular structure was used. Specifically, trans-1,3,3,3-tetrafluoropropene (HFO1234ze) was used, and polypropylene resin foam particles were prepared in the same manner as in Example 1, except that the foaming temperature was changed to the value shown in Table 1. In Table 1, the hydrofluoroolefin used in this example is indicated as "HFO".

[0088] (Examples 4-6) In Examples 4 to 6, polypropylene resin foam particles were produced in the same manner as in Example 1, except that the amount of hydrochlorofluoroolefin added in the first addition step was changed to the value shown in Table 1, and the amount of nitrogen added in the second addition step was changed so that the internal pressure in the container immediately before foaming was the value shown in Table 1.

[0089] (Example 7) In this example, foamed particles were produced in the same manner as in Example 1, except that air was used instead of nitrogen as the foaming agent.

[0090] (Example 8) In this example, polyethylene resin foam particles were produced by foaming polyethylene resin particles using a direct foaming method with hydrochlorofluoroolefin and nitrogen as foaming agents. A more detailed method for producing the foam particles in this example is as follows.

[0091] <Manufacturing of polyethylene resin particles> Polyethylene resin particles were manufactured by the strand-cutting method. An extrusion apparatus equipped with an extruder with an inner diameter of 50 mm and a die attached to the end of the extruder was used to manufacture the polyethylene resin particles. First, polyethylene resin and zinc borate as a bubble nucleating agent were introduced into the extruder and melt-kneaded to form a resin molten mixture. Then, the resin molten mixture was extruded from the extruder die to obtain an extruder. This extruder was cooled by passing it through a water bath, and then cut to an appropriate length using a pelletizer to obtain polyethylene resin particles. The mass of each polyethylene resin particle in this example was approximately 1.5 g.

[0092] The polyethylene resin used in this example is linear low-density polyethylene. The melting point of the linear low-density polyethylene, measured according to JIS K7121:1987, is 120°C. Furthermore, the melt mass flow rate of the linear low-density polyethylene, measured according to JIS K7210-1:2014 at a temperature of 230°C and a load of 2.16 kg, is 1.0 g / 10 min. The density of this linear low-density polyethylene is 925 kg / m³. 3 In Table 1, the linear low-density polyethylene used in this example is abbreviated as "PE".

[0093] Furthermore, the bubble nucleating agent used in this example was zinc borate. The amount of zinc borate added was 200 ppm by mass relative to the mass of the resin particles, as shown in Table 1.

[0094] <Dispersion process> In the dispersion process, 1 kg of polyethylene resin particles were placed in a 5 L sealed container along with 3 L of water as the aqueous medium. Next, 0.3 parts by mass of dispersant and 0.006 parts by mass of surfactant were added to the sealed container per 100 parts by mass of resin particles to disperse the resin particles in the aqueous medium. Mica was used as the dispersant, and sodium dodecylbenzenesulfonate ("Neogen®," manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) was used as the surfactant.

[0095] <Foaming agent addition process> In the foaming agent addition process of this example, the foaming agent was added to the sealed container in two separate steps. First, with the sealed container in place, hydrochlorofluoroolefin was added to the container in the amount shown in Table 1 per 100 parts by mass of polyethylene resin, and nitrogen was added to the container to pressurize it to a pressure of 1.0 MPa (G) (first addition step). Specifically, the hydrochlorofluoroolefin used in this example is trans-1-chloro-3,3,3-trifluoropropene (HCFO1233zd).

[0096] After the first addition step was completed, the sealed container was heated while being stirred, and the temperature inside the container was raised to 0.5°C lower than the foaming temperature shown in Table 1. Next, nitrogen was added to the sealed container, and the pressure inside the container was raised to the internal pressure shown in Table 1 (second addition step).

[0097] <High-temperature peak formation process> After the second addition step was completed, the sealed container was further heated to raise the temperature inside the container to the foaming temperature shown in Table 1. By maintaining this temperature for 15 minutes, the state of the resin particles was adjusted so that a high-temperature peak, described later, would appear in the DSC curve of the resulting foamed particles.

[0098] <Foaming Process> After the high-temperature peak formation process was completed, the contents of the sealed container were released under atmospheric pressure to foam the resin particles. The pressure inside the container at the time of release was the pressure shown in the "Pressure inside the container immediately before foaming" column of Table 1, which was adjusted in the second addition process. In addition, to suppress the decrease in pressure inside the container during foaming, nitrogen was used as an inorganic gas to pressurize the container while the contents were being released from the sealed container, maintaining the pressure inside the container at the pressure shown in Table 1. Through the above process, foamed polyethylene resin particles were obtained by foaming the resin particles.

[0099] (Comparative Example 1) In this example, polypropylene resin foam particles were prepared using the same method as in Example 1, except that hydrofluoroolefin was not used as a foaming agent, and the foaming temperature was changed to the values ​​shown in Table 2.

[0100] (Comparative Example 2) In this example, polypropylene resin foam particles were prepared using the same method as in Example 1, except that nitrogen was not used as a foaming agent, only hydrochlorofluoroolefin was used, the foaming agent was impregnated into the resin particles in a single step, and the foaming temperature was changed to the values ​​shown in Table 2. In this example, the pressure inside the sealed container immediately before foaming was the value shown in Table 2.

[0101] (Comparative Examples 3-5) In Comparative Examples 3 to 5, polypropylene resin foam particles were produced in the same manner as in Example 1, except that the amount of hydrochlorofluoroolefin added in the first addition step was changed to the value shown in Table 2, and the amount of nitrogen added in the second addition step was changed so that the internal pressure in the container immediately before foaming was the value shown in Table 2.

[0102] (Comparative Example 6) In Comparative Example 6, polypropylene resin foam particles were prepared in the same manner as in Example 1, except that carbon dioxide (CO2) was used as the foaming agent instead of nitrogen, and the amount of carbon dioxide added was changed to obtain foam particles with the desired apparent density. In this example, the pressure inside the sealed container immediately before foaming was as shown in Table 2.

[0103] (Comparative Example 7) In Comparative Example 7, polypropylene resin foam particles were prepared in the same manner as in Example 1, except that carbon dioxide (CO2) was used instead of nitrogen as the foaming agent, and the amount of carbon dioxide added was changed so that the internal pressure in the container during the second addition step was the value shown in Table 2.

[0104] Next, we will explain the method for evaluating the various properties of the foamed particles obtained as described above.

[0105] <High-temperature peak heat output> The foamed particles were allowed to stand for more than 24 hours in an environment with a relative humidity of 50%, a temperature of 23°C, and a pressure of 1 atm to adjust their state. Using 1 to 3 mg of the adjusted foamed particles, a differential scanning calorimetry (DSC) curve was obtained by differential scanning calorimetry in accordance with JIS K7121:1987. The measurement start temperature for DSC was 23°C, the measurement end temperature was 200°C, and the heating rate was 10°C / min. A differential scanning calorimetry system "DSC7020" manufactured by Hitachi High-Tech Science Corporation was used as the measurement device. The area of ​​the high-temperature peak in the DSC curve obtained by the method described above was calculated, and the high-temperature peak heat quantity was calculated based on this value. Table 1 shows the high-temperature peak heat quantity of the foamed particles in the example, and Table 2 shows the high-temperature peak heat quantity of the foamed particles in the comparative example.

[0106] <Apparent density of foamed particles> The foamed particles were allowed to stand for more than 24 hours in an environment of 50% relative humidity, 23°C, and 1 atm pressure to adjust their state. After measuring the mass of the adjusted foamed particle group, they were submerged in a graduated cylinder containing ethanol at 23°C using a wire mesh. Then, taking into account the volume of the wire mesh, the volume of the foamed particle group was measured from the rise in water level. The mass (in g) of the foamed particle group obtained in this way was divided by the volume (in L), and the units were converted to obtain the apparent density (in kg / m³) of the foamed particles. 3 The apparent density of foamed particles in the example is shown in Table 1, and the apparent density of foamed particles in the comparative example is shown in Table 2.

[0107] <Bulk density and bulk ratio of foamed particles> The foamed particles were left to stand for more than 24 hours in an environment of 50% relative humidity, 23°C, and 1 atm pressure to adjust their state. The adjusted foamed particles were then filled into a graduated cylinder so that they would naturally accumulate, and the bulk volume (in L) of the foamed particle group was read from the scale of the graduated cylinder. Subsequently, the mass (in g) of the foamed particle group in the graduated cylinder was divided by the aforementioned bulk volume, and the bulk density (in kg / m³) of the foamed particles was obtained by further unit conversion. 3The bulk density was calculated. In addition, the bulk ratio of the foamed particles was calculated by dividing the density of the resin constituting the foamed particles by the bulk density of the foamed particles. Table 1 shows the bulk density and bulk ratio of the foamed particles in the example, and Table 2 shows the bulk density and bulk ratio of the foamed particles in the comparative example.

[0108] <Average bubble diameter of foamed particles> First, the foam particles were cut into approximately two equal parts. Next, magnified photographs were taken so that the entire exposed cut surface was within the field of view. On the resulting magnified photographs, four line segments were drawn from the outermost surface of the foam particle, passing through the center and 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 bubble diameter of each foam particle was calculated by dividing the total length of these four line segments by the total number of bubbles that intersect the line segments.

[0109] The above procedure was performed on 10 or more randomly selected foamed particles, and the arithmetic mean of the resulting bubble diameters was used as the average bubble diameter of the foamed particles. Table 1 shows the average bubble diameter of the foamed particles in the example, and Table 2 shows the average bubble diameter of the foamed particles in the comparative example.

[0110] <Mean value and coefficient of variation of the minor axis> 1000 foamed particles were randomly selected from each of the foamed particle groups in the examples and comparative examples, and the short diameter of each foamed particle was measured using a projection image type particle size distribution analyzer (MicrotracBEL "PartAn3D"). Using the short diameters of the foamed particles obtained in this way, the average value T of the short diameter of the foamed particles was calculated based on the following equations (1) to (3). av (Unit: mm), standard deviation T sd (Unit: mm) and coefficient of variation T cv (Unit: %) was calculated. T av =Σ(T i ) / n ···(1) T cv =T sd / T av ×100 ···(2) T sd=(Σ(T i -T av ) 2 (n-1) 1 / 2 ...(3)

[0111] Note that T in equations (1) and (3) above i T is the value of the minor axis of the i-th measured foam particle (unit: mm), and n is the total number of foam particles measured. Tables 1 and 2 show the average value of the minor axis of the foam particles. av and coefficient of variation T cv This indicates.

[0112] <Shrinkage rate of foamed particles> The foamed particles immediately after manufacturing were dried in an atmosphere at 60°C for 1 hour, and then the bulk ratio M1 of the foamed particles was measured. Next, the foamed particles were placed in a pressure vessel, and inorganic gas was injected into the vessel to pressurize it to 0.3 MPa. By maintaining this pressure for 12 hours, the foamed particles were impregnated with inorganic gas, and internal pressure was applied by increasing the pressure within the bubbles of the foamed particles. After removing the foamed particles with internal pressure from the pressure vessel, they were left at atmospheric pressure for 12 hours to allow the foamed particles to recover their shrinkage. The bulk ratio M2 (unit: kg / m) of the foamed particles after recovering their shrinkage was measured. 3 The following measurements were taken: Note that "foamed particles immediately after manufacturing" refers to foamed particles immediately after the foaming process is completed (more specifically, foamed particles at the point when all the contents of the sealed container have been released during the foaming process).

[0113] Using the bulk ratios M1 and M2 obtained above, the shrinkage rate (unit: %) of the foamed particles was calculated based on the following formula (4). Table 1 shows the shrinkage rate of the foamed particles in the example, and Table 2 shows the shrinkage rate of the foamed particles in the comparative example. Shrinkage rate = (M2-M1) / M2×100 (4)

[0114] <Minimum molding pressure and moldable range> In evaluating the minimum molding pressure and 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 0.08 and 0.38 MPa (G). The minimum molding pressure and moldable range were then determined based on the surface properties, fusion properties, and recovery properties of the resulting molded bodies.

[0115] The method for producing a foam particle molded body is as follows. First, foam particles were filled into a mold using a cracking filling method. In this example, a mold with a cavity capable of forming a flat foam particle molded body measuring 250 mm in length, 200 mm in width, and 50 mm in thickness was used. The amount of cracking was as shown in Tables 1 and 2. In this example, the amount of cracking is expressed as a percentage of the ratio of the size of the cracking gap (unit: mm) in the cavity to the internal dimension (unit: mm) in the thickness direction of the cavity. For example, in the case of a cracking amount of 10%, the foam particles were filled into the mold with a cracking gap of 5 mm in the thickness direction of the molded body, and then the mold was completely closed to mechanically compress the foam particles inside the mold.

[0116] 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).

[0117] 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 smallest molding pressure at which a satisfactory product was obtained (i.e., the molding pressure at which a satisfactory product could be obtained) was defined as the minimum molding pressure. The number of molding pressures at which a satisfactory product (i.e., a good molded body) could be obtained (i.e., the number of molding conditions) was defined as the moldable range. Table 1 shows the minimum molding pressure and moldable range of the foam particles in the example, and Table 2 shows the minimum molding pressure and moldable range of the foam particles in the comparative example. A lower minimum molding pressure and a wider moldable range indicate superior moldability.

[0118] The evaluation methods for surface properties, fusion properties, and recovery properties in the assessment of the minimum molding pressure and moldable range are as follows.

[0119] ·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.

[0120] • Fusion properties The foam particle molded body was fractured so that it was divided into roughly equal parts along its longitudinal direction. More than 100 foam particles were randomly selected from the foam particles exposed on the fracture surface and visually observed to determine whether they were foam particles that fractured internally (i.e., foam particles that underwent material failure) or foam particles that fractured at the interface between foam particles. The ratio of the number of foam particles that fractured internally to the total number of foam particles observed was then calculated as a percentage (i.e., material failure rate), and this value was defined as the fusion rate. A fusion rate of 90% or higher was judged as passing, and a rate below 90% was judged as failing.

[0121] ·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.

[0122] <Hydrofluoroolefin content in foamed particles> Approximately 0.5 mg of effervescent particles, left to stand at room temperature and atmospheric pressure for 3 days immediately after manufacturing, were placed in a vial. The vial was heated to 170°C to vaporize the hydrofluoroolefins in the effervescent particles. The gas containing hydrofluoroolefins in the vial was then introduced into a gas chromatograph-mass spectrometer (Shimadzu GCMS-QP2010) and mass spectrometry was performed to determine the hydrofluoroolefin content in the effervescent particles. A calibration curve prepared using a sample containing a known amount of hydrofluoroolefin was used for the measurement. A VARIAN CP-PoraPLOT.Q column was used.

[0123] <Molded object density> The mass (in g) of the foam particle molded body obtained by in-mold molding at the aforementioned lower limit molding pressure is divided by the volume (in L) determined from the external dimensions of the molded body, and then the density of the molded body (in kg / m³) is calculated by converting the units. 3 The density was calculated. Table 1 shows the molded density of the foamed particle molded bodies of the examples, and Table 2 shows the molded density of the foamed particle molded bodies of the comparative examples.

[0124] <Compression characteristics> From the center of the foam particle molded body obtained by in-mold molding at the lower limit molding pressure, a rectangular parallelepiped specimen measuring 50 mm in length, 50 mm in width, and 25 mm in thickness was taken, ensuring that the skin surface, i.e., the surface that was in contact with the surface without a mold during in-mold molding, was excluded. A compression test was performed on the specimen according to the method specified in JIS K6767:1999, and a stress-strain curve was obtained. Based on the stress-strain curve, the 5% deformation compressive stress σ5 (unit: kPa) and the 50% deformation compressive stress σ5 of the specimen (i.e., the foam particle molded body) were determined. 50 (Unit: kPa) and 70% deformation compressive stress σ 70 The compression was calculated in kPa (unit: kPa). The compression test was conducted in a laboratory at 23°C, with a compression rate of 10 mm / min.

[0125] Figure 2 shows an example of a stress-strain curve. Table 1 shows the 50% deformation compressive stress σ in the foamed particle molded article of the example, calculated based on the stress-strain curve. 50 And, 70% deformation compressive stress σ5 for 5% deformation compressive stress σ 70 ratio σ 70 Table 2 shows the 50% deformation compressive stress σ in the comparative example foam particle molded article, calculated based on the stress-strain curve. 50 And, 70% deformation compressive stress σ5 for 5% deformation compressive stress σ 70 ratio σ 70 This shows / σ5.

[0126] Stress ratio σ 70The smaller the value of / σ5, the smaller the difference between the stress required for deformation of the foam particle molded body when the strain applied is small and the stress required for deformation of the foam particle molded body when the strain applied is large, resulting in superior energy absorption performance. Therefore, the stress ratio σ 70 Foam particle molded articles with a small value of / σ5 are suitable for applications such as shock absorbers. 70 The value of / σ5 is also affected by the type of base resin and the density of the molded product, so only for Comparative Examples 4 and 6, whose molded product densities are roughly the same as those of Examples 1-4, 6 and 7, the stress ratio σ 70 The values ​​of / σ5 are listed, and the stress ratio σ of other comparative examples is listed. 70 I wrote "-" in the / σ5 column.

[0127] [Table 1]

[0128] [Table 2]

[0129] As shown in Table 1, the foamed particles of Examples 1 to 8 use hydrofluoroolefin and nitrogen as blowing agents, and by controlling the pressure inside the container during the blowing agent addition process, the pressure inside the container immediately before foaming is set to within the specified range. By foaming the resin particles under these conditions, even when producing foamed particles with a high apparent density within the specified range using a direct foaming method with hydrofluoroolefin, it is possible to obtain foamed particles with a small coefficient of variation in the short diameter and small particle size variation. Furthermore, the foamed particles of Examples 1 to 8 have a wide moldable range and good moldability.

[0130] Furthermore, the foamed particles produced using hydrochlorofluoroolefins, as in Examples 1 and 2, had a wider moldable range compared to the foamed particles of Example 3, which had a similar apparent density and were produced using hydrofluoroolefins that did not contain chlorine in their molecular structure.

[0131] Furthermore, as shown in Example 8, it was confirmed that the above effects are fully exhibited even when a polyethylene-based resin is used as the base resin.

[0132] On the other hand, the foamed particles of Comparative Example 1 shown in Table 2 were foamed using only nitrogen and not hydrofluoroolefin, resulting in a narrow moldable range and poor moldability.

[0133] In Comparative Example 2, the foamed particles were foamed using only hydrofluoroolefin and no nitrogen to achieve the aforementioned specific range of apparent density, resulting in a large variation in particle size.

[0134] The foamed particles of Comparative Example 3 did not fully benefit from the hydrofluoroolefin due to the low amount of hydrofluoroolefin added. As a result, the foamed particles of Comparative Example 3 had a narrow moldable range and poor moldability, similar to Comparative Example 1.

[0135] The foamed particles of Comparative Examples 4 and 5 had a small amount of nitrogen added and low pressure inside the container immediately before foaming, resulting in a large variation in particle size, similar to Comparative Example 2. Furthermore, the foamed particle molded body obtained by in-mold molding of the foamed particles of Comparative Example 4 had a stress ratio σ lower than that of Examples 1, 2, and 6, which exhibited smaller variation in particle size. 70 The value of / σ5 increased, and the energy absorption performance of the foamed particle molded body decreased.

[0136] The carbon dioxide used in Comparative Example 6 has a superior ability to plasticize polyolefin resins compared to nitrogen, and is a foaming agent that can efficiently foam polyolefin resin particles. Therefore, in order to obtain foamed particles having the specific range of apparent density, it was necessary to reduce the amount of carbon dioxide added. Consequently, in Comparative Example 6, the pressure inside the container immediately before foaming was low, leading to an increase in particle size variation.

[0137] In Comparative Example 7, the amount of carbon dioxide added was increased compared to Comparative Example 6 in order to suppress the variation in particle size. As a result, the pressure inside the container immediately before foaming increased, which suppressed the variation in particle size, but the apparent density decreased, and foamed particles with the desired apparent density could not be obtained. In addition, shrinkage of the foamed particles occurred in this case.

[0138] Although specific embodiments of the polyolefin-based resin foam particles and their manufacturing method according to the present invention have been described above based on the examples, the polyolefin-based resin foam particles and their manufacturing method according to the present invention are not limited to the embodiments described in the examples, and the configuration can be appropriately modified without impairing the spirit of the present invention.

Claims

1. A dispersion step in which polyolefin resin particles are dispersed in an aqueous medium, A foaming agent addition step in which a foaming agent is added to a sealed container, After impregnating the polyolefin resin particles with the foaming agent in the sealed container, the polyolefin resin particles are released from the sealed container together with the aqueous medium, causing the polyolefin resin particles to foam, resulting in an apparent density of 50 kg / m³. ３ More than 300kg / m ３ The following foaming process for producing polyolefin-based resin foam particles is included: The aforementioned blowing agent comprises hydrofluoroolefin and nitrogen. The amount of hydrofluoroolefin added in the foaming agent addition step is 1 part by mass or more and 12 parts by mass or less per 100 parts by mass of the polyolefin resin particles. A method for producing polyolefin resin foam particles, comprising the foaming step, in which the pressure inside the sealed container immediately before foaming is adjusted to a range of 1.8 MPa(G) to 4.5 MPa(G) and the polyolefin resin particles are released from the sealed container.

2. The method for producing polyolefin-based resin foam particles according to claim 1, wherein the hydrofluoroolefin has a carbon skeleton with 3 to 5 carbon atoms.

3. The method for producing polyolefin-based resin foam particles according to claim 1, wherein the hydrofluoroolefin has a chlorine atom in its molecular structure.

4. The method for producing polyolefin resin foam particles according to claim 3, wherein the hydrofluoroolefin is one or more compounds selected from the group consisting of 1-chloro-3,3,3-trifluoropropene and 1-chloro-2,3,3,3-tetrafluoropropene.

5. A method for producing polyolefin-based resin foam particles according to any one of claims 1 to 4, wherein the hydrofluoroolefin content in the polyolefin-based resin foam particles three days after the completion of the foaming process is 1% by mass or less.

6. A method for producing polyolefin resin foam particles according to any one of claims 1 to 4, wherein the average value of the short diameter of the polyolefin resin foam particles is 1.0 mm or more and 5.0 mm or less, and the coefficient of variation is 10% or less.

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