Method for producing expanded particles of polypropylene-based resin and method for producing molded body of expanded particles of polypropylene-based resin

Abstract

The present invention addresses the problem of providing a method for producing expanded particles of a polypropylene-based resin, the method making it possible to conduct in-mold molding at a low molding pressure and obtain a molded body having high compressive strength. The problem is solved by a method for producing expanded particles of a polypropylene-based resin, the method comprising expanding polypropylene-based resin particles to obtain expanded particles, wherein a raw material used for forming the resin particles comprises a polypropylene-based resin (A) having a specific melting point and a propylene homopolymer (B) in a specific mass proportion, the MFR ratio therebetween being 25 or greater, and the resin particles have a specific MFR.

Description

Method for producing polypropylene resin foam particles and method for producing a molded polypropylene resin foam particle article

[0001] This disclosure relates to a method for producing polypropylene resin foam particles and a method for producing a molded article of polypropylene resin foam particles.

[0002] Polypropylene-based foam particle molded products are widely used as shock absorbers, heat insulating materials, and various packaging materials for electrical and electronic components, automotive parts, and other precision parts, as well as food products.

[0003] Polypropylene resin foam particle molded articles are manufactured, for example, by a method called in-mold molding, in which polypropylene resin foam particles are filled into a mold and then heated by supplying a heating medium such as steam into the mold. In in-mold molding, when a heating medium is supplied into the mold, the foam particles undergo secondary foaming and their surfaces melt. As a result, the foam particles in the mold fuse together, and a molded article with a shape corresponding to the shape of the mold cavity can be obtained. Since the molded article is prone to swelling due to secondary foaming immediately after molding, it is cooled with water or air in the mold before being released from the mold. Since the dimensions of the molded article are prone to change after release, a curing process is usually provided in which it is left to stand in a high-temperature atmosphere for a predetermined time to restore its shape to the desired form.

[0004] From the viewpoint of reducing environmental impact, it is preferable to obtain polypropylene resin foam particle molded articles by in-mold molding polypropylene resin foam particles at the lowest possible molding pressure.

[0005] Conventionally, methods have been investigated for obtaining molded products by in-mold molding foamed particles at low molding pressure, and several methods have already been proposed.

[0006] For example, Patent Document 1 describes polypropylene resin foam particles characterized by having a crystalline structure in which, in the first DSC curve obtained when polypropylene resin foam particles are heated from room temperature to 200°C at a heating rate of 2°C / min by thermal flux differential scanning calorimetry, the endothermic peak heat amount is 70 to 95% of the total endothermic peak heat amount, and the peak temperature of the endothermic peak is 100 to 140°C, and two or more endothermic peaks appear on the high-temperature side of the main endothermic peak. It is also stated that such foam particles can be molded by heating with steam at a lower pressure compared to in-mold foaming of conventional polypropylene resin foam particles, thus enabling a significant reduction in energy costs during molding.

[0007] Patent Document 2 describes polypropylene resin foam particles characterized by using a polypropylene resin as the base resin, which satisfies all of the following requirements (a) to (d): (a) a polypropylene resin polymerized with a metallocene polymerization catalyst; (b) a polypropylene resin having at least two melting peaks in a DSC curve obtained by melting point measurement using a differential scanning calorimeter, with the lowest melting peak being between 100°C and 135°C and the highest melting peak being between 140°C and 160°C; (c) a polypropylene resin treated with an organic peroxide; and (d) a polypropylene resin having a melt flow rate of 5 g / 10 min to 30 g / 10 min. Furthermore, it is stated that such foamed particles can be used to produce polypropylene resin molded foam articles at extremely low molding steam pressures, and even when the molding steam pressure is increased, there is little deterioration of surface properties, deformation, or shrinkage. This indicates a wide range of molding conditions, good moldability even when using molds with complex shapes or large molds, and minimal deterioration of physical properties such as compressive strength when used as a molded foam article.

[0008] International Publication No. 2009 / 001626, Japanese Patent Publication No. 2012-184303

[0009] In recent years, there has been a growing social demand to reduce environmental impact, and there is a need for polypropylene-based foamed resin particles that can be molded at even lower molding pressures. Conventionally, when using polypropylene-based foamed resin particles to mold at lower molding pressures, the fusion properties and secondary foaming properties were insufficient, and it was sometimes impossible to obtain a molded product. Furthermore, even when a molded product could be obtained, the compressive strength of a molded product obtained at a lower molding pressure tended to be lower than that of a molded product obtained at a higher molding pressure.

[0010] This disclosure aims to solve the above-mentioned problems. Specifically, this disclosure provides a method for producing polypropylene resin foam particles that can be molded in a mold at a low molding pressure and can produce a molded article with high compressive strength.

[0011] This disclosure relates to a method for producing foamed polypropylene resin particles by foaming polypropylene resin particles, wherein the polypropylene resin raw material used to form the resin particles comprises a polypropylene resin (A) having a melting point of 138°C or lower and a propylene homopolymer (B) having a melting point of 150°C or higher, in a mass ratio of polypropylene resin (A):propylene homopolymer (B) = 99:1 to 85:15, and the melt flow rate (MFR) of the resin (A) is measured under conditions of a temperature of 230°C and a load of 2.16 kg. A and the melt flow rate MFR of the polymer (B) B Ratio to [MFR] A / MFR B The method for producing polypropylene resin foam particles is such that the ratio is 25 or more, and the resin particles have a melt flow rate of 5 g / 10 min or more and 30 g / 10 min or less, as measured under conditions of a temperature of 230°C and a load of 2.16 kg.

[0012] A method for producing such polypropylene resin foam particles (hereinafter also simply referred to as "foam particles") will hereinafter be referred to as "the manufacturing method of the present disclosure."

[0013] According to this disclosure, it is possible to provide a method for producing polypropylene resin foam particles that enables in-mold molding at low molding pressure and yields a molded article with high compressive strength.

[0014] This diagram illustrates the calculation method for the total heat content of foamed particles, the peak temperature and heat of fusion of the main endothermic peak, and the peak temperature and heat of fusion of the high-temperature peak.

[0015] <Physical Properties of Raw Materials> The polypropylene resin raw materials (hereinafter simply referred to as "raw materials") used in the manufacturing method of this disclosure will be described below. The manufacturing method of this disclosure uses at least two raw materials: a polypropylene resin (A) (hereinafter simply referred to as "resin (A)") having a melting point of 138°C or lower, and a propylene homopolymer (B) (hereinafter simply referred to as "polymer (B)") having a melting point of 150°C or higher. In this disclosure, "X to Y" representing a numerical range is synonymous with "X or more and Y or less", and represents a numerical range that includes X and Y, which are the endpoints of the numerical range.

[0016] The melting point of resin (A) is 138°C or lower. If the melting point of resin (A) is too high, it may become difficult to mold the foamed particles in a mold at a low molding pressure. From this viewpoint, the melting point of resin (A) is preferably 135°C or lower, more preferably 130°C or lower, and particularly preferably 128°C or lower. The lower limit of the melting point of resin (A) is not particularly limited, but from the viewpoint of more reliably ensuring the recovery of the foamed particles, it is preferably 115°C or higher, more preferably 118°C or higher, and even more preferably 120°C or higher. In determining the preferred range of the melting point of resin (A), the upper and lower limits of the melting point of resin (A) described above can be arbitrarily combined. For example, the preferred range of the melting point of resin (A) may be 115°C or higher and 138°C or lower, 115°C or higher and 135°C or lower, 118°C or higher and 130°C or lower, or 120°C or higher and 128°C or lower.

[0017] The melting point of the polymer (B) is 150°C or higher. If the melting point of the polymer (B) is too low, the resilience of the expanded beads is impaired, and dents (sink marks) formed on the molded article immediately after molding become excessively large and do not recover even after curing, which may make it difficult to obtain a molded article having a desired shape. In addition, there is a possibility that the compressive strength of the resulting molded article may decrease. From this viewpoint, the melting point of the polymer (B) is preferably 152°C or higher, more preferably 155°C or higher, still more preferably 158°C or higher, particularly preferably 160°C or higher, and most preferably 162°C or higher. The upper limit of the melting point of the polymer (B) is not particularly limited, but it is preferably 170°C or lower, and more preferably 168°C or lower. In defining the preferred range of the melting point of the polymer (B), the above-described upper limit and lower limit of the melting point of the polymer (B) can be arbitrarily combined. For example, the preferred range of the melting point of the polymer (B) may be 150°C or more and 170°C or less, 152°C or more and 170°C or less, 155°C or more and 170°C or less, 158°C or more and 170°C or less, 160°C or more and 170°C or less, or 162°C or more and 168°C or less.

[0018] The melting points of the resin (A) and the polymer (B) are obtained from a DSC curve measured in accordance with "(2) When measuring melting temperature after performing constant heat treatment" in "3. Conditioning of test pieces" in the method for measuring transition temperature of plastics specified in JIS K7121:2012. A detailed method for measuring the melting points of the resin (A) and the polymer (B) will be described in the Examples mentioned later.

[0019] The melt flow rate (MFR A ) of the resin (A) is preferably 20 g / 10 min or more and 40 g / 10 min or less, more preferably 22 g / 10 min or more and 35 g / 10 min or less, and still more preferably 24 g / 10 min or more and 30 g / 10 min or less. When the melt flow rate (MFR A ) of the resin (A) is within the above range, the secondary expandability of the expanded beads can be further improved, and molding at a lower molding pressure becomes easier.

[0020] The melt flow rate (MFR of the polymer (B) B ) is preferably 3 g / 10 minutes or less, more preferably 0.05 g / 10 minutes or more and 3 g / 10 minutes or less, still more preferably 0.1 g / 10 minutes or more and 2 g / 10 minutes or less, and even more preferably 0.2 g / 10 minutes or more and 1 g / 10 minutes or less. The melt flow rate (MFR of the polymer (B) B ) falling within the above range can further improve the resilience of the expanded beads and more reliably increase the compressive strength of the resulting molded article.

[0021] The melt flow rate MFR of resin (A) measured under conditions of a temperature of 230°C and a load of 2.16 kg A and the melt flow rate MFR of polymer (B) measured under conditions of a temperature of 230°C and a load of 2.16 kg B , the ratio [MFR A / MFR B is 25 or more. If the ratio [MFR A / MFR B is too low, there is a risk that it becomes difficult to perform in-mold molding of the expanded beads at a low molding pressure. In addition, even if an attempt is made to lower the molding pressure by, for example, reducing the heat of fusion of the high-temperature peak of the expanded beads described later, this is not sufficient, and furthermore, in this case, the compressive strength of the resulting molded article may significantly decrease. From this viewpoint, the ratio [MFR A / MFR B is preferably 30 or more, more preferably 35 or more, and even more preferably 40 or more. The ratio [MFR A / MFR B is not particularly limited, for example, it is 100 or less, 80 or less, or 60 or less. When configuring the preferred range of the ratio [MFR A / MFR B , the upper limit and lower limit of the aforementioned ratio [MFR A / MFR B can be arbitrarily combined. For example, the ratio [MFR A / MFR Bmay have a preferred range of 25 or more and 100 or less, may be 30 or more and 100 or less, may be 35 or more and 80 or less, or may be 40 or more and 60 or less. Conventionally, there has been a trade-off relationship between reducing the molding pressure of expanded beads and increasing the compressive strength of the resulting molded article, and it has been difficult to achieve both at a high level. On the other hand, according to the production method of the present disclosure, the above-mentioned predetermined resin (A) and polymer (B) are used as raw materials, and the ratio of melt flow rates [MFR A / MFR B being 25 or more provides expanded beads that can be molded at a low molding pressure and can give a molded article having high compressive strength. Although the reason for this is not clear, by including the propylene homopolymer (B) having a high melting point at a predetermined mass ratio, the resilience of the expanded beads and the compressive strength of the molded article are improved, while the ratio [MFR A / MFR B is mainly composed of the low-melting polypropylene-based resin (A) whose melt flow rate is sufficiently higher than that of the polymer (B) so as to be not less than a predetermined value. This is considered to be because the secondary foamability of the resulting expanded beads can be remarkably improved.

[0022] The melt flow rate (MFR) of the resin (A) and the polymer (B) is a value measured under conditions of a temperature of 230°C and a load of 2.16 kg based on JIS K7210-1:2014. A detailed method for measuring the melt flow rate (MFR) of the resin (A) and the polymer (B) will be described in Examples below.

[0023] The flexural modulus of resin (A) is preferably 1000 MPa or less, more preferably 950 MPa or less, even more preferably 900 MPa or less, particularly preferably 850 MPa or less, and most preferably 800 MPa or less. When the flexural modulus of resin (A) is within the above range, molding at lower molding pressures becomes easier. The lower limit of the flexural modulus of resin (A) is not particularly limited, but for example, it is 500 MPa or more, 600 MPa or more, and 700 MPa or more. When configuring the preferred range of the melting point of resin (A), the upper and lower limits of the melting point of resin (A) described above can be arbitrarily combined. For example, the preferred range of the melting point of resin (A) may be 115°C or more and 138°C or less, 115°C or more and 135°C or less, 118°C or more and 130°C or less, or 120°C or more and 128°C or less.

[0024] The flexural modulus of polymer (B) is preferably 1200 MPa or more and 2000 MPa or less, more preferably 1500 MPa or more and 1950 MPa or less, and even more preferably 1600 MPa or more and 1900 MPa or less. When the flexural modulus of polymer (B) is within the above range, the recovery of the foamed particles can be further enhanced, and the compressive strength of the resulting molded article can be more reliably increased.

[0025] The flexural modulus of resin (A) and polymer (B) is determined according to JIS K7171:2008. Detailed methods for measuring the flexural modulus of resin (A) and polymer (B) will be described in the examples below.

[0026] From the viewpoint of reducing the shrinkage rate of the resulting molded article, the crystallization temperature of resin (A) is preferably 100°C or lower, more preferably 98°C or lower, and even more preferably 95°C or lower. The crystallization temperature of resin (A) is preferably 75°C or higher, and preferably 80°C or higher. In determining the preferred range for the crystallization temperature of resin (A), the upper and lower limits of the crystallization temperature of resin (A) described above can be arbitrarily combined. For example, the preferred range for the crystallization temperature of resin (A) may be 75°C or higher and 100°C or lower, 75°C or higher and 98°C or lower, or 80°C or higher and 95°C or lower.

[0027] The crystallization temperature of polymer (B) is preferably 100°C to 120°C, and more preferably 105°C to 118°C.

[0028] The crystallization temperatures of resin (A) and polymer (B) are values ​​measured in accordance with JIS K7121:2012. Detailed methods for measuring the crystallization temperatures of resin (A) and polymer (B) will be described in the examples below.

[0029] Furthermore, if multiple types of propylene resins satisfying the above-mentioned MFR numerical range are used as resin (A), other physical properties other than MFR will be determined by preparing a measurement compound by melt-kneading each polypropylene resin using an extruder or the like at the blending ratio of each propylene resin used during the production of the resin particles, and the various physical properties measured on this measurement compound will be adopted as the various physical properties of resin (A). Similarly, if multiple types of propylene resins satisfying the above-mentioned MFR numerical range are used as polymer (B), other physical properties other than MFR will be determined by preparing a measurement compound by melt-kneading each polypropylene resin using an extruder or the like at the blending ratio of each propylene resin used during the production of the resin particles, and the various physical properties measured on this measurement compound will be adopted as the various physical properties of polymer (B).

[0030] In the manufacturing method of this disclosure, resin particles are obtained by using resin (A) and polymer (B) as raw materials in a mass ratio of resin (A):polymer (B) = 99:1 to 85:15. If the mass ratio of polymer (B) is too low, the recovery of the foamed particles will be impaired, and sink marks that occur in the molded article immediately after molding will become excessively large and will not recover even after curing, making it difficult to obtain a molded article of the desired shape. In addition, the compressive strength of the obtained molded article may decrease. On the other hand, if the mass ratio of polymer (B) is too high, it may be difficult to mold the foamed particles in the mold at a low molding pressure. From this viewpoint, the above mass ratio is preferably resin (A):polymer (B) = 98:2 to 88:12, and more preferably resin (A):polymer (B) = 97:3 to 90:10. Here, the total mass of resin (A) and polymer (B) is 100.

[0031] Resin (A) is not particularly limited as long as it is a polypropylene resin with a melting point of 138°C or lower. For example, it may be a propylene homopolymer, a propylene copolymer which is a copolymer of propylene and ethylene and / or an α-olefin having 4 or more carbon atoms, a propylene-acrylic acid copolymer, a propylene-maleic anhydride copolymer, etc., and may also be a mixture thereof. Furthermore, these copolymers may be block copolymers, random copolymers, or graft copolymers. The propylene copolymer preferably contains 60 mol% or more of propylene component units, more preferably 80 mol% or more, and even more preferably 90 mol% or more. Examples of propylene copolymers include propylene-ethylene copolymers, propylene-butene copolymers, and propylene-ethylene-butene copolymers. Furthermore, the propylene copolymer is preferably a random copolymer. Resin (A) is preferably at least one selected from the group consisting of ethylene-propylene random copolymer, butene-propylene random copolymer, and ethylene-butene-propylene random copolymer.

[0032] Polymer (B) is not particularly limited as long as it is a propylene homopolymer with a melting point of 150°C or higher. Propylene homopolymers are also called homopolypropylene. Examples of polypropylene homopolymers include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene.

[0033] The above-mentioned polypropylene resin may be crosslinked, but it is preferable that it be uncrosslinked. Furthermore, it is preferable that it has not been treated with organic peroxides.

[0034] The manufacturing method of this disclosure includes the above-described resin (A) and polymer (B) as raw materials, but may also include other polymer components and additives that do not fall under the category of resin (A) and polymer (B).

[0035] Other polymer components include polypropylene resins that do not fall under resin (A), and propylene homopolymers that do not fall under polymer (B). Note that propylene homopolymers with a melting point of 138°C or lower fall under resin (A).

[0036] Other polymer components include thermoplastic resins other than polypropylene resins, rubbers, and elastomers. Specifically, thermoplastic resins include, for example, polyethylene resins such as high-density polyethylene, low-density polyethylene, linear low-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, and ethylene-methacrylic acid copolymer; polystyrene resins such as polystyrene and styrene-maleic anhydride copolymer; polybutene resins, polyamide resins, polyester resins, and polycarbonate resins. Rubbers include, for example, ethylene-propylene rubber, ethylene-1-butene rubber, propylene-1-butene rubber, styrene-butadiene rubber and its hydrogenated products, isoprene rubber, neoprene rubber, and nitrile rubber. Elastomers include, for example, thermoplastic elastomers such as olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers and their hydrogenated products.

[0037] The content of other polymer components is not particularly limited as long as it does not hinder the intended effects of this disclosure, but when the total amount of resin (A) and polymer (B) is 100 parts by mass, it is preferably 35 parts by mass or less, more preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less.

[0038] Examples of additives that may be included as raw materials include foam regulators. Examples of foam regulators include inorganic powders such as zinc borate, talc, calcium carbonate, borax, and aluminum hydroxide. By incorporating a foam regulator, it becomes easier to adjust the bulk density and average bubble diameter of the resulting foamed particles to a desired range. The content of the foam regulator is preferably 0.005 to 1 part by mass when the total amount of resin (A) and polymer (B) is 100 parts by mass.

[0039] Examples of additives that may be included as raw materials include antioxidants, ultraviolet absorbers, antistatic agents, metal deactivators, crystal nucleating agents, flame retardants, flame retardant aids, plasticizers, light stabilizers, antibacterial agents, glass fibers, carbon fibers, and colorants.

[0040] The additive content is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, even more preferably 5 parts by mass or less, and particularly preferably 3 parts by mass or less, based on a total amount of resin (A) and polymer (B) of 100 parts by mass.

[0041] <Physical Properties of Resin Particles> In the manufacturing method of this disclosure, polypropylene resin particles (hereinafter also simply referred to as "resin particles") are obtained using the raw materials described above. The resin particles are not particularly limited as long as they can be obtained using the raw materials described above and foamed by conventionally known methods to produce foamed particles.

[0042] The resin particles may be single-layer resin particles obtained by melting, kneading, and granulating the above raw materials in an extruder or the like. By foaming such single-layer resin particles, foamed particles consisting of a foamed layer (foamed particle body) having a buoyant structure containing numerous air bubbles can be obtained. In addition, in the manufacturing method of the present disclosure, foamed particles may be obtained by foaming at least the core layer of a multilayer resin particle having a core layer and a coating layer covering it. This yields foamed particles consisting of a foamed layer (foamed particle body) having a buoyant structure containing numerous air bubbles and a coating layer. In this case, the core layer of the resin particle is obtained by melting and kneading the above raw materials.

[0043] The shape of the resin particles is not particularly limited. For example, they may be cylindrical, rugby ball-shaped, spherical, or tubular.

[0044] The resin particles preferably have a mass of 0.1 to 20 mg per particle, and more preferably 0.5 to 20 mg per particle. The mass per resin particle is the average mass of 200 randomly selected resin particles.

[0045] The resin particles have a melt flow rate of 5 g / 10 min to 30 g / 10 min, measured under conditions of 230°C and a load of 2.16 kg. If the melt flow rate of the resin particles is too low, the foaming properties may be excessively reduced. Conversely, if the melt flow rate of the resin particles is too high, the compressive strength of the resulting molded article may decrease. From this viewpoint, the melt flow rate of the resin particles is preferably 10 g / 10 min to 28 g / 10 min, and more preferably 15 g / 10 min to 25 g / 10 min.

[0046] Furthermore, the MFR of resin particles shall be the value obtained by measuring them using the same method as the MFR of resin (A) and polymer (B), except that the resin particles themselves are the measurement target. In addition, even if the resin particles have a multilayer structure consisting of a core layer and a coating layer covering the core layer, the entire resin particle shall be the measurement target for the MFR.

[0047] The melting point of the resin particles is preferably 108 to 130°C, more preferably 110 to 128°C, and even more preferably 115 to 125°C.

[0048] The melting point of the resin particles shall be the value obtained by measuring them using the same method as the melting points of the resin (A) and polymer (B), except that the resin particles themselves are the object of measurement. Furthermore, even if the resin particles have a multilayer structure consisting of a core layer and a coating layer covering the core layer, the entire resin particle shall be the object of melting point measurement.

[0049] The flexural modulus of the resin particles is preferably 650 MPa or more and 1000 MPa or less, more preferably 700 MPa or more and 980 MPa or less, even more preferably 750 MPa or more and 950 MPa or less, and particularly preferably 800 MPa or more and 900 MPa or less.

[0050] The flexural modulus of the resin particles shall be the value obtained by measuring it using the same method as the flexural modulus of the resin (A) and polymer (B), except that the resin particles themselves are the object of measurement. Furthermore, even if the resin particles have a multilayer structure consisting of a core layer and a coating layer covering the core layer, the entire resin particle shall be the object of measurement for flexural modulus.

[0051] In the manufacturing method of this disclosure, foamed particles may be obtained by foaming resin particles having a core layer and a coating layer covering it. By foaming particles having a multilayer structure including a core layer and a coating layer covering it, foamed particles with a multilayer structure having a foamed core layer (foamed particle body) and a coating layer covering the foamed particle body can be obtained. The coating layer may be in a foamed state or a non-foamed state, but it is preferable that it be in a non-foamed state. The non-foamed state includes a state in which the coating layer does not foam and does not contain air bubbles, and a state in which air bubbles disappear after foaming, meaning that there is almost no air bubble structure in the coating layer.

[0052] The coating layer can be made of, for example, a polyolefin resin. Preferably, the polyolefin resin is a crystalline polyolefin resin having a melting point lower than the melting point of the resin particles constituting the core layer, or an amorphous polyolefin resin having a softening point lower than the melting point of the resin composition constituting the core layer. Furthermore, the polyolefin resin is preferably a polypropylene resin and / or a polyethylene resin, and more preferably a polypropylene resin.

[0053] The polyolefin resin used to form the coating layer preferably has a melting point of 95°C to 130°C, more preferably 98°C to 125°C, and even more preferably 100°C to 120°C. Furthermore, the difference between the melting point of the core layer and the melting point of the coating layer (i.e., the value obtained by subtracting the melting point of the coating layer from the melting point of the core layer) is preferably 1°C to 35°C, more preferably 5°C to 30°C, and even more preferably 8°C to 25°C. In this case, the foam particles fuse together more easily even at low molding steam pressures, and it becomes easier to stably obtain a molded article with good compressible properties.

[0054] When using a polypropylene resin to form a coating layer, a propylene copolymer, which is a copolymer of propylene and ethylene and / or an α-olefin having 4 or more carbon atoms, can be preferably used as the polypropylene resin. The propylene copolymer preferably contains 60 mol% or more of propylene component units, more preferably 80 mol% or more, and even more preferably 90 mol% or more. Examples of propylene copolymers include propylene-ethylene copolymer, propylene-butene copolymer, and propylene-ethylene-butene copolymer. Furthermore, the propylene copolymer is preferably a random copolymer.

[0055] When using polyethylene resin to form a coating layer, examples of polyethylene resin include low-density polyethylene (PE-LD), linear low-density polyethylene (PE-LLD), medium-density polyethylene (PE-MD), and high-density polyethylene (PE-HD). Among these, linear low-density polyethylene (PE-LLD) is preferred.

[0056] <Method for manufacturing resin particles> The method for manufacturing resin particles is not particularly limited other than using the specific raw materials described above, and can be manufactured, for example, by conventionally known methods.

[0057] In the case of single-layer resin particles, the raw materials are heated and kneaded in an extruder or the like to form a molten mixture. This molten mixture is then extruded from the extruder and pelletized using methods such as strand cutting, hot cutting, or underwater cutting to obtain the resin particles.

[0058] In the manufacturing method of this disclosure, foamed particles may be obtained by foaming resin particles having a core layer and a coating layer covering it. Resin particles having a core layer and a coating layer can be manufactured, for example, by the method described below.

[0059] First, an extruder for forming the core layer, an extruder for forming the coating layer, and a die for forming multilayer strands are prepared. All of these may be conventionally known. Here, both the extruder for forming the core layer and the extruder for forming the coating layer are connected downstream to the die for forming multilayer strands. The die for forming multilayer strands has a structure that allows for co-extrusion and has a structure that allows for the layering of molten resin for forming the core layer and the coating layer within the die.

[0060] Next, raw materials for forming the core layer are supplied to a core layer forming extruder, where they are melted and kneaded to form a resin molten material. Similarly, raw materials for forming the coating layer are supplied to a coating layer forming extruder, where they are melted and kneaded to form a resin molten material. Both of these resin molten materials are then continuously extruded from the extruders and continuously introduced into a die for forming multilayer strands. These resin molten materials merge within the die and are continuously extruded through the small holes in the die to form a string-like multilayer strand. The multilayer strand has a two-layer structure consisting of a core layer and a coating layer.

[0061] Here, the mass ratio of the core layer to the coating layer is preferably 99.5:0.5 to 85:15, more preferably 99:1 to 90:10, and even more preferably 98:2 to 92:8. Here, the total mass of the core layer and the coating layer is assumed to be 100. By having the mass ratio within the above range, molding can be performed more reliably at a lower molding pressure, and a molded body with better compressive strength can be manufactured more easily.

[0062] The extruded strands are water-cooled, cut in a pelletizer, and formed into columnar resin particles.

[0063] <Physical Properties of Foamed Particles> In the manufacturing method of this disclosure, foamed resin particles obtained as described above are foamed to obtain foamed particles. When foaming resin particles having a core layer and a coating layer, at least the core layer is foamed to obtain foamed particles.

[0064] The average outer diameter of the foamed particles is preferably 1 to 5 mm, and more preferably 2 to 4 mm. Foamed particles having such an average outer diameter exhibit excellent filling properties into the mold during molding.

[0065] The average outer diameter of the foam particles is determined as follows: First, the foam particle is cut at a position where the area of ​​the cross-section is approximately maximum. Next, a photograph of the cross-section of the foam particle is taken to determine the cross-sectional area of ​​the foam particle. The diameter of a virtual circle with the same area is then calculated and taken as the outer diameter of the foam particle. This operation is then performed for 50 randomly selected foam particles, and the arithmetic mean of the obtained measurements is taken as the average outer diameter of the foam particles.

[0066] The shape of the foam particles is not particularly limited. For example, they may be cylindrical, rugby ball-shaped, spherical, or tubular.

[0067] The foamed particles have a bulk density of 10-200 kg / m³. 3 Preferably, it is 12 to 150 kg / m 3 It is more preferable that the load be 15-100 kg / m 3 It is more preferable that the load is 20-80 kg / m 3 It is even more preferable that this be the case.

[0068] The method for measuring the bulk density of the foamed particles will be explained in the examples described later.

[0069] The foamed particles preferably have a closed-cell ratio of 80% or more, more preferably 85% or more, and even more preferably 90% or more. In this case, the moldability of the foamed particles can be further improved, and the compressive strength of the resulting molded article can be further increased.

[0070] The method for measuring the closed-cell ratio of foamed particles will be explained in the examples described later.

[0071] The foamed particles preferably have a total heat content of 50 to 70 J / g, more preferably 52 to 65 J / g, and even more preferably 55 to 62 J / g.

[0072] The method for measuring the total heat content of the foamed particles will be explained in the examples described later.

[0073] Preferably, the foamed particles have a crystalline structure such that the DSC curve obtained by differential scanning calorimetry (DSC) performed on the foamed particles while heating them from 23°C to 230°C at a heating rate of 10°C / min shows a main endothermic peak and one or more melting peaks (high-temperature peaks) located at a higher temperature than the main endothermic peak. When foamed particles have such a crystalline structure, the in-moldability of the foamed particles can be improved. The main endothermic peak is a melting peak due to the melting of resin-specific components constituting the foamed particles, and can also be said to be a melting peak that appears due to the melting of crystals normally present in the resin constituting the foamed particles. The main endothermic peak is also called the "resin-specific peak". The high-temperature peak is presumed to appear due to the melting of secondary crystals formed in the resin components during the manufacturing process of the foamed particles. That is, if a high-temperature peak appears in the DSC curve, it is presumed that secondary crystals have been formed in the resin components.

[0074] From the viewpoint of further improving moldability at low pressure, the foamed particles preferably have a peak temperature of 115 to 132°C for the main endothermic peak, more preferably 118 to 130°C, and even more preferably 120 to 128°C.

[0075] The foamed particles preferably have a main endothermic peak heat of fusion of 35 to 60 J / g, more preferably 40 to 55 J / g, and even more preferably 42 to 52 J / g.

[0076] The method for measuring the peak temperature and heat of fusion of the main endothermic peak of the foamed particles will be explained in the examples described later.

[0077] From the viewpoint of further enhancing the recovery properties of the foamed particles, the peak temperature of the high-temperature peak is preferably 150 to 178°C, more preferably 152 to 175°C, and even more preferably 155 to 170°C.

[0078] The heat of fusion of the foamed particles at the high temperature peak is preferably 5 to 20 J / g, more preferably 6 to 18 J / g, and more preferably 7 to 15 J / g. Generally, if the heat of fusion of the foamed particles at the high temperature peak is small, the molding pressure will be low, but the compressive strength of the resulting molded article tends to be low. Conversely, if the heat of fusion of the foamed particles at the high temperature peak is small, the molding pressure will be high, but the compressive strength of the resulting molded article tends to be high. From the viewpoint of appropriately balancing these two factors, it is preferable that the heat of fusion of the foamed particles at the high temperature peak is within the above range.

[0079] The methods for measuring the peak temperature and heat of fusion of the foamed particles will be explained in the examples described later.

[0080] Furthermore, it is preferable that the difference between the peak temperatures [Th1 - Th2] is between -10°C and 10°C, when, in the first DSC curve (1st heat) obtained by performing differential scanning calorimetry on heating the foamed particles from 23°C to 230°C at a heating rate of 10°C / min, the peak temperature of one or more melting peaks (high-temperature peaks) located on the high-temperature side is defined as Th1, and in the second DSC curve (2nd heat) obtained by performing differential scanning calorimetry on heating the foamed particles from 23°C to 230°C at a heating rate of 10°C / min, then cooling them from 230°C to 23°C at a cooling rate of 10°C / min, and then heating them again from 23°C to 230°C at a heating rate of 10°C / min, the peak temperature of the melting peak appearing on the highest-temperature side is defined as Th2. In this case, even though a polymer (B) with a high melting point is blended with the resin (A), molding at a lower pressure becomes more reliable compared to the case where polymer (B) is not blended with the resin (A). From this viewpoint, it is more preferable that the difference [Th1-Th2] is between -8°C and 8°C, and even more preferable that it is between -5°C and 5°C. However, the melting peak that appears on the highest temperature side is selected from among melting peaks having a heat of fusion of at least 2 J / g or more.

[0081] Furthermore, the difference [Th1-Tl1] between the peak temperature of the high-temperature peak of the foamed particles (Th1) and the peak temperature of the main endothermic peak of the foamed particles (Tl1) is preferably 20°C or more and 50°C or less, more preferably 25°C or more and 45°C or less, even more preferably 28°C or more and 42°C or less, and particularly preferably 30°C or more and 40°C or less. In this case, the foamed particles can be molded in a mold at a lower molding pressure, and the effects of this disclosure, such as obtaining a molded article with high compressive strength, can be more reliably achieved.

[0082] <Method for Manufacturing Foamed Particles> The method for manufacturing foamed particles is not particularly limited other than using resin particles as described above, and can be manufactured by conventionally known methods, for example. Specifically, foamed particles can be obtained by a direct foaming method, for example, as described below.

[0083] First, the resin particles are placed in a sealed container such as an autoclave and dispersed in a dispersion medium such as water or alcohol (preferably water).

[0084] It is preferable to add the dispersant together with the resin particles into the sealed container. This is because the resin particles are less likely to fuse together within the sealed container. The dispersant can be either organic or inorganic, but finely particulate inorganic materials are preferred for ease of handling. Examples include natural or synthetic clay minerals such as kaolin, mica, and clay, as well as aluminum oxide, tricalcium phosphate, magnesium pyrophosphate, zinc oxide, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, and iron oxide. It is preferable to use 0.001 to 5 parts by mass of the dispersant per 100 parts by mass of the resin particles.

[0085] Alternatively, a dispersion aid may be added to the sealed container along with the resin particles. This is because it can enhance the dispersing power of the dispersant. Examples of such dispersion enhancers include magnesium chloride, magnesium nitrate, magnesium sulfate, aluminum chloride, aluminum nitrate, and aluminum sulfate. It is preferable to use 0.0001 to 1 part by mass of the dispersion enhancer per 100 parts by mass of the resin particles.

[0086] Furthermore, it is preferable to introduce the surfactant into the sealed container together with the resin particles. Examples of surfactants include anionic surfactants such as sodium dodecylbenzenesulfonate.

[0087] Next, a foaming agent is placed inside a sealed container to impregnate the resin particles with the foaming agent. At this time, heating and pressurizing the inside of the sealed container and maintaining it at a predetermined temperature and pressure makes it easier to impregnate the resin particles with the foaming agent. The pressure inside the sealed container is preferably 0.5 to 4.0 MPa(G) in gauge pressure, and more preferably 1.0 to 4.0 MPa(G). It is also preferable to maintain the temperature inside the sealed container at 130 to 170°C. This allows for obtaining resin particles containing the foaming agent.

[0088] Examples of blowing agents include organic physical blowing agents such as aliphatic hydrocarbons like propane, butane, isobutane, pentane, isopentane, hexane, and heptane; cyclic aliphatic hydrocarbons such as cyclobutane and cyclohexane; halogenated hydrocarbons such as chlorofluoromethane, trifluoromethane, 1,2-difluoroethane, 1,2,2,2-tetrafluoroethane, 2,3,3,3-tetrafluoropropene, trans-,3,3,3-tetrafluoropropene, trans-1-chloro-3,3,3-trifluoropropene, methyl chloride, ethyl chloride, and methylene chloride; and so-called inorganic physical blowing agents such as nitrogen, oxygen, air, carbon dioxide, helium, argon, and water.

[0089] The amount of foaming agent added is preferably 0.1 to 30 parts by mass, and more preferably 0.5 to 15 parts by mass, per 100 parts by mass of resin particles.

[0090] Next, one end of the sealed container below the liquid surface is opened, and the contents of the sealed container are released into an atmosphere with lower pressure than that inside the container, thereby obtaining foamed particles.

[0091] Furthermore, a step to adjust the crystalline structure of the resin components constituting the resin particles may be performed between the time the resin particles are dispersed in the aqueous medium and the time the resin particles are foamed. By foaming after adjusting the crystalline structure of the resin components, foamed particles with excellent in-moldability and mechanical strength can be easily obtained. An example of a method for adjusting the crystalline structure of the resin components is as follows: First, a holding step is performed in which the temperature of the resin particles is kept within a temperature range of -15°C or higher from the melting point of the resin particles and +15°C or lower from the melting point of the molten particles for a sufficient time, preferably about 10 to 60 minutes. By foaming the resin particles that have undergone this holding step, foamed particles having a crystalline structure in which the second melting peak described above appears can be obtained. The temperature inside the container during foaming is preferably -5°C or higher from the melting point of the resin particles and +10°C or lower.

[0092] From the viewpoint of obtaining foamed particles with lower bulk density (higher foaming ratio), a two-stage foaming process may be performed to further foam the obtained foamed particles. For example, the first stage of foamed particles (single-stage foamed particles) obtained by foaming resin particles using a direct foaming method, etc., can be pressurized with air or the like to increase the pressure inside the bubbles of the single-stage foamed particles (internal pressure), and then the single-stage foamed particles can be heated with steam or the like to further foam them. In this way, foamed particles with lower bulk density can be obtained.

[0093] <Physical properties of the molded article> A molded article can be obtained using the foamed particles described above, which are obtained by the manufacturing method of this disclosure.

[0094] The density of the molded body (molded body density) is 10 to 200 kg / m³. 3 Preferably, it is 15 to 150 kg / m 3 It is more preferable that the load be 20-100 kg / m 3 It is even more preferable that this be the case.

[0095] The method for measuring the density of the molded body (molded body density) will be explained in the examples described later.

[0096] <Method for Manufacturing Molded Articles> The method for manufacturing molded articles from foamed particles is not particularly limited and can be manufactured by conventionally known methods, for example. Molded articles can be manufactured by a conventionally known method called in-mold heating molding (in-mold molding). Specifically, foamed particles are filled into a molded space (cavity) of a mold that can be heated and cooled, as well as opened, closed, and sealed, after the internal pressure of the foamed particles has been increased as needed. Steam is then supplied to heat the foamed particles in the mold, causing them to expand (secondary foaming) and fuse together. The foamed particles are then cooled and removed from the mold to obtain a molded article that has the shape of the molded space of the mold. Molded articles can be manufactured by a batch-type in-mold heating molding method.

[0097] Furthermore, molded bodies can also be obtained by in-mold molding methods such as the following. For example, a compression molding method can be used in which foam particles are filled into a molding space formed by a pair of molds with uneven surfaces under atmospheric pressure or reduced pressure, the volume of the molding space is compressed to a reduction of 5 to 70%, and then a heat transfer medium such as steam is introduced into the molding space to heat and fuse the foam particles (see, for example, Japanese Patent Publication No. 46-38359).

[0098] Furthermore, there is a pressure molding method in which foamed particles are pre-treated with one or more volatile foaming agents or inorganic gases to enhance their secondary foaming power, and then, while maintaining that secondary foaming power, the foamed particles are filled into a molding space formed by a pair of uneven molds under atmospheric pressure or reduced pressure, and then a heat transfer medium is introduced into the molding space to heat and fuse the foamed particles (for example, Japanese Patent Publication No. 51-22951).

[0099] Furthermore, there is a compression filling method in which a molding space pressurized to above atmospheric pressure using compressed gas is filled with foam particles pressurized to a pressure higher than that, and then a heat transfer medium such as steam is introduced into the molding space to heat and fuse the foam particles (for example, Japanese Patent Publication No. 4-46217).

[0100] When obtaining a molded article, it is preferable to set the lower limit of the molding heating steam pressure (molding pressure) to less than 0.10 MPa(G), more preferably to 0.09 MPa(G) or less, and even more preferably to 0.08 MPa(G) or less. In this case, a significant contribution can be made to reducing the environmental burden. Furthermore, a reduction in the molding cycle due to a reduction in water cooling time can be expected. Note that (G) means gauge pressure. According to the manufacturing method of this disclosure, even when molding at such an extremely low molding pressure, a molded article with high compressive strength can be obtained.

[0101] The method for manufacturing a molded article is, for example, a method for manufacturing a polypropylene resin foam particle molded article, in which the foam particles are filled into a mold, steam is supplied into the mold as a heating medium to heat them, and the foam particles are fused together by in-mold molding to obtain a foam particle molded article, wherein the molding heating steam pressure during in-mold molding is less than 0.10 MPa(G).

[0102] The method for determining the molding heating steam pressure will be explained in the examples described later.

[0103] It is preferable to cure the molded body at a high temperature (e.g., 60-80°C) for a certain period (e.g., 12-24 hours) immediately after demolding from the mold. In this case, even if the molded body is deformed immediately after molding, it tends to recover to the desired shape.

[0104] Conventionally, when polypropylene resin foam particles were used to form molded articles at low molding pressures, the resulting molded articles tended to have lower compressive strength. In other words, there was a trade-off relationship between molding pressure and the compressive strength of the molded articles. Specifically, no manufacturing method had been found that could produce molded articles with a 5% compressive strength of 40 kPa or more, preferably 45 kPa or more, when molded at extremely low molding pressures of less than 0.10 MPa(G). The methods described in the aforementioned Patent Documents 1 and 2 cannot achieve this. For example, when a molded article is used as a packaging material such as a cushioning material, it is desirable to have high energy absorption performance when slightly deformed, so a high 5% compressive strength is preferable. In contrast, when foam particles produced by the manufacturing method of the present disclosure are used, a molded article with high compressive strength can be obtained even at low molding pressures. Specifically, even when molded at a molding pressure of less than 0.10 MPa(G), there is a tendency to obtain molded articles with a 5% compressive strength of 40 kPa or more.

[0105] This disclosure includes the following embodiments (1) to (11): (1) A method for producing foamed polypropylene resin particles by foaming polypropylene resin particles, wherein the polypropylene resin raw material used to form the resin particles contains a polypropylene resin (A) having a melting point of 138°C or less and a propylene homopolymer (B) having a melting point of 150°C or more, in a mass ratio of polypropylene resin (A):propylene homopolymer (B) = 99:1 to 85:15, and the melt flow rate MFR of the resin (A) is measured under the conditions of a temperature of 230°C and a load of 2.16 kg. A and the melt flow rate MFR of the polymer (B) B Ratio to [MFR] A / MFR B A method for producing polypropylene resin foam particles, wherein the ratio is 25 or more, and the melt flow rate of the resin particles, measured under the conditions of a temperature of 230°C and a load of 2.16 kg, is 5 g / 10 min or more and 30 g / 10 min or less. (2) Melt flow rate of the resin (A) MFR AA method for producing polypropylene resin foam particles as described in (1) above, wherein the amount is 20 g / 10 min or more and 40 g / 10 min or less. (3) Melt flow rate MFR of the polymer (B) BA method for producing polypropylene resin foam particles according to (1) or (2) above, wherein the amount is 3 g / 10 min or less. (4) A method for producing polypropylene resin foam particles according to any one of (1) to (3) above, wherein the resin (A) is at least one selected from the group consisting of ethylene-propylene random copolymer, butene-propylene random copolymer and ethylene-butene-propylene random copolymer. (5) A method for producing polypropylene resin foam particles according to any one of (1) to (4) above, wherein the flexural modulus of the resin (A) is 800 MPa or less. (6) A method for producing polypropylene resin foam particles according to any one of (1) to (5) above, wherein the flexural modulus of the resin particles is 650 MPa or more and 1000 MPa or less. (7) A method for producing polypropylene resin foam particles according to any one of (1) to (6) above, wherein the crystallization temperature of the resin (A) is 100°C or less. (8) A method for producing polypropylene resin foam particles according to any one of (1) to (7) above, wherein the foam particles have a crystalline structure in which, in the first DSC curve obtained when the temperature is raised from 23°C to 200°C at a heating rate of 10°C / min by differential scanning calorimetry of heat flux, a main endothermic peak and a high-temperature peak located at a higher temperature than the main endothermic peak appear, and the heat of fusion of the high-temperature peak is 5 J / g or more and 20 J / g or less. (9) A method for producing polypropylene resin foam particles according to any one of (1) to (8) above, wherein the foam particles have a crystalline structure in which, in the first DSC curve obtained when the temperature is raised from 23°C to 200°C at a heating rate of 10°C / min by differential scanning calorimetry of heat flux, a main endothermic peak and a high-temperature peak located at a higher temperature than the main endothermic peak appear, and the difference [Th1-Tl1] between the peak temperature of the high-temperature peak (Th1) and the peak temperature of the main endothermic peak (Tl1) is 20°C or more and 50°C or less.(10) A method for producing polypropylene resin foam particles according to any one of (1) to (9) above, wherein the foam particles have a crystalline structure in which, in the first DSC curve obtained when the temperature is raised from 23°C to 200°C at a heating rate of 10°C / min by differential scanning calorimetry, a main endothermic peak and a high-temperature peak located at a higher temperature than the main endothermic peak appear, and the difference [Th1 - Th2] between the peak temperature of the high-temperature peak (Th1) and the peak temperature of the melting peak appearing at the highest temperature in the second DSC curve obtained when the foam particles are raised from 23°C to 230°C at a heating rate of 10°C / min by differential scanning calorimetry, then cooled from 230°C to 23°C at a cooling rate of 10°C / min, and then raised again from 23°C to 230°C at a heating rate of 10°C / min, is -10°C or more and 10°C or less. (11) A method for producing a polypropylene resin foam particle molded article, comprising in-mold molding of polypropylene resin foam particles obtained by the method for producing polypropylene resin foam particles described in any of (1) to (10) above.

[0106] The present disclosure will be specifically described below with reference to examples. However, the present disclosure is not limited to these examples.

[0107] <Raw Materials> Resins 1 to 6 shown in Table 1 were prepared. Resins 1 to 3 correspond to polypropylene resins (A) with a melting point of 138°C or lower, and resins 4 to 5 correspond to propylene homopolymers (B) with a melting point of 150°C or higher. Resin 6 is linear low-density polyethylene and was used to form a coating layer for the resin particles. The MFR, melting point (°C), crystallization temperature (°C), and flexural modulus (MPa) of each were measured by the following method. The measurement results are shown in Table 1. Melt flow rate MFR of resin (A) A and the melt flow rate MFR of polymer (B) B Ratio to (MFR) A / MFR B The calculation results are shown in Table 2.

[0108]

[0109] <MFR> MFR was measured in accordance with JIS K7210-1 under conditions of a temperature of 230°C and a load of 2.16 kg.

[0110] <Melting Point> The melting point was measured in accordance with "3. Conditioning of Test Specimens" "(2) When measuring the melting temperature after a certain heat treatment" in the method for measuring the transition temperature of plastics specified in JIS K7121:2012, as follows: First, 5 mg of each of resins 1 to 6 was taken and heated from 23°C to 230°C at a heating rate of 10°C / min using a differential scanning calorimeter. Then, it was cooled to 23°C at a cooling rate of 10°C / min, and then heated again from 23°C to 230°C at a heating rate of 10°C / min, and DSC measurement was performed. The temperature at the peak of the melting peak in the DSC curve obtained from the second heating obtained by this measurement was taken as the melting point. The flow rate of nitrogen gas in the measurement environment was 30 mL / min. If two or more melting peaks appeared, the temperature at the peak of the melting peak with the largest melting peak area was adopted as the melting point.

[0111] <Crystallization Temperature> The crystallization temperature of the raw material was measured using a differential scanning calorimeter in accordance with JIS K7121:2012. If multiple crystallization peaks appeared in the DSC curve, the peak temperature of the crystallization peak with the highest peak height was taken as the crystallization temperature.

[0112] <Flexural Modulus> The flexural modulus was measured in accordance with JIS K 7171:2016. First, a 4 mm thick sheet was prepared by heat-pressing the resin to be measured at 230°C, and a rectangular standard test piece measuring 80 mm in length, 10 mm in width, and 4 mm in thickness was cut from this sheet. Using this standard test piece, the indenter radius R1 and the support base radius R2 were both set to 5 mm, the distance between the supports was 64 mm, and the test speed was set to 2 mm / min for the bending test. The flexural modulus of the resin to be measured was measured from the results of the bending test.

[0113] [Multilayered Particles] Next, multilayered particles were formed using resins 1 to 6 in the mixing ratios (mass%) shown in Table 2 by the following method. Note that the amount of resin 6 used is shown as parts by mass when the total amount of resin used to form the core layer is 100 parts by mass.

[0114] A manufacturing apparatus was prepared comprising a core layer forming extruder with an inner diameter of 50 mm, a multilayer strand forming die attached downstream of the core layer forming extruder, and a coating layer forming extruder with an inner diameter of 30 mm. In the manufacturing apparatus, the core layer forming extruder is connected downstream of the coating layer forming extruder and the multilayer strand forming die. Furthermore, the manufacturing apparatus was designed to allow for the lamination of molten resins for forming each layer within the die, as well as co-extrusion. As the resin for forming the core layer, resins 1 to 5 in specific mass formulations shown in Table 2, and zinc borate (0.1 parts by mass per 100 parts by mass of the resin for forming the core layer (total amount of resins 1 to 5)) were supplied to the core layer forming extruder and melt-kneaded. On the other hand, as the resin for forming the coating layer, resin 6 was supplied to the coating layer forming extruder and melt-kneaded. The molten resin materials for each layer, obtained by melt-kneading, were introduced into a die for forming multilayer strands and merged within the die. A multilayer strand having a two-layer structure consisting of a core layer and a coating layer, with a core layer:coating layer mass ratio of 100:3, was extruded. The extruded strands were water-cooled and cut with a pelletizer to obtain multilayer resin particles having a core layer and a coating layer, with an average mass of 1 mg per particle, a particle diameter (length in the extrusion direction) of approximately 2.0 mm, and a length / diameter ratio of approximately 2.0.

[0115] The melting point, MFR, and flexural modulus of the obtained multilayer resin particles were measured using the same method as for resins 1 to 6 described above. The measurement results are shown in Table 2.

[0116] [Foaming Particles] Next, the obtained resin particles were placed in a 5-liter autoclave. Here, 100 parts by mass of resin particles, 300 parts by mass of water, 0.2 parts by mass of sodium dodecylbenzenesulfonate (surfactant) (as an active ingredient), 0.3 parts by mass of kaolin (dispersant), and carbon dioxide (foaming agent) were placed in the autoclave to the pressure indicated in the "Pressure inside the container during foaming" column of Table 2. The mixture was heated while stirring to the temperature indicated in the "Foaming temperature" column of Table 2, and held at that temperature for 15 minutes. The foaming temperature and the pressure inside the container during foaming are shown in Table 2.

[0117] Next, one end of the autoclave was opened to release the contents to atmospheric pressure, causing the core layer of the resin particles to foam and obtain foamed particles. During the release of the contents from the autoclave, carbon dioxide was supplied to the autoclave to maintain the pressure inside the autoclave at the same level as immediately before release.

[0118] Next, the obtained foamed particles were left for 24 hours in an atmospheric pressure environment at a temperature of 23°C and a relative humidity of 50%.

[0119] The foamed particle consists of a foamed particle body (a core layer that is foamed) and a non-foamed coating layer. The mass ratio of the foamed particle body to the coating layer is 97:3.

[0120] The bulk density, closed-cell ratio, total heat content, peak temperature and heat of fusion of the main endothermic peak, and peak temperature and heat of fusion of the high-temperature peak of the obtained foamed particles were measured by the following method. The measurement results are shown in Table 2.

[0121] <Bulk Density> The state of the foamed particles was adjusted by leaving them 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. The adjusted bulk volume was approximately 500 cm³. 3 The foamed particles were filled into a graduated cylinder. Furthermore, the height of the entire group of foamed particles (the foamed particle group) was stabilized by tapping the bottom of the graduated cylinder. The accurate bulk volume of the foamed particle group was measured by reading the scale on the graduated cylinder in this state. Then, the bulk density of the foamed particles (unit: kg / m³) was calculated by dividing the mass of the foamed particle group in the graduated cylinder by its bulk volume and converting the value to units. 3 ) was calculated.

[0122] <Closed-cell ratio> The closed-cell ratio of the foamed particles was measured using an air-comparison hydrometer based on ASTM-D2856-70 Procedure C. Specifically, the measurement was performed as follows: First, 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 the state of the foamed particles. The value of the mark when the foamed particles were naturally deposited in a graduated cylinder after adjustment was approximately 20 cm. 3A sample was collected for measurement in the following manner. This sample was immersed in a graduated cylinder containing ethanol at a temperature of 23°C, and the apparent volume of the sample was measured based on the rise in the liquid level.

[0123] After thoroughly drying the sample whose apparent volume had been measured, the true volume of the sample was measured using a Shimadzu Accupic II 1340 according to procedure C described in ASTM 2856-70. Using these volume values, the closed-cell ratio (in %) of the sample was calculated based on the following formula (X): Closed-cell ratio = (Vx - W / ρ) × 100 / (Va - W / ρ) ... (X)

[0124] However, Vx in the above formula (X) (unit: cm) 3 ) is the true volume of the foamed particle (i.e., the sum of the volume of the resin constituting the foamed particle and the total volume of the closed-cell portion of the foamed particle), and Va (unit: cm) 3 ) is the apparent volume of the foaming particles (i.e., the volume measured from the rise in the liquid level when the foaming particles are submerged in a graduated cylinder containing ethanol), W (unit: g) is the mass of the sample used for measurement, and ρ (unit: g / cm³) is the apparent volume of the foaming particles (i.e., the volume measured from the rise in the liquid level when the foaming particles are submerged in a graduated cylinder containing ethanol), W (unit: g) is the mass of the sample 3 ) is the density of the polypropylene resin that makes up the foamed particles.

[0125] The above procedure was performed five times using different measurement samples, and the arithmetic mean of the closed-cell ratios obtained from these five measurements was defined as the closed-cell ratio of the foamed particles.

[0126] <Total heat quantity, peak temperature and heat of fusion of the main endothermic peak, and peak temperature and heat of fusion of the high-temperature peak> First, 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 the state of the foamed particles. Then, one foamed particle was taken from the group of foamed particles after adjustment. This foamed particle was used as a test specimen, and a DSC curve was obtained when the test specimen was heated from 23°C to 200°C at a heating rate of 10°C / min using a differential thermal scanning calorimeter (specifically, a DSC Q1000 manufactured by T.A. Instruments). An example of a DSC curve is shown in Figure 1. As illustrated in Figure 1, the DSC curve shows a resin-specific peak ΔH1 and a high-temperature peak ΔH2 whose peak is on the higher temperature side than the peak of the resin-specific peak ΔH1. Next, a straight line L1 was obtained by connecting point α at a temperature of 80°C on the DSC curve and point β at the melting end temperature T of the foamed particle. Next, a line L2 parallel to the vertical axis of the graph is drawn from point γ on the DSC curve, which is in the valley between the resin intrinsic peak ΔH1 and the high-temperature peak ΔH2, and the point where lines L1 and L2 intersect is defined as δ. Note that point γ can also be considered the maximum point that exists between the resin intrinsic peak ΔH1 and the high-temperature peak ΔH2. The area of ​​the resin intrinsic peak ΔH1 is the area enclosed by the curve of the resin intrinsic peak ΔH1 portion of the DSC curve, and the line segments α-δ and γ-δ, and this is defined as the heat of fusion of the resin intrinsic peak. The area of ​​the high-temperature peak ΔH2 is the area enclosed by the curve of the high-temperature peak ΔH2 portion of the DSC curve, and the line segments δ-β and γ-δ, and this is defined as the heat of fusion of the high-temperature peak. The area of ​​the total melting peak is the area enclosed by the curve of the resin-specific peak ΔH1 portion of the DSC curve, the curve of the high-temperature peak ΔH2 portion, and the line segment α-β (i.e., the straight line L1). This is defined as the heat of fusion (total heat) of the total melting peak. The above measurement was performed for five foamed particles, and the arithmetic mean values ​​are shown in Table 2.

[0127] [Molded Body] Next, a molded body was manufactured using the obtained foamed particles. First, the foamed particles were dried at 23°C for 24 hours, and then air was impregnated into them to pressurize them so that the internal pressure of the foamed particles (i.e., the pressure inside the bubbles of the foamed particles) was 0.1 MPa(G). Next, the foamed particles were filled into a flat mold measuring 250 mm in length, 200 mm in width, and 50 mm in thickness using the cracking filling method. The amount of cracking during filling (specifically, the ratio of the mold opening amount to the internal dimension in the thickness direction) was 10% (i.e., 5 mm), and after filling was completed, the mold was clamped in the thickness direction to mechanically compress the foamed particles.

[0128] Next, in-mold molding was performed by supplying steam into the mold. In in-mold molding, first, preheating was performed by supplying steam into the mold for 5 seconds with the drain valve of the mold open. Then, the drain valve was closed, and steam was supplied from one side of the mold to perform the first one-sided heating until the pressure reached 0.04 MPa(G) lower than the molding pressure during the main heating. Next, steam was supplied from the other side of the mold to perform the second one-sided heating until the pressure reached 0.02 MPa(G) lower than the molding pressure during the main heating. After that, main heating was performed by supplying steam from both sides of the mold until the lower limit molding pressure (lower limit of molding heating steam pressure) shown in Table 2 was reached. After the main heating was completed, the pressure inside the mold was released, and the molded body was cooled with water inside the mold until the surface pressure due to the foaming force of the molded body reached 0.04 MPa(G).

[0129] The lower limit of the molding steam pressure is the lowest molding steam pressure at which a molded body is obtained in which the fusion properties, secondary foaming properties, and recovery properties, as described later, are evaluated using the molded body after curing, and the evaluation results for all of them are "A".

[0130] Subsequently, the foam particle molded body removed from the mold was left to stand in an 80°C oven for 12 hours. After that, the foam particle molded body was further cured by standing it for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm to obtain the cured molded body.

[0131] (Fusibility) After curing, the molded body was bent and fractured. The total number of foam particles C1 present on the fracture surface and the number of foam particles that fractured (material fracture) C2 were determined, and the ratio of the number of foam particles that fractured (C2) to the total number of foam particles C1 (i.e., the material fracture rate) was calculated. The material fracture rate is calculated using the formula C2 / C1 × 100. The above measurement was performed five times using different test pieces, and the material fracture rate was determined for each. If the arithmetic mean of the material fracture rate was 90% or more, it was evaluated as "A", if it was 70% or more but less than 90%, it was evaluated as "B", and if it was less than 70%, it was evaluated as "C".

[0132] (Secondary foaming properties) Secondary foaming properties were evaluated by visually observing the appearance of the molded product after curing. Specifically, the evaluation was based on the following criteria: A: The molded product shows a good surface condition with almost no interparticle gaps on the surface after curing. B: Some interparticle gaps are observed on the surface of the molded product after curing. C: Significant interparticle gaps are observed on the surface of the molded product after curing.

[0133] (Recovery) The thickness of the molded body after curing was measured near the four corners (specifically, 10 mm inward from the corners towards the center) and in the center (the part that divides the body into two equal parts in both the vertical and horizontal directions). Next, the ratio (in %) of the thickness of the center to the thickness of the thickest part near the four corners was calculated. A ratio of 95% or more was evaluated as "A", a ratio of 90% or more but less than 95% was evaluated as "B", and a ratio of less than 90% was evaluated as "C".

[0134] Next, the 5% compressive strength and shrinkage rate of the molded articles after curing were measured using the following method. The results are shown in Table 2.

[0135] <5% Compressive Strength> The 5% compressive stress σ5 is measured according to the provisions of JIS K6767:1999. First, a test specimen measuring 50 mm in length, 50 mm in width, and 25 mm in thickness is taken from near the center of the molded body after curing, and no skin surface is used. This test specimen is conditioned by being left undisturbed for 24 hours in an environment of 23°C and 50% RH relative humidity, and a compression test is performed using the conditioned test specimen at a compression rate of 10 mm / min. A universal testing machine (Tensilon RTF-1350, manufactured by A&D Co., Ltd.) is used for the compression test. The compressive stress at 5% strain in the obtained compressive stress-strain curve is defined as the 5% compressive stress σ5.

[0136] <Shrinkage Rate> The dimensional change rate (shrinkage rate) of the foam particle molded body relative to the dimensions of the mold used for in-mold molding was measured as follows. First, the dimension of the long side (LB) of the foam particle molded body after curing was measured. The ratio of the difference between the dimension of the long side of the mold (LA) and the dimension of the long side of the foam particle molded body (LB) to the dimension of the long side of the mold (LA) was calculated as (([LA - LB] / LA) × 100) to obtain the shrinkage rate of the foam particle molded body relative to the dimensions of the mold. A smaller shrinkage rate indicates that the molding was performed according to the dimensions of the mold, which is preferable.

[0137]

[0138] In Examples 1 to 4, excellent molded articles with superior fusion properties, secondary foaming properties, and recovery were obtained at extremely low molding pressures of less than 0.10 MPa(G). Furthermore, the obtained molded articles had a 5% compressive strength of 40 kPa or more, possessing sufficient mechanical strength for use as packaging materials.

[0139] In contrast, in Comparative Examples 1 to 7, it was not possible to achieve both a lower limit for the molding heating steam pressure and sufficient mechanical strength.

[0140] This application claims priority based on Japanese Patent Application No. 2025-52141, filed on 26 March 2025, and incorporates all of its disclosures herein.

Claims

1. A method for producing foamed polypropylene resin particles by foaming polypropylene resin particles, wherein the polypropylene resin raw material used to form the resin particles contains a polypropylene resin (A) having a melting point of 138°C or lower and a propylene homopolymer (B) having a melting point of 150°C or higher, in a mass ratio of polypropylene resin (A):propylene homopolymer (B) = 99:1 to 85:15, and the melt flow rate MFR of the resin (A) is measured under the conditions of a temperature of 230°C and a load of 2.16 kg. A and the melt flow rate MFR of the polymer (B) B Ratio to [MFR] A / MFR B A method for producing polypropylene resin foam particles, wherein the ratio is 25 or more, and the melt flow rate of the resin particles, measured under conditions of a temperature of 230°C and a load of 2.16 kg, is 5 g / 10 min or more and 30 g / 10 min or less.

2. Melt flow rate MFR of the resin (A) A A method for producing polypropylene resin foam particles according to claim 1, wherein the amount is 20 g / 10 min or more and 40 g / 10 min or less.

3. Melt flow rate MFR of the polymer (B) B A method for producing polypropylene resin foam particles according to claim 1 or 2, wherein the amount is 3 g / 10 min or less.

4. A method for producing polypropylene resin foam particles according to any one of claims 1 to 3, wherein the resin (A) is at least one selected from the group consisting of ethylene-propylene random copolymer, butene-propylene random copolymer, and ethylene-butene-propylene random copolymer.

5. A method for producing polypropylene resin foam particles according to any one of claims 1 to 4, wherein the flexural modulus of the resin (A) is 800 MPa or less.

6. A method for producing polypropylene resin foam particles according to any one of claims 1 to 5, wherein the flexural modulus of the resin particles is 650 MPa or more and 1000 MPa or less.

7. A method for producing polypropylene resin foam particles according to any one of claims 1 to 6, wherein the crystallization temperature of the resin (A) is 100°C or lower.

8. The method for producing polypropylene resin foam particles according to any one of claims 1 to 7, wherein the foam particles have a crystalline structure in which, in the first DSC curve obtained when the temperature is raised from 23°C to 200°C at a heating rate of 10°C / min by differential scanning calorimetry of heat flux, a main endothermic peak and a high-temperature peak located at a higher temperature than the main endothermic peak appear, and the heat of fusion of the high-temperature peak is 5 J / g or more and 20 J / g or less.

9. The method for producing polypropylene resin foam particles according to any one of claims 1 to 8, wherein the foam particles have a crystalline structure in which, in the first DSC curve obtained when the temperature is raised from 23°C to 200°C at a heating rate of 10°C / min by differential scanning calorimetry of heat flux, a main endothermic peak and a high-temperature peak located at a higher temperature than the main endothermic peak appear, and the difference [Th1-Tl1] between the peak temperature of the high-temperature peak (Th1) and the peak temperature of the main endothermic peak (Tl1) is 20°C or more and 50°C or less.

10. The method for producing polypropylene resin foam particles according to any one of claims 1 to 9, wherein the foam particles have a crystalline structure in which, in the first DSC curve obtained when the temperature is raised from 23°C to 200°C at a heating rate of 10°C / min by differential scanning calorimetry, a main endothermic peak and a high-temperature peak located at a higher temperature than the main endothermic peak appear, and the difference [Th1 - Th2] between the peak temperature of the high-temperature peak (Th1) and the peak temperature of the melting peak appearing at the highest temperature in the second DSC curve obtained when the foam particles are raised from 23°C to 230°C at a heating rate of 10°C / min by differential scanning calorimetry, then cooled from 230°C to 23°C at a cooling rate of 10°C / min, and then raised again from 23°C to 230°C at a heating rate of 10°C / min, is -10°C or more and 10°C or less.

11. A method for producing a molded polypropylene resin foam particle article, comprising obtaining a molded polypropylene resin foam particle article by in-mold molding polypropylene resin foam particles obtained by the method for producing polypropylene resin foam particles according to any one of claims 1 to 10.

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