Polypropylene resin composition, its manufacturing method, manufacturing method of pre-expanded particles, and manufacturing method of foamed molded article

Abstract

The present invention pertains to a polypropylene resin composition that, in at least one embodiment, is characterized by satisfying the following conditions (1) to (4): (1) a melt tension of at least 2.94 cN and not more than 19.6 cN, (2) a melting point Tm greater than 150°C and less than 159°C, (3) a cold crystallization temperature Tc of at least 122°C and less than 130°C, and (4) a heat of fusion ΔH of at least 85 and less than 100 J / g.　The use of the polypropylene resin composition as the base material in the fabrication by extrusion foaming of pre-foamed polypropylene resin particles makes it possible to achieve both of the following at high levels: foam particles that have a high closed cell ratio and exhibit a good secondary moldability, as well as the ability to carry out molding at the low vapor pressures required during secondary molding; and fabrication of a high-strength polypropylene resin foam molded article.

Description

[Technical Field]

[0001] The present invention relates to a polypropylene resin composition that can be suitably used for extrusion foaming, a method for producing the same, a method for producing pre-expanded polypropylene resin particles, and a method for producing a polypropylene resin foam molded article. [Background technology]

[0002] Foamed molded articles obtained using pre-expanded polypropylene resin particles have features such as freedom in shape, cushioning properties, light weight, and heat insulation. Furthermore, because the base material of the foamed molded articles is made of a polypropylene resin, they have excellent chemical resistance, heat resistance, compressive strength, and strain recovery rate after compression. Due to these features, foamed molded articles obtained using pre-expanded polypropylene resin particles are used in a variety of applications, including automotive interior components, core materials for automotive bumpers, heat insulation materials, and cushioning packaging materials.

[0003] The pre-expanded polypropylene resin particles used to obtain a polypropylene resin foam molded article are generally obtained by a so-called "depressurized foaming method," in which polypropylene resin particles are dispersed in water together with a volatile foaming agent in a pressure-resistant container, heated to a temperature near the melting point of the polypropylene resin to impregnate the polypropylene resin particles with the foaming agent, and the dispersion of polypropylene resin particles and water is released into an atmosphere with a lower pressure than that inside the container while maintaining constant the temperature and pressure inside the container under pressure equal to or higher than the vapor pressure of the foaming agent. The pre-expanded polypropylene resin particles obtained by the depressurization foaming method can be easily subjected to secondary molding in a mold using heated steam to form a foamed molded article, and a useful foamed molded article having the above-mentioned characteristics can be obtained. On the other hand, in the manufacturing process of pre-expanded beads using the depressurization expansion method, a two-stage process is required to obtain pre-expanded beads, as they go through a pelletization process in an extruder to make them into a size suitable for expansion, and then move on to an expansion process in a pressure-resistant container. This means that capital investment tends to be large, and there are also issues such as the need for wastewater treatment facilities because a dispersant such as water is used.

[0004] In recent years, in order to overcome these problems, it has been proposed to obtain pre-expanded polypropylene resin particles by an extrusion foaming method. Patent Document 1 proposes a method for obtaining expanded beads by extrusion foaming using a modified polypropylene resin obtained by melt-kneading a polypropylene resin with an aromatic vinyl monomer and / or an isoprene monomer and a radical generator, and imparting extensional viscosity characteristics that indicate high melt elasticity. Patent Document 2 proposes pre-expanded particles obtained by an extrusion foaming method and having excellent secondary moldability, in which a random polypropylene resin having a low melting point is used as the main component of the substrate and a propylene homopolymer is used as part of the substrate. Patent Document 3 proposes pre-expanded particles that have a low internal open cell ratio and excellent secondary moldability, which are obtained by an extrusion foaming method using a polypropylene resin that has been given specific viscoelastic properties as a base material. Patent Document 4 describes pre-expanded particles based on a propylene homopolymer having specific viscoelastic properties, and describes the use of steam at 165°C (equivalent to approximately 0.61 MPa) when secondary molding the pre-expanded particles. Patent Document 5 describes a method for manufacturing pre-expanded particles based on a propylene homopolymer having a specific melt tension and a linear molecular structure, and a method for manufacturing pre-expanded particles using steam at a pressure of 4.5 kg / cm when the pre-expanded particles are secondarily molded. 2 (0.45 MPa). Patent Document 6 describes a method in which a resin composition consisting of a propylene-olefin copolymer having a specific intrinsic viscosity and an ethylene homocopolymer or ethylene-olefin copolymer having a specific intrinsic viscosity is used as a base resin, and pre-expanded particles obtained by an extrusion foaming method are secondary molded using steam at 145°C (corresponding to approximately 0.41 MPa). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 9-302131 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-256460 [Patent Document 3] International Publication No. 2018 / 016399 [Patent Document 4] Japanese Patent Application Publication No. 6-234878 [Patent Document 5] Japanese Patent Application Laid-Open No. 2007-308560 [Patent Document 6] Japanese Patent Application Laid-Open No. 2000-159922 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the case of Patent Document 1, although expanded beads having a low density and a high closed cell content are obtained, there is a problem in that they do not necessarily have excellent secondary moldability. In Patent Document 2, a random polypropylene resin with a low melting point is used as the main component of the substrate, which results in a problem that the strength of the resulting foamed molded article is insufficient. Patent Document 3 also has a problem in that the strength of the foamed molded article obtained is insufficient because random polypropylene is used as the base material. In the cases of Patent Documents 4 to 6, when pre-expanded particles are subjected to secondary molding using heated steam, higher temperature and pressure steam is required, which poses a problem of significantly increasing utility costs. Furthermore, since most molding machines commonly used as in-mold molding machines have a low pressure resistance of 0.40 MPa or less, the melting point of the polypropylene-based resin that can be used as the base material is currently limited.

[0007] In order to solve the above-mentioned problems of the related art, the present invention provides a polypropylene resin composition that uses pre-expanded polypropylene resin beads as a substrate when produced by extrusion foaming, and that achieves at a high level both that the expanded beads have a high closed cell content and good post-molding properties, that molding can be performed at the low vapor pressure required for post-molding, and that polypropylene resin foam molded articles have high strength; a method for producing the same; a method for producing pre-expanded polypropylene resin beads; and a method for producing polypropylene resin foam molded articles. [Means for solving the problem]

[0008] In one or more embodiments, the present invention relates to a polypropylene resin composition that satisfies the following requirements (1) to (4). (1) A melt tension at 230°C of 2.94 cN or more and 19.6 cN or less; (2) A melting point Tm measured by differential scanning calorimetry is greater than 150°C and less than 159°C; (3) A cold crystallization temperature Tc measured by differential scanning calorimetry is 122°C or higher and lower than 130°C; (4) The heat of fusion ΔH measured by differential scanning calorimetry is 85 J / g or more but less than 100 J / g.

[0009] In one or more embodiments, the present invention relates to a method for producing a polypropylene-based resin composition, the method comprising a step of melt-kneading and extruding a polypropylene-based resin, one or more monomers selected from the group consisting of a conjugated diene compound and an aromatic vinyl compound, and a radical polymerization initiator at a temperature at which the polypropylene-based resin melts and the radical polymerization initiator decomposes.

[0010] In one or more embodiments, the present invention relates to a method for producing pre-expanded polypropylene-based resin particles, the method comprising extrusion-foaming the polypropylene-based resin composition to obtain pre-expanded polypropylene-based resin particles.

[0011] In one or more embodiments, the present invention relates to a method for producing a polypropylene-based resin foamed molded article, which comprises steam molding pre-expanded polypropylene resin beads obtained by the method for producing pre-expanded polypropylene resin beads described above to obtain a foamed molded article. [Effects of the Invention]

[0012] According to one or more embodiments of the polypropylene resin composition of the present invention, by using pre-expanded polypropylene resin particles as a substrate when producing them by extrusion foaming, the obtained pre-expanded particles have a high closed cell ratio and exhibit good post-molding properties. In addition, it is possible to achieve at a high level both that they can be molded at the low vapor pressure required for post-molding, and that the pre-expanded particles can be used to obtain a polypropylene resin foam molded article with high strength. According to the method for producing a polypropylene-based resin composition of one or more embodiments of the present invention, it is possible to obtain a polypropylene-based resin composition that has a high closed cell content, good post-formability, and can be used as a base material when producing pre-expanded particles by extrusion foaming, which can give high-strength foamed molded articles at low vapor pressure. According to the method for producing pre-expanded polypropylene resin beads of one or more embodiments of the present invention, it is possible to obtain pre-expanded beads that have a high closed cell content, good post-formability, and can be used to produce high-strength expanded molded articles at low vapor pressure. According to the method for producing a polypropylene resin foam molded article of one or more embodiments of the present invention, a high-strength foam molded article can be molded at a low vapor pressure. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present inventors have conducted extensive research in light of the above-mentioned problems, and have found that by using a polypropylene resin composition as a base material having (1) a melt tension at 230°C (hereinafter simply referred to as "melt tension") of 2.94 cN to 19.6 cN, (2) a melting point Tm measured by differential scanning calorimetry of more than 150°C and less than 159°C, (3) a cold crystallization temperature Tc measured by differential scanning calorimetry of more than 122°C and less than 130°C, and (4) a heat of fusion ΔH measured by differential scanning calorimetry of more than 85 J / g and less than 100 J / g, it is possible to obtain pre-expanded polypropylene resin particles (hereinafter simply referred to as "expanded particles") by extrusion foaming. The pre-expanded polypropylene resin particles (hereinafter simply referred to as "expanded particles") have a high closed cell content and good post-formability, and can be molded at the low vapor pressure required for post-molding, while also achieving increased strength of the polypropylene resin foam molded article (hereinafter simply referred to as "foam molded article").

[0014] In one or more embodiments of the present invention, the melt tension of the polypropylene-based resin composition is 2.94 cN or more and 19.6 cN or less. When the melt tension of the polypropylene-based resin composition is within the above-mentioned range, expanded beads having a low open cell ratio can be obtained by extrusion foaming, and the expanded beads also have good secondary moldability. The melt tension of the polypropylene-based resin composition is more preferably 3.0 cN or more, even more preferably 4.0 cN or more, and particularly preferably 5.0 cN or more. Furthermore, the melt tension of the polypropylene-based resin composition is preferably 15.0 cN or less, more preferably 13.0 cN or less. The melt tension of the polypropylene-based resin composition can be measured by the method described in the examples.

[0015] In one or more embodiments of the present invention, the polypropylene resin composition has a melting point Tm of more than 150°C and less than 159°C. This improves the secondary moldability of the expanded beads obtained by the extrusion foaming method and also increases the strength of the foamed molded article. From the viewpoint of further improving the strength of the foamed molded article, the polypropylene resin composition has a melting point Tm of preferably 152°C or higher, more preferably 153°C or higher.

[0016] In one or more embodiments of the present invention, the cold crystallization temperature Tc of the polypropylene-based resin composition is 122°C or higher and lower than 130°C. This allows expanded beads with a low open cell content to be obtained by extrusion foaming, improves the post-molding properties of the expanded beads, and increases the strength of the foamed molded article. The cold crystallization temperature Tc of the polypropylene-based resin composition is preferably 123°C or higher.

[0017] In one or more embodiments of the present invention, the polypropylene resin composition has a heat of fusion ΔH of 85 J / g or more and less than 100 J / g. This allows expanded beads with a low open cell ratio to be obtained by extrusion foaming, improves the post-molding properties of the expanded beads, and increases the strength of the foamed molded article. From the viewpoint of increasing the strength of the foamed molded article, the polypropylene resin composition has a heat of fusion ΔH of preferably 88 J / g or more, more preferably 91 J / g or more.

[0018] In one or more embodiments of the present invention, the polypropylene resin composition comprises: 13 The mm fraction of propylene unit triads measured by C-NMR (hereinafter simply referred to as "mm fraction") is preferably less than 96%, more preferably 95% or less, and even more preferably 94% or less. This improves the secondary moldability of the expanded beads obtained by the extrusion foaming method. On the other hand, from the viewpoint of improving the strength of the foamed molded article, the mm fraction of the polypropylene resin composition is preferably 91% or more, and more preferably 92% or more.

[0019] In one or more embodiments of the present invention, the polypropylene resin composition preferably has a melt flow rate (MFR) of 1 g / 10 min or more and 20 g / 10 min or less, more preferably 1.5 g / 10 min or more and 15 g / 10 min or less, and even more preferably 2 g / 10 min or more and 12 g / 10 min or less. When the MFR of the polypropylene resin composition is within the above-mentioned range, expanded beads can be suitably obtained by an extrusion foaming method.

[0020] In one or more embodiments of the present invention, the polypropylene-based resin composition is not particularly limited, but is preferably produced, for example, by melt-kneading and extruding a polypropylene-based resin, one or more monomers selected from the group consisting of a conjugated diene compound and an aromatic vinyl compound, and a radical polymerization initiator at a temperature at which the polypropylene-based resin melts and the radical polymerization initiator decomposes.

[0021] The above-described production method facilitates the production of a polypropylene-based resin composition containing a branched polypropylene-based resin having a polypropylene-based resin main chain and side chains derived from one or more monomers selected from the group consisting of conjugated diene compounds and aromatic vinyl compounds. The polypropylene-based resin composition also facilitates the production of a polypropylene-based resin composition having a melt tension, melting point Tm, cold crystallization temperature Tc, heat of fusion ΔH, and mm fraction within the above-described ranges.

[0022] As the polypropylene-based resin, linear polypropylene-based resins can be suitably used, and specific examples include propylene homopolymers, block copolymers, and random copolymers. As the propylene copolymer, a polymer containing 75% by mass or more of propylene is preferred because it maintains the crystallinity, rigidity, chemical resistance, and other properties characteristic of polypropylene-based resins. Monomers copolymerizable with propylene include α-olefins having 2 or 4 to 12 carbon atoms, such as ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 3,4-dimethyl-1-butene, 1-heptene, 3-methyl-1-hexene, 1-octene, and 1-decene; cyclic olefins such as cyclopentene, norbornene, and tetracyclo[6,2,11,8,13,6]-4-dodecene; 5-methylene Examples of suitable polypropylene resins include dienes such as 2-norbornene, 5-ethylidene-2-norbornene, 1,4-hexadiene, methyl-1,4-hexadiene, and 7-methyl-1,6-octadiene; and vinyl monomers such as vinyl chloride, vinylidene chloride, acrylonitrile, vinyl acetate, acrylic acid, methacrylic acid, maleic acid, ethyl acrylate, butyl acrylate, methyl methacrylate, maleic anhydride, styrene, methylstyrene, vinyltoluene, and divinylbenzene. Ethylene and 1-butene are particularly preferred from the viewpoints of improving cold brittleness resistance and being inexpensive. These may be used alone or in combination of two or more. The polypropylene resin is preferably a propylene homopolymer, from the viewpoint of easily obtaining a polypropylene resin composition satisfying the above-mentioned melt tension, melting point Tm, cold crystallization temperature Tc, heat of fusion ΔH, and mm fraction.

[0023] The polypropylene resin is not particularly limited, but preferably has an MFR of 1 g / 10 min to 20 g / 10 min, more preferably 1.5 g / 10 min to 15 g / 10 min, and even more preferably 2 g / 10 min to 12 g / 10 min, which makes it easy to adjust the MFR of the polypropylene resin composition to the above range.

[0024] The polypropylene resin is not particularly limited, but preferably has a melting point Tm of 145° C. or more and 165.5° C. or less, more preferably 155° C. or more and 165° C. or less, and even more preferably 160° C. or more and 165° C. or less, which makes it easy to adjust the melt tension, melting point Tm, cold crystallization temperature Tc, heat of fusion ΔH, and mm fraction, particularly the melting point Tm, of the polypropylene resin composition within the above-mentioned ranges.

[0025] The polypropylene resin preferably has a cold crystallization temperature Tc of 110° C. or higher and 125° C. or lower, more preferably 113° C. or higher and 120° C. or lower, which makes it easy to adjust the melt tension, melting point Tm, cold crystallization temperature Tc, heat of fusion ΔH, and mm fraction, particularly the cold crystallization temperature Tc, of the polypropylene resin composition within the above-mentioned ranges.

[0026] The polypropylene resin preferably has a heat of fusion ΔH of 95 J / g or more and 120 J / g or less, more preferably 100 J / g or more and 120 J / g or less, which makes it easy to adjust the melt tension, melting point Tm, cold crystallization temperature Tc, heat of fusion ΔH and mm fraction, particularly the heat of fusion ΔH, of the polypropylene resin composition within the above-mentioned ranges.

[0027] Although the polypropylene resin is not particularly limited, it is preferable that the mm fraction is 96% or more and 97% or less, which makes it easy to adjust the melt tension, melting point Tm, cold crystallization temperature Tc, heat of fusion ΔH, and mm fraction, especially the mm fraction, of the polypropylene resin composition within the above-mentioned ranges.

[0028] Examples of the conjugated diene compound used as the monomer include butadiene, isoprene, 1,3-heptadiene, 2,3-dimethylbutadiene, and 2,5-dimethyl-2,4-hexadiene.

[0029] Examples of the aromatic vinyl compound used as the monomer include styrene; methylstyrenes such as o-methylstyrene, m-methylstyrene, p-methylstyrene, α-methylstyrene, β-methylstyrene, dimethylstyrene, and trimethylstyrene; chlorostyrenes such as α-chlorostyrene, β-chlorostyrene, o-chlorostyrene, m-chlorostyrene, p-chlorostyrene, dichlorostyrene, and trichlorostyrene; bromostyrenes such as o-bromostyrene, m-bromostyrene, p-bromostyrene, dibromostyrene, and tribromostyrene; o-fluorostyrene, m-fluorostyrene, p-fluorostyrene, and Examples of suitable styrenes include fluorostyrenes such as o-nitrostyrene, m-nitrostyrene, p-nitrostyrene, dinitrostyrene, and trinitrostyrene; nitrostyrenes such as o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, dihydroxystyrene, and trihydroxystyrene; vinylphenols such as o-divinylbenzene, m-divinylbenzene, and p-divinylbenzene; and isopropenylstyrenes such as o-diisopropenylbenzene, m-diisopropenylbenzene, and p-diisopropenylbenzene. Among these, styrene and / or methylstyrene are preferred from the viewpoint of increasing the closed cell ratio and expansion ratio of the foamed molded article.

[0030] Of the above-mentioned monomers, conjugated diene compounds are preferred, and butadiene and / or isoprene are particularly preferred because they are inexpensive, easy to handle, and the reaction tends to proceed uniformly.

[0031] The conjugated diene compound may be used in combination with a copolymerizable monomer, for example, vinyl chloride, vinylidene chloride, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, vinyl acetate, acrylic acid, methacrylic acid, maleic acid, maleic anhydride, metal acrylates, metal methacrylates, acrylic acid esters such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, and stearyl acrylate; or methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, and stearyl methacrylate.

[0032] The amount of the monomer added is preferably 0.04 mol to 0.14 mol, more preferably 0.05 mol to 0.13 mol, and even more preferably 0.06 mol to 0.12 mol per kg of polypropylene resin, which makes it easy to obtain a polypropylene resin composition that satisfies the above-mentioned melt tension, melting point Tm, cold crystallization temperature Tc, heat of fusion ΔH, and mm fraction.

[0033] The amount of the monomer such as the conjugated diene compound added is preferably 0.05 parts by mass or more and 1.5 parts by mass or less, and more preferably 0.1 parts by mass or more and 0.9 parts by mass or less, relative to 100 parts by mass of the polypropylene resin. When the amount of the conjugated diene compound added is within the above-mentioned range, it is easy to obtain a polypropylene resin composition that satisfies the above-mentioned melt tension, melting point Tm, cold crystallization temperature Tc, heat of fusion ΔH, and mm fraction.

[0034] Examples of the radical polymerization initiator generally include peroxides and azo compounds. From the viewpoint of easily obtaining a polypropylene resin composition that satisfies the above-mentioned melt tension, melting point Tm, cold crystallization temperature Tc, heat of fusion ΔH, and mm fraction, it is preferable to use an organic peroxide, and from the viewpoint of effectively causing branching with a small addition amount, it is particularly preferable to use a peroxyester or peroxydicarbonate. Examples of the peroxy ester include cumyl peroxy neodecanoate, 1,1,3,3-tetramethylbutyl peroxy neodecanoate, t-hexyl peroxy neodecanoate, t-butyl peroxy neodecanoate, t-hexyl peroxy pivalate, t-butyl peroxy pivalate, 1,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, t-hexyl peroxy-2-ethylhexanoate, and t-butyl peroxy-2-ethylhexanoate. Suitable peroxycarbonyl compounds that can be used include t-hexylperoxyisopropyl monocarbonate, t-butylperoxy-3,5,5-trimethylhexanoate, t-butylperoxylaurate, t-butylperoxyisopropyl monocarbonate, t-butylperoxy-2-ethylhexyl monocarbonate, t-hexylperoxybenzoate, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, t-butylperoxyacetate, t-butylperoxy-3-methylbenzoate, and t-butylperoxybenzoate. The peroxydicarbonate is not particularly limited, but examples of suitable peroxydicarbonates include di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, di(4-t-butylcyclohexyl) peroxydicarbonate, di(2-ethylhexyl) peroxydicarbonate, di-sec-butyl peroxydicarbonate, and dicetyl peroxydicarbonate. These may be used alone or in combination of two or more.

[0035] The amount of the radical polymerization initiator added is preferably 0.05 to 3 parts by mass, more preferably 0.1 to 2.5 parts by mass, per 100 parts by mass of the polypropylene resin. When the amount of the radical polymerization initiator added is within the above range, it is possible to efficiently copolymerize the monomer into the polypropylene resin as a side chain. This makes it easy to obtain a polypropylene resin composition that satisfies the melt tension, melting point Tm, cold crystallization temperature Tc, heat of fusion ΔH, and mm fraction within the above-mentioned ranges.

[0036] Examples of devices for reacting the polypropylene resin, monomer, and radical polymerization initiator include kneaders such as rolls, co-kneaders, Banbury mixers, Brabenders, single-screw extruders, and twin-screw extruders; horizontal mixers such as twin-screw surface regenerators and twin-screw multi-disc mixers; and vertical mixers such as double helical ribbon mixers. Of these, it is preferable to use a kneader, and extruders such as single-screw extruders and twin-screw extruders are particularly preferred from the viewpoint of productivity.

[0037] There are no particular limitations on the order or method of mixing and kneading (stirring) the polypropylene resin, the monomer, and the radical polymerization initiator. The polypropylene resin, the monomer, and the radical polymerization initiator may be mixed and then melt-kneaded, or the polypropylene resin may be melt-kneaded and then the monomer or the radical initiator may be mixed simultaneously or separately, all at once or in portions, or the polypropylene resin and the radical polymerization initiator may be melt-kneaded and then the monomer may be mixed all at once or in portions.

[0038] The temperature of the kneader may be any temperature at which the polypropylene resin melts and the radical polymerization initiator decomposes. Melt-kneading is preferably performed at 150° C. or higher and 300° C. or lower. The melt-kneading time is generally preferably 1 minute or higher and 60 minutes or lower.

[0039] The obtained polypropylene-based resin composition may contain an alcohol derived from the radical polymerization initiator, specifically, one or more alcohols selected from the group consisting of t-butyl alcohol and isopropanol derived from the radical polymerization initiator. The alcohol present in the polypropylene-based resin composition can be confirmed, for example, by a method in which pellets of the polypropylene-based resin composition are frozen and crushed and used as a sample, and the gas components generated when the sample is heated to 150°C are analyzed by GC-MS.

[0040] In one or more embodiments of the present invention, the polypropylene resin composition is used as a base resin to obtain pre-expanded polypropylene resin particles by extrusion foaming. The base resin may contain, in addition to the polypropylene resin composition, 10% by mass or less, 5% by mass or less, 3% by mass or less, or 1% by mass or less of a resin other than a polypropylene resin, as long as the effects of the present invention are not impaired. More specifically, in one or more embodiments of the present invention, the base resin may be 100% by mass of the polypropylene-based resin composition, or may contain 90% to 100% by mass of the polypropylene-based resin composition and 0% to 10% by mass of other resins, 95% to 100% by mass of the polypropylene-based resin composition and 0% to 5% by mass of other resins, 97% to 100% by mass of the polypropylene-based resin composition and 0% to 3% by mass of other resins, or 99% to 100% by mass of the polypropylene-based resin composition and 0% to 1% by mass of other resins. Examples of the other resins include polyethylene-based resins such as high-density polyethylene, medium-density polyethylene, low-density polyethylene, linear low-density polyethylene, linear very low-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, and ethylene-methacrylic acid copolymer; styrene-based resins such as polystyrene and styrene-maleic anhydride copolymer; and polyamide.

[0041] In one or more embodiments of the present invention, for example, a base resin, a blowing agent, and optional additives are melt-kneaded and then extruded, and the extruded molten mixture is cut to obtain polypropylene-based resin pre-expanded particles. The melt-kneading is preferably performed in two stages, with the temperature of the second stage being lower than that of the first stage. Specifically, the base resin, the blowing agent, and optional additives are fed into a first-stage extruder and melt-kneaded, and the molten mixture is then fed into a second-stage extruder, which has a lower temperature than the first-stage extruder, where it is cooled. The molten mixture (molten resin containing the blowing agent) is then extruded through a die attached to the tip of the second-stage extruder, and the extruded molten mixture is cut, a process known as extrusion foaming. By maintaining the temperature of the second-stage extruder lower than that of the first-stage extruder, the open cell ratio of the expanded particles is easily reduced, resulting in improved moldability. Furthermore, since the second-stage extruder is primarily intended for cooling, any device other than an extruder may be used as long as it can achieve this purpose. For example, a static mixer, melt cooler, or the like may be used instead.

[0042] In one or more embodiments of the present invention, from the viewpoint of reducing the open cell content of the expanded beads and improving moldability, the difference between the first-stage temperature and the second-stage temperature is preferably 5°C or more, more preferably 8°C or more, and even more preferably 10°C or more.

[0043] The shredding methods for obtaining expanded particles in the extrusion foaming method are generally broadly divided into cold cutting and die face cutting. Examples of cold cutting include a method (strand cutting) in which a molten resin containing a blowing agent extruded through a narrow-hole die is foamed, and then the resulting strand-shaped foam is taken out and shredded while being cooled, for example, by passing it through a water tank. The die face cutting method is a method in which the molten resin extruded through a narrow-hole die is cut with a rotating cutter while in contact with the die face or while maintaining a small gap. Die face cutting methods are divided into underwater cutting, watering cutting, and hot cutting, depending on the cooling method.

[0044] In one or more embodiments of the present invention, both physical and decomposable blowing agents can be suitably used as the blowing agent. Specific examples of physical blowing agents include aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, and hexane; alicyclic hydrocarbons such as cyclopentane and cyclobutane; inorganic gases such as air, nitrogen, and carbon dioxide; and water. Specific examples of decomposable blowing agents include inorganic carbonates such as sodium bicarbonate and ammonium carbonate; organic acids such as citric acid or salts thereof (e.g., sodium citrate); azo compounds such as 2,2′-azobisisobutyronitrile and azodicarboxylic acid amide; sulfonyl hydrazide compounds such as benzenesulfonyl hydrazide; nitroso compounds such as N,N′-dinitrosopentamethylenetetramine (DNPT); and azide compounds such as terephthal azide. These blowing agents may be used alone or in combination. Among these, inorganic gases and / or water are preferred from the viewpoint of safety during handling and simplification of required equipment specifications, and inorganic gases, particularly carbon dioxide gas, are preferred from the viewpoint of facilitating the production of foamed molded articles with a high expansion ratio. The amount of foaming agent added varies depending on the type of foaming agent and the target expansion ratio of the pre-expanded polypropylene resin particles, and may be adjusted appropriately, but is preferably 0.1 to 20 parts by mass, more preferably 0.3 to 15 parts by mass, per 100 parts by mass of the base resin.

[0045] In one or more embodiments of the present invention, additives may include cell nucleating agents (also referred to as bubble nucleating agents); colorants; antistatic agents; flame retardants; stabilizers such as antioxidants, metal deactivators, phosphorus-based processing stabilizers, UV absorbers, UV stabilizers, fluorescent brighteners, metal soaps, and antacid adsorbents; crosslinking agents; chain transfer agents; lubricants; plasticizers; fillers; and reinforcing materials. Such additives may be incorporated into a resin at a high concentration to form a masterbatch, and the resulting masterbatch resin may then be added to the polypropylene-based resin mixture. The resin used in such a masterbatch resin is preferably a polyolefin-based resin, more preferably a polypropylene-based resin. It is more preferable to use the same polypropylene-based resin as the base resin of the expanded beads.

[0046] As the additive, a bubble nucleating agent may be added for the purpose of controlling the bubble shape. Examples of bubble nucleating agents include sodium bicarbonate, a sodium bicarbonate-citric acid mixture, monosodium citrate, talc, and calcium carbonate. These may be used alone or in combination of two or more. The amount of bubble nucleating agent added is not particularly limited, but is usually preferably 0.01 to 5 parts by mass per 100 parts by mass of the base resin.

[0047] The extrusion output rate of the second-stage extruder, static mixer, or melt cooler is not particularly limited, and may be, for example, 1 kg / hour to 1,000 kg / hour. In a laboratory type, it may be approximately 1 kg / hour to 50 kg / hour, and in a commercial production type, it is preferably 20 kg / hour to 1,000 kg / hour.

[0048] The shape of the die used in the second-stage extruder, static mixer, or melt cooler is not critical, but from the viewpoint of the appearance of the expanded beads and ease of shaping, the die opening is preferably circular, and the diameter of the opening is preferably 0.1 mm or more and 2.0 mm or less, and more preferably 0.3 mm or more and 1.0 mm or less.

[0049] In one embodiment of the present invention, the mass per expanded bead is preferably 0.2 mg to 10 mg, more preferably 0.5 mg to 6.0 mg. In one embodiment of the present invention, the mass per expanded bead is the average resin particle mass calculated based on the mass of 100 randomly selected polypropylene resin pre-expanded beads. When the mass per polypropylene resin pre-expanded bead is 0.2 mg or more, the dimensional change rate of the foamed molded article does not increase, and when it is 10 mg or less, it tends to be easier to fill into a mold.

[0050] In one or more embodiments of the present invention, the bulk density of the pre-expanded polypropylene resin particles is not particularly limited, but is preferably 20 g / L or more and 450 g / L or less, and more preferably 30 g / L or more and 300 g / L or less. When the bulk density of the expanded particles is within the above-mentioned range, it becomes easy to obtain a foamed molded article having a low open cell ratio and excellent compressive strength. In one or more embodiments of the present invention, the bulk density of the pre-expanded polypropylene resin particles can be measured by the method described in the examples.

[0051] In one or more embodiments of the present invention, a foamed molded article can be produced by steam molding pre-expanded polypropylene resin particles. To mold a foamed molded polypropylene resin from the pre-expanded polypropylene resin particles, for example, (a) the foamed particles are pressurized with an inorganic gas to impregnate the particles with the inorganic gas and impart a predetermined internal particle pressure, and then the particles are packed into a mold and heat-sealed with steam or the like (e.g., JP-B-51-22951); (b) the foamed particles are compressed with gas pressure, packed into a mold, and heat-sealed with steam or the like by utilizing the particle's recovery force (e.g., JP-B-53-33996); or (c) the foamed particles are packed into a mold with an expanded gap, the mold is closed to a predetermined gap, the packed foamed particles are compressed, and the packed foamed particles are heat-sealed with steam or the like. Specifically, a foamed molded polypropylene resin can be obtained by packing the pre-expanded polypropylene resin particles into a mold that can be closed but cannot be sealed, and then heating with steam or the like to perform steam molding.

[0052] From the viewpoint of improving moldability using a general-purpose molding machine and reducing costs, the pressure of the steam (steam pressure) during steam molding is preferably 0.40 MPa or less, more preferably 0.38 MPa or less, and even more preferably 0.36 MPa or less.

[0053] In one or more embodiments of the present invention, from the viewpoint of excellent static compressive strength, it is preferable that the 50% compressive strength of the polypropylene-based resin foam molded article be higher. The 50% compressive strength varies depending on the density of the foam molded article, but for example, for foam molded articles having a density in the range of 60 to 90 g / L, it is preferably 0.40 MPa or more, more preferably 0.50 MPa or more, and particularly preferably 0.60 MPa or more. In one or more embodiments of the present invention, the 50% compressive strength of the polypropylene-based resin foam molded article can be measured by the method described in the examples.

[0054] In one or more embodiments of the present invention, the density of the polypropylene resin foam molded article may be appropriately determined depending on the application and is not particularly limited, but is preferably, for example, 30 g / L or more and 300 g / L or less, and more preferably 40 g / L or more and 300 g / L or less. In one or more embodiments of the present invention, the density of the polypropylene resin foam molded article can be measured by the method described in the examples.

[0055] In one or more embodiments, the present invention may be configured as follows. [1] A polypropylene resin composition characterized by satisfying the following requirements (1) to (4): (1) A melt tension at 230°C of 2.94 cN or more and 19.6 cN or less; (2) A melting point Tm measured by differential scanning calorimetry is greater than 150°C and less than 159°C; (3) A cold crystallization temperature Tc measured by differential scanning calorimetry is 122°C or higher and lower than 130°C; (4) The heat of fusion ΔH measured by differential scanning calorimetry is 85 J / g or more but less than 100 J / g. [2] The polypropylene resin composition comprises: 13 The polypropylene resin composition according to [1], wherein the mm fraction of propylene unit triads determined by C-NMR is less than 96%. [3] The polypropylene resin composition according to [1] or [2], wherein the polypropylene resin composition has a melt flow rate of 1 g / 10 min or more and 20 g / min or less. [4] The polypropylene-based resin composition according to any one of [1] to [3], wherein the polypropylene-based resin composition contains a branched polypropylene-based resin having a polypropylene-based resin main chain and having structural units derived from one or more monomers selected from the group consisting of conjugated diene compounds and aromatic vinyl compounds as side chains. [5] The polypropylene resin composition according to [4], wherein the monomer is at least one selected from the group consisting of isoprene, butadiene, and styrene. [6] The polypropylene resin composition according to any one of [1] to [5], which contains an alcohol derived from a radical polymerization initiator. [7] The polypropylene resin composition according to [6], wherein the alcohol derived from the radical polymerization initiator is at least one selected from the group consisting of t-butyl alcohol and isopropanol. [8] A method for producing the polypropylene-based resin composition according to any one of [1] to [7], comprising a step of melt-kneading and extruding a polypropylene-based resin, one or more monomers selected from the group consisting of conjugated diene compounds and aromatic vinyl compounds, and a radical polymerization initiator at a temperature at which the polypropylene-based resin melts and the radical polymerization initiator decomposes. [9] The method for producing a polypropylene-based resin composition according to [8], wherein the polypropylene-based resin is a propylene homopolymer.

[10] The method for producing a polypropylene-based resin composition according to [8] or [9], wherein the polypropylene-based resin has a melt flow rate of 1 g / 10 min or more and 20 g / min or less.

[11] The method for producing a polypropylene resin composition according to any one of [8] to

[10] , wherein the radical polymerization initiator is at least one selected from the group consisting of peroxydicarbonates and peroxyesters.

[12] The method for producing a polypropylene-based resin composition according to any one of [8] to

[11] , wherein the amount of the monomer used is 0.04 mol or more and 0.14 mol or less per 1 kg of the polypropylene-based resin.

[13] A method for producing pre-expanded polypropylene resin particles, comprising a step of extrusion-expanding the polypropylene resin composition according to any one of [1] to [7] to obtain pre-expanded polypropylene resin particles.

[14] The method for producing pre-expanded polypropylene resin particles according to claim 13, wherein the open cell rate of the pre-expanded polypropylene resin particles is 10.5% or less.

[15] A method for producing a foamed molded polypropylene resin article, comprising steam molding the pre-expanded polypropylene resin particles obtained by the method for producing pre-expanded polypropylene resin particles according to

[13] or

[14] to obtain a foamed molded article.

[16] The method for producing a polypropylene resin foam molded article according to

[15] , wherein the steam pressure during the steam molding is 0.40 MPa or less. [Example]

[0056] One or more embodiments of the present invention will be described in detail below based on examples, although the present invention is not limited to these examples.

[0057] The evaluation and measurement methods carried out in the examples and comparative examples are as follows.

[0058] <Melt tension> A capillary graph (manufactured by Toyo Seiki Seisaku-sho, Ltd.) equipped with an attachment for measuring melt tension and having a cylinder with a diameter of 10 mm with an orifice with a diameter of 1 mm and a length of 10 mm attached to its tip was used. The sample was filled into a cylinder set at 230°C, preheated for 5 minutes, and then, when the piston was lowered at a rate of 10 mm / min, the strand discharged from the orifice was hung on a pulley with a load cell 350 mm below and taken up at a rate of 1 m / min. After stabilization, the take-up rate was increased at a rate that reached a speed of 200 m / min in 4 minutes, and the load (unit: cN) applied to the pulley with a load cell when the strand broke was taken as the melt tension.

[0059] <Melting point Tm, cold crystallization temperature Tc, and heat of fusion ΔH> Using a differential scanning calorimeter DSC [manufactured by Seiko Instruments Inc., model DSC6200], 5 to 6 mg of the sample was heated from 40°C to 220°C at a heating rate of 10°C / min to be melted, and then, after being crystallized by cooling from 220°C to 40°C at a cooling rate of 10°C / min, it was further heated from 40°C to 220°C at a heating rate of 10°C / min to be melted, and DSC curves at the first heating, cooling, and second heating were obtained. 《Melting point Tm》 The melting peak temperature obtained from the DSC curve at the second heating was taken as the melting point Tm. In the DSC curve, when multiple melting peaks were observed, the melting peak on the highest temperature side among them was used. 《Cold crystallization temperature Tc》 The crystallization peak temperature obtained from the DSC curve during cooling was taken as the cold crystallization temperature Tc. In the DSC curve, when multiple crystallization peaks were observed, the crystallization peak on the highest temperature side among them was used. 《Heat of fusion ΔH》 In the DSC curve at the second heating, a line segment connecting the melting end point from the point corresponding to 100°C on the DSC curve was drawn, and the heat of fusion of the portion surrounded by the line segment and the DSC curve was calculated and divided by the mass of the sample to obtain the heat of fusion ΔH.

[0060] <mm fraction> 70-80 mg of sample was dissolved in 0.5 mL of o-dichlorobenzene-d4, and the NMR measurement was performed at 150 °C using an NMR measurement device (VNMRS 600, manufactured by VARIAN). 13 An NMR chart was obtained by performing C-NMR measurement under the following detailed measurement conditions: Flip angle: 90 degrees Pulse interval: 10 seconds Resonance frequency: 150MHz Number of times accumulated: 1024 In the obtained NMR chart, the ratio (%) of the integrated area of ​​the peak in the range of 21.6 to 23.0 ppm due to the mm conformation to the integrated area of ​​the peak appearing between 19.5 and 23.0 ppm was calculated and taken as the mm fraction.

[0061] <Melt flow rate (MFR)> The melt flow rate (MFR) conforms to the provisions of Method B of ISO 1133 (1997), and is determined by measuring the distance a piston moves in a certain time at 230°C under a constant load of 2.16 kg using a Melt Indexer S-01 (manufactured by Toyo Seiki Seisakusho). The MFR is calculated by converting the measured distance and the resin density at the measurement temperature into the mass of resin extruded from an orifice in 10 minutes. The certain time is 120 seconds when the melt flow rate is greater than 0.5 g / 10 min and not greater than 1.0 g / 10 min; 60 seconds when the melt flow rate is greater than 1.0 g / 10 min and not greater than 3.5 g / 10 min; 30 seconds when the melt flow rate is greater than 3.5 g / 10 min and not greater than 10 g / 10 min; 10 seconds when the melt flow rate is greater than 10 g / 10 min and not greater than 25 g / 10 min; 5 seconds when the melt flow rate is greater than 25 g / 10 min and not greater than 100 g / 10 min; and 3 seconds when the melt flow rate is greater than 100 g / 10 min.

[0062] <Bulk density> The polypropylene resin pre-expanded particles were collected in a 1 L measuring cup (mass: W0), and the raised part was leveled off and weighed (mass: W1). The mass of the expanded particles, Wb (g) = (W1 - W0), was calculated from the difference in mass of the measuring cup before and after collecting the expanded particles, and the bulk density was calculated from the accurate internal volume Vk (L) of the measuring cup, which had been measured in advance, according to the following formula. Bulk density (g / L) = Wb / Vk

[0063] <Open cell ratio> The volume of the pre-expanded polypropylene resin particles obtained according to the method described in Procedure C of ASTM D2856-87 is defined as Vc (cm 3 ) and the open cell ratio (%) was calculated according to the following formula: Open cell rate (%) = ((Va-Vc) x 100) / Va Vc was measured using an air comparison hydrometer model 1000 manufactured by Tokyo Science Co., Ltd. The volume Va (cm 3 ) is the apparent volume of the polypropylene resin pre-expanded particles, which is determined by submerging the entire amount of the polypropylene resin pre-expanded particles after measuring Vc with the air comparison type hydrometer in a measuring cylinder containing ethanol, and measuring the rise in the liquid level of the measuring cylinder (submersion method).

[0064] <Fusion rate> A crack about 5 mm deep was made in the surface of the polypropylene resin foam molded article with a knife, and the in-mold foam molded article was split along the crack, and the fracture surface was observed to determine the ratio of the number of broken particles to the total number of particles on the fracture surface, and the fusion rate of the molded article was evaluated. A fusion rate of 80% or more was considered to be acceptable.

[0065] <Deformation and shrinkage> A metal ruler was placed on the center of the largest surface of the polypropylene resin foam molded article, parallel to the longest side, and the largest gap between the ruler and the molded article was measured. This value was taken as the deformation shrinkage. A deformation shrinkage of 1.5 mm or less was considered to be acceptable.

[0066] <Surface elongation evaluation> The polypropylene resin foam molded article was evaluated for the presence of gaps between the foam particles at the center of the surface with the largest area. Those with few gaps between the foam particles and a beautiful surface were rated as passing.

[0067] <density> The mass W (g) of the polypropylene resin foam molded body was measured, and the length, width, and thickness were measured with a vernier caliper to determine the volume V (cm 3 ) was calculated, and the density of the molded product (g / L) was calculated according to the following formula. Molded product density (g / L)=W(g) / V(cm 3 ) x 1000

[0068] <50% compressive strength> Test pieces measuring 50 mm long x 50 mm wide x 25 mm thick were cut out from the polypropylene resin foam molded body, and the compressive stress (MPa) at 50% compression when compressed at a rate of 10 mm / min was measured in accordance with NDZ-Z0504.

[0069] In the examples and comparative examples, the following polypropylene resins were used as raw material resins. Table 1 below shows the melting point Tm, cold crystallization temperature Tc, heat of fusion ΔH, MFR, and mm fraction of the polypropylene resins. (1) PP-1: Propylene homopolymer (Prime Polymer "F113G") (2) PP-2: Propylene homopolymer (Prime Polymer "J106G") (3) PP-3: Propylene and ethylene random copolymer (Prime Polymer Co., Ltd. "F724NPC"), propylene 98% by mass (4) PP-4: Propylene homopolymer (Prime Polymer "J108M")

[0070] [Table 1]

[0071] Example 1 [Preparation of polypropylene resin composition] A mixture of 100 parts by mass of PP-1 as the raw polypropylene resin and 1.4 parts by mass of t-butylperoxyisopropyl monocarbonate ("Perbutyl (registered trademark) I, 1-minute half-life temperature 158.8°C" manufactured by NOF Corporation) as a radical polymerization initiator was fed from a hopper to a 45 mmφ twin-screw extruder (L / D = 40) at 70 kg / hour, and melt-kneaded at a cylinder temperature of 200°C and a rotation speed of 150 rpm. 0.6 parts by mass of isoprene, a conjugated diene compound, was fed as a monomer from an injection port installed midway through the extruder using a metering pump, and the mixture was melt-kneaded in the twin-screw extruder. The extruded strands were water-cooled and chopped to obtain a pellet-shaped polypropylene resin composition. The obtained polypropylene-based resin composition contains a branched polypropylene-based resin having a polypropylene-based resin main chain and a structural unit derived from a conjugated diene compound as a side chain, and also contains an alcohol derived from a radical polymerization initiator, specifically t-butyl alcohol and isopropanol.

[0072] [Preparation of pre-expanded polypropylene resin particles] A mixture of 100 parts by mass of the polypropylene resin composition obtained above and 0.2 parts by mass of talc ("PK-S" manufactured by Hayashi Kasei) was fed from a hopper to a 15 mmφ twin-screw extruder (L / D=30) at a rate of 1.0 kg / hour, and melt-kneaded at a cylinder temperature of 200°C and a rotation speed of 100 rpm. Carbon dioxide gas, a foaming agent, was fed from an injection port provided midway through the extruder using a metering pump at a ratio of 1.7 parts by mass per 100 parts by mass of the polypropylene resin composition, and the mixture was further melt-kneaded. Next, the resulting molten kneaded material was cooled by passing it through a melt cooler connected to the tip of the twin-screw extruder and set at 164°C, and then extruded under atmospheric pressure through a die equipped with two 0.7 mm diameter holes attached to the tip of the melt cooler to foam it, and then cut with a rotary cutter attached to the tip of the die to obtain pre-expanded polypropylene resin particles with a mass per particle of 1.5 to 2.0 mg / particle.

[0073] [Preparation of polypropylene resin foam molded body] The polypropylene resin pre-expanded particles obtained above were filled into a block-shaped mold (400 mm long x 300 mm wide x variable thickness) with a thickness of 52 mm (cracking rate 30%) using a molding machine (KD345) manufactured by Daisen Co., Ltd., and compressed to a mold thickness of 40 mm. Next, the air in the mold was expelled with steam at 0.1 MPa (gauge pressure), and then the mold was heated and molded for 10 seconds using steam with a vapor pressure of 0.40 MPa (gauge pressure), the maximum possible vapor pressure, to obtain a polypropylene resin foam molded article. Subsequently, the heat molding was repeated in the same manner as above, except that the vapor pressure of the water vapor used during heat molding was successively decreased by 0.02 MPa, to obtain foamed molded articles corresponding to each vapor pressure. The resulting foamed molded articles were cured in a curing chamber at 75°C for 24 hours, and then left at room temperature for 4 hours. The fusion rate, deformation and shrinkage, and surface expansion were evaluated using the above-mentioned evaluation methods. The lowest steam pressure at which a foamed molded article that passed these criteria was obtained was defined as the minimum moldable steam pressure.

[0074] Examples 2 to 6 [Preparation of polypropylene resin composition] A polypropylene-based resin composition was prepared in the same manner as in Example 1, except that the polypropylene-based resin shown in Table 2 below was used as the raw material polypropylene-based resin, the monomers shown in Table 2 below, and t-butylperoxyisopropyl monocarbonate as the radical polymerization initiator were used in the blending amounts shown in Table 2 below. The obtained polypropylene-based resin composition contains a branched polypropylene-based resin having a polypropylene-based resin main chain and a structural unit derived from a conjugated diene compound as a side chain, and also contains an alcohol derived from a radical polymerization initiator, specifically t-butyl alcohol and isopropanol. [Preparation of pre-expanded polypropylene resin particles] Pre-expanded polypropylene resin particles were produced in the same manner as in Example 1, except that the polypropylene resin composition obtained above was used. [Preparation of polypropylene resin foam molded body] A polypropylene resin expansion molded article was produced in the same manner as in Example 1, except that the pre-expanded polypropylene resin particles obtained above were used.

[0075] (Comparative Examples 1 to 9) [Preparation of polypropylene resin composition] A polypropylene-based resin composition was prepared in the same manner as in Example 1, except that the polypropylene-based resin shown in Table 3 below was used as the raw material polypropylene-based resin, the monomers shown in Table 3 below, and t-butylperoxyisopropyl monocarbonate as the radical polymerization initiator were used in the blending amounts shown in Table 3 below. [Preparation of pre-expanded polypropylene resin particles] Pre-expanded polypropylene resin particles were produced in the same manner as in Example 1, except that the polypropylene resin composition obtained above was used. [Preparation of polypropylene resin foam molded body] A polypropylene resin expansion molded article was produced in the same manner as in Example 1, except that the pre-expanded polypropylene resin particles obtained above were used.

[0076] Among Comparative Examples 1 to 9, in Comparative Example 4, the pre-expanded particles did not expand sufficiently at any steam pressure, and an evaluable molded product could not be obtained. In Comparative Example 5, although an evaluable molded product was obtained, the pre-expanded particles did not expand sufficiently, so the molded product did not fill the inside of the mold, and a higher steam pressure was required. In Comparative Examples 6 to 8, the pre-expanded particles obtained underwent significant shrinkage immediately after leaving the die and were in a state where they were barely expanded, making them unsuitable for molding evaluation.

[0077] (Reference example 1) [Preparation of pre-expanded polypropylene resin particles] Pre-expanded polypropylene resin particles were prepared in the same manner as in Example 1, except that a branched propylene homopolymer ("Waymax MFX6" manufactured by Japan Polypropylene Corporation) having the properties shown in Table 3 below was used instead of the polypropylene resin composition. [Preparation of polypropylene resin foam molded body] A polypropylene resin foamed molded article was produced in the same manner as in Example 1, except that the polypropylene resin pre-expanded particles obtained above were used. However, the pre-expanded particles did not expand sufficiently at any of the vapor pressures, and no evaluable molded article could be obtained.

[0078] The pre-expanded polypropylene resin particles and expanded polypropylene resin molded articles obtained in the Examples, Comparative Examples, and Reference Examples were evaluated by the evaluation methods described above, and the evaluation results are shown in Tables 2 and 3 below.

[0079] [Table 2]

[0080] [Table 3]

[0081] As can be seen from the data in Table 2, in Examples 1 to 6, by using a polypropylene resin composition having a melt tension at 230°C of 2.94 cN or more and 19.6 cN or less, a melting point Tm of more than 150°C and less than 159°C, a cold crystallization temperature Tc of 122°C or more and less than 130°C, and a heat of fusion ΔH of 85 or more and less than 100 J / g as the base material for the polypropylene resin pre-expanded particles obtained by extrusion foaming, it was possible to achieve at a high level both a high closed cell ratio, good post-molding properties, and the ability to mold at the low vapor pressure required for post-molding, as well as the ability to obtain a polypropylene resin foam molded article with high strength.

[0082] On the other hand, as can be seen from the data in Table 3, in Comparative Examples 1 and 2, in which polypropylene-based resin compositions having a melting point Tm of 150°C or less, a cold crystallization temperature Tc of less than 122°C, and a heat of fusion ΔH of less than 85 J / g were used as the base material for polypropylene-based resin pre-expanded beads obtained by extrusion foaming, the strength of the resulting polypropylene-based resin foam-molded articles was poor. In Comparative Example 3, in which a polypropylene-based resin composition having a heat of fusion ΔH of less than 85 J / g was used as the base material for polypropylene-based resin pre-expanded beads obtained by extrusion foaming, the strength of the resulting polypropylene-based resin foam-molded articles was also poor. In Comparative Example 4, the polypropylene-based resin pre-expanded beads produced by extrusion foaming using a polypropylene-based resin composition having a melt tension at 230°C of less than 2.94 cN, a melting point Tm of 159°C or more, a cold crystallization temperature Tc of 130°C or more, and a heat of fusion ΔH of 100 J / g or more as the base material had an extremely high open cell ratio, and a polypropylene-based resin foam-molded article could not be formed. In Comparative Example 5, polypropylene-based resin pre-expanded beads produced by extrusion foaming using a polypropylene-based resin composition having a melting point Tm of 159°C or higher and a heat of fusion ΔH of 100 J / g or higher as a substrate had a high open-cell ratio, and it was not possible to mold a polypropylene-based resin foam-molded article at a low vapor pressure. In Comparative Examples 6 to 8, in which a polypropylene-based resin composition having a melt tension at 230°C of less than 2.94 cN was used as a substrate, attempts to produce polypropylene-based resin pre-expanded beads by extrusion foaming resulted in severe shrinkage, and expanded beads could not be obtained. In Comparative Example 9, in which a polypropylene-based resin composition having a melt tension at 230°C of less than 2.94 cN was used as a substrate, the die pressure was also insufficient. In Reference Example 1, polypropylene-based resin pre-expanded beads produced by extrusion foaming using a polypropylene-based resin having a cold crystallization temperature Tc of less than 122°C as a substrate had an extremely high open-cell ratio, and it was not possible to mold a polypropylene-based resin foam-molded article.

Claims

1. A method for producing pre-expanded polypropylene resin particles, comprising: The method includes a step of extrusion-foaming a polypropylene-based resin composition to obtain pre-foamed polypropylene-based resin particles, the polypropylene-based resin composition contains a branched polypropylene-based resin having a main chain of a polypropylene-based resin and a side chain of a structural unit derived from one or more monomers selected from the group consisting of a conjugated diene compound and an aromatic vinyl compound, the polypropylene-based resin composition is obtained by a method for producing a polypropylene-based resin composition, the method including a step of melt-kneading and extruding a polypropylene-based resin, one or more monomers selected from the group consisting of a conjugated diene compound and an aromatic vinyl compound, and a radical polymerization initiator at a temperature at which the polypropylene-based resin melts and the radical polymerization initiator decomposes; the polypropylene-based resin is a propylene homopolymer and has a melt flow rate of 1 g / 10 min or more and 20 g / 10 min or less; the amount of the monomer used is 0.04 mol or more and 0.14 mol or less per 1 kg of the polypropylene-based resin, The polypropylene resin composition satisfies the following requirements (1) to (4): The method for producing pre-expanded polypropylene resin particles, wherein the pre-expanded polypropylene resin particles have an open cell ratio of 10.5% or less. (1) A melt tension at 230°C of 2.94 cN or more and 19.6 cN or less; (2) A melting point Tm measured by differential scanning calorimetry is greater than 150°C and less than 159°C; (3) A cold crystallization temperature Tc measured by differential scanning calorimetry is 122°C or higher and lower than 130°C; (4) The heat of fusion ΔH measured by differential scanning calorimetry is 85 J / g or more and less than 100 J / g.

2. A method for producing pre-expanded polypropylene resin particles, comprising: The method includes a step of extrusion-foaming a polypropylene-based resin composition to obtain pre-foamed polypropylene-based resin particles, the polypropylene-based resin composition contains a branched polypropylene-based resin having a main chain of a polypropylene-based resin and a side chain of a structural unit derived from a monomer that is a conjugated diene compound, the polypropylene-based resin composition is obtained by a method for producing a polypropylene-based resin composition, the method including a step of melt-kneading and extruding a polypropylene-based resin, a conjugated diene compound, and a radical polymerization initiator at a temperature at which the polypropylene-based resin melts and the radical polymerization initiator decomposes; the polypropylene-based resin is a propylene homopolymer and has a melt flow rate of 1 g / 10 min or more and 20 g / 10 min or less; the amount of the monomer used is 0.04 mol or more and 0.14 mol or less per 1 kg of the polypropylene-based resin, The polypropylene resin composition satisfies the following requirements (1) to (4): (1) A melt tension at 230°C of 2.94 cN or more and 19.6 cN or less; (2) A melting point Tm measured by differential scanning calorimetry is greater than 150°C and less than 159°C; (3) A cold crystallization temperature Tc measured by differential scanning calorimetry is 122°C or higher and lower than 130°C; (4) The heat of fusion ΔH measured by differential scanning calorimetry is 85 J / g or more and less than 100 J / g.

3. The polypropylene resin composition comprises 13 3. The method for producing pre-expanded polypropylene resin particles according to claim 1, wherein the mm fraction of propylene unit triads determined by C-NMR is less than 96%.

4. The method for producing pre-expanded polypropylene resin particles according to any one of claims 1 to 3, wherein the polypropylene resin composition has a melt flow rate of 1 g / 10 min or more and 20 g / 10 min or less.

5. 2. The method for producing pre-expanded polypropylene resin particles according to claim 1, wherein the monomer is at least one selected from the group consisting of isoprene, butadiene, and styrene.

6. A method for producing polypropylene-based resin pre-expanded particles as described in Claim 2, wherein the monomer is one or more selected from the group consisting of isoprene and butadiene.

7. The method for producing pre-expanded polypropylene resin particles according to any one of claims 1 to 6, wherein the polypropylene resin composition contains an alcohol derived from a radical polymerization initiator.

8. 8. The method for producing pre-expanded polypropylene resin particles according to claim 7, wherein the alcohol derived from the radical polymerization initiator is at least one selected from the group consisting of t-butyl alcohol and isopropanol.

9. The method for producing pre-expanded polypropylene resin particles according to any one of claims 1 to 8, wherein the radical polymerization initiator is at least one selected from the group consisting of peroxydicarbonates and peroxyesters.

10. 3. The method for producing pre-expanded polypropylene resin particles according to claim 2, wherein the pre-expanded polypropylene resin particles have an open cell ratio of 10.5% or less.

11. A method for producing a foamed molded polypropylene resin article, comprising steam molding the pre-expanded polypropylene resin particles obtained by the method for producing pre-expanded polypropylene resin particles according to any one of claims 1 to 10 to obtain a foamed molded article.

12. The method for producing a polypropylene resin foam molded article according to claim 11, wherein the steam pressure during the steam molding is 0.40 MPa or less.

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