Extrusion-foamed polypropylene resin particles, molded article of extrusion-foamed particles, and method for producing extrusion-foamed polypropylene resin particles

Abstract

To provide extrusion-foamed polypropylene resin particles that enable production of a molded article of extrusion-foamed particles having superior surface properties and high compressive strength, a molded article of extrusion-foamed particles made using the particles, and a method for producing the extrusion-foamed polypropylene resin particles.SOLUTION: The present invention provides extrusion-foamed polypropylene resin particles including a polypropylene resin composition that contains a branched polypropylene resin (A) and an alicyclic saturated hydrocarbon resin (C), and further optionally contains a non-branched polypropylene resin (B), wherein the weight ratio of the polypropylene resin (A) to the polypropylene resin (B) is 70:30 to 100:0, and the content of the alicyclic saturated hydrocarbon resin (C) relative to 100 pts.wt. of the total of the polypropylene resin (A) and the polypropylene resin (B) is 1.0 to 5.0 pts.wt.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to polypropylene-based resin extruded foam particles, an extruded foam particle molded body, and a method for producing polypropylene-based resin extruded foam particles.

Background Art

[0002] The extruded foam particle molded body obtained using polypropylene-based resin extruded foam particles is mainly used in various applications such as automotive interior members and core materials for automotive bumpers, as well as heat insulating materials and cushioning packaging materials.

[0003] As a method for producing polypropylene-based resin extruded foam particles, an extrusion foaming method is known in which an inorganic gas is added as a foaming agent to a resin composition containing a polypropylene-based resin having a branched structure, and the obtained melt-kneaded product is extruded from a die and foamed (see Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the polypropylene-based resin foam particles produced by the method described in Patent Document 1 are likely to have the cell membranes inside the foam particles broken and a high closed-cell ratio (also referred to as cell coalescence) during subsequent in-mold foaming molding. Therefore, in addition to the surface property of the obtained extruded foam particle molded body being likely to deteriorate, there is a problem that the compressive strength is likely to be low.

[0006] The present invention has been made in view of the above problems, and provides polypropylene-based resin extrusion foamed particles capable of obtaining an extrusion foamed particle molded body having excellent surface properties and high compressive strength, an extrusion foamed particle molded body obtained by using the extrusion foamed particles, and a method for producing the polypropylene-based resin extrusion foamed particles.

Means for Solving the Problems

[0007] The inventor of the present invention has found that the above problems can be solved by extrusion foaming a polypropylene-based resin composition containing an alicyclic saturated hydrocarbon resin, and has completed the present invention.

[0008] Aspects of the present disclosure relate to the following polypropylene-based resin extrusion foamed particles, extrusion foamed particle molded bodies, and methods for producing polypropylene-based resin extrusion foamed particles.

[0009] [1] Polypropylene-based resin extrusion foamed particles containing a polypropylene-based resin composition (A) having a branched structure and an alicyclic saturated hydrocarbon resin (C), and further optionally containing a polypropylene-based resin (B) into which no branched structure is introduced, wherein the weight ratio of the polypropylene-based resin (A) to the polypropylene-based resin (B) is 70:30 to 100:0, and the content of the alicyclic saturated hydrocarbon resin (C) with respect to a total of 100 parts by weight of the polypropylene-based resin (A) and the polypropylene-based resin (B) is 1.0 to 5.0 parts by weight. [2] The extrusion foamed particles according to [1], wherein the alicyclic saturated hydrocarbon resin (C) is a hydrogenated product of an aromatic petroleum resin. [3] The extrusion foamed particles according to [1] or [2], having a foaming ratio exceeding 10 times and less than 25 times. [4] An extrusion foamed particle molded body obtained by molding the extrusion foamed particles according to any one of [1] to [3]. [5] A method for producing polypropylene-based resin extruded foam particles, comprising supplying a polypropylene-based resin composition to an extruder, melt-kneading it, adding a foaming agent, and further performing melt-kneading to obtain a melt-kneaded product, and extruding and foaming the melt-kneaded product from a die after cooling it. The polypropylene-based resin composition contains a polypropylene-based resin (A) having a branched structure and an alicyclic saturated hydrocarbon resin (C), and may further contain a polypropylene-based resin (B) into which no branched structure is introduced. The weight ratio of the polypropylene-based resin (A) to the polypropylene-based resin (B) is 70:30 to 100:0. The content of the alicyclic saturated hydrocarbon resin (C) with respect to 100 parts by weight in total of the polypropylene-based resin (A) and the polypropylene-based resin (B) is 1.0 to 5.0 parts by weight. [6] The production method according to [5], wherein the alicyclic saturated hydrocarbon resin (C) is a hydrogenated product of an aromatic petroleum resin.

Effect of the Invention

[0010] According to the present invention, it is possible to provide polypropylene-based resin extruded foam particles that can obtain an extruded foam particle molded body excellent in surface properties and having high compressive strength, an extruded foam particle molded body obtained by using the extruded foam particles, and a method for producing the polypropylene-based resin extruded foam particles.

Mode for Carrying Out the Invention

[0011] ≪Polypropylene-based Resin Extruded Foam Particles≫ The polypropylene-based resin extruded foam particles of the present embodiment contain a polypropylene-based resin composition containing a polypropylene-based resin (A) having a branched structure and an alicyclic saturated hydrocarbon resin (C), and may further contain a polypropylene-based resin (B) into which no branched structure is introduced. The weight ratio of the polypropylene-based resin (A) to the polypropylene-based resin (B) is 70:30 to 100:0. The content of the alicyclic saturated hydrocarbon resin (C) with respect to a total of 100 parts by weight of the above polypropylene resin (A) and the above polypropylene resin (B) is 1.0 to 5.0 parts by weight.

[0012] According to the polypropylene resin extrusion foamed particles of this embodiment, an extrusion foamed particle molded body excellent in surface properties and having high compressive strength can be obtained.

[0013] <Polypropylene resin composition> As described above, the polypropylene resin composition includes a polypropylene resin (A) having a branched structure (hereinafter also referred to as "branched polypropylene resin (A)") and an alicyclic saturated hydrocarbon resin (C), and may further include a polypropylene resin (B) into which no branched structure is introduced (hereinafter also referred to as "linear polypropylene resin (B)"). Hereinafter, these components will be described.

[0014] 〔Branched polypropylene resin (A)〕 The branched polypropylene resin (A) is a modified polypropylene resin obtained by introducing a branched structure into a linear polypropylene resin (a). The linear polypropylene resin (a) means a resin containing 50 mol% or more of structural units derived from propylene monomers in 100 mol% of all structural units contained in the resin.

[0015] The method for introducing a branched structure into the linear polypropylene resin (a) is not particularly limited. For example, (1) a method of irradiating the linear polypropylene resin (a) with radiation, and (2) a method of melt-kneading a mixture containing the linear polypropylene resin (a), a monomer (b) selected from the group consisting of conjugated dienes and vinyl aromatic compounds, and a radical polymerization initiator (c) can be mentioned.

[0016] Specific examples of the method (1) include the method described in Japanese Patent Application Laid-Open No. 2002-542360.

[0017] As a specific method of the method described in (2) above, for example, there is a method of melt-kneading a linear polypropylene-based resin (a), a monomer (b) selected from a conjugated diene and a vinyl aromatic compound, and a radical polymerization initiator (c) at a temperature at which the linear polypropylene-based resin (a) is melted and the radical polymerization initiator (c) decomposes.

[0018] [Linear polypropylene-based resin (a)] Examples of the linear polypropylene-based resin (a) include a homopolypropylene-based resin (a1), a random polypropylene-based resin (a2), and a block polypropylene-based resin (a3). Among these, the random polypropylene-based resin (a2) is preferred.

[0019] (Homopolypropylene-based resin (a1)) The homopolypropylene-based resin (a1) refers to a homopolymer of a propylene monomer.

[0020] (Random polypropylene-based resin (a2)) The random polypropylene-based resin (a2) refers to a random copolymer of a propylene monomer and a monomer other than propylene. The random polypropylene-based resin (a2) contains 50 mol% or more of the structural units derived from the propylene monomer and less than 50 mol% of the structural units derived from monomers other than the propylene monomer in 100 mol% of all the structural units contained in the resin. In this specification, the "structural units derived from the propylene monomer" may also be referred to as "propylene units". In this specification, the "structural units derived from monomers other than the propylene monomer" may also be referred to as "comonomer units".

[0021] Examples of the comonomer 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, 1-decene; cyclic olefins such as cyclopentene, norbornene, tetracyclo[6,2,11,8,13,6]-4-dodecene; dienes such as 5-methylene-2-norbornene, 5-ethylidene-2-norbornene, 1,4-hexadiene, methyl-1,4-hexadiene, 7-methyl-1,6-octadiene; and vinyl monomers such as vinyl chloride, vinylidene chloride, acrylonitrile, methacrylonitrile, vinyl acetate, acrylic acid, acrylic ester, methacrylic acid, methacrylic ester, maleic acid, maleic anhydride, styrene monomers, vinyltoluene, divinylbenzene; and the like.

[0022] Examples of the acrylic ester include methyl acrylate, ethyl acrylate, butyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, stearyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and glycidyl acrylate.

[0023] Examples of the methacrylic ester include methyl methacrylate, ethyl methacrylate, butyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, stearyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, and glycidyl methacrylate.

[0024] Examples of the styrene monomer include styrene, methylstyrene, dimethylstyrene, α-methylstyrene, paramethylstyrene, ethylstyrene, diethylstyrene, isopropylstyrene, t-butylstyrene, bromostyrene, dibromostyrene, tribromostyrene, chlorostyrene, dichlorostyrene, and trichlorostyrene.

[0025] The random polypropylene-based resin (a2) preferably has, as comonomer units, structural units derived from an α-olefin having 2 or 4 to 12 carbon atoms. More preferably, it has structural units derived from 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 / or 1-decene. Even more preferably, it has structural units derived from ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene and / or 4-methyl-1-pentene. Even more preferably, it has structural units derived from ethylene, 1-butene, isobutene and / or 1-pentene. Particularly preferably, it has structural units derived from ethylene and / or 1-butene.

[0026] The random polypropylene-based resin (a2) preferably contains 90 mol% or more of propylene units, more preferably 93 mol% or more, even more preferably 94 mol% or more, and particularly preferably 95 mol% or more, based on 100 mol% of all the structural units contained in the random polypropylene-based resin (a2).

[0027] (Block polypropylene-based resin (a3)) Examples of the block polypropylene-based resin (a3) include ethylene-propylene block copolymers, propylene-butene block copolymers, and ethylene-propylene-butene block copolymers. The propylene-ethylene block copolymer is a propylene-based polymer in which a polymer mainly composed of ethylene and an ethylene-propylene rubber-like copolymer are dispersed in a linear polymer mainly composed of propylene to form a sea-island structure.

[0028] [Monomer (b) selected from conjugated dienes and vinyl aromatic compounds] Examples of the conjugated diene include butadiene, isoprene, 1,3 - heptadiene, 2,3 - dimethylbutadiene, 2,5 - dimethyl - 2,4 - hexadiene, and the like. These conjugated diene compounds may be used alone or in combination of two or more. Among these conjugated diene compounds, butadiene and isoprene are particularly preferred from the viewpoints of low cost, ease of handling, and uniform progress of the reaction.

[0029] Examples of the vinyl aromatic compound include styrene; methylstyrenes such as o - methylstyrene, m - methylstyrene, p - methylstyrene, α - methylstyrene, β - methylstyrene, dimethylstyrene, trimethylstyrene; chlorostyrenes such as α - chlorostyrene, β - chlorostyrene, o - chlorostyrene, m - chlorostyrene, p - chlorostyrene, dichlorostyrene, trichlorostyrene; bromostyrenes such as o - bromostyrene, m - bromostyrene, p - bromostyrene, dibromostyrene, tribromostyrene; fluorostyrenes such as o - fluorostyrene, m - fluorostyrene, p - fluorostyrene, difluorostyrene, trifluorostyrene; nitrostyrenes such as o - nitrostyrene, m - nitrostyrene, p - nitrostyrene, dinitrostyrene, trinitrostyrene; vinylphenols such as o - hydroxystyrene, m - hydroxystyrene, p - hydroxystyrene, dihydroxystyrene, trihydroxystyrene; divinylbenzenes such as o - divinylbenzene, m - divinylbenzene, p - divinylbenzene; isopropenylstyrenes such as o - diisopropenylbenzene, m - diisopropenylbenzene, p - diisopropenylbenzene, and the like. Among the above - mentioned vinyl aromatic compounds, styrene and / or methylstyrene are preferred from the viewpoints of low cost, ease of handling, and uniform progress of the reaction.

[0030] The usage amount of the conjugated diene etc. (b) is preferably 0.01 to 5.00 parts by weight, more preferably 0.10 to 3.00 parts by weight, and still more preferably 0.10 to 2.00 parts by weight with respect to 100 parts by weight of the linear polypropylene - based resin (a).

[0031] [Radical polymerization initiator (c)] The radical polymerization initiator (c) is an organic peroxide having the ability to abstract hydrogen from the linear polypropylene-based resin (a) and the conjugated diene compound etc. (b). Examples of the radical polymerization initiator (c) include organic peroxides such as ketone peroxide, peroxyketal, hydroperoxide, dialkyl peroxide, diacyl peroxide, peroxydicarbonate, and peroxyester.

[0032] As the organic peroxide, those having a particularly high hydrogen abstraction ability are preferred. Examples of the organic peroxide having a high hydrogen abstraction ability include peroxyketals such as 1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, n-butyl 4,4-bis(t-butylperoxy)valerate, 2,2-bis(t-butylperoxy)butane; dialkyl peroxides such as dicumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, α,α'-bis(t-butylperoxy-m-isopropyl)benzene, t-butylcumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexyne; diacyl peroxides such as benzoyl peroxide; peroxyesters such as t-butyl peroxy octoate, t-butyl peroxy isobutyrate, t-butyl peroxy laurate, t-butyl peroxy 3,5,5-trimethylhexanoate, t-butyl peroxy isopropyl carbonate, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, t-butyl peroxy acetate, t-butyl peroxy benzoate, di-t-butyl peroxy isophthalate; etc. Among these, t-butyl peroxy isopropyl carbonate and / or t-butyl peroxy benzoate are preferred. These organic peroxides may be used alone or in combination of two or more.

[0033] The amount of the radical polymerization initiator (c) used is preferably 0.01 to 5.00 parts by weight, more preferably 0.10 to 3.00 parts by weight, still more preferably 0.10 to 2.00 parts by weight, and particularly preferably 0.10 to 1.50 parts by weight with respect to 100 parts by weight of the linear polypropylene resin (a).

[0034] In the method of (2) above, examples of the apparatus for melt-kneading the linear polypropylene resin (a), the monomer (b) such as conjugated diene, and the radical polymerization initiator (c) include kneaders such as rolls, conical kneaders, Banbury mixers, Brabenders, single-screw extruders, and twin-screw extruders; horizontal stirrers such as twin-screw surface renewal machines and twin-screw multi-disc devices; and vertical stirrers such as double-helical ribbon stirrers. Among these, it is preferable to use a kneader, and particularly, extruders such as single-screw extruders and twin-screw extruders are preferable from the viewpoint of productivity.

[0035] There are no particular restrictions on the order and method of mixing and kneading the linear polypropylene resin (a), the monomer (b) such as conjugated diene, and the radical polymerization initiator (c). After mixing the linear polypropylene resin (a), the monomer (b) such as conjugated diene, and the radical polymerization initiator (c), they may be melt-kneaded, or after melt-kneading the linear polypropylene resin (a), the monomer (b) such as conjugated diene or the radical polymerization initiator (c) may be mixed simultaneously or separately, either all at once or in portions. The temperature of the kneader is preferably 130°C or higher and 300°C or lower. Also, the melt-kneading time is generally preferably 1 minute or longer and 60 minutes or shorter.

[0036] (Physical properties of the branched polypropylene resin (A)) The melting point (Tm) of the branched polypropylene resin (A) is not particularly limited, but is preferably 130.0°C to 165.0°C, and more preferably 135.0°C to 163.0°C. In this specification, the melting point is a value determined by measurement by differential scanning calorimetry.

[0037] The melt flow rate (MFR) of the branched polypropylene resin (A) is not particularly limited, but is preferably 0.5 g / 10 min or more and 50 g / 10 min or less, and more preferably 0.5 g / 10 min or more and 30 g / 10 min or less. In this specification, the MFR is a value determined by measuring at a temperature of 230 °C in accordance with ISO 1133.

[0038] (Linear polypropylene resin (B)) The linear polypropylene resin (B) is a linear resin that contains 50 mol% or more of structural units derived from propylene monomers in 100 mol% of all structural units contained in the resin and has no branched structure. Examples of the linear polypropylene resin (B) include a propylene homopolymer, a block polypropylene resin, and a random polypropylene resin. A propylene homopolymer or a random polypropylene resin is preferable, and a random polypropylene resin is more preferable. The random polypropylene resin is the same as the embodiment described in the item [(Random polypropylene resin (a2))] described above. In the polypropylene resin extrusion foamed particles of the present embodiment, the linear polypropylene resin (B) is an optional component. However, from the viewpoint of obtaining an extrusion foamed particle molded body having excellent surface properties and high compressive strength, it is preferable that the polypropylene resin extrusion foamed particles contain the linear polypropylene resin (B).

[0039] (Physical properties of linear polypropylene resin (B)) The melting point (Tm) of the linear polypropylene resin (B) is not particularly limited, but is preferably 130.0 °C to 165.0 °C, and more preferably 135.0 °C to 163.0 °C.

[0040] The upper limit of the melt flow rate (MFR) of the linear polypropylene resin (B) is preferably 5 g / 10 min or less, and more preferably 1 g / 10 min or less, from the viewpoint of obtaining an extrusion foamed particle molded body having excellent surface properties and high compressive strength. The lower limit of the MFR of the linear polypropylene resin (B) is not particularly limited, but it is preferably 0.1 g / 10 min or more, and more preferably 0.2 g / 10 min or more.

[0041] In the polypropylene resin extrusion foamed particles of the present embodiment, the weight ratio of the branched polypropylene resin (A) to the linear polypropylene resin (B) is 70:30 to 100:0, preferably 75:25 to 98:2, and more preferably 80:20 to 95:5 from the viewpoint of obtaining an extrusion foamed particle molded body having excellent surface properties and high compressive strength.

[0042] (alicyclic saturated hydrocarbon resin (C)) The alicyclic saturated hydrocarbon resin (C) has an effect of suppressing the cell coalescence of the extrusion foamed particles when the polypropylene resin extrusion foamed particles of the present embodiment are foam-molded in a mold. As a result, an extrusion foamed particle molded body having excellent surface properties and high compressive strength can be obtained.

[0043] The alicyclic saturated hydrocarbon resin (C) refers to a hydrocarbon resin obtained by polymerizing an alicyclic fraction generated when cracking petroleum, naphtha, etc. as a main raw material, or a hydrocarbon resin obtained by hydrogenating an aromatic petroleum resin.

[0044] Examples of the alicyclic fraction include one or a mixture of two or more of cyclopentadiene, dicyclopentadiene, methyldicyclopentadiene, dimethyldicyclopentadiene, etc.

[0045] Examples of the aromatic petroleum resin include resins mainly composed of one or two or more of C9 or C10 fractions such as vinyltoluene, indene, and methylindene.

[0046] Specific examples of the alicyclic saturated hydrocarbon resin (C) include, for example, Alcon (manufactured by Arakawa Chemical Industries, Ltd., trade name), Quintone 1500, Quintone 1700 (manufactured by Nippon Zeon Co., Ltd., trade name), etc.

[0047] As the alicyclic saturated hydrocarbon resin (C), from the viewpoint of obtaining an extrusion foamed particle molded body having excellent surface properties and high compressive strength, a hydrocarbon resin obtained by hydrogenating an aromatic petroleum resin (hereinafter, also referred to as "hydrogenated aromatic petroleum resin") is more preferable. The softening point of the alicyclic saturated hydrocarbon resin (C) is preferably -10°C or lower, more preferably -15°C or lower, than the melting point of the polypropylene-based resin (A) having a branched structure. The lower limit of the softening point of the alicyclic saturated hydrocarbon resin (C) is preferably -60°C or higher than the melting point of the polypropylene-based resin (A) having a branched structure.

[0048] From the viewpoint of ensuring fluidity during in-mold foaming molding, the MFR of the alicyclic saturated hydrocarbon resin (C) is preferably 50 g / 10 min or more, more preferably 100 g / 10 min or more, and even more preferably 200 g / 10 min or more, as measured under the conditions of 160°C and 2.16 kg.

[0049] The alicyclic saturated hydrocarbon resin (C) may be used alone or in combination of two or more.

[0050] The content of the alicyclic saturated hydrocarbon resin (C) with respect to a total of 100 parts by weight of the polypropylene-based resin (A) and the polypropylene-based resin (B) is 1.0 to 5.0 parts by weight, preferably 1.0 to 4.5 parts by weight, and more preferably 1.0 to 4.0 parts by weight.

[0051] (Other components) The polypropylene-based resin composition may contain components other than the above-described branched-chain polypropylene-based resin (A), linear polypropylene-based resin (B), and alicyclic saturated hydrocarbon resin (C) (hereinafter also referred to as "other components") as long as the effects of the present invention are not impaired. Examples of other components include other resins or rubbers (hereinafter also referred to as "other resin etc.") other than the above resin (A) and the above resin (B), a bubble nucleating agent; stabilizers such as an antioxidant, a metal deactivator, a phosphorus-based processing stabilizer, an ultraviolet absorber, an ultraviolet stabilizer, a fluorescent brightening agent, a metal soap, and an antacid adsorbent; and / or additives such as a crosslinking agent, a chain transfer agent, a lubricant, a plasticizer, a filler, a reinforcing material, a flame retardant, a colorant, and an antistatic agent; and the like. These other components may be used alone or in combination of two or more. The total content of other components in the polypropylene-based resin composition is not particularly limited. The total content of other components in the polypropylene-based resin composition is preferably 0.01 part by weight to 50.00 parts by weight, more preferably 0.05 part by weight to 30.00 parts by weight, based on 100 parts by weight in total of the above polypropylene-based resin (A) and the above polypropylene-based resin (B).

[0052] (other resin etc.) Examples of other resins include ethylene-based resins such as high-density polyethylene, medium-density polyethylene, low-density polyethylene, linear low-density polyethylene, linear ultra-low-density polyethylene, ethylene / vinyl acetate copolymer, ethylene / acrylic acid copolymer, and ethylene / methacrylic acid copolymer; and styrene-based resins such as polystyrene, styrene / maleic anhydride copolymer, and styrene / ethylene copolymer. Examples of the rubber include olefin-based rubbers such as ethylene / propylene rubber, ethylene / butene rubber, ethylene / hexene rubber, and ethylene / octene rubber. The total content of other resins and rubbers in the base resin is not particularly limited. In the polypropylene-based resin composition, the total content of other resins and rubbers is preferably 1 to 30 parts by weight, more preferably 2 to 15 parts by weight, based on 100 parts by weight in total of the above polypropylene-based resin (A) and the above polypropylene-based resin (B).

[0053] (Bubble nucleating agent) Examples of the bubble nucleating agent include sodium bicarbonate-citric acid mixture, sodium citrate, talc, and calcium carbonate. These bubble nucleating agents may be used alone or in combination of two or more.

[0054] The content of the bubble nucleating agent in the polypropylene-based resin composition is not particularly limited. The content of the bubble nucleating agent is preferably 0.01 to 5.00 parts by weight, more preferably 0.01 to 3.50 parts by weight, still more preferably 0.01 to 1.00 parts by weight, and particularly preferably 0.01 to 0.50 parts by weight, based on 100 parts by weight in total of the above polypropylene-based resin (A) and the above polypropylene-based resin (B). According to this configuration, there is an advantage that the average bubble diameter and the shape of the bubbles of the extruded foam particles become uniform, and as a result, the foamability at the time of extrusion foaming tends to be stable.

[0055] (Physical properties of extruded foam particles) (Expansion ratio) The expansion ratio of the polypropylene resin extruded foam particles is not particularly limited, and is preferably more than 10 times and less than 25 times, and more preferably 12 times or more and 23 times or less.

[0056] (Average cell diameter) The average cell diameter (also referred to as the average bubble diameter) of the polypropylene resin extruded foam particles is not particularly limited, but is preferably 200 μm or more and 600 μm or less, and more preferably 300 μm or more and 550 μm or less. When the average bubble diameter of the polypropylene resin extruded foam particles is 200 μm or more, the shrinkage of the extruded foam particle molded body is likely to be small, and when it is 600 μm or less, it is easy to shorten the molding cycle during in-mold foaming.

[0057] (Closed cell ratio) The lower the closed cell ratio of the polypropylene resin extruded foam particles, the more preferable. The closed cell ratio of the extruded foam particles is preferably 12% or less, more preferably 11% or less, further preferably 10% or less, and even more preferably 9% or less. The lower limit value of the closed cell ratio of the present polypropylene resin extruded foam particles is not particularly limited, and is, for example, 0.0% or more. According to the above configuration, when the extruded foam particles are molded, the cells are hardly broken and shrunk, so that the extruded foam particles have the advantage of excellent moldability, and in the extruded foam particle molded body obtained by using the extruded foam particles, features such as arbitrary shape, cushioning property, light weight, compressive strength and heat insulation property are more exhibited.

[0058] In this specification, the closed cell ratio of the polypropylene resin extruded foam particles is a value obtained by measuring according to the method described in PROCEDURE C of ASTM D2856-87 using an air comparison type specific gravity meter [manufactured by Tokyo Science Co., Ltd., model 1000]. Specifically, the closed cell ratio of the extruded foam particles is calculated by sequentially performing the following (1) to (3): (1) Using an air comparison type specific gravity meter, the volume Vc (cm of the extruded foam particles 3Measure (2); then, immerse the total amount of the extruded foamed particles after measuring Vc into the ethanol in the graduated cylinder; (3) thereafter, from the amount of increase in the position of the ethanol in the graduated cylinder, determine the apparent volume Va (cm 3 ) of the extruded foamed particles; (4) calculate the closed cell ratio of the extruded foamed particles by the following formula: Closed cell ratio (%) = ((Va - Vc) × 100) / Va. Note that the method for measuring the volume Va is also referred to as the water immersion method.

[0059] ≪Method for Producing Polypropylene - Based Resin Extruded Foamed Particles≫ The method for producing polypropylene - based resin extruded foamed particles of this embodiment is feeding a polypropylene - based resin composition to an extruder, melt - kneading it, then adding a foaming agent and further performing melt - kneading to obtain a melt - kneaded product, and extruding and foaming the melt - kneaded product from a die after cooling it, wherein the polypropylene - based resin composition includes a polypropylene - based resin (A) having a branched structure and an alicyclic saturated hydrocarbon resin (C), and may further include a polypropylene - based resin (B) into which no branched structure is introduced, the weight ratio of the polypropylene - based resin (A) to the polypropylene - based resin (B) is 70:30 to 100:0, and the content of the alicyclic saturated hydrocarbon resin (C) with respect to 100 parts by weight in total of the polypropylene - based resin (A) and the polypropylene - based resin (B) is 1.0 to 5.0 parts by weight.

[0060] According to the method for producing polypropylene - based resin extruded foamed particles of this embodiment, polypropylene - based resin extruded foamed particles can be produced, from which an extruded foamed particle molded body excellent in surface properties and having high compressive strength can be obtained.

[0061] <Step of obtaining a melt - kneaded product> (Polypropylene - based resin composition) The polypropylene-based resin composition used in the production method of this embodiment is the same as the embodiment described in the item of <polypropylene-based resin composition> in the above-mentioned <<polypropylene-based resin extrusion foamed particles>>.

[0062] (Blowing agent) The blowing agent used in the production method of this embodiment is not particularly limited as long as it is a commonly used blowing agent for extrusion foaming. Examples of the blowing agent include aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, and hexane; alicyclic hydrocarbons such as cyclopentane and cyclobutane; ethers such as dimethyl ether, diethyl ether, and methyl ethyl ether; alcohols such as methanol and ethanol; inorganic gases such as air, nitrogen, and carbon dioxide gas; and physical blowing agents such as water; and chemical blowing agents including thermal decomposition type blowing agents such as sodium bicarbonate, azodicarbonamide, and dinitrosopentamethylenetetramine; and the like. Among these, inorganic gases and physical blowing agents are preferable from the viewpoint of environmental load.

[0063] The amount of the blowing agent used is preferably 0.5 to 7.0 parts by weight, more preferably 0.5 to 6.0 parts by weight, and even more preferably 0.5 to 5.0 parts by weight with respect to 100.0 parts by weight of the polypropylene-based resin composition.

[0064] Examples of the extruder used to obtain the melt-kneaded product include a single-screw extruder and a twin-screw extruder. Among these, a twin-screw extruder is preferable in terms of excellent melt-kneading properties.

[0065] The cylinder temperature of the extruder can be appropriately set according to the melting point of the polypropylene-based resin composition used, the type and amount of the blowing agent, etc., from the viewpoint of sufficiently melt-kneading the polypropylene-based resin composition and the blowing agent, and is not particularly limited. For example, the cylinder temperature of the extruder is preferably 150°C or higher and 250°C or lower, and more preferably 170°C or higher and 230°C or lower.

[0066] <Step of extruding and foaming the melt-kneaded product after cooling it> As a cooling device used for cooling the obtained melt-kneaded product, for example, a single-screw extruder, a static mixer, a melt cooler, etc. can be mentioned. In this cooling device, a die for extruding the cooled melt-kneaded product is provided at the tip in the extrusion direction of the melt-kneaded product. The temperature of the cooling device may be a temperature suitable for foaming the obtained melt-kneaded product and is not particularly limited. For example, the temperature of the cooling device is preferably 120°C or higher and 180°C or lower, and more preferably 130°C or higher and 170°C or lower.

[0067] The cooled melt-kneaded product is extruded through the holes provided in the die into a region that is at a lower pressure than the internal pressure of the cooling device (hereinafter also referred to as the "low-pressure region"). The melt-kneaded product may be extruded into the gas phase or into the liquid phase. The pressure of the low-pressure region is not particularly limited, but is preferably 5 MPa or lower, and more preferably 1 MPa or lower. The extruded melt-kneaded product immediately starts to foam, and by the end of foaming, a polypropylene-based resin extrusion foam is obtained.

[0068] In the above foaming step, a comminution step of cutting the extruded melt-kneaded product into particles may be further included. In the comminution step, the melt-kneaded product (extrusion foam) during foaming may be comminuted, or the melt-kneaded product (extrusion foam) after foaming may be comminuted. The method of comminuting the extruded melt-kneaded product is not particularly limited. For example, a method of comminuting the melt-kneaded product with a cutter provided next to the die along the extrusion direction can be mentioned. Through the above steps, polypropylene-based resin extrusion foam particles are obtained.

[0069] The above low-pressure region may be in the gas phase or in the liquid phase, but is preferably in the gas phase because it is easy to obtain foamed particles with a relatively high foaming ratio. Water or the like may be sprayed to cool the surface of the foamed particles cut in the gas phase.

[0070] ≪Extrusion foam particle molded body≫ The polypropylene-based resin extrusion foamed particle molded body of this embodiment is the in-mold molded body of the polypropylene-based resin extrusion foamed particles described above. The polypropylene-based resin extrusion foamed particle molded body can be obtained by filling the above extrusion foamed particles into a mold that can close but not seal them, and heating and molding them with steam or the like. As a method for manufacturing the above extrusion foamed particle molded body, for example, (a) a method in which the foamed particles are pressurized with an inorganic gas to impregnate the inorganic gas into the particles to impart a predetermined internal particle pressure, then filled into a mold, and heat-sealed with steam or the like (for example, Japanese Patent Publication No. 51-22951), (b) a method in which the foamed particles are compressed with gas pressure and filled into a mold, and heat-sealed with steam or the like by utilizing the restoring force of the particles (for example, Japanese Patent Publication No. 53-33996), (c) a method in which the foamed particles are filled into a mold with an expanded gap, then the mold is closed to a predetermined gap to compress the filled foamed particles, and heat-sealed with steam or the like can be used.

[0071] <Physical properties of the extrusion foamed particle molded body> (Expansion ratio) The expansion ratio of the polypropylene-based resin extrusion foamed particle molded body is not particularly limited, and is preferably 12 times or more and 40 times or less, and more preferably 15 times or more and 30 times or less.

[0072] (Closed cell ratio) The lower the closed cell ratio of the polypropylene-based resin extrusion foamed particle molded body, the more preferable. From the viewpoint of obtaining an extrusion foamed particle molded body with high compressive strength, the closed cell ratio of the extrusion foamed particle molded body is preferably 40% or less, more preferably 36% or less, further preferably 33% or less, and even more preferably 30% or less. The lower limit value of the closed cell ratio of the foamed molded body is not particularly limited, and is, for example, 0.0% or more, and preferably 2% or more.

[0073] (Compressive strength) The compressive strength of the polypropylene-based resin extrusion foamed particle molded body is not particularly limited, and is preferably 0.100 MPa or more, more preferably 0.120 MPa or more, and further preferably 0.140 MPa or more.

[0074] In this specification, the compressive strength of the extruded foamed particle molded body is the value of the compressive stress at 50% strain when compressed at a compression rate of 10 mm / min using a tensile-compression testing machine.

[0075] The extruded foamed particle molded body is used for automobile interior members, core materials for automobile bumpers, heat insulating materials, cushioning packaging materials, etc.

Examples

[0076] Hereinafter, the present invention will be described more specifically based on examples and comparative examples, but the present invention is not limited to the following examples.

[0077] [Examples 1 to 4, Comparative Examples 1 to 2] (Materials) In the examples and comparative examples, the following resin A was produced as the branched polypropylene-based resin (A).

[0078] (Production Example of Resin A) First, the raw material resin RD265CF (manufactured by Borouge, linear random polypropylene-based resin, melting point: 152 °C, MFR: 8 g / 10 min) was supplied to a twin-screw extruder at 70 kg / h, and then 1.0 part by weight of t-butyl peroxyisopropyl carbonate (manufactured by NOF Corporation, PERBUTYL (registered trademark) I), which is a radical polymerization initiator, was supplied to the twin-screw extruder with respect to 100 parts by weight of the raw material resin. Then, 0.41 part by weight of isoprene (manufactured by Kuraray Co., Ltd., isoprene monomer), which is a conjugated diene compound, was supplied to the twin-screw extruder containing the molten-kneaded raw material resin and the radical polymerization initiator, and a resin mixture was prepared in the twin-screw extruder. The prepared resin mixture was melt-kneaded in a twin-screw extruder at a cylinder temperature of 200 °C and a screw rotation speed of 230 rpm, and discharged in a strand shape from the die. The discharged strand was cooled with water and then cut into pellets (cylindrical shape) to obtain resin A. The melting point of the obtained resin A was 148 °C and the MFR was 2.0 g / 10 min.

[0079] In the examples and comparative examples, the following resin B was used as the linear polypropylene-based resin (B). Resin B: Linear random polypropylene-based resin (B221WC, melting point 149°C, MFR: 0.5 g / 10 min) manufactured by Prime Polymer Co., Ltd.

[0080] In the examples and comparative examples, the following C-1 to C-2 were used as the alicyclic saturated hydrocarbon resin (C), and the following D was used as the cell nucleating agent (D). C-1: Alcon P-100 manufactured by Arakawa Chemical Industries, Ltd. C-2: Alcon P-125 manufactured by Arakawa Chemical Industries, Ltd. D: Talc (Talkan Powder (registered trademark) PK-S) manufactured by Hayashi Kasei Co., Ltd.

[0081] (Production Example of Polypropylene-Based Resin Extrusion Foamed Particles) (Examples 1 to 4 and Comparative Examples 1 to 2) As the apparatus used for producing the extrusion foamed particles, an apparatus in which a twin-screw extruder with a shaft diameter of Φ15 mm, a melt cooler, a diverter valve, and a die were connected in series was used. Resins (A), resin (B), alicyclic saturated hydrocarbon resin (C), and cell nucleating agent (D) of the types shown in Table 1 were dry blended in the amounts shown in Table 1 to prepare each polypropylene-based resin composition for extrusion foaming. Next, the resin composition was supplied to the twin-screw extruder at 0.75 kg / h, and the resin composition was melt-kneaded at a cylinder temperature of 200°C and a screw rotation speed of 40 rpm. Further, carbon dioxide gas, which is a foaming agent, was supplied from the injection part provided in the middle of the extruder in the amount described in Table 1 with respect to 100 parts by weight of the resin composition using a metering pump, and each of the obtained compositions was further melt-kneaded. The obtained melt-kneaded product was passed through a melt cooler connected to the tip of a twin-screw extruder and set at the temperature shown in Table 1 for cooling. Then, it was extruded into the atmosphere through a die (φ0.7 mm × 2 holes) attached to the tip of the melt cooler to cause foaming. Each extruded foam was quickly cut with a cutter (4 blades, 750 rpm) to obtain extruded foam particles of 2.1 mg / granule. The granulation method at this time was carried out by the watering cut method (hereinafter also referred to as the "WRC method") in which water was made to flow on the wall surface so that the foam particles after cutting the foam touched water and were discharged outside the system. The water temperature of the water used in the WRC method was 25 to 35°C. The obtained extruded foam particles were dried at 75°C for 2 hours or more and allowed to stand at 23°C for 24 hours or more, and then the foaming ratio, average cell diameter, and continuous bubble ratio were measured. The results are shown in Table 1.

[0082] (Application of internal pressure) The obtained polypropylene-based resin extruded foam particles were put into a pressure-resistant container and pressurized with air at a pressure increase rate of 0.05 MPa / h to 0.20 MPa·G and held at that pressure for 20 hours. Then, the pressure of the pressure-resistant container was adjusted if necessary so that the internal pressure of the above foam particles became 0.20 MPa (absolute pressure).

[0083] (Manufacturing example of extruded foam particle molded body) Using polypropylene-based resin extrusion foamed particles to which internal pressure was applied, a polypropylene-based resin extrusion foamed particle molded body was produced by the following method. A block-shaped mold (the molding space is 381 mm in length × 320 mm in width × variable thickness) was set in a state where the thickness of the molding space was 44 mm (cracking rate 10%). Next, the polypropylene-based resin extrusion foamed particles were filled into the molding space of the mold. Then, the mold was moved so that the thickness of the molding space in the mold became 40 mm, and the molding space was compressed. Next, the air in the mold was expelled with steam at 0.10 MPa·G, and then, the foamed particles were fused together to form an extrusion foamed particle molded body by heating and molding for 7 seconds using steam showing a steam pressure of 0.26 MPa·G. Next, after removing the steam in the mold from the drain valve over 10 seconds, it was water-cooled until the surface pressure gauge attached to the mold reached 0.05 MPa·G to obtain an extrusion foamed particle molded body. The obtained polypropylene-based resin extrusion foamed particle molded body was dried at 75°C for 16 hours or more and left standing at 23°C for 24 hours or more, and then various evaluations were carried out.

[0084] <Evaluation> For Resin A and Resin B, the MFR and melting point (Tm) were measured according to the following method. The results are shown in Table 1. For the polypropylene-based resin extrusion foamed particles, the foaming ratio, continuous cell ratio, and average cell diameter were measured according to the following method. The results are shown in Table 1. For the extrusion foamed particle molded body, the foaming ratio and continuous cell ratio were measured according to the following method. Also, in order to evaluate the surface property of the extrusion foamed particle molded body, the fusion ratio, between particles, surface beauty, and shrinkage ratio were measured and evaluated according to the following method. In addition, a compression test of the extrusion foamed particle molded body was carried out, and the molded body density and compression strength at 50% strain were measured. The results are shown in Table 1.

[0085] [Resin A, Resin B] (MFR) In accordance with the provisions of Method B described in ISO 1133 (1997), using a melt indexer S-01 (manufactured by Toyo Seiki Seisaku-sho, Ltd.), under the conditions of 230 °C and 2.16 kg, the amount of resin extruded from the die over a certain period of time was converted to the amount extruded in 10 minutes. The said certain period of time is 120 seconds when the melt flow rate exceeds 0.5 g / 10 min and is 1.0 g / 10 min or less; 60 seconds when it exceeds 1.0 g / 10 min and is 3.5 g / 10 min or less; and 30 seconds when it exceeds 3.5 g / 10 min and is 10 g / 10 min or less.

[0086] (Melting point) As a differential scanning calorimeter, the DSC6200 type manufactured by Seiko Instruments Inc. was used, and the following steps (1) to (3) were carried out. (1) The temperature of the sample was raised from 40 °C to 220 °C at a heating rate of 10 °C / min to melt the sample. (2) Then, the temperature of the obtained sample was lowered from 220 °C to 40 °C at a cooling rate of 10 °C / min to crystallize the sample. (3) Then, further, the temperature of the crystallized sample was raised from 40 °C to 220 °C at a heating rate of 10 °C / min. The temperature of the peak (melting peak) of the DSC curve of the said sample obtained during the second heating (i.e., in (3)) was defined as the melting point Tm.

[0087] [Extruded foam particles] (Expansion ratio) The expansion ratio of polypropylene-based resin extruded foam particles was measured by measuring the mass w1 (g) of the foam particles, then submerging the foam particles in a graduated cylinder containing ethanol, and measuring the volume v1 (cm 3 ) of the rise in the water level of the graduated cylinder (water immersion method). The true specific gravity ρb = w1 / v1 of the foam particles was calculated, and it can be further calculated as the ratio (ρr / ρb) to the resin density ρr before foaming. The resin density ρr before foaming was measured using a resin obtained by melt-kneading a resin with the same formulation as the foam particles in a state without a foaming agent. After measuring the mass w2 (g) of the resin, the resin was submerged in a graduated cylinder containing ethanol, and the volume v2 (cm 3) is measured and calculated at ρr = w2 / v2.

[0088] (Average cell diameter) Ten extrusion foamed particles were arbitrarily taken out, and for each cut surface of the samples cut out with sufficient care so that the cell membranes were not broken, observations were made with a microscope (manufactured by Keyence: VHX digital microscope). A line segment corresponding to 1 mm in length was drawn on the part excluding the surface layer, the number of cells passing through the outer line segment was measured, and thereafter the average cell diameter was measured in accordance with ASTM D3576.

[0089] (Closed cell ratio) It was calculated by performing the following (1) to (3) in order according to the method described in Procedure C of ASTM D2856-87. (1) Using an air comparison type pycnometer, the volume Vc (cm 3 ) of the extrusion foamed particles was measured. (2) Next, the total amount of the extrusion foamed particles after measuring Vc was immersed in ethanol in a graduated cylinder. (3) Thereafter, from the amount of increase in the position of ethanol in the graduated cylinder, the apparent volume Va (cm 3 ) of the extrusion foamed particles was determined. (4) The closed cell ratio of the extrusion foamed particles was calculated by the following formula: Closed cell ratio (%) = ((Va - Vc) × 100) / Va

[0090] [Extrusion foamed particle molded body] (Expansion ratio) From a polypropylene resin extrusion foamed particle molded body, a sample piece with a length / width / thickness = 25 / 25 / 25 mm (without a full-surface skin layer) was cut out. After standing at 23°C for 24 hours or more, the weight of the sample piece was measured, and the volume of the sample piece was measured by the water immersion method in the same manner as the expansion ratio of the foamed particles, and the expansion ratio was calculated.

[0091] (Closed cell ratio) A sample piece with a size of length / width / thickness = 25 / 25 / 25 mm (without a full-surface skin layer) was cut out from a polypropylene-based resin extrusion foamed particle molded body. After standing at 23°C for 24 hours or more, the continuous cell ratio of the extrusion foamed particle molded body was measured by the same method as the continuous cell ratio measurement method of the foamed particles.

[0092] (Surface property) Regarding the inter-particle, surface beauty, and shrinkage rate described below, if all were qualified, it was marked as ○; if any one was unqualified, it was marked as △; if two or more were unqualified, it was marked as ×.

[0093] (Inter-particle) Among the surfaces of the polypropylene-based resin extrusion foamed particle molded body, near the center of both sides (the surfaces perpendicular to the thickness direction) with a size of length × width = 100 mm × 100 mm, the gaps between the extrusion foamed particles were visually observed, and the number of gaps (the sunken parts on the surface) between the extrusion foamed particles was counted. Those with 20 or fewer gaps between the extrusion foamed particles were considered qualified, and those with 21 or more gaps were considered unqualified.

[0094] (Surface beauty) Regarding the polypropylene-based resin extrusion foamed particle molded body, among the longest pieces at the ends, the gaps between the extrusion foamed particles were visually observed, and the number of gaps (the sunken parts on the surface) between the extrusion foamed particles was counted. Those with 3 or fewer gaps between the extrusion foamed particles were considered qualified, and those with 4 or more gaps were considered unqualified.

[0095] (Shrinkage rate) The dimensions of the length, width, and thickness of the polypropylene-based resin extrusion foamed particle molded body were measured. Using the obtained results, for each of the length, width, and thickness dimensions of the polypropylene-based resin extrusion foamed particle molded body, the shrinkage rate (%) was evaluated by the following formula: {(Dimensions of the molding space in the mold) - (Dimensions of the molded body)} × 100 / Dimensions of the molding space in the mold The case where the shrinkage rates in the longitudinal, transverse, and thickness directions are all 5% or less is considered qualified, and if any one of them is greater than 5%, it is considered unqualified. Incidentally, the dimensions of the molding space in the mold were 381 mm in length, 320 mm in width, and 40 mm in thickness.

[0096] (Compression test) From the obtained polypropylene-based resin extrusion foam particle molded body, a rectangular parallelepiped with a length / width / thickness of 50 / 50 / 25 mm (without a full-surface skin layer) was cut out and used as a test piece. After leaving the test piece standing in an environment of 23°C and 50% humidity for 24 hours or more, the weight and dimensions were measured, and the density of the test piece was calculated. Then, for the test piece, using a tensile compression testing machine (for example, manufactured by Minebea Mitsumi, TG-50kN), it was compressed at a compression rate of 10 mm / min, and the value of the compression stress at 50% strain was measured. The obtained value was taken as the compression strength of the extrusion foam particle molded body.

[0097] (Cooling time in the molding process) In the molding process, after the heat molding process using steam, the steam in the mold was removed from the drain valve over 10 seconds, and then the cooling time required for the pressure to reach 0.05 MPa·G as measured by the surface pressure gauge attached to the mold was recorded.

[0098] [Table 1]

[0099] From Table 1, it was found that by extruding and foaming a polypropylene-based resin composition containing 1.0 to 5.0 parts by weight of an alicyclic saturated hydrocarbon resin (C) with respect to a total of 100 parts by weight of a polypropylene-based resin (A) and a polypropylene-based resin (B), an extrusion foam particle molded body with excellent surface properties and high compression strength can be obtained.

Claims

1. A polypropylene-based resin extruded foam particle containing a polypropylene-based resin (A) having a branched structure and an alicyclic saturated hydrocarbon resin (C), and optionally further containing a polypropylene-based resin (B) into which no branched structure is introduced, wherein the weight ratio of the polypropylene-based resin (A) to the polypropylene-based resin (B) is 70:30 to 100:0, and the content of the alicyclic saturated hydrocarbon resin (C) is 1.0 to 5.0 parts by weight with respect to 100 parts by weight in total of the polypropylene-based resin (A) and the polypropylene-based resin (B).

2. The extruded foam particle according to Claim 1, wherein the alicyclic saturated hydrocarbon resin (C) is a hydrogenated product of an aromatic petroleum resin.

3. The extruded foam particle according to Claim 1 or 2, having a foaming ratio exceeding 10 times and less than 25 times.

4. An extruded foam particle molded body formed by molding the extruded foam particle according to Claim 1 or 2.

5. A method for producing polypropylene-based resin extruded foam particles, comprising supplying a polypropylene-based resin composition to an extruder, melt-kneading it, adding a foaming agent, further performing melt-kneading to obtain a melt-kneaded product, cooling the melt-kneaded product, and then extruding and foaming it from a die, wherein the polypropylene-based resin composition contains a polypropylene-based resin (A) having a branched structure and an alicyclic saturated hydrocarbon resin (C), and may further contain a polypropylene-based resin (B) into which no branched structure is introduced, the weight ratio of the polypropylene-based resin (A) to the polypropylene-based resin (B) is 70:30 to 100:0, and the content of the alicyclic saturated hydrocarbon resin (C) is 1.0 to 5.0 parts by weight with respect to 100 parts by weight in total of the polypropylene-based resin (A) and the polypropylene-based resin (B).

6. The production method according to Claim 5, wherein the alicyclic saturated hydrocarbon resin (C) is a hydrogenated product of an aromatic petroleum resin.

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