Polypropylene resin extrusion foamed particle, foamed molding, and method for producing polypropylene resin extrusion foamed particle

Abstract

To provide a polypropylene resin extrusion foamed particle capable of obtaining a foamed molding having a high compression strength, a foamed molding obtained using the extrusion foamed particle, and a method for producing the polypropylene resin extrusion foamed particle.SOLUTION: A polypropylene resin extrusion foamed particle is obtained by an extrusion foaming method, where the foamed particle has a shape of a column formed from one column side face and two cut faces, the length of the column side face L is 8.0 mm or less, and the cut face ratio expressed by the following formula (1) is 0.30 or less when a length of the column side face, a major axis of the cut face, and a minor axis of the cut face are measured, and measured values are written as L, D1, and D2 respectively. Cut face ratio=surface area of two cut faces / (surface area of two cut faces+surface area of column side face) (1).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to extruded polypropylene resin foam beads, foamed molded articles, and a method for producing extruded polypropylene resin foam beads. [Background technology]

[0002] BACKGROUND ART Foamed molded articles obtained using extruded polypropylene resin foam particles are used mainly as automobile interior components and core materials for automobile bumpers, as well as for various other applications such as heat insulating materials and cushioning packaging materials.

[0003] A known method for producing extruded polypropylene resin foam particles is an extrusion foaming method in which an inorganic gas is added as a foaming agent to a resin composition containing a polypropylene resin having a branched structure, and the resulting molten mixture is extruded through a die to foam (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2022 / 163627 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the expanded polypropylene resin beads produced by the method described in Patent Document 1 have a problem in that the cell membranes in the expanded beads are easily broken during the subsequent in-mold foam molding, resulting in a high open cell ratio (also called open cell formation), and therefore the compressive strength is likely to be low.

[0006] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide extruded polypropylene resin beads that can give foamed molded articles having high compressive strength, foamed molded articles obtained using the extruded foamed beads, and a method for producing the extruded polypropylene resin beads. [Means for solving the problem]

[0007] The present inventors have found that the above-mentioned problems can be solved by reducing the ratio of cut surfaces in cylindrical extruded foamed polypropylene resin beads obtained by cutting resin extruded into strands through a die, and have completed the present invention.

[0008] Aspects of the present disclosure relate to the following extruded polypropylene resin foam beads, foamed molded articles, and methods for producing extruded polypropylene resin foam beads.

[0009] [1] Extruded polypropylene resin foam particles obtained by an extrusion foaming method, the expanded beads contain a random polypropylene-based resin (A) having a branched structure as a base resin, the expanded beads have a cylindrical shape formed of one cylindrical side surface and two cut surfaces, the cut surfaces of which are substantially circular and perpendicular to the longitudinal direction of the cylindrical side surface; When the measured values ​​of the length of the cylinder side surface, the major axis of the cut surface, and the minor axis of the cut surface are expressed as the length L of the cylinder side surface, the major axis D1 of the cut surface, and the minor axis D2 of the cut surface, respectively, The length L of the cylindrical side surface is 8.0 mm or less, and The extruded polypropylene resin foam particles have a cut surface ratio represented by the following formula (1) of 0.30 or less. Cut surface ratio = surface area of ​​two cut surfaces / (surface area of ​​two cut surfaces + surface area of ​​the cylinder side) (1) In formula (1), the surface area of ​​the two cut surfaces is expressed by formula (2) below, and the surface area of ​​the side surface of the cylinder is expressed by formula (3) below. Surface area of ​​two cut faces = (π×D1×D2) / 2 (2) Surface area of ​​the cylinder side = (π × (D1 + D2) × L) / 2 (3) [2] The extruded polypropylene resin foam particles according to [1], wherein the length L of the cylindrical side surface is 1.0 mm or more and 8.0 mm or less. [3] A polypropylene resin foam molded article obtained by molding the extruded polypropylene resin foam beads according to [1] or [2]. [4] A method for producing extruded foamed polypropylene resin beads, comprising: feeding a polypropylene resin composition to an extruder, melt-kneading the composition, adding a foaming agent, and further melt-kneading the composition to form a melt-kneaded mixture; cooling the melt-kneaded mixture, and then cutting the extruded foamed resin into strands from a die; The polypropylene resin composition contains a random polypropylene resin (A) having a branched structure, The die diameter of the die is less than 1.0 mm, the cut resin has an expansion ratio of 40 times or less, and a strand length of 8.0 mm or less. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide extruded polypropylene resin beads from which foamed molded articles having high compressive strength can be obtained, foamed molded articles obtained using the extruded foamed beads, and a method for producing the extruded polypropylene resin beads. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram illustrating an example of extruded polypropylene resin foam beads according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] <Polypropylene resin extruded foam particles> The extruded polypropylene resin foamed beads of this embodiment (hereinafter sometimes referred to as "expanded beads") are obtained by an extrusion foaming method. The expanded beads contain a random polypropylene resin (A) having a branched structure as a base resin. The expanded beads have a cylindrical shape formed from one cylindrical side surface and two cut surfaces, the cut surfaces of which are substantially circular and perpendicular to the longitudinal direction of the cylindrical side surface. When the measured values ​​of the length of the cylinder side surface, the major axis of the cut surface, and the minor axis of the cut surface are expressed as the length L of the cylinder side surface, the major axis D1 of the cut surface, and the minor axis D2 of the cut surface, respectively, The length L of the cylinder side surface is 8.0 mm or less, and The cut surface ratio described below is 0.30 or less.

[0013] The extruded polypropylene resin foam beads have a cylindrical side length L of 8.0 mm or less and a cut surface ratio of 0.30 or less, so that the foamed molded article obtained by molding the foamed beads has high compressive strength.

[0014] The extruded polypropylene resin foam particles will be described below with reference to the drawings.

[0015] <Cut surface ratio> Fig. 1 is a schematic diagram showing an example of an extruded polypropylene resin foamed bead of this embodiment. In Fig. 1, the foamed bead 1 is cylindrical and has one cylindrical side surface 10 and two cut surfaces 11, 11, each of which is substantially circular when cut perpendicular to the longitudinal direction of the cylindrical side surface 10.

[0016] The method for producing expanded beads by extrusion foaming, which will be described later, includes a step of cooling a molten mixture containing a polypropylene resin composition, and then cutting the resin extruded into strands from a die to a predetermined length. The expanded beads 1 in Fig. 1 are strands with a substantially circular cross section that have been cut to a predetermined length. Therefore, the expanded beads 1 are formed into a cylindrical shape.

[0017] When the extruded strand is cut, the cylindrical side surface 10 corresponds to the curved surface that forms the outline of the cut strand. When the extruded strand is cut, the cut surfaces 11, 11 correspond to the cut surfaces of the cut strand.

[0018] In the expanded beads 1, the length direction of the cylindrical side surface 10 refers to the direction connecting the centers of the two cut surfaces 11, 11 (i.e., the height direction of the cylinder), as indicated by the double arrow in Fig. 1. The cut surfaces 11, 11 are cut surfaces perpendicular to the length direction of the cylindrical side surface 10.

[0019] 1, the length of the cylindrical side surface 10, the major axis of the cut surface 11, and the minor axis of the cut surface are measured and expressed as the length L of the cylindrical side surface, the major axis D1 of the cut surface, and the minor axis D2 of the cut surface, respectively. From the viewpoint of obtaining a foamed molded article with high compressive strength, the upper limit of the length L of the cylindrical side surface is 8.0 mm or less, preferably 7.5 mm or less, and more preferably 7.0 mm or less. The lower limit of the length L of the cylindrical side surface is preferably 1.0 mm or more.

[0020] The length L of the cylindrical side surface can be any combination of the above upper and lower limits, and is preferably 1.0 mm or more and 8.0 mm or less, more preferably 1.0 mm or more and 7.5 mm or less, and even more preferably 1.0 mm or more and 7.0 mm or less.

[0021] As mentioned above, the cut surface 11 is substantially circular, but not a perfect circle. Therefore, the cut surface 11 is regarded as an ellipse, and the measurement is performed by taking the longest diameter of the diameters of the cut surface 11 as the major diameter D1 and the diameter perpendicular to the major diameter D1 as the minor diameter D2.

[0022] When determining the cylindrical side length L, the cut surface major axis D1, and the cut surface minor axis D2, the cylindrical side length, the cut surface major axis, and the cut surface minor axis may be measured for one expanded bead and these measurements may be used as the cylindrical side length L, the cut surface major axis D1, and the cut surface minor axis D2. However, it is preferable to measure the cylindrical side length, the cut surface major axis, and the cut surface minor axis for two or more expanded beads and use the average values ​​as the cylindrical side length L, the cut surface major axis D1, and the cut surface minor axis D2. The number of expanded beads to be measured is not particularly limited, but is preferably 10, more preferably 15, and even more preferably 20. The cut surface major axis D1 is preferably 2 mm or more and 6 mm or less, more preferably 2.5 mm or more and 5.5 mm or less. The cut surface minor axis D2 is preferably 2 mm or more and 6 mm or less, more preferably 2.5 mm or more and 5.5 mm or less.

[0023] The cut surface ratio is expressed by the following formula (1). Cut surface ratio = surface area of ​​two cut surfaces / (surface area of ​​two cut surfaces + surface area of ​​the cylinder side) (1) In formula (1), the surface area of ​​the two cut surfaces is expressed by formula (2) below, and the surface area of ​​the side surface of the cylinder is expressed by formula (3) below. Surface area of ​​two cut faces = (π×D1×D2) / 2 (2) Surface area of ​​the cylinder side = (π × (D1 + D2) × L) / 2 (3)

[0024] In equation (3), the surface area of ​​the cylinder side surface should be calculated as the perimeter of the ellipse on the cut surface × L. However, since the formula for the perimeter of an ellipse is complicated, it is approximated by the perimeter of a circle.

[0025] The cut surface ratio represented by formula (1) is 0.30 or less, preferably 0.29 or less, and more preferably 0.28 or less, from the viewpoint of obtaining a foamed molded article with high compressive strength.

[0026] <Random polypropylene resin (A) having a branched structure> The extruded polypropylene resin foam particles of this embodiment contain a polypropylene resin composition. The polypropylene resin composition preferably contains a random polypropylene resin (A) having a branched structure (hereinafter also referred to as "branched polypropylene resin (A)") as a base resin. The extruded polypropylene resin foam particles of this embodiment may also contain a random polypropylene resin (B) having no branched structure (hereinafter also referred to as "linear polypropylene resin (B)"). These components will be described below.

[0027] [Branched polypropylene resin (A)] The branched polypropylene resin (A) is a modified polypropylene resin obtained by introducing a branched structure into the random polypropylene resin (a). The random polypropylene resin (a) refers to a resin containing 50 mol % or more of structural units derived from propylene monomers, out of 100 mol % of all structural units contained in the resin.

[0028] The method for introducing a branched structure into the random polypropylene resin (a) is not particularly limited, and examples thereof include (1) a method of irradiating the random polypropylene resin (a) with radiation, and (2) a method of melt-kneading a mixture containing the random polypropylene resin (a), a monomer (b) selected from the group consisting of conjugated dienes and vinyl aromatic compounds, and a radical polymerization initiator (c).

[0029] A specific example of the method (1) above is the method described in JP-A-2002-542360.

[0030] A specific example of the method (2) above is a method in which a random polypropylene resin (a), a monomer (b) selected from a conjugated diene and a vinyl aromatic compound, and a radical polymerization initiator (c) are melt-kneaded at a temperature at which the random polypropylene resin (a) melts and the radical polymerization initiator (c) decomposes.

[0031] (Random polypropylene resin (a)) Random polypropylene resin (a) refers to a random copolymer of a propylene monomer and a monomer other than propylene. Random polypropylene resin (a) contains 50 mol% or more of structural units derived from propylene monomers and less than 50 mol% of structural units derived from monomers other than propylene monomers, out of 100 mol% of all structural units contained in the resin. In this specification, "structural units derived from propylene monomers" may also be referred to as "propylene units." In this specification, "structural units derived from monomers other than propylene monomers" may also be referred to as "comonomer units."

[0032] 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, and 1-decene; cyclic olefins, such as cyclopentene, norbornene, and tetracyclo[6,2,11,8,13,6]-4-dodecene; and 5-methylene-2-norbornene. Examples of suitable vinyl monomers include dienes such as bornene, 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, methacrylonitrile, vinyl acetate, acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters, maleic acid, maleic anhydride, styrene-based monomers, vinyltoluene, and divinylbenzene.

[0033] Examples of acrylic acid esters 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.

[0034] Examples of methacrylic acid esters 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.

[0035] Styrenic monomers include styrene, methylstyrene, dimethylstyrene, alpha-methylstyrene, para-methylstyrene, ethylstyrene, diethylstyrene, isopropylstyrene, t-butylstyrene, bromostyrene, dibromostyrene, tribromostyrene, chlorostyrene, dichlorostyrene, and trichlorostyrene.

[0036] The random polypropylene resin (a) preferably has, as a comonomer unit, a structural unit derived from an α-olefin having 2 or 4 to 12 carbon atoms, more preferably a structural unit 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, more preferably a structural unit derived from ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, and / or 4-methyl-1-pentene, even more preferably a structural unit derived from ethylene, 1-butene, isobutene, and / or 1-pentene, and particularly preferably a structural unit derived from ethylene and / or 1-butene.

[0037] The random polypropylene resin (a) preferably contains propylene units in an amount of 90 mol% or more, more preferably 93 mol% or more, even more preferably 94 mol% or more, and particularly preferably 95 mol% or more, of 100 mol% of all structural units contained in the random polypropylene resin (a).

[0038] [Monomer (b) selected from conjugated dienes and vinyl aromatic compounds] Examples of conjugated dienes include butadiene, isoprene, 1,3-heptadiene, 2,3-dimethylbutadiene, and 2,5-dimethyl-2,4-hexadiene. 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 because they are inexpensive, easy to handle, and the reaction proceeds uniformly.

[0039] Examples of vinyl aromatic compounds 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 difluorostyrene. Examples of suitable vinyl aromatic compounds include fluorostyrenes such as fluorostyrene and trifluorostyrene; nitrostyrenes such as o-nitrostyrene, m-nitrostyrene, p-nitrostyrene, dinitrostyrene and trinitrostyrene; vinylphenols such as o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, dihydroxystyrene and trihydroxystyrene; divinylbenzenes such as o-divinylbenzene, m-divinylbenzene and p-divinylbenzene; and isopropenylstyrenes such as o-diisopropenylbenzene, m-diisopropenylbenzene and p-diisopropenylbenzene. Among the above-mentioned vinyl aromatic compounds, styrene and / or methylstyrene are preferred because of their low cost, ease of handling, and tendency for the reaction to proceed uniformly.

[0040] The amount of the conjugated diene etc. (b) used 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, relative to 100 parts by weight of the linear polypropylene resin (a).

[0041] [Radical polymerization initiator (c)] The radical polymerization initiator (c) is an organic peroxide capable of abstracting hydrogen from the linear polypropylene resin (a) and the conjugated diene compound (b). Examples of the radical polymerization initiator (c) include organic peroxides such as ketone peroxides, peroxyketals, hydroperoxides, dialkyl peroxides, diacyl peroxides, peroxydicarbonates, and peroxyesters.

[0042] As the organic peroxide, those with particularly high hydrogen abstraction ability are preferred. Examples of organic peroxides with 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, and 2,2-bis(t-butylperoxy)butane; 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, and 2,5-dimethyl-2,5- Suitable examples of organic peroxides include dialkyl peroxides such as di(t-butylperoxy)-3-hexyne; diacyl peroxides such as benzoyl peroxide; and peroxyesters such as t-butylperoxyoctate, t-butylperoxyisobutyrate, t-butylperoxylaurate, t-butylperoxy3,5,5-trimethylhexanoate, t-butylperoxyisopropylcarbonate, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, t-butylperoxyacetate, t-butylperoxybenzoate, and di-t-butylperoxyisophthalate. Among these, t-butylperoxyisopropylcarbonate and / or t-butylperoxybenzoate are preferred. These organic peroxides may be used alone or in combination of two or more.

[0043] The amount of 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, more preferably 0.10 to 2.00 parts by weight, and particularly preferably 0.10 to 1.50 parts by weight, relative to 100 parts by weight of linear polypropylene resin (a).

[0044] In the above method (2), examples of the apparatus for melt-kneading the random polypropylene resin (a), the monomer (b) such as a conjugated diene, and the radical polymerization initiator (c) 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 devices; and vertical mixers such as double helical ribbon mixers. Among 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.

[0045] There are no particular restrictions on the order or method of mixing and kneading the random polypropylene resin (a), the monomer (b) such as a conjugated diene, and the radical polymerization initiator (c). The random polypropylene resin (a), the monomer (b) such as a conjugated diene, and the radical polymerization initiator (c) may be mixed and then melt-kneaded. Alternatively, the random polypropylene resin (a) may be melt-kneaded, and then the monomer (b) such as a conjugated diene or the radical polymerization initiator (c) may be mixed simultaneously or separately, all at once or in portions. The kneader temperature is preferably 130°C or higher and 300°C or lower. The melt-kneading time is generally preferably 1 minute or higher and 60 minutes or lower.

[0046] (Physical properties of branched polypropylene resin (A)) The melting point (Tm) of the branched polypropylene resin (A) is not particularly limited, but is preferably from 130.0°C to 165.0°C, and more preferably from 135.0°C to 163.0°C. In this specification, the melting point is a value determined by measurement using differential scanning calorimetry.

[0047] The melt flow rate (MFR) of the branched polypropylene resin (A) is not particularly limited, but is preferably 0.1 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, MFR is a value determined by measurement at a temperature of 230°C in accordance with ISO1133.

[0048] (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 out of 100 mol % of all structural units contained in the resin, and has no branched structure. As described above, the linear polypropylene resin (B) is a random polypropylene resin without a branched structure. The random polypropylene resin is the same as the embodiment described above in the section [(Random polypropylene resin (a))]. In the extruded polypropylene resin foam beads of this embodiment, the linear polypropylene resin (B) is an optional component, but from the viewpoint of obtaining extruded foam bead moldings with good surface properties and high compressive strength, it is preferred that the extruded polypropylene resin foam beads contain the linear polypropylene resin (B).

[0049] (Physical properties of linear polypropylene resin (B)) The melting point (Tm) of the linear polypropylene resin (B) is not particularly limited, but is preferably from 130.0°C to 165.0°C, and more preferably from 135.0°C to 163.0°C.

[0050] The upper limit of the melt flow rate (MFR) of the linear polypropylene resin (B) is preferably 5 g / 10 min or less, more preferably 1 g / 10 min or less, from the viewpoint of obtaining an extruded foamed bead molding having good surface properties and high compressive strength. The lower limit of the MFR of the linear polypropylene resin (B) is not particularly limited, but is preferably 0.1 g / 10 min or more, and more preferably 0.2 g / 10 min or more.

[0051] When the extruded polypropylene resin foam beads of this embodiment contain the linear polypropylene resin (B), 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 a foamed molded article with high compressive strength.

[0052] (Other ingredients) The polypropylene resin composition may contain components other than the branched polypropylene resin (A) and linear polypropylene resin (B) described above (hereinafter also referred to as "other components"), as long as the effects of the present invention are not impaired. Examples of other components include resins or rubbers other than the resins (A) and (B) (hereinafter also referred to as "other resins, etc."), bubble nucleating agents, stabilizers such as antioxidants, metal deactivators, phosphorus-based processing stabilizers, UV absorbers, UV stabilizers, fluorescent brighteners, metal soaps, and antacid adsorbents, and / or additives such as crosslinkers, chain transfer agents, lubricants, plasticizers, fillers, reinforcing agents, flame retardants, colorants, and antistatic agents. These other components may be used alone or in combination of two or more. The total content of the other components in the polypropylene resin composition is not particularly limited. The total content of other components in the polypropylene-based resin composition is, for example, preferably 0.01 to 50.00 parts by weight, and more preferably 0.05 to 30.00 parts by weight, relative to 100 parts by weight of the total of the polypropylene-based resin (A) and the polypropylene-based resin (B).

[0053] (Other resins, 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 very-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 the other resins and rubbers in the base resin is not particularly limited. The total content of the other resins and rubbers in the polypropylene-based resin composition is, for example, preferably 1 to 30 parts by weight, more preferably 2 to 15 parts by weight, per 100 parts by weight of the total of the polypropylene-based resin (A) and the polypropylene-based resin (B).

[0054] (bubble nucleating agent) Examples of the bubble nucleating agent include sodium bicarbonate-citric acid mixture, monosodium citrate, talc, calcium carbonate, etc. These bubble nucleating agents may be used alone or in combination of two or more.

[0055] The content of the bubble nucleating agent in the polypropylene resin composition is not particularly limited. The content of the bubble nucleating agent is, for example, preferably 0.01 to 5.00 parts by weight, more preferably 0.01 to 3.50 parts by weight, even more preferably 0.01 to 1.00 parts by weight, and particularly preferably 0.01 to 0.50 parts by weight, relative to 100 parts by weight of the total of the polypropylene resin (A) and the polypropylene resin (B). This configuration has the advantage that the average cell diameter and cell shape of the extruded foamed beads become uniform, and as a result, the foamability during extrusion foaming tends to be more stable.

[0056] <Physical properties of expanded beads> (Expansion ratio) The expansion ratio of the extruded polypropylene resin foamed beads is preferably 40 times or less, and more preferably 38 times or less. In this specification, the expansion ratio of extruded polypropylene resin foamed beads is calculated by carrying out the following steps (1) to (5) in order: (1) measuring the weight w (g) of the foamed beads; (2) immersing the foamed beads used for the weight measurement in ethanol contained in a measuring cylinder, and calculating the volume v (cm) of the foamed beads based on the rise in the liquid level in the measuring cylinder. 3 ) to measure the weight w (g) and the volume v (cm 3 ) to calculate the density ρ1 of the expanded beads; (4) By performing the same operations as (1) to (3) using a base resin instead of the expanded beads, the density ρ2 of the base resin is calculated; (5) The density ρ2 of the base resin of the expanded beads is divided by the density ρ1 of the expanded beads (ρ2 / ρ1), and the obtained value is the expansion ratio.

[0057] (average cell diameter) The average cell diameter (also referred to as the average bubble diameter) of the extruded polypropylene resin foamed beads 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 extruded polypropylene resin foamed beads is 200 μm or more, shrinkage of the extruded foamed beads tends to be small, and when it is 600 μm or less, the molding cycle during in-mold foam molding tends to be shortened. In this specification, the average cell diameter of the extruded polypropylene resin foam particles is measured in accordance with ASTM D3576.

[0058] (Open cell ratio) The lower the open cell ratio of the extruded polypropylene resin foamed beads, the better. The open cell ratio of the extruded polypropylene resin foamed beads is preferably 12% or less, more preferably 11% or less, even more preferably 10% or less, and even more preferably 9% or less. The lower limit of the open cell ratio of the extruded polypropylene resin foamed beads is not particularly limited, and is, for example, 0.0% or more. According to the above-mentioned configuration, the cells of the extruded polypropylene resin foamed beads are less likely to break and shrink during molding, which has the advantage that the extruded polypropylene resin foamed beads have excellent moldability, and the extruded polypropylene resin foamed beads obtained from the extruded polypropylene resin foamed beads exhibit characteristics such as flexibility in shape, cushioning properties, light weight, compressive strength, and heat insulation.

[0059] In this specification, the open cell ratio of extruded polypropylene resin foamed beads is a value determined by measurement using an air-comparison type hydrometer [Tokyo Science Co., Ltd., Model 1000] in accordance with the method described in Procedure C of ASTM D2856-87. Specifically, the open cell ratio of extruded foamed beads is calculated by carrying out the following steps (1) to (3) in order: (1) Using an air-comparison type hydrometer, the volume Vc (cm) of the extruded foamed beads is measured. 3 (2) Next, the entire amount of the extruded foam particles after measuring Vc is submerged in ethanol contained in a measuring cylinder; (3) After that, the apparent volume Va (cm 3 ) of the extruded foam particles is calculated from the amount of rise in the position of the ethanol in the measuring cylinder. 3 (4) The open cell ratio of the extruded foam particles is calculated using the following formula: Open cell ratio (%) = ((Va - Vc) x 100) / Va. The method for measuring the volume Va is also called the submersion method.

[0060] <Method for producing extruded polypropylene resin foam beads> The method for producing extruded polypropylene resin foam beads of the present embodiment includes the steps of: The method includes feeding a polypropylene-based resin composition to an extruder, melt-kneading the composition, adding a foaming agent, and further melt-kneading the composition to form a melt-kneaded product; cooling the melt-kneaded product, and then cutting the extruded and foamed resin into strands from a die. The polypropylene resin composition contains a random polypropylene resin (A) having a branched structure. The die diameter of the die is less than 1.0 mm, the cut resin has an expansion ratio of 40 times or less, and a strand length of 8.0 mm or less.

[0061] According to the method for producing extruded polypropylene resin foam beads of this embodiment, extruded polypropylene resin foam beads that can give foamed molded articles with high compressive strength can be produced.

[0062] <Step of Obtaining Melt-Kneaded Product> (Polypropylene Resin Composition) The polypropylene resin composition used in the production method of this embodiment contains a polypropylene resin (A) having a branched structure. The aspect of the polypropylene resin (A) having a branched structure is the same as the embodiment described above in the section "Extruded Polypropylene Resin Foamed Particles." The aspects of the components other than the branched polypropylene resin (A) are also the same as those described above in the section <<Extruded Polypropylene Resin Foam Particles>>.

[0063] (foaming agent) The blowing agent used in the production method of this embodiment is not particularly limited as long as it is a blowing agent commonly used in extrusion foaming. Examples of blowing agents 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; physical blowing agents such as water; and chemical blowing agents including thermal decomposition blowing agents such as sodium bicarbonate, azodicarbonamide, and dinitrosopentamethylenetetramine. Among these, inorganic gases and physical blowing agents are preferred from the standpoint of environmental impact.

[0064] The amount of the foaming 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, per 100.0 parts by weight of the polypropylene resin composition.

[0065] Examples of extruders used to obtain the melt-kneaded product include single-screw extruders and twin-screw extruders, among which twin-screw extruders are preferred because of their excellent melt-kneading properties.

[0066] The cylinder temperature of the extruder is not particularly limited and can be appropriately set depending on the melting point of the polypropylene resin composition used, the type and amount of the blowing agent used, etc., from the viewpoint of sufficiently melt-kneading the polypropylene resin composition and the blowing agent. For example, the cylinder temperature of the extruder is preferably 150°C or higher and 250°C or lower, more preferably 170°C or higher and 230°C or lower.

[0067] <Step of cooling the molten mixture and then extruding and foaming it through a die> Examples of cooling devices used to cool the resulting melt-kneaded product include single-screw extruders, static mixers, and melt coolers. A die for extruding the cooled melt-kneaded product is provided at the tip of the cooling device in the extrusion direction of the melt-kneaded product. The temperature of the cooling device is not particularly limited as long as it is a temperature suitable for foaming the resulting melt-kneaded product. 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.

[0068] The cooled molten kneaded material is extruded and foamed through a circular hole provided in the die into a region where the pressure is lower than the internal pressure of the cooling device (hereinafter also referred to as the "low-pressure region"). The molten kneaded material may be extruded into a gas phase or a liquid phase. The pressure in the low-pressure region is not particularly limited, but is preferably 5 MPa or less, and more preferably 1 MPa or less. The extruded molten kneaded material immediately begins to foam, and when foaming is completed, a resin (extruded polypropylene-based resin foam) is obtained.

[0069] The diameter of the circular hole in the die (hereinafter sometimes referred to as the "die diameter") is less than 1.0 mm, preferably 0.9 mm or less and 0.3 mm or more, and more preferably 0.8 mm or less, from the viewpoint of obtaining a foamed molded product with high compressive strength.

[0070] <Process for cutting extruded foamed resin> In the cutting step, the molten kneaded product (extruded foam) may be cut during foaming, or the resin (extruded foam) may be shredded after foaming. The method for shredding the extruded resin is not particularly limited. For example, the resin may be cut along the extrusion direction using a rotary cutter or the like provided next to the die. Through the above steps, extruded polypropylene resin foam particles are obtained.

[0071] The length of the cut strands of the resin is 8.0 mm or less, preferably 2.0 mm or more and 7.5 mm or less, and more preferably 7.0 mm or less, from the viewpoint of obtaining a foamed molded article with high compressive strength. The strand length can be appropriately adjusted by adjusting the rotation speed of the rotary cutter.

[0072] The expansion ratio of the cut resin is 40 times or less, and more preferably 38 times or less. The method for measuring the expansion ratio is as explained above in the section <Physical properties of expanded beads>.

[0073] The low-pressure region may be a gas phase or a liquid phase, but is preferably a gas phase because it is easier to obtain expanded beads with a relatively high expansion ratio. Water or the like may be sprayed onto the surface of the expanded beads cut in the gas phase to cool them.

[0074] The resulting extruded polypropylene resin foamed beads are cylindrical, and the form of the foamed beads is the same as that described above in the section <<Extruded Polypropylene Resin Foamed Beads>>.

[0075] <Foam molded body> The polypropylene-based resin foam molded article of this embodiment is a molded article made from the extruded polypropylene-based resin foam beads described above. The polypropylene-based resin foam molded article is preferably an in-mold molded article made from the extruded polypropylene-based resin foam beads described above. The in-mold molded article can be obtained by filling a mold that can be closed but cannot be sealed with the extruded foam beads and then heating and molding with steam or the like. Examples of methods for producing the foam molded article include: (a) a method in which the foam beads are pressurized with an inorganic gas to impregnate the particles with the inorganic gas and apply a predetermined internal particle pressure, and then the particles are filled into a mold and heat-sealed with steam or the like (e.g., JP-B-51-22951); (b) a method in which the foam beads are compressed with gas pressure and filled into a mold, and then heat-sealed with steam or the like by utilizing the recovery force of the particles (e.g., JP-B-53-33996); and (c) a method in which the foam beads are filled into a mold with an expanded gap, the mold is closed to a predetermined gap, the filled foam beads are compressed, and then heat-sealed with steam or the like.

[0076] <Physical properties of foamed molded products>

[0077] (density) The density of the polypropylene resin foam molded article is not particularly limited, but is preferably 15 g / L or more and 100 g / L or less, and more preferably 20 g / L or more and 70 g / L or less.

[0078] (Expansion ratio) The expansion ratio of the polypropylene resin foam molded article is not particularly limited, but is preferably 12 times or more and 40 times or less, and more preferably 15 times or more and 38 times or less.

[0079] In this specification, the density and expansion ratio of the polypropylene resin foam molded article are calculated by the following method. (1) The weight w (g) of the foamed molded article is measured. (2) The foamed molded article used for measuring the weight was submerged in ethanol contained in a measuring cylinder, and the volume v (cm) of the foamed molded article was calculated based on the rise in the liquid level of the measuring cylinder. 3 ) is measured. (3) Weight w (g) to volume v (cm 3 ) to calculate the density ρ1 of the foamed molded article. (4) The density ρ2 of the base resin is calculated by carrying out the same operations as in (1) to (3) using the base resin instead of the foamed molded article. (5) The density ρ2 of the base resin of the foamed molded article is divided by the density ρ1 of the foamed molded article (ρ2 / ρ1), and the resulting value is the expansion ratio.

[0080] (Compressive strength) The compressive strength of the polypropylene resin foam molded article is preferably 0.120 MPa or more, more preferably 0.130 MPa or more, and even more preferably 0.140 MPa or more. In this specification, the compressive strength of a foam molded article is the value of compressive stress at 50% compression when compressed at a compression rate of 10 mm / min using a tension and compression tester.

[0081] The foamed molded articles are used for automobile interior parts, core materials for automobile bumpers, heat insulating materials, cushioning packaging materials, etc. [Example]

[0082] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0083] The test methods used to measure and evaluate various physical properties of the extruded polypropylene resin foam beads are as follows.

[0084] [Cylinder side length L] The length of the cylindrical side surface of the extruded polypropylene resin foam particles was measured using a digital caliper that displays to two decimal places. The length of the cylindrical side surface of 10 extruded polypropylene resin foam particles was measured, and the average value was taken as the "length of the cylindrical side surface L."

[0085] [Long diameter of the cut surface D1 and short diameter of the cut surface D2] The major diameter D1 and minor diameter D2 of the cut surface of the extruded polypropylene resin foam particles were measured using a digital caliper that displays to two decimal places. The major diameter and minor diameter of the cut surface of 10 extruded polypropylene resin foam particles were measured, and the average values ​​were taken as the "major diameter D1 of the cut surface" and the "minor diameter D2 of the cut surface," respectively.

[0086] [Cut surface ratio] The cut surface ratio of the extruded polypropylene resin foam beads was calculated based on the following formula (1). Cut surface ratio = surface area of ​​two cut surfaces / (surface area of ​​two cut surfaces + surface area of ​​the cylinder side) (1) In formula (1), the surface area of ​​the two cut surfaces is expressed by formula (2) below, and the surface area of ​​the side surface of the cylinder is expressed by formula (3) below. Surface area of ​​two cut faces = (π×D1×D2) / 2 (2) Surface area of ​​the cylinder side = (π × (D1 + D2) × L) / 2 (3)

[0087] Example 1 [Production Example of Branched Polypropylene Resin (A)] A branched polypropylene resin was produced as follows. First, the raw resin RD265CF (Borouge, linear random polypropylene resin, melting point: 152°C, MFR: 8g / 10min) was fed to a twin-screw extruder at 70 kg / h. Next, 1.0 part by weight of a radical polymerization initiator (t-butylperoxyisopropyl carbonate: NOF Corporation, Perbutyl I (registered trademark)) was fed to the twin-screw extruder per 100 parts by weight of the raw resin. Subsequently, 0.35 part by weight of a conjugated diene compound (isoprene: Kuraray Co., Ltd.) was fed to the twin-screw extruder containing the melt-kneaded raw resin and 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 to obtain a branched polypropylene resin (A). The obtained branched polypropylene resin (A) was extruded in the form of strands from a die. The extruded branched polypropylene resin (strands) were (a) water-cooled and then (b) chopped into pellets (cylindrical).

[0088] [Example of production of extruded polypropylene resin foam particles] Extruded polypropylene resin particles as pre-expanded polypropylene resin particles were produced as follows: 90% by weight of branched polypropylene resin (A), 10% by weight of linear polypropylene resin (B) (Prime Polymer Corporation, linear random polypropylene resin, B221WC, melting point: 145°C, MFR: 0.5g / 10 min), and 0.02 parts by weight of talc (Hayashi Kasei Co., Ltd., Talcan PK-S) as a bubble nucleating agent relative to the total weight of the branched polypropylene resin (A) and linear polypropylene resin (B) were dry-blended to form a resin composition. The equipment used to produce the extruded foamed particles consisted of a twin-screw extruder with a shaft diameter of 15 mm, a melt cooler, and a hot-cut device equipped with a rotary cutter next to the die along the extrusion direction, all connected in series. The resin composition was fed into the twin-screw extruder and melt-kneaded at 200°C and a discharge rate of 0.75 kg / hr. Furthermore, carbon dioxide gas, a foaming agent, was fed using a metering pump at a rate of 4 parts by weight relative to the resin discharge rate through an injection port installed midway through the extruder, and the resulting composition was further melt-kneaded. The melt-kneaded composition obtained through the melt-kneading process was cooled by passing it through a melt cooler set at 155°C. It was then passed through a two-hole die with a hole diameter of 0.7 mm in a hot cutter and discharged into an air phase where the pressure was lower than the internal pressure of the manufacturing equipment. The extruded composition was shredded in the air phase with the above-mentioned rotary cutter with four blades at a cutter rotation speed of 500 rpm to obtain cylindrical extruded polypropylene resin foam particles. The obtained extruded polypropylene resin foam particles were allowed to land on the surface of water flowing along the wall of the hot cutter and collected by the water flow.

[0089] [Example of manufacturing foam molded article] The resulting extruded polypropylene resin foam particles were impregnated with air using an air pressure treatment to apply an internal pressure of 0.06-0.08 MPa (gauge pressure). After that, a 4 mm crack was created in a mold measuring length, width, and thickness of 370 mm, 320 mm, and 40 mm, and the extruded polypropylene resin foam particles were filled into the mold. The extruded polypropylene resin foam particles were then foam-molded in-mold at a vapor pressure of 0.26 MPa·G to obtain a foamed molded product. The resulting foamed molded product was dried at 75°C for 24 hours, then aged at 23°C for at least 24 hours. The compressive strength was measured using the method described below.

[0090] [Compression strength] Test pieces measuring 50 mm in length, 50 mm in width, and 25 mm in thickness were cut out from the obtained foamed molded article, so as not to include the surface that had been in contact with the mold during in-mold foaming (also called the skin layer). The weight W (g) of the test piece was measured, and the value obtained by dividing the measured weight W (g) by the volume V (0.0625 L) of the test piece (length / width / thickness = 50 / 50 / 25 mm) was used as the density (g / L) of the foamed molded body. The test piece was compressed at a rate of 10 mm / min using a tension and compression tester (TG-50kN, manufactured by MinebeaMitsumi Inc.) in accordance with NDZ-Z0504, and the compressive stress (MPa) at 50% compression was measured. The obtained value was defined as the compressive strength of the foam-molded article. The manufacturing conditions and measurement results are shown in Table 1.

[0091] <Example 2> Except for the fact that the cutter rotation speed of the hot cutting device was set to 375 rpm, the same method as in Example 1 was used. The manufacturing conditions and measurement results are shown in Table 1.

[0092] Example 3 The same method as in Example 1 was used, except that a die with a hole diameter of 0.5 mm was used and the cutter rotation speed of the hot cutting device was set to 500 rpm. The manufacturing conditions and measurement results are shown in Table 1.

[0093] Example 4 Except for the fact that the cutter rotation speed of the hot cutting device was set to 375 rpm, the same method as in Example 3 was used. The manufacturing conditions and measurement results are shown in Table 1.

[0094] <Example 5> Except for the fact that the cutter rotation speed of the hot cutting device was set to 250 rpm, the same method as in Example 3 was used. The manufacturing conditions and measurement results are shown in Table 1.

[0095] <Comparative Example 1> Except for changing the cutter rotation speed of the hot cutting device to 1000 rpm, the same method as in Example 1 was used. The manufacturing conditions and measurement results are shown in Table 1.

[0096] <Comparative Example 2> Except for the fact that the cutter rotation speed of the hot cutting device was set to 750 rpm, the same method as in Example 1 was used. The manufacturing conditions and measurement results are shown in Table 1.

[0097] <Comparative Example 3> Except for the fact that the cutter rotation speed of the hot cutting device was set to 250 rpm, the same method as in Example 1 was used. The manufacturing conditions and measurement results are shown in Table 1.

[0098] <Comparative Example 4> The same method as in Example 1 was used, except that a die with a hole diameter of 0.5 mm was used and the cutter rotation speed of the hot cutting device was set to 150 rpm. The manufacturing conditions and measurement results are shown in Table 1.

[0099] [Table 1]

[0100] From the results in Table 1, it can be seen that the compressive strength of the examples is 0.14 MPa or more, while the compressive strength of the comparative examples is less than 0.14 MPa. Therefore, it can be seen that the compressive strength of the foamed molded article is improved when the extruded foamed beads satisfy the following conditions: "The length L of the cylindrical side surface is 8.0 mm or less" and "The cut surface ratio is 0.30 or less." [Explanation of symbols]

[0101] 1. Foam particles 10 Cylinder side 11 Cut surface L length of the cylinder side D1 Long diameter of cut surface D2 Minor diameter of cut surface

Claims

1. Extruded polypropylene resin foam particles obtained by an extrusion foaming method, the expanded beads contain a random polypropylene-based resin (A) having a branched structure as a base resin, the expanded beads have a cylindrical shape formed of one cylindrical side surface and two cut surfaces, the cut surfaces of which are substantially circular and perpendicular to the longitudinal direction of the cylindrical side surface; When the measured values ​​of the length of the cylindrical side surface, the major axis of the cut surface, and the minor axis of the cut surface are expressed as the length L of the cylindrical side surface, the major axis D1 of the cut surface, and the minor axis D2 of the cut surface, respectively, The length L of the cylindrical side surface is 8.0 mm or less, and The extruded polypropylene resin foam particles have a cut surface ratio represented by the following formula (1) of 0.30 or less. Cut surface ratio = surface area of ​​two cut surfaces / (surface area of ​​two cut surfaces + surface area of ​​the cylinder side surface) (1) In formula (1), the surface area of ​​the two cut surfaces is expressed by the following formula (2), and the surface area of ​​the side surface of the cylinder is expressed by the following formula (3). Surface area of ​​two cut surfaces = (π × D1 × D2) / 2 (2) Surface area of ​​the cylinder side surface = (π × (D1 + D2) × L) / 2 (3)

2. The extruded polypropylene resin foam beads according to claim 1, wherein the length L of the cylindrical side surface is 1.0 mm or more and 8.0 mm or less.

3. A polypropylene resin foam molded article obtained by molding the extruded polypropylene resin foam beads according to claim 1 or 2.

4. A method for producing extruded foamed polypropylene resin beads, comprising: supplying a polypropylene resin composition to an extruder, melt-kneading the composition, adding a foaming agent, and further melt-kneading the composition to form a melt-kneaded mixture; cooling the melt-kneaded mixture, and then cutting the extruded foamed resin into strands from a die; The polypropylene resin composition contains a random polypropylene resin (A) having a branched structure, The die diameter of the die is less than 1.0 mm, the cut resin has an expansion ratio of 40 times or less, and a strand length of 8.0 mm or less.

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