Method for producing extruded polypropylene resin particles

Abstract

To provide a method for manufacturing polypropylene resin extrusion foamed particles that can provide polypropylene resin extrusion foamed particles with excellent moldability and high productivity.SOLUTION: The method for manufacturing polypropylene resin extruded foamed particles includes a melting and kneading process for melting and kneading a composition containing a resin mixture containing branched polypropylene resin and carbon dioxide gas, and a granulation process for granulating polypropylene resin extruded foamed particles by the hot cut method, where the discharge volume of the composition per die hole and the composition pressure at the die inlet are specific values.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] The polypropylene resin in-mold foamed article obtained by using the expanded polypropylene resin particles has the advantages of an in-mold foamed article, such as the freedom in shape, cushioning properties, light weight, and heat insulation properties.

[0003] Methods for producing expanded polypropylene resin beads include a batch expansion method, which is a discontinuous process, and an extrusion expansion method, which is a continuous process, etc. The extrusion expansion method has many advantages in terms of efficiency and environment, but has a problem in that it is difficult to obtain expanded polypropylene resin beads with good moldability.

[0004] As a means for solving this problem, the use of modified polypropylene resins with improved melt tension as raw materials has been considered.

[0005] For example, Patent Document 1 discloses a method for producing expanded polypropylene resin particles by extrusion foaming using a specific radical polymerization initiator. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2020 / 004429 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above-mentioned conventional techniques are not sufficient from the viewpoint of further improving productivity while maintaining good moldability of the extruded polypropylene resin foam beads, and there is room for further improvement.

[0008] One embodiment of the present invention has been made in consideration of the above problems, and an object of the present invention is to provide a method for producing extruded polypropylene resin foamed beads, which can provide extruded polypropylene resin foamed beads having excellent moldability with good productivity. [Means for solving the problem]

[0009] As a result of intensive research into solving the above problems, the present inventors have found that by controlling the pressure at the die inlet of a composition containing a branched polypropylene resin and carbon dioxide gas as a foaming agent within an appropriate range, extruded polypropylene resin foamed beads having excellent moldability can be obtained even under high productivity conditions in a hot cut method, and have completed the present invention.

[0010] An embodiment of the present invention includes the following configuration.

[0011] [1] A melt-kneading step of melt-kneading a composition containing a resin mixture containing a polypropylene-based resin having a branched structure and carbon dioxide gas as a foaming agent; A granulation step of granulating the polypropylene-based resin extruded particles by a hot cut method, The hot cutting method is a process of extruding the composition into air through holes in a die, cutting the composition extruded from the holes in the die with a cutter in the air, and cooling the cut composition in the air while foaming or after foaming, The discharge rate of the composition per hole of the die is 0.350 kg / hr or more; A method for producing extruded polypropylene resin foamed beads, wherein the pressure of the composition at the inlet of the die is higher than the saturation pressure of the carbon dioxide gas and is equal to or lower than the saturation pressure of the carbon dioxide gas + 4 MPa.

[0012] [2] The method for producing extruded polypropylene resin foam particles according to [1], wherein the die hole has a cylindrical flow path.

[0013] [3] The method for producing extruded polypropylene resin foamed beads according to [1] or [2], wherein the diameter of the hole in the die is 0.5 mm to 2.0 mm.

[0014] [4] The method for producing extruded polypropylene resin foamed beads according to any one of [1] to [3], wherein the temperature of the composition immediately after entering the die is from a melting point of the resin mixture + 0°C to the melting point + 25°C.

[0015] [5] The method for producing extruded polypropylene resin foam beads according to any one of [1] to [4], wherein the extruded polypropylene resin foam beads have an expansion ratio of 9 to 45 times. Effect of the Invention

[0016] According to one embodiment of the present invention, there can be provided a method for producing extruded polypropylene resin foamed beads, which can provide extruded polypropylene resin foamed beads having excellent moldability with good productivity. [Brief description of the drawings]

[0017] [Figure 1] FIG. 2 is a cross-sectional view showing a configuration in the vicinity of an outlet of a die used in one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] An embodiment of the present invention will be described below, but the present invention is not limited thereto. The present invention is not limited to each configuration described below, and various modifications are possible within the scope of the claims. In addition, embodiments or examples obtained by appropriately combining the technical means disclosed in different embodiments or examples are also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment. All academic documents and patent documents described in this specification are incorporated herein by reference. In addition, unless otherwise specified in this specification, "A to B" representing a numerical range means "A or more (including A and greater than A) and B or less (including B and smaller than B)".

[0019] 1. Technical Concept of the Present Invention Among the methods for producing expanded polypropylene resin particles, the depressurization expansion method, which is a batch expansion method, requires a step of obtaining polypropylene resin particles by melt-kneading and extruding a polypropylene resin, and a step of pressurizing and heating the obtained polypropylene resin particles together with an inorganic dispersant and a foaming agent in an aqueous solvent to impregnate the resin with the foaming agent, and then releasing the foaming agent under low pressure to cause it to expand. The depressurization expansion method has problems in that the process is complicated because it is a discontinuous process, and problems in that wastewater treatment equipment is required because an inorganic dispersant is used, and a large facility site is required.

[0020] In contrast, the extrusion foaming method is a method in which a composition containing a thermoplastic resin and a foaming agent is fed to an extruder, the composition is melt-kneaded and optionally cooled to obtain a melt-kneaded composition (hereinafter also referred to as "foamable molten resin").The composition is then extruded into a low-pressure region through a die attached to the tip of the extruder, and chopped to obtain expanded particles of the thermoplastic resin.

[0021] Unlike the depressurization foaming method, the extrusion foaming method (a) is a continuous process, so the process can be simplified, and (b) does not require a large facility site such as a wastewater treatment facility, so a production base can be set up near the demand area, and transportation costs and labor costs can be reduced. Therefore, the extrusion foaming method has many advantages in terms of efficiency and environment, and can produce polypropylene-based resin foamed beads at low cost. However, there is a problem that it is difficult to obtain polypropylene-based resin foamed beads with good moldability by the extrusion foaming method. In particular, the extrusion foaming method using carbon dioxide gas, which has a low environmental impact as a foaming agent, has lower gas retention during foaming than the extrusion foaming method using an organic foaming agent. For this reason, it is particularly difficult to obtain polypropylene-based resin foamed beads with good moldability by the extrusion foaming method using carbon dioxide gas.

[0022] The cutting method (granulation method) for obtaining expanded particles in the extrusion foaming method is roughly divided into the cold cut method and the die face cut method. The die face cut method is a method in which the composition extruded from a die is cut by a rotating cutter while in contact with the face surface of the die or while keeping a small gap.

[0023] The die face cutting method is further divided into three methods based on the difference in cooling method: underwater cutting (hereinafter sometimes referred to as UWC), watering cutting (hereinafter sometimes referred to as WRC), and hot cutting (hereinafter sometimes referred to as HC).

[0024] In the HC method, the composition cut by the cutter is cooled in air while foaming or after foaming. The conveying method is generally carried out by air transportation. The HC method has the advantage that it can produce extruded foam particles with a high expansion ratio because the particles are expanded in an atmospheric pressure environment.

[0025] In the HC method, in order to increase productivity, there are methods for increasing the amount of composition discharged per die hole and / or increasing the number of die holes. Here, when the number of die holes is increased, the equipment is enlarged or additional production equipment is required, which requires high costs. On the other hand, when the amount of composition discharged per die hole is increased, the cost of introducing additional equipment is not required. However, the present inventor has found for the first time that when the amount of composition discharged per die hole is increased, the moldability of the extruded polypropylene resin beads may decrease. That is, it has been found that it has been difficult to improve productivity while maintaining good moldability of the extruded polypropylene resin beads.

[0026] Therefore, the present inventors have conducted extensive research in order to improve productivity (for example, increase the amount of composition discharged per die hole) while maintaining good moldability of the polypropylene resin extruded foam particles in the HC method. As a result, the present inventors have independently found the following findings, and have completed the present invention: (a) when increasing the amount of composition discharged per die hole, by using a composition containing a branched polypropylene resin and carbon dioxide gas as the composition and controlling the pressure at the die inlet of the composition within an appropriate range, the moldability of the resulting extruded foam particles is surprisingly improved; (b) the appropriate range of the pressure at the die inlet of the composition is a range in which the pressure of the composition at the die inlet is higher than the saturation pressure of carbon dioxide gas and is equal to or lower than the saturation pressure of carbon dioxide gas + 4 MPa.

[0027] Furthermore, the present inventors have investigated the reason why the moldability of the extruded foamed beads can be improved even when the amount of composition discharged per hole of the die is increased by controlling the pressure at the inlet of the die. As a result, the present inventors have inferred the reason as follows: (a) Under high production conditions where the amount of foamable molten resin discharged per die hole is large, it is presumed that the time it takes for the foamable molten resin to start foaming after being discharged into the air (hereinafter also referred to as the "foaming preparation time") becomes long. When the foaming preparation time is long, it is presumed that the foamable molten resin starts to solidify due to cooling before foaming of the foamable molten resin begins (i.e., before the cells start to expand). If the resin becomes difficult to stretch due to solidification and the cells expand due to foaming, the cell membrane is likely to break. Extruded polypropylene resin foam particles containing many broken cell membranes have poor moldability; (b) It is presumed that the foaming preparation time can be shortened by using a polypropylene resin having a branched structure and a foaming molten resin containing carbon dioxide as the foaming molten resin, and controlling the pressure in the die of the foaming molten resin to an appropriate range. When the foaming preparation time is short, it is presumed that the foaming molten resin is cooled while foaming, and solidification begins while the cells are expanding due to foaming, or after the cells have sufficiently expanded due to foaming. Therefore, the obtained extruded polypropylene resin foam particles retain the cell membrane and show excellent moldability. The above-mentioned "foaming preparation time" can be said to be the time required for the pressure of the foamable molten resin discharged into the air to decrease to a pressure at which foaming can start (saturation pressure of carbon dioxide gas). Note that the present invention is not limited by such an assumption.

[0028] In this specification, "moldability" refers to the properties of the polypropylene resin extruded foam particles and means the ease of molding. In this specification, "good moldability" means that the polypropylene resin extruded foam particles can be further expanded and the extruded foam particles can be easily fused together during in-mold foaming. In other words, in this specification, "good moldability" means that a polypropylene resin in-mold foamed molded product having a high expansion ratio and excellent fusion properties can be easily obtained. In this specification, the moldability is evaluated by an internal pressure decay test of the polypropylene resin extruded foam particles. The internal pressure decay test will be described in detail in the examples below.

[0029] 2. Method for producing extruded polypropylene resin foam particles A method for producing extruded polypropylene resin foam particles according to one embodiment of the present invention includes a melt-kneading step of melt-kneading a composition containing a resin mixture containing a branched polypropylene resin and carbon dioxide gas as a foaming agent, and a granulation step of granulating extruded polypropylene resin foam particles by a hot-cut method, in which the composition is extruded through a die hole, the composition extruded from the die hole is cut with a cutter in air, and the cut composition is cooled in air while foaming or after foaming, and the discharge rate of the composition per die hole is 0.350 kg / hr or more, and the pressure of the composition at the inlet of the die is higher than the saturation pressure of carbon dioxide gas and is equal to or lower than the saturation pressure of carbon dioxide gas + 4 MPa.

[0030] In this specification, "polypropylene-based resin having a branched structure" may be referred to as "branched polypropylene-based resin", "extruded polypropylene-based resin beads" may be referred to as "extruded foam beads", "a composition containing a resin mixture containing a polypropylene-based resin having a branched structure and carbon dioxide gas as a foaming agent" may be referred to as "composition", and "a method for producing extruded polypropylene-based resin beads according to one embodiment of the present invention" may be referred to as "the present production method".

[0031] Since the present production method has the above-mentioned configuration, it has an advantage that extruded polypropylene resin foamed beads having excellent moldability can be obtained with high productivity. More specifically, according to the present production method, even when the discharge amount of the composition per hole of the die is set to 0.350 kg / hr or more in the hot cut method, it has an advantage that extruded polypropylene resin foamed beads having excellent moldability can be obtained.

[0032] First, the raw materials (ingredients) and production equipment used in this production method will be described, and then each step will be described.

[0033] (2-1.Resin mixture) In the present production method, the resin mixture containing the branched polypropylene resin can be said to be a component other than the foaming agent in the composition. The resin mixture contains the branched polypropylene resin and may further contain additives such as a bubble nucleating agent.

[0034] In this specification, the term "polypropylene resin having a branched structure" refers to (a) a polypropylene resin in which molecules of a polypropylene resin not having a branched structure are partially crosslinked with each other, and (b) a polypropylene resin in which a diene compound other than (poly)propylene is introduced as a branched chain into a polypropylene resin not having a branched structure. In this specification, the term "polypropylene resin having a branched structure" refers to a polypropylene resin having a loss tangent tanδ of 5 or less at 200°C and 0.1 rad / s. Other aspects of the loss tangent tanδ (such as a measurement method) will be described in detail in the section (polypropylene resin having a branched structure) described later.

[0035] In this specification, the "polypropylene resin without a branched structure" may be referred to as a "linear polypropylene resin", the "polypropylene resin with a branched structure" may be referred to as a "branched polypropylene resin", and the "linear polypropylene resin" and the "branched polypropylene resin" may be collectively referred to as a "polypropylene resin". The linear polypropylene resin may also be considered as a raw material for the branched polypropylene resin.

[0036] In this specification, a linear polypropylene resin refers to a resin that contains 50 mol % or more of structural units derived from propylene monomers, based on 100 mol % of all structural units contained in the resin. In this specification, the "structural units derived from propylene monomers" may be referred to as "propylene units."

[0037] (Linear polypropylene resin) The linear polypropylene resin may be (a) a homopolymer of propylene, (b) a block copolymer or random copolymer of propylene and a monomer other than propylene, or (c) a mixture of two or more of these.

[0038] A linear polypropylene-based resin may have, in addition to a propylene unit, one or more structural units derived from a monomer other than a propylene monomer, or may have one or more types. The "monomer other than a propylene monomer" used in the production of a linear polypropylene-based resin may be referred to as a "comonomer", and the "structural unit derived from a monomer other than a propylene monomer" contained in a linear polypropylene-based resin may be referred to as a "comonomer unit".

[0039] Examples of the comonomer include the following monomers: (a) 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; (b) cyclic olefins, such as cyclopentene, norbornene, and tetracyclo[6,2,11,8,13,6]-4-dodecene; (c) cyclic olefins, such as cyclopentene, norbornene, and 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 (d) 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 monomers, vinyl toluene, divinyl benzene, and the like.

[0040] Examples of the acrylic acid 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.

[0041] 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.

[0042] Styrenic monomers include styrene, methylstyrene, dimethylstyrene, alpha-methylstyrene, para-methylstyrene, ethylstyrene, diethylstyrene, isopropylstyrene, t-butylstyrene, bromostyrene, dibromostyrene, tribromostyrene, chlorostyrene, dichlorostyrene, and trichlorostyrene.

[0043] The linear polypropylene resin preferably has a structural unit derived from an α-olefin having 2 or 4 to 12 carbon atoms as a comonomer unit, 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. This configuration has the advantages of (a) obtaining a branched polypropylene-based resin having high melt tension and a low gel fraction, and (b) providing extruded polypropylene-based resin beads in which the obtained branched polypropylene-based resin has excellent moldability.

[0044] The linear polypropylene resin is preferably a propylene homopolymer, a polypropylene block copolymer, and / or a polypropylene random copolymer, and more preferably a propylene homopolymer and / or a polypropylene random copolymer. This configuration has the advantages of (a) being able to obtain a branched polypropylene resin having high melt tension and low gel fraction, and (b) being able to provide extruded polypropylene resin beads in which the obtained branched polypropylene resin has excellent moldability.

[0045] The linear polypropylene resin preferably contains propylene units in an amount of 90 mol % or more, more preferably 93 mol % or more, even more preferably 95 mol % or more, and particularly preferably 97 mol % or more, of all structural units contained in the linear polypropylene resin (100 mol %). This configuration has the advantage that a branched polypropylene resin having high melt tension and low gel fraction can be obtained.

[0046] The melting point of the linear polypropylene resin is not particularly limited. The melting point of the linear polypropylene resin is, for example, preferably 130°C to 165°C, more preferably 135°C to 164°C, further preferably 138°C to 163°C, and particularly preferably 140°C to 162°C. When the melting point of the linear polypropylene resin is within the above-mentioned range, (a) the obtained extruded foam particles have excellent moldability, and (b) the extruded foam particles can provide a foamed molded product having excellent breakage resistance. When the melting point of the linear polypropylene resin is (a) 130°C or higher, there is an advantage that there is no risk of the dimensional stability of the foamed molded product decreasing, there is no risk of the heat resistance of the foamed molded product becoming insufficient, and the compressive strength of the foamed molded product tends to be strong, and when (b) 165°C or lower, there is an advantage that the extruded foam particles can be molded at a relatively low vapor pressure, so that the extruded foam particles can be molded using a general-purpose molding machine for polypropylene resin extruded foam particles.

[0047] In this specification, the melting point of the linear polypropylene resin is a value obtained by measuring by a differential scanning calorimeter method (hereinafter referred to as the "DSC method"). The specific operation procedure is as follows: (1) the temperature of 5 to 6 mg of linear polypropylene resin is raised from 40°C to 220°C at a heating rate of 10°C / min to melt the linear polypropylene resin; (2) the temperature of the melted linear polypropylene resin is then lowered from 220°C to 40°C at a heating rate of 10°C / min to crystallize the linear polypropylene resin; (3) the temperature of the crystallized linear polypropylene resin is then further 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 linear polypropylene resin obtained at the second heating (i.e., at the time of (3)) can be determined as the melting point of the linear polypropylene resin. In addition, when there are multiple peaks (melting peaks) in the DSC curve of the linear polypropylene resin obtained at the second temperature rise by the above-mentioned method, the temperature of the peak (melting peak) with the maximum heat of fusion is taken as the melting point of the linear polypropylene resin. As the differential scanning calorimeter, for example, DSC6200 manufactured by Seiko Instruments Inc. can be used.

[0048] The melt flow rate (MFR) of the linear polypropylene resin is not particularly limited. The MFR of the linear polypropylene resin is, for example, preferably 0.5g / 10min to 50.0g / 10min, more preferably 1.0g / 10min to 30.0g / 10min, further preferably 2.0g / 10min to 20.0g / 10min, and particularly preferably 2.0g / 10min to 10.0g / 10min. When the MFR of the linear polypropylene resin is within the above-mentioned range, it has the advantage that a branched polypropylene resin having an MFR of 0.5g / 10min to 20g / 10min can be easily obtained.

[0049] In this specification, the MFR of a linear polypropylene-based resin is a value determined by measurement using an MFR measuring device described in JIS K7210 under conditions of an orifice diameter of 2.0959±0.0050 mmφ, an orifice length of 8.000±0.025 mm, a load of 2160 g, and 230±0.2°C.

[0050] (Polypropylene resin having a branched structure) A polypropylene resin having a branched structure (branched polypropylene resin) can be obtained by introducing a branched structure into a linear polypropylene resin. The method for introducing a branched structure into a linear polypropylene resin is not particularly limited, and examples thereof include (a1) a method of irradiating a linear polypropylene resin with radiation, and (a2) a method of melt-kneading a mixture containing a linear polypropylene resin, a conjugated diene compound, and a radical polymerization initiator.

[0051] A specific example of the method (a1) is the method described in JP-T No. 2002-542360.

[0052] The method (a2) will be further described. In the method (a2), for example, the following steps (i) to (iv) are performed in order to obtain a branched polypropylene resin: (i) a mixture containing a linear polypropylene resin, a conjugated diene compound, and a radical polymerization initiator is melt-kneaded in an apparatus equipped with a die; (ii) the resulting melt-kneaded product is extruded from the die; (iii) the extruded melt-kneaded product (also called a strand) is cooled; (iv) the strand is chopped at the same time as or after cooling the strand. Specific examples of the method (a2) include the method described in WO2020 / 004429.

[0053] In one embodiment of the present invention, the branched polypropylene resin is preferably a branched polypropylene resin obtained by the above-mentioned method (a2), because (i) a branched structure can be stably introduced into a linear polypropylene resin and the introduction of the branched structure is highly reproducible, and / or (ii) no complicated equipment is required and the branched polypropylene resin can be obtained with high productivity.

[0054] The branched polypropylene resin may have the same composition as the linear polypropylene resin, which is the raw material. That is, the branched polypropylene resin may be (a) a homopolymer of propylene, (b) a block copolymer or random copolymer of propylene and a monomer other than propylene, or (c) a mixture of two or more of these. The branched polypropylene resin may also contain a structural unit (comonomer unit) derived from the above-mentioned comonomer. The branched polypropylene resin is preferably a branched propylene homopolymer, a branched polypropylene block copolymer, and / or a branched polypropylene random copolymer, and more preferably a branched polypropylene homopolymer and / or a branched polypropylene random copolymer. The advantages obtained by the branched polypropylene resin having these structures are the same as the advantages obtained by the linear polypropylene resin having these structures.

[0055] The loss tangent tanδ of the branched polypropylene resin can be smaller than that of the linear polypropylene resin. The loss tangent tanδ is the ratio (G'' / G') of the storage modulus G' to the loss modulus G''. The smaller the loss tangent tanδ of the resin, the higher the moldability of the resulting extruded polypropylene resin beads. The loss tangent tanδ of the branched polypropylene resin at 200°C and 0.1 rad / s is preferably 4 or less, more preferably 3 or less, and even more preferably 2 or less.

[0056] The method for measuring the loss tangent tanδ in this specification is described below. In this specification, the loss tangent tanδ, storage modulus G', and loss modulus G'' are measured by a vibration experiment using a rotational rheometer (manufactured by TA Instruments, product name: ARES) under a nitrogen atmosphere, at a measurement temperature of 200°C, and at a measurement angular frequency of 0.1 rad / s. Specifically, the method is as follows (1) to (3): (1) a sample resin to be measured (branched polypropylene resin) is sandwiched between two parallel plates (gap 1 mm); (2) one of the plates is driven to apply a periodic strain due to vibration to the sample resin between the plates; (3) the storage modulus G', loss modulus G'', and loss tangent tanδ are obtained from the waveform of the shear stress as a response to the applied strain and the phase difference between them.

[0057] The melt flow rate of the branched polypropylene resin is not particularly limited. The MFR of the branched polypropylene resin is, for example, preferably 0.5 g / 10 min to 20.0 g / 10 min, more preferably 1.0 g / 10 min to 15.0 g / 10 min, and particularly preferably 2.0 g / 10 min to 10.0 g / 10 min.

[0058] In this specification, the MFR of the branched polypropylene resin is a value determined by measurement using an MFR measuring device described in JIS K7210 under conditions of an orifice diameter of 2.0959±0.0050 mmφ, an orifice length of 8.000±0.025 mm, a load of 2160 g, and 230±0.2°C.

[0059] The melt tension of the branched polypropylene resin is not particularly limited, and is preferably, for example, 3 cN to 20 cN, more preferably 3 cN to 15 cN, and particularly preferably 3 cN to 10 cN.

[0060] In this specification, the melt tension of the branched polypropylene resin is measured using a Capilograph 1D (manufactured by Toyo Seiki Seisakusho, Japan). Specifically, the steps are as follows: (1) A sample resin (branched polypropylene resin) for measurement is filled into a barrel with a diameter of 9.55 mm heated to a test temperature (200°C); (2) The sample resin is then heated for 10 minutes in the barrel heated to the test temperature (200°C); (3) The sample resin is then discharged in a string-like form from a capillary die (diameter 1.0 mm, length 10 mm) at a constant piston descending speed (10 mm / min), while passing this string-like material through a tension detection pulley located 350 mm below the capillary die, and then winding using a winding roll is started; (4) After the take-up of the string-like material becomes stable, the winding speed of the string-like material is increased at a constant rate from an initial speed of 1.0 m / min until it reaches a speed of 200 m / min in 4 minutes; (5) The load applied to the pulley with the load cell when the string-like material breaks is measured as the melt tension.

[0061] (Other resins or rubber) The resin mixture may further contain resins other than the branched polypropylene-based resin (sometimes referred to as "other resins") or rubber, as long as the effect of one embodiment of the present invention is not impaired. Examples of the other resins include (a) linear polypropylene-based resins such as ethylene / propylene random copolymers, ethylene / propylene block copolymers, and propylene homopolymers, (b) 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 copolymers, ethylene / acrylic acid copolymers, and ethylene / methacrylic acid copolymers, and (c) styrene-based resins such as polystyrene, styrene / maleic anhydride copolymers, and styrene / ethylene copolymers. Examples of the rubbers include olefin-based rubbers such as ethylene / propylene rubber, ethylene / butene rubber, ethylene / hexene rubber, and ethylene / octene rubber.

[0062] The content of the other resin in the resin mixture is, for example, preferably 60 parts by weight or less, more preferably 40 parts by weight or less, and even more preferably 20 parts by weight or less, relative to 100 parts by weight of the resin mixture. The lower limit of the content of the other resin is not particularly limited, and may be, for example, 0 part by weight relative to 100 parts by weight of the resin mixture.

[0063] (bubble nucleating agent) The resin mixture may contain a bubble nucleating agent for the purpose of controlling the number and shape of bubbles in the resulting extruded foam particles. Examples of the bubble nucleating agent include a sodium bicarbonate-citric acid mixture, monosodium citrate, talc, and calcium carbonate. These bubble nucleating agents may be used alone or in combination of two or more.

[0064] The amount of the bubble nucleating agent used, in other words, the content of the bubble nucleating agent in the resin mixture, is not particularly limited. The amount of the bubble nucleating agent used 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 branched polypropylene resin.

[0065] (Other ingredients) The resin mixture may further contain, as other components as necessary, (a) stabilizers such as antioxidants, metal deactivators, phosphorus-based processing stabilizers, ultraviolet absorbers, ultraviolet stabilizers, fluorescent brighteners, metal soaps, and antacid adsorbents, and / or (b) additives such as crosslinkers, chain transfer agents, lubricants, plasticizers, fillers, reinforcing materials, flame retardants, colorants, and antistatic agents. These other components may be used alone or in combination of two or more.

[0066] (Melting point of resin mixture) The melting point of the resin mixture is not particularly limited. The melting point of the resin mixture is, for example, preferably 130°C to 165°C, more preferably 135°C to 164°C, further preferably 138°C to 163°C, and particularly preferably 140°C to 162°C. When the melting point of the resin mixture is within the above-mentioned range, (a) the obtained extruded foam particles have excellent moldability, and (b) the extruded foam particles can provide a foamed molded product having excellent breakage resistance. When the melting point of the resin mixture is (a) 130°C or higher, there is an advantage that there is no risk of the dimensional stability of the foamed molded product decreasing, there is no risk of the heat resistance of the foamed molded product becoming insufficient, and the compressive strength of the foamed molded product tends to be strong, and when (b) 165°C or lower, there is an advantage that the extruded foam particles can be molded using a general-purpose molding machine for polypropylene resin extruded foam particles, since it is possible to mold the extruded foam particles at a relatively low vapor pressure.

[0067] In this specification, the melting point of the resin mixture is a value determined by measurement using a DSC method. Specifically, the melting point can be determined by the same method as the above-mentioned method for measuring the melting point of the linear polypropylene-based resin, except that the resin mixture is used instead of the linear polypropylene-based resin.

[0068] (2-2. Composition) In this manufacturing method, the substance obtained by adding a foaming agent to the above-mentioned resin mixture may be referred to as a "composition."

[0069] In this production method, carbon dioxide gas is used as the foaming agent, which has the advantages of low production costs and low environmental impact.

[0070] It is preferable that the composition is substantially free of any substance other than carbon dioxide gas that can function as a foaming agent. Specifically, the content of the substance other than carbon dioxide gas that can function as a foaming agent in the composition is preferably 0.01 parts by weight or less, more preferably 0.001 parts by weight or less, even more preferably 0.0001 parts by weight or less, and particularly preferably 0 parts by weight, relative to 100 parts by weight of the composition.

[0071] Examples of substances other than carbon dioxide gas that can function as a blowing agent include (a) aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, and hexane; (b) aliphatic cyclic hydrogens such as cyclopentane and cyclobutane; (c) inorganic gases such as air and nitrogen; and (d) water.

[0072] The amount of the foaming agent used may be adjusted appropriately depending on the expansion ratio of the target foam. In the present production method, the total amount of the foaming agent used is preferably 1 to 20 parts by weight, more preferably 1 to 15 parts by weight, even more preferably 1 to 10 parts by weight, and particularly preferably 2 to 10 parts by weight, relative to 100 parts by weight of the composition. The "amount of the foaming agent used" can also be called the "content of the foaming agent in the composition."

[0073] (2-3. Manufacturing equipment) The manufacturing apparatus used in this manufacturing method will be described below. The manufacturing apparatus used in this embodiment is an apparatus for granulating polypropylene resin extruded foam particles by melt-kneading the composition, extruding the melt-kneaded composition, and cutting it in air. Such a manufacturing apparatus may be one equipped with an extruder and a granulating section having a die. The extruder and the granulating section having a die are connected to each other and are arranged in this order from the upstream side to the downstream side in the extrusion direction of the composition. A raw material supplying device and a foaming agent supplying device are connected to the extruder. A cooling section for lowering the temperature of the composition may be provided between the extruder and the granulating section. A transporting section for transporting the resin may be provided between the extruder and the cooling section and / or between the cooling section and the granulating section.

[0074] In the manufacturing apparatus, (a) the raw material of the resin mixture such as branched polypropylene resin is supplied to the extruder through the raw material supply device, and (b) carbon dioxide gas as a foaming agent is supplied to the extruder through the foaming agent supply device. The resin mixture and carbon dioxide gas as a foaming agent in the extruder are melt-kneaded. Then, the melt-kneaded composition passes through a transport section and a cooling section as necessary and reaches the granulation section. Then, the composition extruded from the die hole in the granulation section is cut by a cutter, and is foamed and granulated to produce extruded polypropylene resin foam particles. Here, with respect to the flow of the composition from the extruder to the granulation section, the extruder side is the upstream side, and the granulation section side is the downstream side.

[0075] The extruder can be appropriately selected from conventionally known extruders, and examples thereof include extruders using screws. As the extruder using screws, for example, a single screw extruder or a twin screw extruder can be used. When a twin screw extruder is used, the screw rotation directions may be the same or different.

[0076] The raw material supplying device is composed of members for supplying the branched polypropylene-based resin, etc. The number of raw material supplying devices can be appropriately set depending on the characteristics, types, number, etc. of the raw materials of the resin mixture.

[0077] The foaming agent supplying device is composed of a member that supplies carbon dioxide gas as a foaming agent to the resin mixture melted and kneaded in the extruder. More specifically, the foaming agent supplying device includes a foaming agent storage section (carbon dioxide gas cylinder) and a pump (high-pressure pump). In the foaming agent supplying device, the foaming agent stored in the foaming agent storage section is supplied to the extruder by the pump.

[0078] The transport section is composed of a transport member for transporting the composition from the extruder to the granulation section. The transport member may be a known transport member used in the extrusion foaming method, such as a gear pump. The gear pump is a member useful for maintaining or appropriately increasing the pressure of the flow of the composition.

[0079] The cooling section is composed of a cooling member that cools the composition transported from the transport section (extruder when the transport section is omitted). The cooling member may be a known cooling member used in the extrusion foaming method. Examples of the cooling member include a single screw extruder, a static mixer, and a melt cooler. The above-mentioned cooling members may be used alone or in combination of two or more.

[0080] The granulating section having the die is disposed on the most downstream side in the flow of the composition. Fig. 1 is a cross-sectional view showing the configuration in the vicinity of the outlet of the die used in one embodiment of the present invention.

[0081] The die 10 has a face surface 11 at its end. The face surface 11 is the surface of the die 10 that contacts the outside, and in one embodiment of the present invention, is the surface that contacts the gas phase. At least one hole 12 is formed in the face surface 11 of the die 10 as an outlet for the composition. The hole 12 communicates with a resin passage 13 inside the die 10. In the die 10, the diameter of the hole 12 is smaller than the diameter of the resin passage 13. Between the hole 12 and the resin passage 13, a land portion 15 is formed that has a flow path 14 having the same diameter as the diameter of the hole 12 and a diameter smaller than the diameter of the resin passage 13. In other words, the hole 12 can be said to have the flow path 14. The land portion 15 refers to a region in the die 10 that has a flow path 14 having a diameter smaller than the diameter of the resin passage 13.

[0082] The diameter of the hole 12 is indicated by the symbol b in FIG. 1. In this specification, when the shape of the hole 12 is not a perfect circle, the diameter of the hole 12 refers to the diameter of an inscribed circle of the shape of the hole 12. When the shape of the cross section of the resin passage 13 perpendicular to the extrusion direction (hereinafter, may be simply referred to as the "shape of the resin passage") is a perfect circle, the diameter of the resin passage 13 refers to the diameter of the shape of the resin passage 13. When the shape of the resin passage 13 is not a perfect circle, the diameter of the resin passage 13 refers to the diameter of an inscribed circle of the shape of the resin passage 13. When the shape of the cross section of the flow passage 14 perpendicular to the extrusion direction (hereinafter, may be simply referred to as the "shape of the flow passage 14") is a perfect circle, the diameter of the flow passage 14 refers to the diameter of the shape of the flow passage 14. When the shape of the flow passage 14 is not a perfect circle, the diameter of the flow passage 14 refers to the diameter of the inscribed circle of the shape of the flow passage 14.

[0083] The resin passage 13 also communicates with the outlet at the tip of the extruder. That is, the composition melted and kneaded in the extruder passes through the resin passage 13 in the die 10 from the tip of the extruder, passes through the flow path 14 of the land portion 15, reaches the hole 12, and is extruded from the hole 12 into the air.

[0084] The cross-sectional shape of the holes 12 of the die 10 perpendicular to the extrusion direction (hereinafter, may be simply referred to as "hole shape") is not particularly limited. Since extruded foamed particles having a spherical or nearly spherical shape can be obtained, the shape of the holes of the die is preferably a perfect circle, a nearly circle, an ellipse, etc., and more preferably a perfect circle or a nearly circle. The number of holes provided in the die is not particularly limited.

[0085] The diameter of the hole 12 is not particularly limited, but is preferably 0.5 mm to 2.0 mm, more preferably 0.5 mm to 1.5 mm, even more preferably 0.5 mm to 1.2 mm, and particularly preferably 0.5 mm to 1.0 mm. If the diameter of the hole 12 is less than 0.5 mm, the pressure in the composition section at the inlet of the die becomes high, which is not preferable from the viewpoint of the pressure resistance of the die. Also, if the diameter of the hole 12 is 2.0 mm or more, the pressure in the composition section at the inlet of the die becomes low, making it difficult to make the pressure higher than the saturation pressure of carbon dioxide gas.

[0086] Moreover, the flow paths 14 in the land portions 15 are preferably cylindrical. This makes it easier to push the composition out without stagnation to the holes 12. When the flow paths 14 in the land portions 15 are cylindrical, it can be said that the holes 12 have cylindrical flow paths.

[0087] By adjusting the length of the land portion 15, that is, the length of the flow path 14 (symbol a in FIG. 1), the pressure of the composition in the die 10 can be controlled to be in a desired range. The length of the land portion 15 can also be said to be the length of the land portion 15 in the extrusion direction. The longer the length of the land portion 15, the more the pressure of the composition in the die 10 tends to increase, and the shorter the length of the land portion 15, the more the pressure of the composition in the die 10 tends to decrease. The preferred length of the land portion 15 varies depending on the physical properties of the branched polypropylene resin, the temperature of the composition, the discharge amount of the composition, and the like, and is not particularly limited. In one embodiment, the length of the land portion 15 is, for example, preferably 0.5 mm to 5.5 mm, more preferably 1.0 mm to 3.5 mm, and even more preferably 1.5 mm to 2.5 mm. If the length of the land portion 15 is 0.5 mm or more, the land portion 15 has good pressure resistance, which is advantageous in that the shape and size of the hole 12 are easily maintained. If the length of the land portion 15 is 5.5 mm or less, there is an advantage that the pressure of the composition in the die 10 can be easily controlled within a suitable range. The diameter of the flow path 14 is the same as the diameter of the hole 12. Therefore, the length of the land portion 15 can also be said to be the length of the hole 12 having the above-mentioned diameter.

[0088] The length and diameter of the resin passage 13 are not particularly limited, and can be adjusted appropriately depending on the total amount of composition discharged according to the number of holes 12, etc.

[0089] The granulation section further includes a cutter. In the granulation section, polypropylene resin extrusion foam particles are granulated by the HC method. The cutter is a member that cuts the composition extruded from the hole of the die in the air. The cutter may include one or more cutter blades and a rotating shaft. The cutter blade is configured to press against the face surface of the die or to secure a small gap while rotating around the axis of the rotating shaft.

[0090] The granulating section may further include an air outlet for conveying the granulated polypropylene resin extrusion foam particles. In one embodiment, the air outlet is preferably arranged behind the rotation direction of the cutter blade and configured to discharge air forward in the rotation direction of the cutter blade. As a result, in the granulating section, air is blown from the air outlet to the composition immediately after cutting with the cutter blade. By providing the air outlet in this way, the composition is easily released from the cutter blade after cutting with the cutter blade. Also, a water outlet can be arranged instead of the air outlet.

[0091] The granulation section may further include a mist nozzle that sprays cooling water onto the face of the die. In one embodiment, the mist nozzle is preferably provided at a position where it can spray cooling water toward the die hole. In other words, the mist nozzle preferably has a spray nozzle that is arranged toward the die hole. This allows the mixed mist of water and air sprayed from the spray nozzle to fill the space near the die hole, preventing mutual adhesion of the extruded foam particles. In this specification, "in the air" includes the space filled with the mixed mist.

[0092] The granulating unit may further include a cutter case that houses a cutter. In one embodiment, the cutter case may include a discharge port through which the extruded foam particles produced by the HC method are discharged in order to collect the extruded foam particles.

[0093] (2-4. Melt kneading process) The present production method includes a melt-kneading step of melt-kneading a composition containing a resin mixture containing a branched polypropylene resin and carbon dioxide gas as a foaming agent. The melt-kneading step can also be said to be a step of melting the branched polypropylene resin in an extruder of a production device and dissolving the foaming agent in the branched polypropylene resin. The melt-kneading step can also be said to be a step of preparing a melt-kneaded product of the composition containing the resin mixture containing the branched polypropylene resin and the foaming agent.

[0094] In the melt-kneading step, it is sufficient that the foaming agent is finally dissolved in the branched polypropylene resin. In the melt-kneading step, the order and method of supplying the branched polypropylene resin and the foaming agent to the melt-kneading section are not particularly limited, and examples thereof include the following methods: (1) supplying the branched polypropylene resin from a raw material supply device to an extruder, and melt-kneading the branched polypropylene resin; (2) thereafter, supplying the foaming agent from a foaming agent supply device located in the middle of the extruder to the melt-kneaded branched polypropylene resin, that is, preparing (completing) a composition in the extruder, and further melt-kneading the composition.

[0095] In the above-mentioned method, when other resins, bubble nucleating agents and other components are added to the composition as required, the method and order of supplying these raw materials to the extruder are not particularly limited. The other resins, bubble nucleating agents and other components may be added simultaneously with the branched polypropylene resin and / or the foaming agent, or may be added separately and in any order.

[0096] In the melt-kneading step, the barrel temperature in the extruder for melting the raw material such as the branched polypropylene resin is not particularly limited, and may be within a range that does not interfere with the supply of the foaming agent to the raw material. If the branched polypropylene resin is not melted at the supply position of the foaming agent in the extruder, the foaming agent may escape to the upstream side of the extruder. For this reason, it is preferable to set the barrel temperature so that the branched polypropylene resin is completely melted while preventing the foaming agent from vaporizing due to the high resin temperature. For example, the branched polypropylene resin is preferably melt-kneaded at a barrel temperature of 160°C to 260°C, more preferably at a barrel temperature of 170°C to 240°C, and even more preferably at a barrel temperature of 180°C to 220°C.

[0097] The melt-kneading step may further include a cooling step of lowering the temperature of the melt-kneaded composition within a temperature range in which the melt-kneaded composition does not solidify after melt-kneading the composition by, for example, the method described above. The cooling step is performed by sending the composition melt-kneaded in the extruder to a cooling section and cooling the composition to a desired temperature in the cooling section.

[0098] When the cooling step is performed in the cooling section, the temperature of the composition at the outlet of the cooling section is preferably in the range of, for example, the melting point of the resin mixture containing the branched polypropylene resin +0°C to the melting point +25°C, more preferably in the range of the melting point of the resin mixture +0°C to the melting point +20°C, and even more preferably in the range of the melting point of the resin mixture +0°C to the melting point +15°C. If the temperature of the composition at the outlet of the cooling section is in the above range, it has the advantage that the temperature of the composition immediately after entering the die can be easily adjusted to a preferred range. The temperature of the composition at the outlet of the cooling section can be controlled to a desired range by adjusting the temperature and heat transfer area of ​​the cooling member, and the residence time of the composition in the cooling member.

[0099] (2-5. Granulation process) This manufacturing method includes a granulation step of granulating polypropylene-based resin extruded foam particles by a hot-cut method. The hot-cut method is a step of extruding the composition through a die hole, cutting the composition extruded from the die hole with a cutter in air, and cooling the cut composition in air while foaming or after foaming. The granulation step can also be said to be a step of extruding the composition obtained in the melt-kneading step, i.e., the melt-kneaded composition, through a die into air and cutting the extruded composition.

[0100] In this manufacturing method, the discharge amount of the composition per hole of the die is 0.350 kg / hr or more. Due to this configuration, the manufacturing method has the advantage of high productivity. From the viewpoint of increasing productivity, the larger the discharge amount of the composition per hole of the die in the granulation process, the more preferable it is. The discharge amount of the composition per hole of the die is not particularly limited, but may be, for example, 0.400 kg / hr or more, 0.500 kg / hr or more, 0.600 kg / hr or more, or 0.650 kg / hr or more. On the other hand, the pressure of the composition at the inlet of the die tends to increase as the discharge amount of the composition per hole of the die increases. The upper limit of the discharge amount of the composition per hole of the die can be determined by the pressure value of the composition at the inlet of the die, and is not particularly limited. From the viewpoint of maintaining a high level of moldability of the obtained extruded foamed beads, the upper limit of the extrusion rate of the composition per hole of the die is preferably 2,500 kg / hr or less, more preferably 2,000 kg / hr or less, and even more preferably 1,500 kg / hr or less.

[0101] Methods for adjusting the amount of composition discharged per hole of the die within a desired range include a method of adjusting the total amount of composition discharged, or a method of adjusting the number of holes in the die.

[0102] In this manufacturing method, the pressure of the composition at the inlet of the die is adjusted to be higher than the saturation pressure of carbon dioxide gas and to be equal to or lower than the saturation pressure of carbon dioxide gas + 4 MPa. The pressure is preferably higher than the saturation pressure of carbon dioxide gas + 0 MPa and equal to or lower than the saturation pressure of carbon dioxide gas + 4.0 MPa, more preferably higher than the saturation pressure of carbon dioxide gas + 0 MPa and equal to or lower than the saturation pressure of carbon dioxide gas + 3.5 MPa, and even more preferably higher than the saturation pressure of carbon dioxide gas + 0 MPa and equal to or lower than the saturation pressure of carbon dioxide gas + 3.0 MPa. By making the pressure of the composition at the inlet of the die higher than the saturation pressure of carbon dioxide gas, it is possible to prevent the phenomenon in which carbon dioxide gas is vaporized at the inlet of the die, the foamable molten resin foams, and the cell membrane breaks (hereinafter also referred to as "foaming in the die"). In addition, by making the pressure of the composition at the inlet of the die equal to or lower than the saturation pressure of carbon dioxide gas + 4 MPa, it is possible to reduce the foaming preparation time to almost zero even under high production conditions. As a result, the cell membrane is maintained, and extruded polypropylene resin foamed particles exhibiting excellent moldability can be obtained.

[0103] The saturation pressure of carbon dioxide varies depending on the content of carbon dioxide in the composition and the temperature of the composition, but can be measured based on the presence or absence of foaming in the die. More specifically, under conditions where the content of carbon dioxide in the composition and the temperature of the composition are constant, the pressure of the composition at the inlet of the die is changed by changing the discharge amount of the composition, and the pressure of the composition at the inlet of the die at the time when foaming in the die occurs is defined as the saturation pressure of carbon dioxide. In one embodiment, when the content of carbon dioxide in the composition is 5% by weight and the temperature of the composition is 145°C, the saturation pressure of carbon dioxide is within the range of 5.5 MPa to 6.5 MPa. It is determined that "foaming in the die occurs" when at least one of the following phenomena (1) to (3) occurs: (1) the pressure of the composition at the inlet of the die is not stable at a constant value (e.g., it fluctuates by ±5.0 MPa or more), (2) the composition extruded into the air from the die hole shrinks immediately after foaming (e.g., it shrinks by 50% or more), and (3) a crackling sound occurs when the composition is extruded into the air from the die hole.

[0104] A method for adjusting the pressure of the composition at the die inlet to be higher than the saturation pressure of carbon dioxide gas and not higher than 4 MPa higher than the saturation pressure of carbon dioxide gas includes adjusting various conditions that may affect the pressure of the composition at the die inlet and the saturation pressure of carbon dioxide gas. The various conditions that may affect these pressures include (a) the length of the land portion, (b) the amount of composition discharged per hole in the die, (c) the temperature of the composition in the die (for example, the temperature of the composition immediately after entering the die), (d) the content of carbon dioxide gas in the composition, and (e) the diameter of the hole.

[0105] The pressure of the composition at the inlet of the die can be measured by (a) a pressure gauge installed near the inlet of the die, or (b) a pressure gauge installed in contact with the composition immediately before entering the die, for example, near the outlet of the transport section. The pressure of the composition may change while the composition passes through the die. The pressure gauge for measuring the pressure of the composition immediately after entering the die is preferably installed, for example, within 20 mm upstream or downstream from the inlet of the die along the extrusion direction.

[0106] In this production method, the temperature of the composition immediately after entering the die is preferably adjusted to be the melting point of the resin mixture +0°C to the melting point +25°C, more preferably adjusted to be the melting point of the resin mixture +0°C to the melting point +20°C, and even more preferably adjusted to be the melting point of the resin mixture +0°C to the melting point +15°C. This configuration suppresses cell rupture during foaming, and makes it possible to obtain extruded polypropylene resin foamed beads that exhibit excellent moldability. The lower the temperature of the composition immediately after entering the die, the higher the viscosity of the composition and the higher the pressure of the composition at the inlet of the die.

[0107] Methods for adjusting the temperature of the composition immediately after entering the die to within the range of the melting point of the resin mixture +0°C to the melting point +25°C include methods for adjusting various conditions that may affect the temperature. Such various conditions include (a) the set temperature of the members before the die (extruder, cooling section, transport section), (b) the set temperature of the die, (c) the length of the die in the extrusion direction (sometimes referred to as the thickness of the die), (d) the size of the face surface of the die (contact area with the air), and (e) the temperature of the air (the area from which the composition is extruded).

[0108] The temperature of the composition immediately after entering the die can be measured by (a) a thermometer installed near the inlet of the die, or (b) a thermometer installed in contact with the composition immediately before entering the die, for example, near the outlet of the transport section. The temperature of the composition may change while the composition passes through the die. The thermometer for measuring the temperature of the composition immediately after entering the die is preferably installed, for example, within 20 mm upstream or downstream from the inlet of the die along the extrusion direction.

[0109] In the granulation step, the temperature of the region where the composition is extruded is not particularly limited, but since extruded foamed beads with few mutual adhesions are easily obtained, the temperature is preferably 20° C. to 90° C., more preferably 20° C. to 75° C., even more preferably 20° C. to 65° C., and particularly preferably 20° C. to 50° C. In this specification, the temperature of the region refers to, for example, the temperature inside a cutter case that contains the produced extruded foamed beads.

[0110] [3. Extruded polypropylene resin particles] In this specification, "extruded polypropylene resin foamed beads obtained by the present production method" may also be referred to as "the present extruded foamed beads".

[0111] (Moldability) The extruded foamed particles have the advantage of excellent moldability, specifically, that the internal pressure 60 minutes after inclusion in the internal pressure decay test is high (e.g., 1.30 MPa or more). Note that "inclusion" refers to increasing the pressure inside the extruded foamed particles by placing the extruded foamed particles in a sealed container and holding them under pressure for a certain period of time.

[0112] The internal pressure decay test is performed by measuring the internal pressure 60 minutes after the initiation of the test. (1) 5.0 g of extruded foam particles are placed in a closed container (internal volume 1 m 3 (1) The extruded beads are placed in a container and pressurized to a gauge pressure of 0.32 MPa with air; (2) The pressure inside the extruded beads is increased by holding the container for 18 hours; (3) The extruded beads are removed from the container and the pressure inside the extruded beads is measured after 60 minutes at 25°C. The pressure inside the extruded beads can be calculated by measuring the weight a (g) of the air inside the extruded beads and using the following formula:

[0113] 1 + a / 28.8 x 0.082 x temperature (K) x 1000 / 5.0 x density of base resin of extruded foam particles (g / cm 3 ) / Expansion ratio of extruded foam particles The "weight a of air inside the extruded foam beads" can be calculated from the difference between the weight of the extruded foam beads left to stand at normal pressure and temperature for at least one day and the weight of the extruded foam beads 60 minutes after inclusion in the internal pressure decay test. The "density of the base resin of the extruded foam beads" and the "expansion ratio of the extruded foam beads" are calculated by the method described in the section (expansion ratio) below. The higher the pressure inside the extruded foam beads after 60 minutes, the better the moldability.

[0114] The internal pressure of the present extruded foam particles 60 minutes after inclusion is preferably 1.30 MPa or more, more preferably 1.35 MPa or more, and even more preferably 1.45 MPa or more. The extruded foam particles having an internal pressure of 1.30 MPa or more 60 minutes after inclusion have excellent moldability, high expansion ratio, and excellent fusion properties. Therefore, by molding the present extruded foam particles (for example, in-mold foam molding), a foam molded product having excellent compression strength and excellent fusion properties can be obtained.

[0115] (Expansion Ratio) The present extruded foamed beads can exhibit a high expansion ratio, preferably 9 to 45. The expansion ratio of the present extruded foamed beads is more preferably 9 to 30, and further preferably 10 to 25. The above-mentioned configuration has the advantage that the polypropylene resin in-mold foamed article obtained by using the extruded foamed beads exhibits characteristics such as arbitrary shape, cushioning properties, light weight, and heat insulation properties.

[0116] In this specification, the expansion ratio of the extruded polypropylene resin foamed beads is calculated by the following method: (1) The weight w (g) of the extruded foamed beads is measured; (2) Next, the extruded foamed beads used for the weight measurement are submerged in ethanol contained in a graduated cylinder, and the volume v (cm) of the extruded foamed beads is calculated based on the rise in the liquid level position of the graduated cylinder. 3 (3) measure the weight w (g) and the volume v (cm 3 ) to calculate the density ρ1 of the extruded foamed beads; (4) By carrying out the same operations as (1) to (3) except that a base resin is used instead of the extruded foamed beads, the density ρ2 of the base resin is calculated; (5) The density ρ2 of the base resin of the extruded foamed beads is divided by the density ρ1 of the extruded foamed beads (ρ2 / ρ1), and the obtained value is the expansion ratio.

[0117] In this specification, the base resin can be said to be a resin component that essentially constitutes the extruded foamed beads. The density of the base resin of the extruded foamed beads does not substantially change even when the extruded foamed beads are melted under reduced pressure and returned to a resin mass. Therefore, the density of the resin mass obtained by melting the extruded foamed beads under reduced pressure can be regarded as the density of the base resin of the extruded foamed beads. In this specification, melting the extruded foamed beads under reduced pressure to obtain a resin mass is sometimes referred to as "resin return", and the resin mass obtained by resin return is sometimes referred to as "returned resin".

[0118] The specific method for resin return is not particularly limited, and examples include a method of performing the following (b1) to (b5) in order: (b1) placing the extruded foam particles in a dryer adjusted to a temperature of the melting point of the linear polypropylene resin + 10°C; (b2) then using a vacuum pump to reduce the pressure inside the dryer to -0.05 MPa (gauge pressure) to -0.10 MPa (gauge pressure) over a period of 5 to 10 minutes; (b3) then leaving the extruded foam particles in the dryer for 30 minutes to prepare a resin lump (returned resin); (b4) then cooling the temperature inside the dryer to room temperature, and then returning the pressure inside the dryer to normal pressure; (b5) then removing the resin lump from the dryer. EXAMPLES

[0119] Hereinafter, one embodiment of the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples in any way.

[0120] (Measurement and Evaluation Methods) [Melting point] The melting point of the resin mixture was measured by the DSC method according to the following method: (1) the temperature of 5 to 6 mg of the resin mixture was raised from 40°C to 220°C at a heating rate of 10°C / min to melt the resin mixture; (2) the temperature of the molten resin mixture was then lowered from 220°C to 40°C at a heating rate of 10°C / min to crystallize the resin mixture; (3) the temperature of the crystallized resin mixture was then further 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 resin mixture obtained at the second heating (i.e., at the time of (3)) was taken as the melting point of the resin mixture. Note that, when there are multiple peaks (melting peaks) in the DSC curve of the resin mixture obtained at the second heating by the above-mentioned method, the temperature of the peak (melting peak) with the maximum heat of fusion was taken as the melting point of the resin mixture. The differential scanning calorimeter used was a DSC6200 model manufactured by Seiko Instruments Inc.

[0121] [MFR of branched polypropylene resin] The MFR of the branched polypropylene resin was measured using an MFR measuring device specified in JIS K7210 under the following conditions: orifice diameter of 2.0959±0.0050 mmφ, orifice length of 8.000±0.025 mm, and load of 2160 g and 230±0.2°C.

[0122] [Expansion ratio] The expansion ratio of the extruded polypropylene resin foam particles was calculated by the following method: (1) The weight w (g) of the extruded foam particles was measured; (2) Next, the extruded foam particles used for the weight measurement were submerged in ethanol contained in a measuring cylinder, and the volume v (cm) of the extruded foam particles was calculated based on the rise in the liquid level position of the measuring cylinder. 3 (3) The weight w (g) and the volume v (cm 3 ) to calculate the density ρ1 of the extruded foamed beads; (4) The density ρ2 of the base resin was calculated by carrying out the same operations as (1) to (3) except that the base resin (returned resin) was used instead of the extruded foamed beads; (5) The density ρ2 of the base resin of the extruded foamed beads was divided by the density ρ1 of the extruded foamed beads (ρ2 / ρ1) to obtain the expansion ratio.

[0123] In the examples and comparative examples, the density of the resin mass obtained by resin-returning the extruded foam particles was regarded as the density of the base resin. The following (b1) to (b5) were carried out in order, and the obtained resin mass was used as the resin-returned mass of the extruded foam particles: (b1) the extruded foam particles were placed in a dryer adjusted to a temperature of 160°C; (b2) the pressure in the dryer was then reduced to -0.05MPa (gauge pressure) to -0.10MPa (gauge pressure) using a vacuum pump over 5 to 10 minutes; (b3) the extruded foam particles were then left in the dryer for 30 minutes to prepare a resin mass (returned resin); (b4) the temperature in the dryer was then cooled to room temperature, and the pressure in the dryer was then returned to normal pressure; (b5) the resin mass was then taken out of the dryer.

[0124] [loss tangent tanδ] The loss tangent tanδ (loss modulus G'' / storage modulus G') of branched polypropylene resin was measured by vibration experiment as follows: (1) A sample resin (branched polypropylene resin) for measurement was sandwiched between two parallel plates (gap 1 mm); (2) Periodic strain was applied to the sample resin between the plates by driving one of the plates; (3) The storage modulus G', loss modulus G'', and loss tangent tanδ were obtained from the shear stress waveforms and their phase difference as a response to the applied strain. The vibration experiment was performed using a rotational rheometer (TA Instruments, product name: ARES) under nitrogen atmosphere at a measurement temperature of 200°C and a measurement angular frequency of 0.1 rad / s.

[0125] [Melt tension] The melt tension of the branched polypropylene resins used in the examples and comparative examples was measured using a Capillograph 1D (manufactured by Toyo Seiki Seisakusho, Japan). Specifically, the steps were as follows (1) to (5): (1) A barrel with a diameter of 9.55 mm heated to 200°C was filled with the branched polypropylene resin used in the examples and comparative examples; (2) The branched polypropylene resin was then heated in the barrel heated to 200°C for 10 minutes; (3) The branched polypropylene resin was then discharged in a string-like form from a capillary die (diameter 1.0 mm, length 10 mm) at a piston descending speed (10 mm / min) that was kept constant, and the string-like material was passed through a tension detection pulley located 350 mm below the capillary die, after which winding using a winding roll was started; (4) After the take-up of the string-like material was stabilized, the winding speed of the string-like material was increased at a constant rate from an initial speed of 1.0 m / min until it reached a speed of 200 m / min in 4 minutes; (5) The load applied to the pulley with the load cell when the string-like material broke was measured as the melt tension.

[0126] [Moldability] The moldability was evaluated by measuring the internal pressure of the extruded foam particles 60 minutes after incorporation in an internal pressure decay test by the following method: (1) 5.0 g of the extruded foam particles were placed in a closed container (internal volume 1 m 3(2) The extruded beads were held in the container for 18 hours to increase the pressure inside the extruded beads; (3) The extruded beads were taken out of the container and the pressure inside the extruded beads was measured after 60 minutes at 25° C. If the internal pressure of the extruded beads 60 minutes after composition was 1.30 MPa or more, the moldability was evaluated as good, and if it was less than 1.30 MPa, the moldability was evaluated as poor.

[0127] (manufacturing equipment) In the following examples and comparative examples, a manufacturing apparatus used for manufacturing polypropylene-based resin extruded foam particles was an apparatus in which an extruder, a first conveying section, a cooling section, a second conveying section, and a granulating section equipped with a die were connected in series in this order. A twin-screw extruder was used as the extruder. Gear pumps were used as the first and second conveying sections. A static mixer was used as the cooling section. As the die, a die was used in which the diameter of the resin passage was 10 mm, the shape of the hole was a perfect circle, the diameter of the hole was 0.7 mm, the length of the land section having a cylindrical flow path was 1.6 mm or 2.1 mm, and the number of holes was 1. As the granulating section, a granulating section equipped with a cutter and a cutter case was used.

[0128] (Polypropylene resin) Resins 1 to 3 were prepared as polypropylene resins.

[0129] Resin 1 (branched polypropylene resin) was produced by carrying out the following steps (1) to (5) in order: (1) 100 parts by weight of random polypropylene resin (F-724NPC, manufactured by Prime Polymer Co., Ltd.) and 1.4 parts by weight of a radical polymerization initiator were fed to a twin-screw extruder having a shaft diameter of φ45 mm at 70 kg / h; (2) 0.56 parts by weight of isoprene was fed as a conjugated diene compound relative to 100 parts by weight of the random polypropylene resin from an inlet section provided midway through the twin-screw extruder; (3) The mixture in the twin-screw extruder was melt-kneaded at a cylinder temperature of 200°C and a screw rotation speed of 230 rpm; (4) The molten mixture was extruded from a die provided at the end of the twin-screw extruder, and the extruded molten mixture (strands) was water-cooled in a water tank; (5) The strands were chopped by a pelletizer provided at the end of the water tank to obtain Resin 1 (branched polypropylene resin). The MFR of the obtained resin 1 (branched polypropylene resin) was 7.2 g / 10 min, and the melt tension was 7.1 cN. The loss tangent tanδ was 5 or less. As the radical polymerization initiator, a mixture of 2,2-di-t-butylperoxybutane (DT50) and t-butylperoxyisopropyl monocarbonate (PBI) with a DT50 / PBI ratio of 1 / 2 was used.

[0130] Resin 2 (branched polypropylene resin) was produced in the same manner as Resin 1, except that the amount of radical polymerization initiator was 1.33 parts by weight and the amount of conjugated diene compound was 0.58 parts by weight relative to 100 parts by weight of random polypropylene resin. The MFR of the obtained Resin 2 was 7.0 g / 10 min, the loss tangent tanδ was 1.7, and the melt tension was 9.9 cN.

[0131] A random polypropylene resin (F-724NPC, manufactured by Prime Polymer Co., Ltd.) was used as Resin 3 (linear polypropylene resin). The MFR of Resin 3 was 7.0 g / 10 min, and the loss tangent tan δ was 6.8.

[0132] Example 1 The manufacturing apparatus used had a land length of 2.1 mm, and extruded foam particles were manufactured by the following method.

[0133] (Melting and kneading process) A resin mixture was prepared by blending 100 parts by weight of resin 1 (branched polypropylene resin) and 0.02 parts by weight of talc as a bubble nucleating agent. The melting point of the prepared resin mixture was measured by the above-mentioned method and found to be 145°C. Next, the resin mixture was fed from the raw material feeder to a twin-screw extruder at 0.350 kg / h, and melt-kneading of the resin mixture was started at a barrel temperature of 180°C and a screw rotation speed of 35 rpm. During the melt-kneading of the resin mixture, 5 parts by weight of carbon dioxide gas was pressed into the twin-screw extruder as a foaming agent relative to 100 parts by weight of the resin mixture to prepare a composition. The melt-kneaded composition was passed through a first conveying section (set temperature at the outlet of the first conveying section: 180°C), a cooling section (set temperature at the outlet of the cooling section: 148°C), and a second conveying section (set temperature at the outlet of the second conveying section: 146°C) in this order.

[0134] (granulation process) The composition that passed through the second conveying section was allowed to enter the die with a set temperature of 142°C. The temperature of the composition immediately after entering the die was 146°C, and the melting point of the resin mixture was 145°C + 1°C. At the inlet of the die, the saturation pressure of carbon dioxide was 6.0MPa, and the pressure of the composition was 6.7MPa. The composition was extruded through the land section at a discharge rate of 0.350kg / hr from the hole. The extruded composition was cut with a cutter (rotation speed 175 rpm) in air (temperature 35°C). The cut composition was foamed and cooled to obtain extruded polypropylene resin beads. The obtained extruded polypropylene resin beads were measured for the expansion ratio and the internal pressure value after 60 minutes of exposure. The manufacturing conditions and the measurement results are shown in Table 1.

[0135] Example 2 Except for changing the feeding rate of the resin mixture in the raw material supply device to 0.675 kg / hr, the screw rotation speed to 68 rpm, the cutter rotation speed to 338 rpm, and the discharge rate from the hole to 0.675 kg / hr, polypropylene resin extruded foamed beads were obtained in the same manner as in Example 1. At the inlet of the die, the saturation pressure of the carbon dioxide gas was 6.0 MPa, and the pressure of the composition was 8.8 MPa.

[0136] Example 3 Except for using a production apparatus with a land length of 1.6 mm and using Resin 2 as the branched polypropylene resin, the same production method as in Example 2 was used to obtain extruded polypropylene resin foamed particles. At the inlet of the die, the saturation pressure of the carbon dioxide gas was 6.0 MPa, and the pressure of the composition was 6.6 MPa.

[0137] Example 4 Except for changing the feeding rate of the resin mixture in the raw material supply device to 1.000 kg / hr, the screw rotation speed to 100 rpm, the cutter rotation speed to 500 rpm, and the discharge rate from the hole to 1.000 kg / hr, polypropylene resin extruded foamed particles were obtained in the same manner as in Example 3. At the inlet of the die, the saturation pressure of the carbon dioxide gas was 6.0 MPa, and the pressure of the composition was 8.1 MPa.

[0138] Example 5 Except for changing the feeding rate of the resin mixture in the raw material supply device to 1,500 kg / hr, the screw rotation speed to 150 rpm, the cutter rotation speed to 750 rpm, and the discharge rate from the hole to 1,500 kg / hr, polypropylene resin extruded foamed particles were obtained in the same manner as in Example 3. At the inlet of the die, the saturation pressure of carbon dioxide gas was 6.0 MPa, and the pressure of the composition was 9.5 MPa.

[0139] Comparative Example 1 Except for changing the feeding rate of the resin mixture in the raw material supply device to 1.000 kg / hr, the screw rotation speed to 100 rpm, the cutter rotation speed to 500 rpm, and the discharge rate from the hole to 1.000 kg / hr, polypropylene resin extruded foamed beads were obtained in the same manner as in Example 1. At the inlet of the die, the saturation pressure of the carbon dioxide gas was 6.0 MPa, and the pressure of the composition was 10.3 MPa.

[0140] Comparative Example 2 Except for changing the feeding rate of the resin mixture in the raw material supply device to 2,000 kg / hr, the screw rotation speed to 200 rpm, the cutter rotation speed to 1,000 rpm, and the discharge rate from the hole to 2,000 kg / hr, polypropylene resin extruded foamed particles were obtained in the same manner as in Example 3. At the inlet of the die, the saturation pressure of the carbon dioxide gas was 6.0 MPa, and the pressure of the composition was 10.9 MPa.

[0141] Comparative Example 3 The melt kneading process and the granulation process were carried out in the same manner as in Example 4, except that Resin 3 (linear polypropylene resin) was used instead of Resin 2. The composition shrunk immediately after being discharged from the die hole, and extruded foam particles could not be obtained. In addition, the pressure in the device was not stable, and the initial pressure of the composition at the inlet of the die was 9.8 MPa, but increased over time. In addition, because the pressure was unstable, the composition could not be cooled to the same temperature as in Example 4 in the cooling section, and the temperature at the outlet of the cooling section was 152°C, and the temperature of the composition immediately after entering the die with a set temperature of 147°C was 152°C.

[0142] [Table 1]

[0143] From Table 1, the extruded polypropylene resin beads of Examples 1 to 5 produced by this production method showed good expansion ratios, and the internal pressure after 60 minutes of composition was 1.30 MPa or more, and they were excellent in moldability. Therefore, it was found that this production method can provide extruded polypropylene resin beads with excellent expansion ratios and moldability even under high production conditions in which the discharge amount per hole is 0.350 kg / hr or more in the hot cut method. In contrast, the extruded polypropylene resin beads of Comparative Example 1 and Comparative Example 2, in which the pressure of the composition at the inlet of the die exceeds the saturated pressure of carbon dioxide gas + 4 MPa, had an internal pressure of less than 1.30 MPa after 60 minutes of composition, and were poor in moldability. In addition, in Comparative Example 3, in which a linear polypropylene resin was used instead of a branched polypropylene resin as a raw material, extruded polypropylene resin beads could not be produced. [Industrial Applicability]

[0144] According to one embodiment of the present invention, polypropylene resin extruded foam particles having excellent moldability can be provided with high productivity. Therefore, one embodiment of the present invention can be suitably used to obtain polypropylene resin in-mold foamed molded articles having good physical properties such as arbitrary shape, cushioning properties, light weight, compressive strength, and heat insulation properties. Therefore, one embodiment of the present invention can be suitably used in the fields of automobile interior parts, cushioning materials, packaging materials, heat insulation materials, etc. [Explanation of symbols]

[0145] 10 Die 11 Face surface 12 holes 13 Resin passage 14 Flow path in land section 15 Land Department a Land length b Hole diameter

Claims

1. A melt-kneading step of melt-kneading a composition containing a resin mixture containing a polypropylene-based resin having a branched structure and carbon dioxide gas as a foaming agent; A granulation step of granulating the polypropylene-based resin extruded particles by a hot cut method, The hot cutting method is a process of extruding the composition into air through holes in a die, cutting the composition extruded from the holes in the die with a cutter in the air, and cooling the cut composition in the air while foaming or after foaming, The discharge rate of the composition per hole of the die is 0.350 kg / hr or more, The pressure of the composition at the inlet of the die is greater than the saturation pressure of the carbon dioxide gas and is equal to or less than the saturation pressure of the carbon dioxide gas + 4 MPa, the die hole has a cylindrical flow passage; The method for producing extruded polypropylene resin foam particles, wherein the length of the flow path is 0.5 mm to 5.5 mm.

2. The method for producing extruded polypropylene resin foam beads according to claim 1, wherein the diameter of the hole in the die is 0.5 mm to 2.0 mm.

3. 3. The method for producing extruded polypropylene resin foam beads according to claim 1, wherein the temperature of the composition immediately after entering the die is from a melting point of the resin mixture + 0°C to the melting point + 25°C.

4. The method for producing extruded polypropylene resin foam beads according to any one of claims 1 to 3, wherein the expansion ratio of the extruded polypropylene resin foam beads is 9 to 45 times.

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