WO2023176911A1 - Method for producing polypropylene resin extruded foam particles - Google Patents

Abstract

The purpose of the present invention is to provide a method for producing polypropylene resin extruded foam particles having a low open-cell rate. Provided is a method for producing polypropylene resin extruded foam particles, the method comprising: a melt-kneading step for melt-kneading a polypropylene resin and a foaming agent in a melt-kneading part having a plurality of screws; and an extrusion foaming step for discharging the composition obtained in the melt-kneading step from a die, wherein a ratio (L/D) between the screw length L and the screw diameter D is 20-24.

Description

Method for producing polypropylene resin extruded foam particles

The present invention relates to a method for producing extruded polypropylene resin foam particles.

Polypropylene resin in-mold foamed molded products obtained using polypropylene resin foam particles have the advantages of in-mold foamed molded products such as excellent shape flexibility, cushioning properties, lightness, and heat insulation properties. .

Examples of methods for producing expanded polypropylene resin particles include a batch foaming method that is a discontinuous process, and an extrusion foaming method that is a continuous process (for example, Patent Documents 1 and 2). The extrusion foaming method has many advantages in terms of efficiency and environment.

Japanese Patent Application Publication No. 2019-038967 International Publication No. 2021/106795

However, the above-mentioned conventional techniques are not sufficient from the viewpoint of the closed cell ratio of expanded particles, and there is room for further improvement.

One embodiment of the present invention has been made in view of the above problems, and its purpose is to provide a new manufacturing method capable of providing extruded polypropylene resin foam particles having a low open cell ratio.

The present inventor has completed the present invention as a result of intensive research to solve the above problems.

A method for producing extruded polypropylene resin foam particles according to an embodiment of the present invention uses a production apparatus equipped with a melt-kneading section having a plurality of screws and a granulation section having a die. A melt-kneading step of melt-kneading a resin and a foaming agent in the melt-kneading section, and an extrusion foaming step of discharging the composition obtained in the melt-kneading step through the die into an area whose pressure is lower than the internal pressure of the manufacturing device. In the melt-kneading section, the ratio (L/D) of the effective length L of the screw to the screw diameter D is 20 to 24.

According to one embodiment of the present invention, it is possible to provide a novel manufacturing method capable of providing extruded polypropylene resin foam particles having a low open cell ratio.

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 changes can be made within the scope of the claims. Furthermore, embodiments or examples obtained by combining 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. Note that all academic literature and patent literature described in this specification are incorporated as references in this specification. In addition, unless otherwise specified in this specification, the numerical range "A to B" is intended to be "above A (including A and greater than A) and less than or equal to B (including B and less than B)."

In this specification, a "constituent unit derived from an X monomer" contained in a polymer, copolymer, or resin may be referred to as an "X unit."

[1. Technical idea of one embodiment of the present invention]
When the present inventor studied methods for manufacturing polypropylene resin foam particles using conventionally known extrusion foaming methods, the present inventor found that the polypropylene resin foam particles obtained by these methods have a low closed cell ratio. We independently obtained new knowledge.

The present inventor conducted extensive research into the reason why the closed cell ratio of extruded polypropylene resin foam particles is low. First, the present inventor attempted to observe changes in resin due to extrusion foaming. Normally, in the extrusion foaming process, the composition extruded from the extruder into a region with a lower pressure than the inside of the extruder immediately begins to foam, so it is not possible to take out the resin during the extrusion foaming process and analyze it. Therefore, the present inventors returned the obtained extruded foamed polypropylene resin particles to the resin by subjecting them to heat treatment under reduced pressure, and analyzed the obtained resin. The physical properties of the resin thus obtained after extrusion foaming were compared with the physical properties of the polypropylene resin used as the material for the extruded polypropylene resin foam particles. As a result, the present inventor surprisingly found that the melt tension of the resin obtained by returning the extruded polypropylene resin foam particles to the resin is significantly lower than the melt tension of the polypropylene resin before extrusion foaming. We have independently obtained new knowledge that the resin obtained by returning extruded foam particles to resin deteriorates during the extrusion foaming process.

There are two steps in the method for producing extruded polypropylene resin foam particles: a step of melt-kneading the raw material polypropylene resin and a foaming agent, and a step of melt-kneading the resulting composition (melt-kneaded product) in an area of lower pressure than the inside of the extruder. This is a step of extruding and foaming. The present inventor assumed that resin deterioration occurred during the melt-kneading process. However, melt-kneading of polypropylene resin is also performed in the process of producing polypropylene resin particles by a batch foaming method (for example, depressurized foaming method), which is a discontinuous process. Conventionally, there is no knowledge that polypropylene resin deteriorates significantly during the process of manufacturing polypropylene resin particles.

Here, in order to produce polypropylene resin foam particles with good moldability by an extrusion foaming method, in the extrusion foaming method, a polypropylene resin having a high melt tension, such as a polypropylene resin having a branched structure, is used as a raw material. There is. Surprisingly, the present inventor found that polypropylene resins that do not have a branched structure do not deteriorate when melt-kneaded, or only slightly deteriorate when melt-kneaded, but polypropylene resins that have a branched structure do not deteriorate when melt-kneaded. We independently obtained new knowledge that the resin deteriorates significantly. In other words, the knowledge that polypropylene resins having a branched structure are significantly degraded by melt-kneading is the knowledge obtained from (a) techniques using polypropylene resins that do not have a branched structure (for example, depressurized foaming techniques); This finding was first obtained through the ingenuity of (b) returning the obtained extruded polypropylene resin foam particles to the resin.

Based on this new knowledge, the present inventor further conducted intensive studies in order to obtain extruded polypropylene resin foam particles having a low open cell ratio. As a result, the present inventor independently discovered the following findings and completed the present invention: By setting the length of the screw in the melt-kneading section of a twin-screw extruder within a specific range, a surprising In particular, it is possible to obtain extruded polypropylene resin foam particles with a low open cell ratio.

[2. Method for producing polypropylene resin with branched structure]
A method for producing foamed polypropylene particles having a branched structure according to an embodiment of the present invention is a method for producing a polypropylene resin having a branched structure, and includes a melt kneading section having a plurality of screws and a granulation part having a die. a melt-kneading step of melt-kneading a polypropylene resin having a branched structure and a blowing agent in the melt-kneading section using a manufacturing apparatus comprising a part; an extrusion foaming step of discharging into a region whose pressure is lower than the internal pressure of the manufacturing device, and in the melt-kneading section, the ratio (L/D) of the effective length L of the screw to the screw diameter D is 20 to 24. be.

In this specification, the "effective length L of the screw" refers to the length (mm) from the upstream end in the extrusion direction of the screw element that has the function of melting the polypropylene resin to the downstream end of the screw. Moreover, in this specification, "screw diameter D" intends the diameter (mm) of the circumscribed circle in the cross section perpendicular to the extrusion direction of the screw element.

In this specification, "polypropylene resin having a branched structure" may be referred to as "branched polypropylene resin", "polypropylene resin extruded foam particles" may be referred to as "extruded foam particles", and "polypropylene resin extruded foam particles" may be referred to as "polypropylene foam particles". "Method for producing extruded polypropylene resin foam particles according to an embodiment of the present invention" may be referred to as "the production method", and "method for producing extruded polypropylene resin foam particles according to an embodiment of the present invention" may be referred to as "the present production method". be. The extruded foam particles obtained by this production method can be made into a polypropylene resin foam molded article by molding the extruded foam particles (for example, in-mold foam molding). In this specification, a "polypropylene resin foam molded product" may be referred to as a "foamed molded product".

Since the present manufacturing method has the above-described configuration, it has the advantage that there is little deterioration of the resin, and as a result, it is possible to provide extruded foam particles having a low open cell ratio. As mentioned above, the deterioration of the resin can be determined by comparing the physical properties of the resin obtained by returning the extruded foamed particles to the resin and the physical properties of the branched polypropylene resin that is the raw material for the extruded foamed particles. It can be evaluated. Here, the term "resin return" refers to melting the extruded foam particles under reduced pressure to obtain a resin mass. A resin mass obtained by returning the resin is sometimes referred to as a "restored resin."

The specific method for returning the resin is not particularly limited, but examples include a method in which the following steps (b1) to (b5) are carried out in order: (b1) Melting point of the raw material polypropylene resin (for example, branched polypropylene resin) Place the expanded particles into a dryer adjusted to +10°C; (b2) Then, over a period of 5 to 10 minutes, use a vacuum pump to reduce the pressure inside the dryer to -0.05 MPa (gauge pressure). - Reduce the pressure to 0.10 MPa (gauge pressure); (b3) Then, leave the extruded foam particles in the dryer for 30 minutes to prepare a resin mass (reconstituted resin); (b4) Next, After cooling the temperature to room temperature, the pressure inside the dryer is returned to normal pressure; (b5) After that, the resin mass is taken out from the dryer.

It can also be said that extruded foamed particles having a low open cell ratio are extruded foamed particles with excellent moldability. That is, since the present manufacturing method has the above-described configuration, it also has the advantage of being able to provide extruded foam particles with excellent moldability.

First, the raw materials (components) used in this manufacturing method will be explained, and then each step will be explained.

(2-1. Resin mixture)
In this manufacturing method, raw materials (components) other than the blowing agent may be referred to as a "resin mixture." The resin mixture contains a polypropylene resin having a branched structure, and may optionally contain additives such as a cell nucleating agent.

In this specification, "polypropylene resin having a branched structure" refers to (a) a polypropylene resin in which molecules of a polypropylene resin into which no branched structure has been introduced are partially crosslinked, and (b) A polypropylene resin in which a diene compound other than (poly)propylene or the like is introduced as a branched chain is intended as opposed to a polypropylene resin in which a branched structure is not introduced. In this specification, a "polypropylene resin into which a branched structure has not been introduced" may be referred to as a "linear polypropylene resin." Moreover, in this specification, "linear polypropylene resin" and "branched polypropylene resin" may be collectively referred to as "polypropylene resin." The linear polypropylene resin can also be said to be a raw material for branched polypropylene resin.

In this specification, a "constituent unit derived from a propylene monomer" may be referred to as a "propylene unit."

As used herein, the term "polypropylene resin" refers to a resin containing 50 mol% or more of propylene units out of 100 mol% of all structural units constituting the resin.

(branched polypropylene resin)
In branched polypropylene resins, the structure derived from the raw material linear polypropylene resin is also referred to as the "main chain." Therefore, by referring to each aspect regarding the main chain of the branched polypropylene resin detailed in this section (for example, the structural units included in the main chain and the bonding order of the structural units), the branched polypropylene resin Each aspect regarding the structure and/or composition of the linear polypropylene resin that is the raw material can be explained.

The main chain of the branched polypropylene resin may be (a) a homopolymer of propylene, or (b) a block copolymer, alternating copolymer, or random copolymer of propylene and a monomer other than propylene. It may be a combination or graft copolymer, or (c) a mixture of two or more thereof.

The main chain of polypropylene resins with a branched structure consists of a propylene homopolymer, a block copolymer of propylene and a monomer other than propylene, and a random copolymer of propylene and a monomer other than propylene. Preferably, it is one or more selected from the group.

In addition to propylene units, the main chain of the branched polypropylene resin has one or more structural units (sometimes referred to as "structural units") derived from monomers other than propylene monomers. It is also possible to have one or more types. In this specification, monomers other than propylene monomers contained in the main chain of branched polypropylene resin may be referred to as "comonomers", and "propylene monomers contained in the main chain of branched polypropylene resin "Constituent units derived from monomers other than monomers" are sometimes referred to as "comonomer units."

Comonomers include the following monomers: (a) ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, α-olefins having 2 or 4 to 12 carbon atoms such as 3,4-dimethyl-1-butene, 1-heptene, 3-methyl-1-hexene, 1-octene, 1-decene, (b) cyclopentene, norbornene, Cyclic olefins such as tetracyclo[6,2,11,8,13,6]-4-dodecene, (c) 5-methylene-2-norbornene, 5-ethylidene-2-norbornene, 1,4-hexadiene, methyl- Dienes such as 1,4-hexadiene, 7-methyl-1,6-octadiene, and (d) vinyl chloride, vinylidene chloride, acrylonitrile, methacrylonitrile, vinyl acetate, acrylic acid, acrylic ester, methacrylic acid, methacrylic acid Ester, maleic acid, maleic anhydride, styrene monomers, vinyl monomers such as vinyltoluene, divinylbenzene, etc.

Examples of acrylic esters include methyl acrylate, ethyl acrylate, butyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, stearyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and Examples include glycidyl acrylate.

Examples of methacrylate esters include methyl methacrylate, ethyl methacrylate, butyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, stearyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, and Examples include glycidyl methacrylate.

Styrenic monomers include styrene, methylstyrene, dimethylstyrene, alpha-methylstyrene, para-methylstyrene, ethylstyrene, diethylstyrene, isopropylstyrene, t-butylstyrene, bromostyrene, dibromostyrene, tribromostyrene, and chlorostyrene. , dichlorostyrene and trichlorostyrene. The main chain of the branched polypropylene resin preferably has a structural unit derived from an α-olefin having 2 or 4 to 12 carbon atoms as a comonomer unit, and includes 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, etc. It is more preferable to have a structural unit derived from ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene and/or 4-methyl-1-pentene. It is more preferable to have a structural unit derived from ethylene, 1-butene, isobutene and/or 1-pentene, even more preferably to have a structural unit derived from ethylene and/or 1-butene. Particularly preferred. According to this configuration, since the branched polypropylene resin can have high melt tension and low gel fraction, it is possible to provide extruded polypropylene resin foam particles with a lower open cell ratio, and the extruded polypropylene resin has better moldability. It has the advantage of providing expanded particles.

The main chain of the branched polypropylene resin is preferably a propylene homopolymer, a polypropylene block copolymer, a polypropylene alternating copolymer, and/or a polypropylene random copolymer; More preferably, it is a polypropylene random copolymer. According to this configuration, since the branched polypropylene resin can have high melt tension and low gel fraction, it is possible to provide extruded polypropylene resin foam particles with a lower open cell ratio, and the extruded polypropylene resin has better moldability. It has the advantage of providing expanded particles.

The main chain of the branched polypropylene resin preferably contains 90 mol% or more of propylene units, more preferably 93 mol% or more, and 95 mol% of the total structural units contained in the polypropylene resin. It is more preferable to contain at least 97 mol%, particularly preferably at least 97 mol%. This configuration has the advantage that the branched polypropylene resin can have high melt tension and low gel fraction.

The melting point of the branched polypropylene resin is not particularly limited. The melting point of the polypropylene resin is, for example, preferably 130°C to 165°C, more preferably 135°C to 164°C, even more preferably 138°C to 163°C, and even more preferably 140°C to 162°C. It is particularly preferable that there be. When the melting point of the branched polypropylene resin is (a) 130°C or higher, there is no risk that the dimensional stability of the foamed molded product will decrease, there is no risk that the heat resistance of the foamed molded product will be insufficient, and It has the advantage that the compressive strength of the molded product tends to be strong, and (b) when the temperature is 165°C or lower, it is possible to mold extruded foam particles with a relatively low vapor pressure, so polypropylene resin foaming It has the advantage that extruded foam particles can be formed using a general-purpose particle forming machine.

In this specification, the melting point of the branched polypropylene resin is measured by differential scanning calorimetry (hereinafter referred to as "DSC method"). The specific operating procedure is as follows: (1) The branched polypropylene resin is heated by increasing the temperature of 5 to 6 mg of the branched polypropylene resin from 40°C to 220°C at a heating rate of 10°C/min. (2) Then, the temperature of the melted branched polypropylene resin is lowered from 220°C to 40°C at a cooling rate of 10°C/min to crystallize the branched polypropylene resin. (3) Thereafter, the temperature of the crystallized branched polypropylene resin is further increased from 40°C to 220°C at a rate of 10°C/min. The temperature of the peak (melting peak) of the DSC curve of the branched polypropylene resin obtained during the second temperature increase (that is, at the time of (3)) can be determined as the melting point of the branched polypropylene resin. In addition, if there are multiple peaks (melting peaks) in the DSC curve of the branched polypropylene resin obtained during the second temperature increase by the above method, the temperature of the peak with the maximum heat of fusion (melting peak) is , is the melting point of the branched polypropylene resin. As the differential scanning calorimeter, for example, DSC6200 model manufactured by Seiko Instruments Inc. can be used.

The melt tension of the branched polypropylene resin is not particularly limited, but it can usually be higher than the melt tension of the raw material linear polypropylene resin. The melt tension of the branched polypropylene resin at 200° C. is preferably 8.0 cN or more, more preferably 9.0 cN or more, and even more preferably 10.0 cN or more. By using a branched polypropylene resin having a high melt tension as a raw material, it is possible to widen the molding width of the obtained extruded polypropylene resin foam particles.

The method for measuring melt tension in this specification will be explained below. In this specification, melt tension is measured using Capillograph 1D (manufactured by Toyo Seiki Seisakusho Co., Ltd., Japan). Specifically, the following are (1) to (5): (1) Sample resin for measurement (branched polypropylene resin) is placed in a barrel with a diameter of 9.55 mm heated to the test temperature (200°C). (2) Then, the sample resin is heated for 10 minutes in a barrel heated to the test temperature (200 ° C.); (3) Then, from a capillary die (caliber 1.0 mm, length 10 mm), At a constant piston descending speed (10 mm/min), the sample resin is drawn out in a string, passed through a tension detection pulley located 350 mm below the capillary die, and then wound up. Start winding using a roll; (4) After the string-like material is taken up stably, the string-like material is wound at a constant speed from an initial speed of 1.0 m/min until it reaches a speed of 200 m/min in 4 minutes. (5) Measure the load applied to the pulley with a load cell when the string-like material breaks as the melt tension.

The melt flow rate (MFR) of the branched polypropylene resin at 230°C is not particularly limited. The MFR of the branched polypropylene resin at 230°C is, for example, preferably 0.5 g/10 minutes to 20.0 g/10 minutes, and preferably 1.0 g/10 minutes to 15.0 g/10 minutes. It is more preferable, and particularly preferably 2.0 g/10 minutes to 10.0 g/10 minutes. When the MFR at 230°C of the branched polypropylene resin is (a) 0.5 g/10 minutes or more, the resulting extruded foamed particles have little deformation and a foamed molded product with good (beautiful) surface properties. (b) If it is 20.0 g/10 minutes or less, the composition has an advantage that the foamability of the composition becomes good during extrusion foaming.

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

(Method for producing branched polypropylene resin)
A branched polypropylene resin can be obtained by introducing a branched structure into a linear polypropylene resin. Methods for introducing a branched structure into a linear polypropylene resin are not particularly limited, but examples include (a1) a method of irradiating a linear polypropylene resin with radiation, and (a2) a method of introducing a linear polypropylene resin and a conjugated diene compound. Examples include a method of melt-kneading a mixture containing a radical polymerization initiator and a radical polymerization initiator.

Specific examples of the method (a1) include the method described in Japanese Patent Application Publication No. 2002-542360.

The method (a2) above will be further explained. In the method (a2) above, for example, a branched polypropylene resin can be obtained by sequentially performing the following (i) to (iv): (i) a linear polypropylene resin, a conjugated diene compound, and a radical polymerization initiator; (ii) extrude the obtained melt-kneaded product from the die; (iii) cool the extruded melt-kneaded product (also referred to as strand); (iv) Shredding the strands simultaneously with or after cooling the strands. Specific examples of the method (a2) include the method described in WO2020/004429.

(i) It is possible to stably introduce a branched structure into a linear polypropylene resin and the reproducibility of introducing the branched structure is high, and/or (ii) it does not require complicated equipment and has high productivity. Since a branched polypropylene resin can be obtained, in one embodiment of the present invention, the branched polypropylene resin is preferably a branched polypropylene resin obtained by the method (a2) described above.

(conjugated diene compound)
Examples of the conjugated diene compound in method (a2) 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 (a) they are inexpensive and easy to handle, and (b) the reaction tends to proceed uniformly.

The amount of the conjugated diene compound used in the method (a2) is 0.30 parts by weight to 1.50 parts by weight, and 0.30 parts by weight to 0.80 parts by weight, based on 100 parts by weight of the linear polypropylene resin. parts by weight, and more preferably from 0.30 parts by weight to 0.60 parts by weight. When the amount of the conjugated diene compound used is 0.30 parts by weight or more per 100 parts by weight of the linear polypropylene resin, the degree of modification to the polypropylene resin (the number of crosslinks introduced into the linear polypropylene resin) ) becomes sufficient, and as a result, the melt tension of the branched polypropylene resin obtained can be sufficiently increased (for example, to 8.0 cN or more). When the amount of the conjugated diene compound used is 1.50 parts by weight or less based on 100 parts by weight of the linear polypropylene resin, the crosslinking between the polypropylene resins by the conjugated diene compound will not become excessive, so that the There is no risk of the viscosity of the branched polypropylene resin being increased. As a result, it becomes easy to obtain extruded polypropylene resin foam particles with a high magnification from the obtained branched polypropylene resin.

(radical polymerization initiator)
The radical polymerization initiator used in method (a2) is an organic peroxide having the ability to abstract hydrogen from a linear polypropylene resin and a conjugated diene compound. Examples of radical polymerization initiators suitably used in one embodiment of the present invention include organic polymerization initiators such as ketone peroxide, peroxyketal, hydroperoxide, dialkyl peroxide, diacyl peroxide, peroxydicarbonate, and peroxyester. Examples include oxides.

As the organic peroxide, one having particularly high hydrogen abstraction ability is preferable. Examples of organic peroxides with high hydrogen abstraction ability include 1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, n- Peroxyketals such as butyl 4,4-bis(t-butylperoxy)valerate, 2,2-bis(t-butylperoxy)butane; dicumyl peroxide, 2,5-dimethyl-2,5-dimethyl (t-butylperoxy)hexane, α,α'-bis(t-butylperoxy-m-isopropyl)benzene, t-butylcumyl peroxide, di-t-butylperoxide, 2,5-dimethyl-2 , 5-di(t-butylperoxy)-3-hexyne and other dialkyl peroxides; benzoyl peroxide and other diacyl peroxides; t-butyl peroxyoctate, t-butyl peroxyisobutyrate, t-butyl Peroxylaurate, t-butylperoxy 3,5,5-trimethylhexanoate, t-butylperoxyisopropyl carbonate, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, t-butyl Preferred examples include peroxyesters such as peroxyacetate, t-butylperoxybenzoate, and di-t-butylperoxyisophthalate. Among these, t-butylperoxyisopropyl carbonate, t-butylperoxybenzoate and 2,2-bis(t-butylperoxy)butane are preferred. These organic peroxides may be used alone or in combination of two or more.

The amount of the radical polymerization initiator used in the method (a2) is 0.50 to 2.00 parts by weight, and 0.60 to 1.80 parts by weight based on 100 parts by weight of the linear polypropylene resin. parts by weight are preferable, 0.70 parts by weight to 1.60 parts by weight are more preferable, 0.70 parts by weight to 1.50 parts by weight are more preferable, 0.70 parts by weight to 1.30 parts by weight are more preferable, It is more preferably 0.70 parts by weight to 1.10 parts by weight, even more preferably 0.75 parts by weight to 1.00 parts by weight, and particularly preferably 0.75 parts by weight to 0.90 parts by weight. If the amount of the radical polymerization initiator used is less than 0.50 parts by weight per 100 parts by weight of the linear polypropylene resin, the degree of modification of the linear polypropylene resin may be insufficient. As a result, when extrusion foaming is performed using the obtained branched polypropylene resin, the branched polypropylene resin does not exhibit sufficient strain hardening properties, and therefore only extruded foam particles with a high open cell ratio tend to be obtained. When the amount of the radical polymerization initiator used exceeds 2.00 parts by weight per 100 parts by weight of the linear polypropylene resin, hydrogen is obtained by increasing the abstraction of hydrogen from the linear polypropylene resin by the radical polymerization initiator. The amount of gel of the branched polypropylene resin can be increased. As a result, when extrusion foaming is performed using the branched polypropylene resin, there is a tendency that only extruded foam particles with a low expansion ratio and a high open cell ratio are obtained.

(Other resins and rubbers)
The resin mixture may further contain a resin other than the branched polypropylene resin (sometimes referred to as "other resin") and/or rubber, as long as the effects of the embodiment of the present invention are not impaired. good. Other resins and rubbers may be collectively referred to as "other resins, etc.". Other resins include (a) linear polypropylene resins such as ethylene/propylene random copolymers, ethylene/propylene block copolymers, ethylene/propylene alternating copolymers, and propylene homopolymers; 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 (c) styrenic resins such as polystyrene, styrene/maleic anhydride copolymers, and styrene/ethylene copolymers. Examples of the rubber include olefin rubbers such as ethylene/propylene rubber, ethylene/butene rubber, ethylene/hexene rubber, and ethylene/octene rubber.

The content of other resins in the resin mixture is, for example, preferably 60 parts by weight or less, more preferably 40 parts by weight or less, and 20 parts by weight or less based on 100 parts by weight of the resin mixture. It is even more preferable that the amount is 10 parts by weight or less. The lower limit of the content of other resins, etc. is not particularly limited, and may be, for example, 0 parts by weight based on 100 parts by weight of the resin mixture.

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

The amount of the cell nucleating agent used, in other words, the content of the cell nucleating agent in the resin mixture, is not particularly limited. The amount of the cell nucleating agent used is, for example, preferably 0.01 parts by weight to 5.00 parts by weight, and 0.01 parts by weight to 3.50 parts by weight, based on 100 parts by weight of the branched polypropylene resin. parts by weight, more preferably from 0.01 parts by weight to 1.00 parts by weight, and particularly preferably from 0.01 parts by weight to 0.50 parts by weight.

(Other ingredients)
The resin mixture may contain other ingredients as necessary (a) antioxidant, metal deactivator, phosphorus processing stabilizer, ultraviolet absorber, ultraviolet stabilizer, optical brightener, metal soap, and antacid adsorbent. and/or (b) additives such as crosslinkers, chain transfer agents, lubricants, plasticizers, fillers, reinforcements, flame retardants, colorants, and antistatic agents. You can stay there. These other components may be used alone or in combination of two or more. The amount of additives such as the above-mentioned antioxidants and metal deactivators used is not particularly limited. The amount of the additive used is, for example, preferably 0.01 parts by weight to 20.00 parts by weight, and 0.10 parts by weight to 5.00 parts by weight, based on 100 parts by weight of the branched polypropylene resin. It is more preferable that it is part.

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

The blowing agent that can be used in this manufacturing method is not particularly limited as long as it is a commonly used blowing agent used in extrusion foaming. Examples of the blowing agent include (a) (a-1) aliphatic hydrocarbons such as propane, n-butane, isobutane, n-pentane, isopentane, and hexane; (a-2) alicyclics such as cyclopentane and cyclobutane; (a-3) Ethers such as dimethyl ether, diethyl ether, methyl ethyl ether; (a-4) Fluorinated hydrocarbons such as difluoroethane; (a-5) Alcohols such as methanol and ethanol; (a-6) Inorganic gases such as air, nitrogen, and carbon dioxide; (a-7) Physical blowing agents such as water; and (b) Inorganic gases such as sodium bicarbonate, azodicarbonamide, and dinitrosopentamethylenetetramine. Examples include chemical foaming agents including pyrolytic foaming agents.

In this manufacturing method, as the blowing agent, inorganic gas is preferred, and carbon dioxide gas is more preferred, since the production cost and environmental impact are low. In addition, since the production cost and environmental burden are smaller, it is preferable to use only carbon dioxide gas as the blowing agent, and not substantially contain any blowing agent other than carbon dioxide gas, which is mentioned above as an example of the blowing agent. Specifically, it is preferable that the content of the blowing agent other than carbon dioxide, which is mentioned above as an example of the blowing agent, in the composition is 0.01 parts by weight or less, and 0.001 parts by weight or less, based on 100 parts by weight of the composition. It is more preferably at most 0.0001 parts by weight, even more preferably at most 0.0001 parts by weight, and particularly preferably at most 0 parts by weight.

The amount of the blowing agent used may be adjusted as appropriate depending on the type of blowing agent and the target expansion ratio of the foam. In this production method, the total amount of blowing agents used is preferably 1 to 20 parts by weight, more preferably 1 to 15 parts by weight, based on 100 parts by weight of the composition. , more preferably from 1 part by weight to 10 parts by weight, particularly preferably from 2 parts by weight to 10 parts by weight. The "amount of blowing agent used" can also be said to be the "content of the blowing agent in the composition."

(2-3. Manufacturing equipment)
The manufacturing apparatus used in this manufacturing method includes a melt-kneading section having a plurality of screws and a granulation section having a die. The manufacturing apparatus may further include a transport section and/or a cooling section. When the manufacturing apparatus includes a transport section and a cooling section, (a) the melt-kneading section, the transport section, the cooling section, and the granulation section are connected, and (b) from the upstream side to the downstream side in the extrusion direction of the composition. A melt-kneading section, a transport section, a cooling section, and a granulation section are arranged in this order. When the manufacturing apparatus includes a transport section and a cooling section, (a) the transport section and the cooling section may be provided with the order changed between the upstream side and the downstream side, and (b) the transport section is provided on the cooling section. It may be provided on both the upstream side and the downstream side. The transport section can be omitted if the pressure of the composition in the pipe from the melt-kneading section to the granulation section is sufficiently low. Further, if the temperature of the composition is sufficiently lowered at the outlet of the melt-kneading section, the cooling section may be omitted.

As the melt-kneading section having a plurality of screws, for example, a twin-screw extruder having two screws is preferable in order to have good mixing properties. Further, it is preferable that the melt-kneading section has a screw configuration so that the blowing agent press-injected into the melt-kneading section does not flow back to the upstream side. That is, as the melt-kneading section having a plurality of screws, a twin-screw extruder having a two-screw configuration with a backflow prevention function is more preferable.

As a more specific embodiment, the melt-kneading section includes a kneading disk, a notched screw, which has a forward shift angle (for example, 45 degrees) and/or a shift angle of 90 degrees and/or a reverse shift angle (for example, -45 degrees). It may have elements such as a forward-threaded flight screw and a counter-threaded flight screw. Kneading disks and notched screws having a forward shift angle (for example, 45 degrees) and/or a shift angle of 90 degrees and/or a reverse shift angle (for example, -45 degrees) can be said to be elements having a relatively strong kneading action. Elements with relatively strong kneading effects include rotors, gear mixing elements, pin elements, and the like. The normally threaded flight screw can be said to be an element with relatively weak kneading action. The counter-threaded flight screw can be said to be an element that has the function of securing a filling area. Such elements having the function of ensuring a filling area include kneading discs, torpedo rings, seal rings, etc. with a 90 degree offset angle and/or a reverse offset angle (for example -45 degrees).

Specific configurations of the melt-kneading section include the following configurations:
(1) A configuration in which a kneading disk, a forward flight screw, a kneading disk, a reverse flight screw, a forward flight screw, a kneading disk, a notch screw, and a forward flight screw are connected in this order;
(2) A configuration in which a kneading disk, a counter-thread flight screw, a normal-thread flight screw, a kneading disk, a notch screw, and a normal-thread flight screw are connected in this order;
(3) A configuration in which a kneading disk, a forward flight screw, a kneading disk, a reverse flight screw, a gear mixing element, a forward flight screw, a gear mixing element, and a forward flight screw are connected in this order;
(4) A configuration in which a kneading disk, a reverse thread flight screw, a gear mixing element, a normal thread flight screw, a gear mixing element, and a normal thread flight screw are connected in this order;
(5) A configuration in which a kneading disk, a reverse flight screw, a seal ring, a normal flight screw, a kneading disk, a notch screw, and a normal flight screw are connected in this order.

The transport section is constituted by a transport member for transporting the composition from the melt-kneading section to the granulation section. The transport member may be any known transport member used in the extrusion foaming method, such as a gear pump. A gear pump is a useful member for maintaining or optionally increasing the pressure of the composition stream.

The cooling section is composed of a cooling member that cools the composition transported from the transport section (or the melt-kneading section if the transport section is omitted). The cooling member may be any known cooling member used in the extrusion foaming method. Examples of the cooling member include a single screw extruder, a static mixer, a melt cooler, and the like. The composition is cooled to a predetermined temperature by slow cooling while mixing at a low shear rate using a single screw extruder, static mixer, or melt cooler.

When the manufacturing apparatus further includes a cooling section, it is preferable that the cooling section includes a static mixer, and it is preferable that the cooling section is composed only of a static mixer. The present inventor independently obtained the following new knowledge in the process of intensive study: Compared to the case of using a manufacturing device equipped with a cooling section equipped with a single-screw extruder, the present inventor has a cooling section equipped with a static mixer. Using the production equipment, it is surprisingly possible to obtain extruded foam particles with a significantly lower open cell content.

The die in the granulation section is equipped with at least one hole (sometimes referred to as a discharge hole) for discharging the melt-kneaded material. The shape of the cross section of the hole provided in the die perpendicular to the extrusion direction (hereinafter sometimes simply referred to as "the shape of the hole of the die") is not particularly limited. Since extruded foamed particles having a spherical or approximately spherical shape can be obtained, the shape of the hole in the die is preferably a perfect circle, a substantially circle, an ellipse, a square, or the like. The number and diameter of holes provided in the die are not particularly limited. The die preferably has a hole with a hole diameter of 0.1 mm to 2.0 mm, for example, and a hole diameter of 0.4 mm to 1.5 mm, from the viewpoint of maintaining an appropriate size of expanded particles in the in-mold foam molding process. It is more preferable to have holes. More preferably, the die has a plurality of holes (for example, two or more). In this specification, when the shape of the hole of the die is not a perfect circle, the diameter of the hole of the die is intended to be the diameter of the inscribed circle of the shape of the hole of the die.

(2-4. Melt-kneading process)
The melt-kneading step is a step in which the branched polypropylene resin is melted and the blowing agent is dissolved in the branched polypropylene resin in the melt-kneading section of the manufacturing device. The melt-kneading step can also be said to be a step of preparing a melt-kneaded product of a composition containing a resin mixture having a branched polypropylene resin and a blowing agent.

In the melt-kneading step, it is sufficient that the blowing 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 blowing agent to the melt-kneading section are not particularly limited, and include, for example, the following methods (d) or (e):
(d) (d-1) Mix or blend the branched polypropylene resin and a blowing agent to prepare a composition; (d-2) Then, supply the composition to a melt-kneading section, and A method of melting and kneading things;
(e) (e-1) Supply the branched polypropylene resin to the melt-kneading section, and melt-knead the branched polypropylene-based resin; (e-2) After that, the branched polypropylene resin that has been melt-kneaded is A blowing agent is supplied from a raw material supply port located in the middle of the melt-kneading section, that is, a composition is prepared (completed) in the melt-kneading section, and the composition is further melt-kneaded. In the above-mentioned method (d) or (e), when adding other resins, cell nucleating agents, and other components used as necessary to the composition, a method of supplying these raw materials to the melt-kneading section. and the order is not particularly limited. Other resins, cell nucleating agents, and other components used as necessary may be added at the same time as the branched polypropylene resin and/or blowing agent, or may be added separately and in random order. When using a blowing agent that is liquid at room temperature (for example, aliphatic hydrocarbons, alicyclic hydrocarbons, ethers, and alcohols) as a blowing agent, use the methods (d) and (e) above. be able to. When using a foaming agent that is gaseous at room temperature, such as carbon dioxide gas, the above-mentioned method (e) can be used.

In the melt-kneading step, for example, after melt-kneading the composition by the method (d) or (e) described above, the temperature of the melt-kneaded composition is controlled within a temperature range at which the melt-kneaded composition does not solidify. It may further include a lowering step.

In this manufacturing method, the ratio of the effective length L of the screw to the screw diameter D (L/D) is 20 to 24 in the melt-kneading section. The present inventor independently obtained the following new findings in the course of intensive studies: A manufacturing apparatus equipped with a melt-kneading section in which the ratio of the effective length L of the screw to the screw diameter D (L/D) is 24. Through its use, it is surprisingly possible to obtain extruded foamed particles with a significantly lower open cell content due to a significantly reduced deterioration of the resin. Moreover, when the above-mentioned (L/D) is 20 or more, there is an advantage that the blowing agent can be sufficiently uniformly dispersed in the melt-kneaded branched polypropylene resin.

Since it is possible to obtain extruded foam particles having an even lower open cell ratio, the above (L/D) is preferably 20 or more and less than 24, more preferably 20 to 23, and 20 or more and less than 23. It is more preferably 1, and particularly preferably 20 to 22.

As mentioned above, the "effective length L of the screw" refers to the length (mm) from the upstream end in the extrusion direction of the screw element that has the function of melting the polypropylene resin to the downstream end of the screw. The "effective length L of the screw" includes the lengths of the elements that have a relatively strong kneading action, the elements that have a relatively weak kneading action, and the elements that have the function of ensuring a filled region.

In the present manufacturing method, the ratio of the kneading element length (mm) of the screw to the screw diameter D (mm) (kneading element length/D) is preferably less than 14 in the melt-kneading section. In this specification, the "kneading element length of the screw" refers to the length (mm) of elements with strong kneading action such as kneading discs and notched screws, and the length (mm) of elements with strong kneading action, such as kneading disks and notched screws, out of the "effective length of the screw". It is intended to be the sum total of the length (mm) of the element that has the function of securing a filled area. Deterioration of polypropylene resin is suppressed by using a manufacturing device equipped with a melt-kneading section in which the ratio of the screw kneading element length (mm) to the screw diameter D (mm) (kneading element length/D) is less than 14. It has the advantage of being

Furthermore, from the viewpoint of suppressing deterioration of the polypropylene resin and balancing the plasticizing function and the dispersion function of the blowing agent, the ratio of the kneading element length to the screw diameter D (kneading element length/D) of the screw is less than 14. It is preferably less than 12, even more preferably less than 11, and particularly preferably less than 10. The lower limit of kneading element length/D is preferably 4 or more.

In the melt-kneading step, the temperature of the melt-kneading section is not particularly limited. For example, the temperature of the melt-kneading section is preferably within a range that does not interfere with the supply of the blowing agent to the raw materials. When the blowing agent is a gas, if the branched polypropylene resin is not melted at the blowing agent supply position in the melt-kneading section, the blowing agent may escape to the upstream side of the melt-kneading section. For this reason, it is preferable to set the temperature of the melt-kneading section so that the branched polypropylene resin is completely melted and the blowing agent is not vaporized due to the high resin temperature. The temperature of the melt-kneading section is, for example, preferably in the range of 170°C to 230°C, more preferably in the range of 180°C to 220°C, even more preferably in the range of 180°C to 210°C. . The temperature of the melt-kneading section includes, for example, the temperature of the cylinder (barrel) in which the screw is housed. When the temperature of the melt-kneading section is within the above range, there is an advantage that the branched polypropylene resin and other resins are melted and not thermally decomposed.

The temperature of the cooling section and the temperature of the die are not particularly limited. The temperature of the cooling section and the temperature of the die may be independently set as appropriate depending on the melting point of the branched polypropylene resin, the type and amount of blowing agent used, the mode of the extrusion foaming process, etc. good. When a single screw extruder is used as the cooling section, the temperature of the cooling section includes, for example, the temperature of the cylinder (barrel) in which the screw is housed in the single screw extruder. When a static mixer is used as the cooling section, the temperature of the cooling section includes, for example, the temperature of the housing in which the static mixer is housed.

The discharge amount Q of the melt-kneaded composition is not particularly limited. The discharge amount Q of the melt-kneaded composition may be set as appropriate depending on the size of the melt-kneading section (for example, the above (L/D)).

Both the melt-kneading time and the rotation speed N of the screw in the melt-kneading section are not particularly limited. The melt-kneading time and the rotation speed N of the screw in the melt-kneading section are both independent depending on the size of the melt-kneading section (for example, (L/D)) and/or the discharge amount Q, etc. You may set it as appropriate.

The discharge amount Q of the melt-kneaded composition, the melt-kneading time, and the rotation speed N of the screw in the melt-kneading section all have no effect on the deterioration of the branched polypropylene resin due to melt-kneading, or if they do. There are also very few.

(2-5. Extrusion foaming process)
The extrusion foaming process is a process in which the composition obtained in the melt-kneading process, that is, the melt-kneaded composition, is extruded through a die into an area whose pressure is lower than the internal pressure of the manufacturing equipment, and the extruded composition is shredded. be. Extruded foam particles are obtained by the extrusion foaming process. Therefore, the extrusion foaming process can also be said to be a granulation process of granulating extruded polypropylene resin foam particles.

In the extrusion foaming step, the area where the composition obtained in the melt-kneading step is extruded is not particularly limited as long as the pressure is lower than the internal pressure of the manufacturing equipment. For example, in the extrusion foaming process, the composition obtained in the melt-kneading process may be extruded into the gas phase or into the liquid phase.

In the extrusion and foaming process, the composition extruded into a region whose pressure is lower than the internal pressure of the manufacturing device immediately begins to foam. In the extrusion foaming process, the composition being foamed may be shredded, or the foamed composition may be shredded. If the foaming composition is shredded, the shredded composition may complete foaming in the extruded region.

Depending on the region from which the composition obtained in the melt-kneading step is extruded and the method for shredding the composition, the extrusion foaming step (granulation step) can be roughly divided into two types: a cold cut method and a die face cut method. Examples of the cold cut method include a method in which a composition containing a foaming agent extruded from a die is foamed, the foam is cooled through a water tank, and the strand-shaped foam is taken out and shredded (strand cut method). . The die face cutting method is a method in which a composition extruded from a hole in a die is cut with a rotating cutter while contacting the surface of the die or while maintaining a slight gap.

The die face cutting method can be further divided into the following three methods depending on the cooling method. That is, they are an underground cut (hereinafter sometimes referred to as UWC) method, a watering cut (hereinafter sometimes referred to as WRC) method, and a hot cut (hereinafter sometimes referred to as HC) method. The UWC method is a method in which a chamber attached to the tip of a die is filled with cooling water adjusted to a predetermined pressure so as to be in contact with the resin discharge surface of the die, and the composition extruded from the holes of the die is cut underwater. . In addition, in the WRC method, a cooling drum is connected to the die and cooling water flows along the inner peripheral surface of the drum, which is placed downstream from the die, so that the composition cut by the cutter foams in the air. This method involves cooling in the cooling water while foaming or after foaming. The HC method is a method in which a composition is cut with a cutter in the air, and the cut composition is cooled in the air while or after foaming. The HC method also includes a mist cut method that further includes a step of spraying a mixed mist of water and air.

In the extrusion foaming process, the case where the composition obtained in the melt-kneading process is extruded into the liquid phase (for example, UWC) will be explained. The liquid phase is not particularly limited, but water is preferred because it can be produced inexpensively and safely. The temperature of the liquid phase is not particularly limited, but it is preferably between 20°C and 90°C, and between 25°C and 85°C, since it is easier to obtain extruded foamed particles with fewer extruded foamed particles adhering to each other. The temperature is preferably 30°C to 80°C, even more preferably 35°C to 80°C, and particularly preferably 40°C to 80°C. In this specification, the temperature of the liquid phase can be measured by a thermometer placed in contact with the liquid phase. The pressure of the liquid phase on the composition within the region is not particularly limited, but it is easy to keep the open cell ratio of the obtained extruded foam particles low and also to keep the stickiness of the obtained extruded foam particles low, so it is 0. It is preferably from 0.05 MPa.G to 0.60 MPa.G, more preferably from 0.07 MPa.G to 0.55 MPa.G, and more preferably from 0.10 MPa.G to 0.50 MPa.G. , more preferably 0.10 MPa·G to 0.45 MPa·G, particularly preferably 0.10 MPa·G to 0.40 MPa·G. In this specification, "MPa.G" is intended to mean that MPa indicates gauge pressure.

[3. Polypropylene resin extruded foam particles]
In this specification, "polypropylene resin extruded foamed particles obtained by this production method" may be referred to as "this extruded foamed particles."

(Open cell rate)
The extruded foamed particles have the advantage of having a low open cell ratio. The lower the open cell ratio of the extruded foamed particles, the more preferable. The open cell ratio of the extruded foam particles is preferably less than 4.2%, more preferably 4.1% or less, more preferably 4.0% or less, and 3.9% or less. It is more preferably 3.8% or less, more preferably 3.7% or less, more preferably 3.6% or less, and 3.5% or less. It is more preferable that the content is 3.4% or less, and particularly preferably 3.4% or less.
The lower limit of the open cell ratio of the extruded foamed particles is not particularly limited, and is, for example, 0.0% or more. According to this configuration, (a) the extruded foamed particles have excellent moldability because the cells hardly break and shrink during molding of the extruded foamed particles, and (b) the extruded foamed particles have excellent moldability. The foamed molded article obtained using this method has the advantage that characteristics such as shape flexibility, cushioning properties, lightness, compressive strength, and heat insulation properties are better exhibited. The method for measuring the open cell ratio of extruded polypropylene resin foam particles will be explained in detail in Examples below.

(Foaming ratio)
The expansion ratio of the extruded foam particles is preferably 2.0 times to 45.0 times, more preferably 2.5 times to 40.0 times, and 2.5 times to 35.0 times. More preferably, it is 3.0 times to 30.0 times, more preferably 3.5 times to 25.0 times, and 3.5 times to 20.0 times. is more preferable, more preferably 3.5 times to 15.0 times, even more preferably 3.5 times to 10.0 times, particularly 4.0 times to 8.0 times. preferable. According to the above structure, there is an advantage that the polypropylene resin foam molded product obtained using the extruded foam particles exhibits characteristics such as shape arbitrariness, cushioning properties, lightness, and heat insulation properties. . If the expansion ratio of the extruded foamed particles obtained by manufacturing the extruded foamed particles does not reach the above range, the extruded foamed particles obtained are pressurized with an inert gas, and then the extruded foamed particles are A method of increasing the expansion ratio by heating expanded particles (for example, the method described in JP-A-10-237212) can also be used. The method for measuring the expansion ratio of extruded polypropylene resin foam particles will be explained in detail in Examples below.

[4. Manufacturing method of polypropylene resin foam molding]
[2. Extruded polypropylene resin foam particles obtained by the manufacturing method described in section [3. After filling the extruded polypropylene resin foam particles described in the section [Extruded polypropylene resin foam particles] into a molding space formed by at least two molds provided in a mold, extruding the polypropylene resin in the molding space. It has a heating step of heating the expanded particles.

In this specification, the "polypropylene resin foam molded article according to an embodiment of the present invention" may be referred to as the "present foam molded article". Moreover, a foamed molded product obtained by manufacturing using a mold is sometimes referred to as an in-mold foamed molded product.

Since the method for producing a polypropylene resin foam molded article according to an embodiment of the present invention has the above-described configuration, the polypropylene resin foam molding has excellent compressive strength, excellent fusion properties, and/or a beautiful surface. It has the advantage of being able to provide the body.

In the method for manufacturing a polypropylene resin foam molded article according to an embodiment of the present invention, the mold used is not particularly limited. The mold may include at least two molds, for example, a fixed mold that cannot be driven and a movable mold that can be driven. As the movable mold approaches the fixed mold, a molding space is formed inside the fixed mold and the movable mold. Note that when the extruded foam particles in the molding space are heated, the fixed mold and the movable mold can come into contact (that is, the mold can be sealed). On the other hand, when filling extruded foam particles into the molding space, the fixed formwork and the movable formwork do not need to be in contact with each other, and there is a slight gap between the fixed formwork and the movable formwork. A gap (also referred to as a crack) may be formed.

In the method for manufacturing a polypropylene resin foam molded article according to an embodiment of the present invention, a method for filling extruded polypropylene resin foam particles into a molding space and a method for heating extruded polypropylene resin foam particles in a mold are as follows: Not particularly limited. Examples of these methods include methods (b1) to (b4) below.

(b1) The extruded foam particles are pressurized with an inorganic gas in a container to impregnate the extruded foam particles with the inorganic gas and apply a predetermined internal pressure to the extruded foam particles. Thereafter, a method of filling the extruded foam particles into a molding space of a mold and heating the extruded foam particles in the molding space with steam;
(b2) Filling the molding space of the mold with extruded foam particles. Next, after compressing the extruded foam particles in the molding space so as to reduce the volume in the molding space by 10% to 75%, a method of heating the extruded foam particles in the molding space with steam;
(b3) The extruded foam particles are compressed by gas pressure and filled into the molding space of the mold. Thereafter, a method of heating the extruded foam particles in the molding space with water vapor using the recovery force of the extruded foam particles in the molding space;
(b4) Filling the molding space of the mold with extruded foam particles without any particular pretreatment. Then, the extruded foam particles in the molding space are heated with steam.

The heating step preferably includes a step of heating the extruded polypropylene resin foam particles with steam.

In the method for manufacturing a polypropylene resin foam molded article according to an embodiment of the present invention, the pressure of water vapor for heating extruded foam particles (hereinafter sometimes referred to as vapor pressure) varies depending on the characteristics of the extruded foam particles used, etc. , cannot be defined in general terms.

In the above method (b1), at least one kind selected from the group consisting of air, nitrogen, oxygen, carbon dioxide, helium, neon, argon, etc. can be used as the inorganic gas. Among these inorganic gases, air and/or carbon dioxide are preferred.

The internal pressure of the expanded particles in the method (b1) is preferably 0.05 MPa to 0.30 MPa (absolute pressure), and preferably 0.06 MPa to 0.25 MPa (absolute pressure).

In the method (b1), the temperature inside the container when impregnating the expanded particles with an inorganic gas is preferably 10°C to 90°C, more preferably 40°C to 90°C.

An embodiment of the present invention may have the following configuration.

[1] Melt-kneading in which a polypropylene resin having a branched structure and a blowing agent are melt-kneaded in the melt-kneading part using a manufacturing device equipped with a melt-kneading part having a plurality of screws and a granulation part having a die. and an extrusion foaming step of discharging the composition obtained in the melt-kneading step through the die into an area whose pressure is lower than the internal pressure of the manufacturing apparatus, and in the melt-kneading section, the effective length of the screw is A method for producing extruded polypropylene resin foam particles, wherein the ratio of L to screw diameter D (L/D) is 20 to 24.

[2] The method for producing extruded polypropylene resin foam particles according to [1], wherein the production apparatus further includes a cooling section.

[3] The method for producing extruded polypropylene resin foam particles according to [2], wherein the cooling section includes a static mixer.

[4] The method for producing extruded polypropylene resin foam particles according to any one of [1] to [3], wherein the production apparatus further includes a transport section.

[5] In the melt-kneading section, the ratio of the kneading element length of the screw to the screw diameter D (kneading element length/D) is less than 14, and the kneading element includes a kneading disk, a notched screw, a rotor, The extruded polypropylene resin foam according to any one of [1] to [4], which is one or more selected from the group consisting of gear mixing elements, pin elements, reverse thread flight screws, torpedo rings, and seal rings. Method of manufacturing particles.

[6] The method for producing extruded polypropylene resin foam particles according to any one of [1] to [5], wherein the die has holes with a hole diameter of 0.1 mm to 2.0 mm.

[7] The polypropylene resin having a branched structure has a melt flow rate of 0.5 g/10 minutes to 20.0 g/10 minutes at 230°C, and a melt tension of 8.0 cN or more at 200°C, [1 ] to [6]. The method for producing extruded polypropylene resin foam particles according to any one of [6].

[8] The method for producing extruded polypropylene resin foam particles according to any one of [1] to [7], wherein the open cell ratio of the extruded polypropylene resin foam particles is 4.1% or less.

[9] The method for producing extruded polypropylene resin foam particles according to any one of [1] to [8], wherein the blowing agent is an inorganic gas.

[10] The method for producing extruded polypropylene resin foam particles according to any one of [1] to [9], wherein the blowing agent is carbon dioxide gas.

[11] The method for producing extruded polypropylene resin foam particles according to any one of [1] to [10], wherein a cell nucleating agent is further added in the melt-kneading step.

[12] Production of extruded polypropylene resin foam particles according to any one of [1] to [11], wherein the expansion ratio of the extruded polypropylene resin foam particles is 3.5 times to 10.0 times. Method.

[13] The main chain of the polypropylene resin having a branched structure may be a propylene homopolymer, a block copolymer of propylene and a monomer other than propylene, or a random copolymer of propylene and a monomer other than propylene. The method for producing extruded polypropylene resin foam particles according to any one of [1] to [12], which is one or more selected from the group consisting of polymers.

[14] The polypropylene resin having a branched structure is obtained by melt-kneading a linear polypropylene resin, a conjugated diene compound, and a radical polymerization initiator, [1] to [13] The method for producing extruded polypropylene resin foam particles according to any one of the above.

[15] Filling the extruded polypropylene resin foam particles obtained by the production method according to any one of [1] to [14] into a molding space formed by at least two molds provided in a mold. . A method for manufacturing a polypropylene resin foam molded article, comprising a heating step of heating the extruded polypropylene resin foam particles in the molding space.

An embodiment of the present invention will be described below in more detail with reference to Examples, but the present invention is not limited to these Examples in any way.

(Measurement and evaluation method)
[Foaming ratio]
The expansion ratio of the extruded foamed polypropylene resin particles was calculated by the following method: (1) The weight w (g) of the extruded foamed particles was measured; (2) Next, the extruded foamed particles used to measure the weight were measured. , the extruded foam particles were submerged in ethanol contained in a graduated cylinder, and the volume v (cm ³ ) of the extruded foam particles was measured based on the rise in the liquid level in the graduated cylinder; (3) the weight w (g) was expressed as the volume v ( cm ³ ) to calculate the density ρ ₁ of the extruded foam particles; (4) Perform the same operations as (1) to (3) using the base resin (restored resin) instead of the extruded foam particles. By this, the density ρ ₂ of the base resin was calculated; (5) The density ρ ₂ of the base resin of the extruded foam particles was divided by the density ρ ₁ of the extruded foam particles (ρ ₂ /ρ ₁ ), and the expansion ratio and did.

In the Examples and Comparative Examples, the density of the resin mass obtained by returning the extruded foam particles to the resin was regarded as the density of the base resin. The following steps (b1) to (b5) were performed in order, and the resulting resin mass was used as a return resin for extruded foam particles: (b1) The extruded foam particles were placed in a dryer whose temperature was adjusted to 160°C; (b2 ) Next, the pressure inside the dryer was reduced to -0.05 MPa (gauge pressure) to -0.10 MPa (gauge pressure) using a vacuum pump over a period of 5 to 10 minutes; (b3) After that, The extruded foam particles were left in the dryer for 30 minutes to prepare a resin mass (reconstituted resin); (b4) Next, after cooling the temperature in the dryer to room temperature, the pressure in the dryer was returned to normal pressure. (b5) Thereafter, the resin mass was taken out from the dryer.

[Open cell rate]
The open cell ratio of the extruded foam particles was measured using an air comparison hydrometer [manufactured by Tokyo Science Co., Ltd., Model 1000] according to the method described in ASTM D2856-87 Procedure C (PROSEDURE C). Specifically, the open cell ratio of the extruded foam particles was calculated by performing the following (1) to (3) in order: (1) Using an air comparison type hydrometer, the volume Vc (cm ³ ) was measured; (2) Next, the entire amount of extruded foam particles after measuring Vc was submerged in ethanol contained in the graduated cylinder; (3) Then, from the amount of increase in the position of ethanol in the graduated cylinder, The apparent volume Va (cm ³ ) of the extruded foam particles was determined; (4) The open cell ratio of the extruded foam particles was calculated using the following formula:
Open cell rate (%) = ((Va-Vc) x 100)/Va.

(Manufacturing equipment)
In the following Examples and Comparative Examples, the manufacturing equipment used for manufacturing extruded polypropylene resin foam particles is an equipment in which a melt-kneading section, a cooling section, a transport section, a diverter valve, and a granulation section are connected in series. used. As the melt-kneading section, a twin-screw extruder having two screws, a raw material supply section at one end, and a blowing agent supply section in the middle of the screws was used. In addition, the structure of the melt-kneading section is a kneading disk, a forward flight screw, a kneading disk, a reverse flight screw, a forward flight screw, a kneading disk, a notch screw, and a forward flight screw connected in this order. It was the composition. In the melt-kneading section, the outer diameter D of the screw was 26 mm. The effective length (mm) of the screw was different for each example and comparative example. The kneading element length (mm) of the screw was different between Example 1, Example 2, and Comparative Example 1. A static mixer was used as the cooling section. The granulation section had a die with three holes with a hole diameter of 0.8 mm.

(Production of polypropylene resin with branched structure)
Resin A was produced as a branched polypropylene resin as follows. First, a linear polypropylene resin (F-724NPC (manufactured by Prime Polymer)), which is a raw material resin, was supplied to a twin-screw extruder. Next, 0.81 parts by weight of t-butylperoxyisopropyl monocarbonate as a radical polymerization initiator was supplied to the twin-screw extruder based on 100 parts by weight of the raw material resin. Next, 0.36 parts by weight of isoprene as a conjugated diene compound was supplied to a twin-screw extruder containing the melt-kneaded raw resin and a radical polymerization initiator for 100 parts by weight of the raw resin, and A resin mixture was prepared. The amount of resin mixture supplied to the twin screw extruder was 70 kg/h. Note that the amount of resin mixture supplied refers to the amount of resin mixture prepared within the twin-screw extruder at the time when the conjugated diene compound is supplied to the twin-screw extruder per unit time. 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. The obtained branched polypropylene resin was discharged from the die in the form of a strand at a discharge rate of 70 kg/h. The discharged branched polypropylene resin (strand) was cooled with water and shredded into pellets (cylindrical shapes). Resin B was obtained in the same manner as above except that the raw resin was changed to F227D (manufactured by Prime Polymer) and the amount of the conjugated diene compound added was changed to 0.27 parts by weight.

The MFR of the obtained resin A at 230°C was 3.0 g/10 minutes, and the melt tension was 10.2 cN. The MFR of the obtained resin B at 230° C. was 2.2 g/10 minutes, and the melt tension was 9.7 cN.

(Production of polypropylene resin without branched structure)
A mixture was prepared by mixing 60% by weight of random polypropylene resin (manufactured by Borealis, trade name RD734MO) and 40% by weight of carbon black. The resulting mixture was used as a polypropylene resin without a branched structure (linear polypropylene resin) in the following Examples and Comparative Examples.

Furthermore, in the following Examples and Comparative Examples, talc was used as a bubble nucleating agent.

[Example 1]
(melt kneading process)
75.55 parts by weight of resin A as a polypropylene resin having a branched structure (branched polypropylene resin), 20 parts by weight of resin B, and 4.25 parts by weight of a polypropylene resin without a branched structure (linear polypropylene resin) 1 part by weight, and 0.2 part by weight of talc as a cell nucleating agent to prepare a resin mixture. Thereafter, the resin mixture was supplied from the raw material supply section to a twin-screw extruder (melt-kneading section), and melt-kneading of the resin mixture was started at a cylinder temperature of 200° C. and a screw rotation speed of 120 rpm. The amount of resin mixture supplied to the twin screw extruder was 10 kg/h. During the melt-kneading of the resin mixture, carbon dioxide gas was forced into the twin-screw extruder as a blowing agent from the blowing agent supply section, and the resulting composition was further melt-kneaded. The amount of blowing agent supplied to the twin-screw extruder was 0.25 kg/h. The ratio (L/D) between the effective length L (mm) of the screw and the screw diameter D (mm) was 21. The ratio (L/D) of the screw kneading element length (mm) to the screw diameter D (mm) was 6.

(Extrusion foaming process)
The melt-kneaded composition obtained through the melt-kneading process is passed through a die included in the granulation unit, and is discharged into an area filled with water at a temperature of 70°C as a liquid phase at a pressure lower than the internal pressure of the manufacturing equipment. It was discharged at a rate of 10.25 kg/h. In this region, the pressure of water on the composition was 0.2 MPa·G. The extruded composition was shredded with a cutter in a region filled with water (liquid phase) to obtain spherical or approximately spherical extruded polypropylene resin foam particles. The obtained extruded polypropylene resin foam particles were collected by subjecting them to a centrifugal dehydrator. Table 1 shows each manufacturing condition and manufacturing result. The obtained extruded polypropylene resin foam particles were evaluated for expansion ratio and open cell ratio using the methods described above. As a result, the open cell ratio was 3.4%, and other physical properties were as shown in Table 1.

[Example 2]
The ratio (L/D) between the effective length L and the outer diameter D of the screw of the twin screw extruder is 22, and the ratio (L/D) between the screw kneading element length (mm) and the screw diameter D (mm). Polypropylene resin extruded foam particles and in-mold foamed molded products were obtained by the same manufacturing method as in Example 1, except that an apparatus equipped with a melt-kneading section with ) of 10 was used. The obtained extruded polypropylene resin foam particles and in-mold foam molded articles were evaluated for expansion ratio and open cell ratio using the methods described above. As a result, the open cell ratio was 3.7%, and other physical properties were as shown in Table 1.

[Comparative example 1]
The ratio (L/D) between the effective length L and the outer diameter D of the screw of the twin screw extruder is 25, and the ratio (L/D) between the screw kneading element length (mm) and the screw diameter D (mm). Polypropylene resin extruded foam particles and in-mold foamed molded products were obtained by the same manufacturing method as in Example 1, except that an apparatus equipped with a melt-kneading section with ) of 10 was used. The obtained extruded polypropylene resin foam particles and in-mold foam molded articles were evaluated for expansion ratio and open cell ratio using the methods described above. As a result, the open cell ratio was 4.2%, and other physical properties were as shown in Table 1.

〔result〕
Table 1 shows that Examples 1 and 2 have lower open cell ratios than Comparative Example 1. That is, it was found that in Examples 1 and 2, the deterioration of the resin was reduced compared to Comparative Example 1, in which the value of effective length L/outer diameter D of the screw was larger.

The extruded foamed polypropylene resin particles according to one embodiment of the present invention provide a method for producing extruded foamed polypropylene resin particles having a low open cell ratio. The extruded foam particles obtained by this production method can be suitably used in agriculture, fisheries, forestry, medicine, sanitary products, clothing, packaging, and other fields.

Claims (13)

1. Using a manufacturing device equipped with a melt kneading section having a plurality of screws and a granulation section having a die,
   a melt-kneading step of melt-kneading a polypropylene resin having a branched structure and a blowing agent in the melt-kneading section;
   an extrusion foaming step in which the composition obtained in the melt-kneading step is discharged through the die into a region whose pressure is lower than the internal pressure of the manufacturing device,
   A method for producing extruded polypropylene resin foam particles, wherein in the melt-kneading section, the ratio (L/D) of the effective length L of the screw to the screw diameter D is 20 to 24.
2. The method for producing extruded polypropylene resin foam particles according to claim 1, wherein the production apparatus further includes a cooling section.
3. The method for producing extruded polypropylene resin foam particles according to claim 2, wherein the cooling section includes a static mixer.
4. The method for producing extruded polypropylene resin foam particles according to any one of claims 1 to 3, wherein the production apparatus further comprises a transport section.
5. In the melt-kneading section, the ratio of the kneading element length of the screw to the screw diameter D (kneading element length/D) is less than 14, and the kneading element includes a kneading disk, a notched screw, a rotor, a gear mixing element. , a pin element, a reverse threaded flight screw, a torpedo ring, and a seal ring.
6. The method for producing extruded polypropylene resin foam particles according to any one of claims 1 to 5, wherein the die has holes with a hole diameter of 0.1 mm to 2.0 mm.
7. Claims 1 to 6, wherein the polypropylene resin having a branched structure has a melt flow rate of 0.5 g/10 minutes to 20.0 g/10 minutes at 230°C, and a melt tension of 8.0 cN or more at 200°C. The method for producing extruded polypropylene resin foam particles according to any one of the above.
8. The method for producing extruded polypropylene resin foam particles according to any one of claims 1 to 7, wherein the open cell ratio of the extruded polypropylene resin foam particles is 4.1% or less.
9. The method for producing extruded polypropylene resin foam particles according to any one of claims 1 to 8, wherein the blowing agent is an inorganic gas.
10. The method for producing extruded polypropylene resin foam particles according to any one of claims 1 to 9, wherein the blowing agent is carbon dioxide gas.
11. The method for producing extruded polypropylene resin foam particles according to any one of claims 1 to 10, wherein a cell nucleating agent is further added in the melt-kneading step.
12. The method for producing extruded polypropylene resin foam particles according to any one of claims 1 to 11, wherein the expansion ratio of the extruded polypropylene resin foam particles is 3.5 times to 10.0 times.
13. The extruded polypropylene resin foam particles obtained by the production method according to any one of claims 1 to 12 are filled into a molding space formed by at least two molds included in a mold, and the molding space is filled with A method for producing a polypropylene resin foam molded article, comprising a heating step of heating the extruded polypropylene resin foam particles.