CN117355564A

CN117355564A - Polypropylene resin expanded beads, process for producing the same, and molded article of polypropylene resin expanded beads

Info

Publication number: CN117355564A
Application number: CN202280036863.7A
Authority: CN
Inventors: 北原泰三; 千叶琢也
Original assignee: JSP Corp
Current assignee: JSP Corp
Priority date: 2021-06-09
Filing date: 2022-03-29
Publication date: 2024-01-05
Also published as: JP7137099B1; WO2022259726A1; JP2022188401A

Abstract

The polypropylene resin foam particles (1) have a core layer (2) and a coating layer (3) that coats the core layer (2). The base resin (II) of the coating layer (3) is a composition (X) composed of a polypropylene resin (B) having a melting point of 125-150 ℃, carbon nanotubes (C), and a polypropylene resin (D) having a melting point of 70-100 ℃. The amount of the polypropylene resin (D) is 3 to 20 parts by mass and the amount of the carbon nanotube (C) is 6 to 120 parts by mass based on 100 parts by mass of the polypropylene resin (B). The mass ratio (D)/(C) of the blending amount of the polypropylene resin (D) to the blending amount of the carbon nanotube (C) is 2 to 10.

Description

Polypropylene resin expanded beads, process for producing the same, and molded article of polypropylene resin expanded beads

Technical Field

The present invention relates to polypropylene resin expanded beads, a method for producing the same, and a polypropylene resin expanded bead molded article.

Background

The expanded granular molded article is used as a packing material for spacers, boxes, and the like by effectively utilizing the characteristic of excellent impact absorbability. Examples of objects to be protected by the packaging material include precision equipment, electronic equipment, and electronic components.

For a packaging material used for packaging of electronic devices and electronic components, for example, a property called electrostatic diffusivity or other electric characteristics capable of slowly discharging static electricity is sometimes demanded in addition to impact absorbability. In the present specification, "electrostatic diffusivity" specifically means that the surface resistivity is 1×10 ⁵ Omega above 1×10 ⁹ Electrical characteristics in the range of Ω or less. The foamed particles used for producing such a packaging material contain conductive substances such as conductive carbon black and carbon nanotubes.

For example, patent document 1 describes a foam molding material in which an aqueous gel obtained by dispersing a plurality of layers of carbon nanotubes in water is added to pre-expanded polystyrene beads and mixed by heating. Patent document 2 describes conductive expanded beads as follows: the resin composition comprises a polyolefin resin, a carbon nanotube aggregate comprising a plurality of carbon nanotubes having an average outer diameter of 8-50 nm and an average inner diameter of 40% or more of the average outer diameter, and a foaming agent.

Prior art literature

Patent literature

Patent document 1: japanese patent application laid-open No. 2012-87041

Patent document 2: japanese patent application laid-open No. 2019-108540

Disclosure of Invention

Problems to be solved by the invention

However, in the case of applying a conductive material to expanded beads as in patent document 1, after the expandable resin beads are expanded to form expanded beads, an operation of applying a conductive material to the expanded beads is required, and the process for producing expanded beads becomes complicated. In addition, when the amount of the conductive material applied to the surface of the expanded beads is large in this way, the conductive material may be peeled off from the surface of the expanded beads during, for example, in-mold molding.

On the other hand, as in patent document 2, when the conductive material is blended into the expanded beads, it is necessary to make the blending amount of the conductive material large in order to exhibit desired electric characteristics. However, if the amount of the conductive material blended into the resin increases, the foamability of the resin particles tends to be poor when the resin particles are foamed, the cell diameters of the foamed particles tend to be small, or the variation in cell diameters tends to be large. In addition, if the secondary foamability of the expanded beads is deteriorated, for example, the cells of the expanded beads may be easily broken during in-mold molding or it may become difficult to reduce the apparent density of the expanded bead molded article.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide polypropylene resin expanded beads having excellent foamability, a method for producing the same, and a polypropylene resin expanded bead molded article, which can impart desired electrical characteristics to the expanded bead molded article and can suppress the shedding of carbon nanotubes.

Means for solving the problems

The present invention relates to a method for producing polypropylene resin foam particles as described in the following [1] to [6 ].

[1] A process for producing polypropylene resin foam particles, which comprises foaming polypropylene resin particles having a core layer and a coating layer covering the core layer, wherein,

the base resin (I) of the core layer is a polypropylene resin (A),

the base resin (II) of the coating layer is composed of a melting point Tm _(B) Polypropylene resin (B) with a temperature of 125-150 ℃, carbon nanotubes (C) and melting point Tm _(D) A composition (X) comprising a polypropylene resin (D) having a temperature of 70 to 100 ℃,

the amount of the carbon nanotubes (C) to be blended is 3 to 20 parts by mass based on 100 parts by mass of the polypropylene resin (B),

the amount of the polypropylene resin (D) to be blended is 6 to 120 parts by mass based on 100 parts by mass of the polypropylene resin (B),

The mass ratio (D)/(C) of the blending amount of the polypropylene resin (D) to the blending amount of the carbon nanotube (C) is 2 to 10.

[2][1]The method for producing polypropylene resin foam particles, wherein the heat of fusion Q of the polypropylene resin (B) _(B) [J/g]Heat of fusion Q with the base resin (II) _(II) [J/g]Ratio Q of _(B) /Q _(II) 1.2 to 3.5.

[3] The method for producing expanded polypropylene resin particles according to [1] or [2], wherein the polypropylene resin (D) has a weight average molecular weight of 4 to 10 ten thousand.

[4][1]To [3]]The method for producing polypropylene resin foam particles according to any one of the preceding claims, wherein the heat of fusion Q of the polypropylene resin (D) _(D) Is 0J/g to 50J/g.

[5][1]To [4 ]]The method for producing polypropylene resin foam particles according to any one of the preceding claims, wherein the base resin (II) has a melting point Tm _(II) Is 120-140 ℃ and the heat of fusion Q _(II) Is 5J/g to 50J/g.

[6][1]To [5 ]]The method for producing polypropylene resin foam particles according to any one of the preceding claims, wherein the substrate resin (II) has a melt flow rate MFR at a temperature of 230℃and a load of 2.16kg _(II) Is 1g/10 min to 30g/10 min, and the melt flow rate MFR of the base resin (II) _(II) Melt flow Rate MFR with the base resin (I) _(I) Specific MFR of _(II) /MFR _(I) Is 0.2 to 4.5 inclusive.

Another embodiment of the present invention is the polypropylene resin foam particles according to the following [7] to [12 ].

[7] A polypropylene resin foam particle comprising a core layer in a foamed state and a coating layer coating the core layer, wherein,

the base resin (I) of the core layer is a polypropylene resin (A),

[8][7]The polypropylene resin foam particles, wherein the heat of fusion Q of the polypropylene resin (B) _(B) [J/g]Heat of fusion Q with the base resin (II) _(II) [J/g]Ratio Q of _(B) /Q _(II) 1.2 to 3.5.

[9] The polypropylene resin foam particles according to [7] or [8], wherein the polypropylene resin (D) has a weight average molecular weight of 4 to 10 ten thousand.

[10][7]To [9]]The polypropylene resin foam particles according to any one of the preceding claims, wherein the heat of fusion Q of the polypropylene resin (D) _(D) Is 0J/g to 50J/g.

[11][7]～[10]The polypropylene resin foam particles according to any one of the preceding claims, wherein the base resin (II) has a melting point Tm _(II) Is 120-140 ℃ and the heat of fusion Q _(II) Is 5J/g to 50J/g.

[12][7]To [11 ]]The polypropylene resin foam particles according to any one of the preceding claims, wherein the substrate resin (II) has a melt flow rate MFR at a temperature of 230℃and a load of 2.16kg _(II) Is 1g/10 min to 30g/10 min and,melt flow Rate MFR of the base resin (II) _(II) Melt flow Rate MFR with the base resin (I) _(I) Specific MFR of _(II) /MFR _(I) Is 0.2 to 4.5 inclusive.

Another embodiment of the present invention is the polypropylene resin foam molded article according to the following [13 ].

[13] A polypropylene resin foam particle molded article obtained by molding polypropylene resin foam particles according to any one of [7] to [12], wherein,

Surface resistivity of 1X 10 ⁵ Omega above 1×10 ⁹ Omega or less.

Effects of the invention

In the method for producing polypropylene resin expanded beads (hereinafter referred to as "expanded beads"), a polypropylene resin (a) is formed as a base resin (I) in a core layer of the resin beads, and a composition (X) comprising the polypropylene resin (B), the carbon nanotubes (C), and the polypropylene resin (D) is formed as a base resin (II) in a coating layer that coats the core layer. In this way, by forming the coating layer with the specific base resin (II), it is possible to easily coat the core layer of the resin particle with the coating layer and uniformly disperse the carbon nanotubes in the coating layer. In addition, in the expanded particles obtained by expanding the resin particles, the coating layer is coated on the core layer of the expanded particles and the carbon nanotubes are uniformly dispersed in the coating layer.

In the expanded beads, the core layer after foaming is covered with the covering layer, so that the electrical resistance of the surface of the expanded beads can be reduced. Further, since the carbon nanotubes (C) are contained in the base resin (II) constituting the coating layer, the expanded particles can suppress the carbon nanotubes (C) from falling off from the expanded particles and can exhibit desired electric characteristics.

Then, the expanded beads can be molded in a mold to easily impart desired electrical characteristics to a polypropylene resin expanded bead molded article (hereinafter referred to as "expanded bead molded article").

As described above, according to the above aspect, the desired electrical characteristics of the expanded beads can be imparted to the molded article, and the carbon nanotubes can be prevented from falling off, and the expanded polypropylene resin beads having excellent foamability, the method for producing the same, and the molded article of expanded polypropylene resin beads can be provided.

Drawings

Fig. 1 is an explanatory diagram showing a method of calculating the area of the high temperature peak.

FIG. 2 is a cross-sectional view of polypropylene resin foam particles in examples.

Detailed Description

In the present specification, when the upper limit and the lower limit of the numerical range are defined with respect to one physical property, or the like, the numerical ranges defining both the upper limit and the lower limit can be produced by arbitrarily combining the values of the upper limit and the lower limit.

(method for producing Polypropylene resin foam particles)

In the method for producing the expanded beads, resin beads having a core layer and a coating layer coating the core layer are used. The base resin (I) of the core layer of the resin particles is a polypropylene resin (A), and the base resin (II) of the coating layer is a composition (X) composed of a polypropylene resin (B), carbon nanotubes (C) and a polypropylene resin (D). The base resin (I) for the core layer and the base resin (II) for the cladding layer will be described in detail below.

Further, by foaming the resin particles, foamed particles having a core layer in a foamed state and a coating layer coating the core layer are obtained. It is considered that the resin component constituting the core layer of the resin particles and the resin component constituting the core layer of the foamed particles do not change before and after foaming the resin particles. Therefore, the core layer of the expanded particles has the same resin composition as the core layer of the resin particles. Likewise, the coating layer of the expanded particles has the same resin composition as the coating layer of the resin particles.

[ core layer ]

The base resin (I) of the core layer is a polypropylene resin (A). In the present invention, the base resin (I) of the core layer is the polypropylene resin (a), and the polypropylene resin (a) is the basic resin component constituting the core layer. More specifically, the proportion of the polypropylene resin (a) as the base resin (I) is preferably 70 mass% or more, more preferably 80 mass% or more, still more preferably 90 mass% or more, and most preferably 95 mass% or more, relative to the total mass of the core layer.

Polypropylene resin (A)

As the polypropylene resin (a), one or two or more kinds of polypropylene resins selected from the group consisting of homopolymers of propylene monomers and propylene copolymers can be used. In the present specification, the polypropylene resin means a homopolymer of a propylene monomer and a propylene copolymer containing 50 mass% or more of a structural unit derived from propylene.

As the homopolymer of the propylene monomer, for example, isotactic polypropylene, syndiotactic polypropylene, atactic polypropylene and the like can be used.

Examples of the propylene-based copolymer include copolymers of propylene and an α -olefin having 4 to 10 carbon atoms such as ethylene, 1-butene, isobutylene, 1-pentene, 3-methyl-1-butene, 1-hexene, 3, 4-dimethyl-1-butene, 3-methyl-1-hexene, propylene-acrylic acid copolymers, and propylene-maleic anhydride copolymers. These copolymers may be random copolymers or block copolymers. The copolymer may be a binary copolymer, or may be a ternary or higher order copolymer. The content of structural units other than structural units derived from propylene in the copolymer is preferably 25% by mass or less, more preferably 15% by mass or less, and still more preferably 10% by mass or less. The lower limit of the content of the structural unit other than the structural unit derived from propylene in the copolymer is approximately 1% by mass.

Other ingredients

In addition to the base resin (I), additives such as a bubble regulator, a catalyst neutralizer, a lubricant, and a crystallization nucleating agent may be added to the core layer. The amount of the additive to be added to the core layer is, for example, preferably 15 parts by mass or less, more preferably 10 parts by mass or less, still more preferably 5 parts by mass or less, and particularly preferably 1 part by mass or less, based on 100 parts by mass of the base resin (I) of the core layer. In addition, a colorant can be added to the core layer for the purpose of adjusting the degree of coloration with the coating layer. In such a case, the amount of the colorant to be added is preferably 1 part by mass or more and 5 parts by mass or less, more preferably 1.5 parts by mass or more and 3 parts by mass or less, relative to 100 parts by mass of the base resin (I) of the core layer.

In addition, a resin, an elastomer, or the like other than the polypropylene resin (a) may be added to the core layer in a range not to impair the object and the effect of the present invention, in addition to the polypropylene resin (a) as the base resin (I). Examples of the resins other than the polypropylene resin (a) include thermoplastic resins such as polyethylene resins, polystyrene resins, polybutylene resins, polyamide resins, and polyester resins, olefinic thermoplastic elastomers, and styrenic thermoplastic elastomers. The amount of the resin, elastomer, or the like other than the polypropylene resin (a) added to the core layer is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and still more preferably 5 parts by mass or less, relative to 100 parts by mass of the base resin (I) of the core layer. The core layer may be composed of only the polypropylene resin (a) as the base resin (I), but when a resin other than the polypropylene resin (a) and/or an elastomer is added to the core layer, the lower limit of the total amount of the resin other than the polypropylene resin (a) and the elastomer to be added may be approximately 1 part by mass.

[ coating layer ]

The base resin (II) of the coating layer is a composition (X) composed of a polypropylene resin (B), carbon nanotubes (C) and a polypropylene resin (D). In the present invention, the base resin (II) of the coating layer being the composition (X) means that a basic resin component constituting the coating layer is the composition (X). More specifically, the proportion of the composition (X) as the base resin (II) is preferably 70 mass% or more, more preferably 80 mass% or more, still more preferably 90 mass% or more, and most preferably 95 mass% or more, relative to the total mass of the coating layer. In the present invention, the base resin (II) is defined as a concept including a system in which carbon nanotubes other than the resin are incorporated.

The composition (X) is composed of a melting point Tm _(B) Polypropylene resin (B) with a temperature of 125-150 ℃, carbon nanotubes (C) and melting point Tm _(D) A polypropylene resin (D) having a temperature of 70 to 100 ℃. The amount of the carbon nanotubes (C) blended in the base resin (II) is 3 to 20 parts by mass based on 100 parts by mass of the polypropylene resin (B). The amount of the polypropylene resin (D) blended in the base resin (II) is 6 to 120 parts by mass based on 100 parts by mass of the polypropylene resin (B). The mass ratio (D)/(C) of the blending amount of the polypropylene resin (D) to the blending amount of the carbon nanotube (C) in the base resin (II) is 2 to 10. The polypropylene resin (B), the carbon nanotubes (C) and the polypropylene resin (D) in the base resin (II) are contained in the same amounts as the polypropylene resin (B), the carbon nanotubes (C) and the polypropylene resin (D) contained in the base resin (II), respectively.

Polypropylene resin (B)

The melting point Tm is blended with the base resin (II) _(B) A polypropylene resin (B) having a temperature of 125-150 ℃. One type of polypropylene resin (B) may be blended with the base resin (II), or two or more types of polypropylene resins (B) may be blended. By mixing the melting point Tm of the polypropylene resin (B) _(B) The specific range can improve the adhesion between the core layer and the clad layer.

Melting Point Tm of Polypropylene resin (B) _(B) Can pass JIS K7121: the heat flux difference was measured by scanning calorimeter measurement described in 1987. Specifically, first, the condition of the test piece made of the polypropylene resin (B) was adjusted. Thereafter, the temperature was increased from 23℃to 200℃at a heating rate of 10℃per minute, and then the temperature was decreased to 23℃at a cooling rate of 10℃per minute. Thereafter, the melting point Tm of the polypropylene resin (B) can be determined as the peak temperature of the endothermic peak determined by a DSC curve obtained when the temperature is raised from 23℃to 200℃again at a heating rate of 10℃per minute _(B) . When a plurality of endothermic peaks appear in the DSC curve, the peak temperature of the endothermic peak having the largest area is used as the melting point.

Carbon nanotube (C)

The carbon nanotubes (C) are blended in the base resin (II) in an amount of 3 to 20 parts by mass based on 100 parts by mass of the polypropylene resin (B). When the amount of carbon nanotubes (C) blended in the base resin (II) is within the above-described specific range, expanded beads having desired electrical characteristics can be easily obtained. Then, by in-mold molding the expanded beads, desired electrical characteristics can be easily imparted to the expanded bead molded article. The lower limit of the amount of the carbon nanotubes (C) to be blended is preferably 3 parts by mass, more preferably 4 parts by mass, and even more preferably 6 parts by mass, per 100 parts by mass of the polypropylene resin (B) from the viewpoint of more reliably imparting desired electrical characteristics to the expanded particle molded product. On the other hand, the upper limit of the amount of the carbon nanotubes (C) to be blended is preferably 18 parts by mass, more preferably 16 parts by mass, and still more preferably 14 parts by mass, based on 100 parts by mass of the polypropylene resin (B). From the same viewpoint, the amount of the carbon nanotube (C) to be blended is preferably 3 parts by mass or more and 18 parts by mass or less, more preferably 4 parts by mass or more and 16 parts by mass or less, and still more preferably 6 parts by mass or more and 14 parts by mass or less.

When the amount of the carbon nanotubes (C) to be blended is too small, it is difficult to form a conductive network in the coating layer, and there is a possibility that desired electrical characteristics cannot be imparted to the foamed particles. On the other hand, when the blending amount of the carbon nanotubes (C) is too large, an increase in melt viscosity of the base resin (II) blended with the carbon nanotubes (C) is likely to occur when the resin particles are produced in the process of producing the expanded particles. As a result, it is difficult to laminate the resin melt for forming the coating layer on the resin melt for forming the core layer, and there is a possibility that the foamed particles of the multilayer structure cannot be obtained.

The carbon nanotubes (C) may be single-layer carbon nanotubes or multi-layer carbon nanotubes. In addition, the base resin (II) may contain both single-layer carbon nanotubes and multi-layer carbon nanotubes as the carbon nanotubes (C). As the carbon nanotube (C), for example, a carbon nanotube having an aspect ratio of 80 to 1000 and an outer diameter of 9.5 to 25nm is preferably used, and a carbon nanotube having an aspect ratio of 100 to 200 and an outer diameter of 9.5 to 12nm is more preferably used.

Polypropylene resin (D)

The melting point Tm is blended in the base resin (II) _(D) A polypropylene resin (D) having a temperature of 70 to 100 ℃. In addition, melting point Tm of the polypropylene resin (D) _(D) The measurement method of (2) is to use a test piece composed of a polypropylene resin (D) instead of a test piece composed of a polypropylene resin (B), and the melting point Tm of the polypropylene resin (B) is the same as that of the above-mentioned test piece _(B) The measurement method is the same.

One kind of polypropylene resin (D) may be blended with the base resin (II), or two or more kinds of polypropylene resins (D) may be blended. The amount of the polypropylene resin (D) to be blended is 6 parts by mass to 120 parts by mass based on 100 parts by mass of the polypropylene resin (B). The mass ratio (D)/(C) of the blending amount of the polypropylene resin (D) to the blending amount of the carbon nanotube (C) is 2 to 10.

The polypropylene resin (D) has a property of being softened more easily than the polypropylene resin (B). By blending the polypropylene resin (D) with the base resin (II), crystallization of the polypropylene resin (B) due to addition of the carbon nanotubes (C) can be suppressed. Therefore, the rapid viscosity increase of the base resin (II) caused by crystallization of the polypropylene resin (B) can be suppressed, and the lamination of the coating layer to the core layer can be easily performed and the exposure of the core layer can be avoided. Further, since the dispersibility of the carbon nanotubes (C) in the coating layer is improved by blending the polypropylene resin (D) with the base resin (II), desired electrical characteristics can be easily imparted to the expanded beads and the expanded bead molded article.

The blending amount of the polypropylene resin (D) is preferably 10 parts by mass or more and 100 parts by mass or less, more preferably 15 parts by mass or more and 90 parts by mass or less, still more preferably 20 parts by mass or more and 80 parts by mass or less, still more preferably 30 parts by mass or more and 60 parts by mass or less, relative to 100 parts by mass of the polypropylene resin (B), from the viewpoint of easier obtaining of the expanded beads having a multilayer structure. From the same viewpoint, the mass ratio (D)/(C) of the amount of the polypropylene resin (D) to the amount of the carbon nanotubes (C) is preferably 3 to 8.

When the amount of the polypropylene resin (D) blended is too small relative to the polypropylene resin (B), or when the ratio (D)/(C) of the amount of the polypropylene resin (D) blended to the amount of the carbon nanotubes (C) is too low, the fluidity of the base resin (II) may extremely decrease, and the coating layer may not be uniformly formed on the core layer.

On the other hand, even when the amount of the polypropylene resin (D) blended is too large relative to the polypropylene resin (B), or when the ratio (D)/(C) of the amount of the polypropylene resin (D) blended to the amount of the carbon nanotubes (C) is too large, it may be difficult to uniformly form the coating layer on the core layer, and it may be difficult to produce foamed particles having a desired laminated structure.

The weight average molecular weight of the polypropylene resin (D) is preferably 4 to 10 tens of thousands. At the melting point Tm _(D) The polypropylene resin (D) having a weight average molecular weight within the above specific range has an effect of reducing interaction between the polypropylene resin (B) and the carbon nanotubes (C), and can more effectively suppress crystallization of the polypropylene resin (B) accompanying addition of the carbon nanotubes (C). Therefore, by forming the coating layer using the base resin (II) to which the low molecular weight polypropylene resin (D) is added, it is possible to uniformly laminate the coating layer on the core layer while suppressing the change in fluidity due to crystallization of the polypropylene resin (B). As a result, good expanded particles in which the carbon nanotubes are uniformly dispersed in the coating layer can be produced. From the viewpoint of more reliably obtaining the above-described effect, the weight average molecular weight of the polypropylene resin (D) is more preferably 4.2 to 8 tens of thousands, and still more preferably 4.3 to 6 tens of thousands.

The weight average molecular weight of the polypropylene resin (D) is a molecular weight in terms of polystyrene measured by Gel Permeation Chromatography (GPC) using polystyrene as a standard substance.

Heat of fusion Q of polypropylene resin (D) _(D) Preferably from 0J/g to 50J/g. By compounding the base resin (II) with a compound having a melting point Tm _(D) Based on (1), the heat of fusion Q _(D) The polypropylene resin (D) within the above specific range can form the clad layer on the core layer more easily. From the viewpoint of obtaining the above-mentioned effect more reliably, the heat of fusion Q of the polypropylene resin (D) _(D) More preferably 1J/g to 20J/g, still more preferably 2J/g to 10J/g, still more preferably 2.5J/g to 7J/g.

Heat of fusion Q of polypropylene resin (D) _(D) The measurement can be performed by the following method. First, the melting point Tm of the polypropylene resin (D) is used as a melting point Tm of the polypropylene resin (D) _(D) The condition of the test piece was adjusted in the same manner as the measurement method of (a). Thereafter, the temperature was increased from 23℃to 200℃at a heating rate of 10℃per minute, and then the temperature was decreased to 23℃at a cooling rate of 10℃per minute. Thereafter, the temperature was again raised from 23℃to 200℃at a heating rate of 10℃per minute, to obtain a DSC curve. The value obtained by dividing the area (unit: J) of the endothermic peak appearing in the DSC curve by the mass (unit: g) of the test piece is the heat of fusion Q of the polypropylene resin (D) _(D) (unit: J/g).

Other ingredients

In addition to the base resin (II), additives such as a catalyst neutralizer, a lubricant, a crystal nucleating agent, and an antistatic agent may be added to the coating layer. Further, conductive carbon black may be added to the coating layer within a range that does not impair the aforementioned operational effects. The amount of the additive to be blended in the coating layer is preferably 15 parts by mass or less, more preferably 10 parts by mass or less, and still more preferably 5 parts by mass or less, based on 100 parts by mass of the base resin (II).

In addition to the polypropylene resin (B) and the polypropylene resin (D) blended in the base resin (II), the coating layer may contain other materials such as resins and elastomers within a range that does not impair the aforementioned effects. The amount of the polypropylene resin (B) and the resin other than the polypropylene resin (D), the elastomer, and the like added in the coating layer is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, per 100 parts by mass of the base resin (II). The coating layer may be composed of only the composition (X), or when the polypropylene resin (B) and the resin other than the polypropylene resin (D) and/or the elastomer contained in the composition (X) are added to the coating layer, the lower limit of the total amount of the resins other than the polypropylene resin (B) and the polypropylene resin (D) and the elastomer may be set to approximately 1 part by mass.

Physical Properties of the base resin (II)

The base resin (II) preferably has the following characteristics. The base resin (II) having the characteristics shown below is particularly suitable for the formation of a multilayer structure of the expanded particles.

Melting Point Tm of base resin (II) for coating layer _(II) Preferably 120 ℃ to 140 ℃. By using melting point Tm _(II) The coating layer is formed for the base resin (II) within the specific range, and the foamed particles of the multilayer structure can be produced more easily. In addition, melting point Tm of the base resin (II) _(II) Preferably a melting point Tm of the base resin (I) _(I) Low. In this case, the weldability of the expanded beads can be further improved. From the viewpoint of further improving the above-mentioned action effect, the melting point Tm of the base resin (II) _(II) More preferably 135 ℃ to 138 ℃.

In addition, the melting point Tm of the base resin (I) _(I) Melting Point Tm of base resin (II) _(II) The measurement method of (2) is to use a test piece composed of a base resin (I) or a base resin (II) instead of a test piece composed of a polypropylene resin (B), and the melting point Tm of the polypropylene resin (B) is the same as that of the above-mentioned test piece _(B) The measurement method is the same.

Heat of fusion Q of base resin (II) _(II) Preferably 5J/g to 50J/g. By using the heat of fusion Q _(II) The coating layer is formed for the base resin (II) within the above specific range, so that the change in fluidity due to crystallization of the polypropylene resin (B) in the base resin (II) can be further reduced, and the core layer can be coated with the coating layer more uniformly. From the viewpoint of further improving the above-mentioned action effect, the heat of fusion Q of the base resin (II) _(II) More preferably 15J/g to 45J/g, still more preferably 20J/g to 40J/g, still more preferably 25J/g to 39J/g.

In addition, the heat of fusion Q of the base resin (II) _(II) The measurement method (2) is not limited to using a test piece composed of the base resin (II) instead of a test piece composed of the polypropylene resin (D), and the heat of fusion Q of the test piece and the polypropylene resin (D) is the same as that of the test piece composed of the polypropylene resin (D) _(D) The measurement method is the same.

Heat of fusion Q of base resin (I) for core layer _(I) Heat of fusion Q with base resin (II) for coating layer _(II) Difference Q of _(II) -Q _(I) The absolute value of (2) is preferably 10J/g or more. By taking the difference Q between the melting heat _(II) -Q _(I) The absolute value of (c) is set within the above specific range, and the foamability of the expanded beads can be further improved. From the viewpoint of further improving the above-mentioned action effect, the heat of fusion Q of the base resin (I) _(I) Heat of fusion Q with base resin (II) _(II) Difference Q of _(II) -Q _(I) The absolute value of (2) is more preferably 40J/g or more.

In addition, the heat of fusion Q of the base resin (I) _(I) The measurement method of (2) is to use a test piece composed of a base resin (I) instead of a test piece composed of a polypropylene resin (D), and the heat of fusion Q of the test piece and the polypropylene resin (D) _(D) The measurement method is the same.

Melt flow Rate MFR of base resin (II) at 230℃under a load of 2.16kg _(II) Preferably 1g/10 min to 30g/10 min, and the melt flow rate MFR of the base resin (II) _(II) Melt flow Rate MFR with base resin (I) _(I) Ratio (MFR) _(II) /MFR _(I) ) Preferably from 0.2 to 4.5. Thus, by allowing the melt flow rate MFR of the base resin (II) _(II) Within the specified range and with the melt flow rate MFR of the base resin (II) _(II) Melt flow Rate MFR with base resin (I) _(I) Specific MFR of _(II) /MFR _(I) Within the above specific range, the coating layer can be coated on the core layer more uniformly during the production of the expanded beads.

From the viewpoint of further improving the above-mentioned effect, the melt flow rate MFR of the base resin (II) _(II) More preferably 3 g/min to 30 g/min, still more preferably 5g/10 min to 15g/10 min. In addition, the melt flow rate MFR of the base resin (II) _(II) Melt flow Rate MFR with base resin (I) _(I) Specific MFR of _(II) /MFR _(I) More preferably from 0.2 to 4.0, particularly preferably from 0.4 to 3.8, and most preferably from 0.5 to 3.

In addition, the melt flow rate MFR of the base resin (II) _(II) Melt flow Rate MFR of base resin (I) _(I) Is based on JIS K7210-1:2014 under a test temperature of 230 ℃ and a load of 2.16 kg.

Heat of fusion Q of the polypropylene resin (B) _(B) Heat of fusion Q with base resin (II) _(II) Ratio Q of _(B) /Q _(II) Preferably 1.2 to 3.5, more preferably 1.3 to 3.2, and still more preferably 1.5 to 3. By making the ratio Q of the heat of fusion _(B) /Q _(II) Within the above specific range, the coating layer can be coated on the core layer more uniformly during the production of the expanded beads.

In addition, the heat of fusion Q of the polypropylene resin (B) _(B) The measurement method of (2) is to use a test piece composed of a polypropylene resin (B) instead of a test piece composed of a polypropylene resin (D), and the heat of fusion Q of the test piece and the polypropylene resin (D) is the same as that of the test piece composed of the polypropylene resin (D) _(D) The measurement method is the same.

[ manufacturing Process ]

The method for producing the expanded beads comprises, for example, the following steps:

a granulation step of producing polypropylene resin pellets (hereinafter referred to as "resin pellets") having a core layer obtained by coextruding a resin melt for forming a core layer containing a base resin (I) in a molten state and a resin melt for forming a coating layer containing a base resin (II) in a molten state, and then cutting the extrudate to obtain a non-foamed state in which the polypropylene resin (a) is the base resin (I), and a coating layer obtained by coextruding the resin melt for forming a core layer containing the base resin (I) in a molten state and the resin melt for forming a coating layer containing the base resin (II) in a molten state, wherein the coating layer is formed by coating the core layer with the composition (X); and

And a foaming step of foaming the core layer of the polypropylene resin particles.

Granulation step

In the granulating step, first, the core layer-forming resin melt and the coating layer-forming resin melt are co-extruded so that the coating layer-forming resin melt coats the core layer-forming resin melt, and an extrudate is obtained. By cutting the extrudate, resin particles in an unfoamed state can be obtained. For example, an extrusion molding machine for forming a core layer for extruding a resin melt for forming a core layer, an extrusion molding machine for forming a cladding layer for extruding a resin melt for forming a cladding layer, and a coextrusion device having an extrusion die connected to the extrusion ports of these extrusion molding machines can be used for producing the extrudate.

The cutting of the extrudate may be performed immediately after coextrusion or may be performed after cooling the linear extrudate extruded from the extrusion die. More specifically, as the method of cutting the extrudate, various methods such as a dicing method of cutting the extrudate after completion of cooling, a thermal cutting method of cutting the extrudate before completion of cooling, and an underwater cutting method of cutting the extrudate extruded into water can be used.

The core layer of the resin particles obtained after cutting is covered with the clad layer. For example, when the resin particles are formed as described above, cylindrical resin particles having a coating layer coated on the side surface of the cylindrical core layer can be formed. The mass ratio of the core layer to the coating layer in the resin particles can be determined from the core layer: cladding = 70: 30-99: 1, preferably from the core layer: cladding = 80: 20-98: 2 is appropriately set in the range of 2.

Foaming step

In the foaming step, the resin particles are foamed to obtain foamed particles. In foaming the resin particles, the resin particles may be foamed to a desired bulk density by one-time foaming, or may be foamed to a desired bulk density by multiple foaming. Hereinafter, the first foaming step is referred to as a "primary foaming step", and the second foaming step is referred to as a "secondary foaming step".

First-stage foaming step

In the primary foaming step, first, resin particles are placed in a closed vessel and dispersed in an aqueous dispersion medium such as water. In this case, a dispersing agent for dispersing the resin particles may be added to the dispersion medium in the closed vessel as needed.

As the dispersant, for example, a surfactant such as alumina, aluminum sulfate, tricalcium phosphate, magnesium pyrophosphate, zinc oxide, inorganic fine particles such as kaolin, mica, or the like, sodium alkylbenzenesulfonate, sodium dodecylbenzenesulfonate, sodium alkane sulfonate, or the like can be used. As the dispersant, one kind selected from these inorganic fine particles and surfactants may be used alone, or two or more kinds may be used in combination.

After the sealed container is sealed, an inorganic physical blowing agent is added into the container, and the inorganic physical blowing agent is impregnated into the resin particles. At this time, the impregnation of the inorganic physical blowing agent into the resin particles can be promoted by pressurizing and simultaneously heating the inside of the closed container. Then, after the foaming agent is sufficiently impregnated into the resin particles, the content of the closed container is released at a pressure lower than the internal pressure of the container, and the resin particles can be foamed to form foamed particles.

As the inorganic physical foaming agent, for example, an inorganic gas such as carbon dioxide, nitrogen, air, or the like, water, or the like can be used. The inorganic physical foaming agent may be used alone or two or more of them may be used together. The inorganic physical blowing agent in the primary foaming step is preferably carbon dioxide, from the viewpoint of easier production of expanded beads having a high expansion ratio and a narrow particle size distribution.

The amount of the inorganic physical blowing agent to be added can be appropriately set according to the type of the polypropylene resin (a) contained in the core layer, the type of the blowing agent, the intended stacking ratio of the expanded beads, and the like. The amount of the inorganic physical blowing agent to be added may be appropriately set in a range of, for example, 0.1 to 10 parts by mass, preferably 1 to 9 parts by mass, and more preferably 3 to 8 parts by mass, based on 100 parts by mass of the polypropylene resin (a) contained in the core layer.

The primary foaming step may include a step of generating a high temperature peak described later before foaming the resin particles. As a method for generating the high temperature peak, for example, a method of heat-treating the resin particles in a dispersion medium in a closed container while keeping the resin particles in a specific temperature range can be used. The time point of the heat treatment is not particularly limited, and the heat treatment may be performed at any one of the time points before, during and after the impregnation of the foaming agent, or may be performed from any one of the time points to another time point. By this heat treatment, expanded particles having a crystal structure that shows a melting peak (intrinsic peak) derived from the intrinsic crystals of the polypropylene resin (a) and a melting peak (high temperature peak) located on the high temperature side of the intrinsic peak can be obtained.

In the above-described production method, the resin particles may be foamed to a desired bulk density of the foamed particles by the primary foaming step, or may be foamed to a bulk density higher than the desired bulk density of the foamed particles. In the former case, the obtained expanded beads can be used for in-mold molding as they are. In the latter case, the expanded beads may be further subjected to a secondary foaming step or the like to reduce the bulk density to a desired value.

Secondary foaming step

In the secondary foaming step, first, the pressure vessel is filled with the expanded beads obtained in the primary foaming step. Then, the inside of the pressure-resistant container is pressurized with the inorganic gas, so that the inorganic gas is impregnated into the expanded particles. In this way, by impregnating the inorganic gas, the pressure ratio in the cells of the expanded beads can be increased before impregnation. In the case of performing the secondary foaming step, the resin particles obtained in the primary foaming step may be referred to as "primary foam particles".

In the secondary foaming step, the primary foam particles in the pressure-resistant vessel may be heatedPressurizing. In this case, impregnation of the primary expanded particles with the inorganic gas can be further promoted. In the case of heating the primary expanded beads in the secondary foaming step, from the viewpoint of suppressing agglomeration, in other words, from the viewpoint of suppressing the phenomenon that the primary expanded beads are welded to each other to form a block, it is preferable to set the heating temperature of the primary expanded beads to be higher than the melting point Tm of the base resin (II) constituting the coating layer _(II) Low.

As the inorganic gas used in the secondary foaming step, carbon dioxide, nitrogen, air, steam, and the like can be used. The inorganic gas may be used alone or in combination of two or more. The inorganic gas used in the secondary foaming step is preferably a mixed gas of steam and air. In this case, the primary expanded particles can be warmed appropriately to promote the impregnation of the inorganic gas and to suppress the agglomeration of the primary expanded particles more effectively.

The pressure (internal pressure) in the air bubble can be measured by the method described in japanese patent application laid-open No. 2003-201661, for example.

After the impregnation of the primary expanded particles with the inorganic gas is completed, the primary expanded particles are taken out of the pressure-resistant container. The primary expanded beads can be heated with steam or the like at a pressure lower than the pressure inside the cells to expand the cells. As a result, the primary expanded beads can be further expanded to obtain expanded beads having a desired bulk density.

In the present specification, the vessel used in the primary foaming step is referred to as a "closed vessel", and the vessel used in the secondary foaming step is referred to as a "pressure-resistant vessel", but the vessels may be closed and pressure-applied. The closed vessel in the primary foaming step and the pressure vessel in the secondary foaming step may be the same vessel or may be different vessels.

(Polypropylene resin foam particles)

The expanded particles have a multilayer structure including a core layer in a foamed state and a coating layer covering the core layer. The coating layer may cover the entire core layer or a part of the core layer, but it is preferable that a part of the core layer is covered with the coating layer. Particularly preferably, the core layer in a foamed state has a columnar shape, and the coating layer is uniformly formed on the side surface of the core layer. Further, it is more preferable that the carbon nanotubes (C) are uniformly dispersed in the coating layer. The side surface of the core layer is also sometimes referred to as a side circumferential surface.

The core layer comprises the polypropylene resin (A) as a base resin (I), and the coating layer comprises a polymer having a melting point Tm _(B) Polypropylene resin (B) with a temperature of 125-150 ℃, carbon nanotubes (C) and melting point Tm _(D) The polypropylene resin (D) having a temperature of 70 to 100 ℃ is used as the base resin (II). The content of the carbon nanotubes (C) is 3 to 20 parts by mass based on 100 parts by mass of the polypropylene resin (B). The content of the polypropylene resin (D) is 6 to 120 parts by mass based on 100 parts by mass of the polypropylene resin (B). The mass ratio (D)/(C) of the blending amount of the polypropylene resin (D) to the content of the carbon nanotubes (C) is 2 to 10.

The expanded beads having the above-described structure contain carbon nanotubes in the coating layer, and thus can effectively suppress the shedding of the carbon nanotubes. Further, since the carbon nanotubes are uniformly dispersed in the core layer and have a multilayer structure in which the cladding layers are uniformly laminated with respect to the core layer, stable electrical characteristics can be exhibited. The expanded beads having the above-described structure are also excellent in foamability such as secondary foamability during molding.

[ bulk Density ]

The bulk density of the expanded beads is preferably 18g/L to 180g/L, more preferably 20g/L to 150g/L, still more preferably 25g/L to 120g/L, still more preferably 30g/L to 100 g/L. In general, the higher the expansion ratio at the time of foaming the resin particles and the lower the bulk density of the foamed particles, the more difficult it is to uniformly form the coating layer at the time of foaming. In contrast, in the expanded beads, since the composition (X) is used as the base resin (II) for the coating layer, the coating layer can be uniformly formed on the core layer even when the bulk density is low, and the effect of the present invention can be easily obtained.

The bulk density of the expanded beads is a value calculated by the following method. First, a pellet group composed of 500 or more expanded pellets is left for 24 hours or more in an atmosphere having a temperature of 23 ℃ and a relative humidity of 50% and 1 atm. The thus obtained pellet group was naturally stacked in a measuring cylinder and filled, and the stacking volume (unit: L) of the pellet group was read based on the scale of the measuring cylinder. Then, the bulk density (unit: g/L) of the expanded beads was obtained by dividing the mass (unit: g) of the bead group in the measuring cylinder by the aforementioned bulk volume.

[ high temperature Peak ]

The expanded beads preferably have a crystal structure in which one or more endothermic peaks (hereinafter, referred to as "high temperature peaks") appear on the high temperature side of the peaks of the endothermic peaks (hereinafter, referred to as "intrinsic peaks") inherent to the polypropylene resin (a) in a DSC curve obtained by thermal energy differential scanning calorimetry. In this case, the independent cell ratio of the expanded beads can be further improved, and the molding conditions for molding the expanded beads can be selected from a wide range. Further, the rigidity of the obtained expanded molded article can be further improved. From the above viewpoints, the amount of heat absorption at the high temperature peak (hereinafter referred to as "high temperature peak heat") is preferably 5J/g or more, more preferably 8J/g or more. The high temperature peak heat amount is preferably 50J/g or less, more preferably 40J/g or less. From the same viewpoint, the high temperature peak heat amount is preferably 5J/g to 50J/g, more preferably 8J/g to 40J/g.

The high temperature peak heat of the expanded particles can be calculated by the following method. First, a DSC curve of a test piece obtained by heating and melting the test piece was obtained by a method of measuring the transfer heat of a plastic prescribed in JIS K7122-1987 using about 1 to 3mg of foamed particles as the test piece. The DSC curve has a temperature ranging from 30 ℃ to a temperature 30 ℃ higher than the temperature at the end of the melting peak, and the heating rate during heating is 10 ℃/min. When the expanded beads have a high temperature peak, as shown in fig. 1, an intrinsic peak Δh1 appears in the DSC curve, and a height Wen Feng H2 having a peak on the high temperature side from the peak of the intrinsic peak Δh1 appears.

Next, a straight line L1 connecting a point α corresponding to 80 ℃ on the DSC curve and a point β corresponding to the melting end temperature T of the expanded beads is drawn. The melting end temperature T is an end point on the high temperature side at the high temperature peak Δh2, in other words, an intersection point of the high temperature peak Δh2 and a base line on the high temperature side in the DSC curve, which is higher than the high temperature peak Δh2.

After drawing the straight line L1, a straight line L2 parallel to the longitudinal axis of the graph is drawn by a maximum point γ existing between the intrinsic peak Δh1 and the high temperature peak Δh2. The intrinsic peak Δh1 and the high temperature peak Δh2 are divided by the straight line L2. The heat absorption amount of the high temperature peak Δh2 can be calculated based on the area of the portion surrounded by the portion constituting the high Wen Feng H2, the straight line L1, and the straight line L2 in the DSC curve.

Further, in the case where the DSC curve is obtained by the aforementioned method, and then the expanded beads are temporarily cooled, and the DSC curve is obtained again, only the intrinsic peak Δh1 appears in the DSC curve, and the high temperature peak Δh2 disappears from the DSC curve.

[ core layer ]

The core layer of the expanded beads is formed using the base resin (I). The polypropylene resin (a) used as the base resin (I) is the same as the polypropylene resin (a) used in the above-described method for producing expanded beads.

[ coating layer ]

The core layer of the foaming particles is coated by the coating layer. The clad layer may cover the entire core layer or a part of the core layer. The coating layer may be in a foamed state or in a non-foamed state. The mass ratio of the core layer to the coating layer in the foamed particles can be determined from the core layer: cladding = 70: 30-99: 1 is appropriately set in the range of 1.

The coating layer is formed using a base resin (II). The composition of the base resin (II) is the same as that of the base resin (II) used in the above-described method for producing expanded beads.

In addition, the base resin (II) for the coating layer is a resin having a melting point Tm _(B) Polypropylene resin (B) with a temperature of 125-150 ℃, carbon nanotubes (C) and melting point Tm _(D) A composition (X) comprising a polypropylene resin (D) having a temperature of 70 ℃ to 100 ℃. The content of the carbon nanotube (C) relative to the polypropylene100 parts by mass of the resin (B) is 3 to 20 parts by mass. The content of the polypropylene resin (D) is 6 to 120 parts by mass based on 100 parts by mass of the polypropylene resin (B). The mass ratio (D)/(C) of the blending amount of the polypropylene resin (D) to the content of the carbon nanotubes (C) is 2 to 10. That is, it is considered that the content of the polypropylene resin (B), the carbon nanotube (C), and the polypropylene resin (D) in the base resin (II) is substantially the same as the content of the polypropylene resin (B), the carbon nanotube (C), and the polypropylene resin (D) blended in the base resin (II), respectively.

(Polypropylene resin foam molded article)

The expanded beads are molded in a mold to obtain a polypropylene resin expanded beads molded article. The surface resistivity of the molded foam of expanded beads obtained by in-mold molding the expanded beads was 1X 10 ⁵ Omega above 1×10 ⁹ Omega or less. The expanded granular molded article having the surface resistivity in the above range is suitable as a cushioning material or a packaging material for electronic parts, electronic devices, or the like, for example, because it can slowly discharge static electricity charged in the object to be packaged or the like.

The surface resistivity of the expanded granular molded article was determined by following JIS K6271-1:2015, a value measured by a measuring method. Specifically, first, test pieces having a rectangular parallelepiped shape of 100mm in length, 100mm in width and 20mm in thickness were collected from the expanded granular molded body. At this time, the test piece was collected so that at least one of the two surfaces having the dimensions of 100mm long by 100mm wide was a surface of the surface, in other words, a surface that was in contact with the mold at the time of in-mold molding. After the electrodes were attached to the skin surface of the test piece, a voltage of 10V was applied between the electrodes in an atmosphere having a temperature of 23 ℃ and a relative humidity of 50%. Then, the surface resistivity (unit: Ω) at the time point when 30 seconds passed after the voltage was applied was used as the surface resistivity of the expanded granular molded product.

Examples

Examples of the polypropylene resin expanded beads and molded articles of the polypropylene resin expanded beads will be described. The specific embodiment of the polypropylene resin expanded beads and the polypropylene resin expanded beads molded article according to the present invention is not limited to the embodiment shown below, and the configuration may be appropriately changed within a range not impairing the gist of the present invention. In the following examples, "polypropylene" may be omitted as "PP" and carbon nanotubes may be omitted as "CNT".

The polypropylene resin used in this example has the following melting point, melt flow rate, heat of fusion and weight average molecular weight.

PP1; propylene-1-butene-ethylene copolymer (density: 900 g/cm) ³ MFR:6g/10 min, melting point: heat of fusion at 133 ℃): 63J/g, flexural modulus: 650MPa, ethylene component content: 3.1%)

PP2; low melting point polypropylene resin (L-MODU (registered trademark) S400, manufactured by Nippon Denshoku Co., ltd., density: 870 g/cm) ³ MFR:2600g/10 min, melting point: heat of fusion at 84 ℃): 3J/g, weight average molecular weight: 45000)

PP3; ethylene-propylene random copolymer (Density: 900 g/cm) ³ MFR:8g/10 min, melting point: 143 ℃, heat of fusion: 79J/g, flexural modulus: 950MPa, ethylene component content; 3.1%)

The physical properties of the polypropylene resin were measured as follows.

[ melting Point and Heat of melting of Polypropylene-based resin ]

Based on JIS K7121: the melting point and the heat of fusion of the polypropylene resin were measured by the calorimetric measurement of heat flux difference as described in 1987. First, a test piece made of polypropylene resin was prepared, and the state of the test piece was adjusted by scanning a calorimeter with a heat flux difference and standing for 1 day or more in an environment of a temperature of 23 ℃ and a relative humidity of 50% rh. The temperature of the test piece after the state adjustment was raised from 23℃to 200℃at a temperature-raising rate of 10℃per minute, then cooled to 23℃at a cooling rate of 10℃per minute, and again raised from 23℃to 200℃at a temperature-raising rate of 10℃per minute. Then, the melting point of the resin was defined as the peak temperature of the endothermic peak determined from the DSC curve obtained at the time of the second temperature rise. In the case where a plurality of endothermic peaks appear in the second DSC curve, the peak temperature of the endothermic peak having the largest area is used as the melting point. For obtaining the DSC curve, a calorimeter (DSC 7020, manufactured by SII NanoTechnology Co., ltd.) was used.

In addition, the area (unit: J) of the endothermic peak corresponding to the melting of the polypropylene resin was calculated in the DSC curve of the second time obtained by the above-mentioned method. The heat of fusion (unit: J/g) of the polypropylene resin was obtained by dividing the area of the endothermic peak by the mass (unit: g) of the test piece.

[ melt flow Rate of Polypropylene-based resin ]

The melt flow rate of the polypropylene resin was determined by following JIS K7210-1:2014, measured under conditions of a temperature of 230℃and a load of 2.16kg (unit: g/10 min).

[ weight average molecular weight of Polypropylene resin ]

The weight average molecular weight of the polypropylene resin was calculated based on a chromatogram obtained by Gel Permeation Chromatography (GPC) using polystyrene as a standard substance.

For the acquisition of the chromatograph, 150C manufactured by water company was used. After a sample solution having a concentration of 2.2mg/ml was prepared by dissolving a resin as a measurement sample in 1,2, 4-trichlorobenzene, the sample solution was eluted with TSKgel (registered trademark) GMHHR-H (S) HT as a column: 1,2, 4-trichlorobenzene, flow: 1.0 ml/min, temperature: under 145 ℃ separation conditions, the measurement sample was separated by a Gel Permeation Chromatography (GPC) using a difference in molecular weight, and a chromatogram was obtained.

Then, the retention time in the chromatogram obtained by using the calibration curve prepared with the standard polystyrene was converted into a molecular weight, and a differential molecular weight distribution curve was obtained. The weight average molecular weight Mw of the polypropylene resin was calculated from the differential molecular weight distribution curve.

Table 1 shows the types, average lengths, average outer diameters, and aspect ratios of the carbon nanotubes used in this example.

TABLE 1

CNT symbol	CNT1	CNT2	CNT3	CNT4
					Species of type	Multilayer CNT	Multilayer CNT	Multilayer CNT	Multilayer CNT
Average length (μm)	1.5	1	10	3～12
					Average outer diameter (nm)	9.5	12	10～25	12～25
Aspect ratio (L/D)	158	83	400～1000	200～1000

Example 1

As shown in fig. 2, the expanded particle 1 of this example has a core layer 2 in a foamed state and a coating layer 3 coating the core layer 2. As shown in table 2, the core layer 2 uses PP3 as the polypropylene resin (a) as the base resin (I). The coating layer 3 has a melting point Tm _(B) PP1 of polypropylene resin (B) with a temperature of 125-150 ℃, carbon nanotube (C), melting point Tm _(D) The polypropylene resin (D) having a temperature of 70 to 100 ℃ inclusive and the PP2 composition (X) being a base resin (II).

In producing the expanded beads 1 of the present example, first, a base resin (II) for forming a coating layer was prepared by the following method. A masterbatch of carbon nanotubes (C) in which the carbon nanotubes (C) shown in table 2 were dispersed in a polypropylene resin PP1 was prepared. The content of the carbon nanotube (C) in the master batch was 15 mass%. The master batch was kneaded with the polypropylene resin (B) and the polypropylene resin (D) shown in table 1 in an extruder to prepare pellets of the base resin (II) having the mass ratio of the polypropylene resin (B), the carbon nanotubes (C), and the polypropylene resin (D) shown in table 2.

Next, an extrusion molding machine for forming a core layer having an inner diameter of 26mm and an extrusion molding machine for forming a clad layer having an inner diameter of 25mm were arranged in parallel, and a strand was produced using a coextrusion apparatus having a die capable of coextrusion in a multi-layer wire form attached to an outlet side. The polypropylene resin (a) shown in table 2 and the bubble regulator in an amount of 1000 mass ppm relative to the mass of the polypropylene resin (a) were supplied to the core layer forming extruder, and both were kneaded in the core layer forming extruder. Further, particles of the base resin (II) having the composition shown in the column "coating layer" in table 2, prepared by the above-described method, were supplied to the extrusion molding machine for forming the coating layer. In addition, zinc borate powder was used as the bubble regulator.

Then, "core layer" in table 2 was obtained as the mass ratio of core layer to clad layer: the core layer-forming resin melt and the cladding layer-forming resin melt were co-extruded from the extrusion molding machines so that the values indicated in the column of the cladding layer were obtained. The melt-kneaded materials extruded from the respective extrusion molding machines are joined in a die, and extruded into a multi-layered linear shape in which the side surfaces of the core layer are covered with the covering layer from the fine holes of the joint attached to the tip of the extrusion molding machine. By water-cooling the extrudate, a multi-layered strand is obtained.

The obtained strand was cut with a fan cutter so that the mass of the strand became about 1.0 mg. Thus, resin particles having a core layer in an unfoamed state and a coating layer coating the side surface of the core layer were obtained.

Next, a primary foaming step is performed as follows, and the resin particles are foamed. In the closed vessel, 1000g of resin particles, 3L of water as a dispersion medium, 3g of kaolin as a dispersant, 0.2g of a surfactant, and 0.1g of aluminum sulfate were enclosed. As the surfactant, specifically, a 20% aqueous solution of sodium alkylbenzenesulfonate (manufactured by first Industrial pharmaceutical Co., ltd. "Neogen (registered trademark) S-20F") was used.

Thereafter, carbon dioxide as a blowing agent was supplied into the closed vessel so that the pressure in the vessel was 2.1MPa (G) under the gauge pressure, and the vessel was pressurized. In this state, the inside of the container was heated while stirring, and the temperature in the container was raised to 149.8 ℃. After the foaming temperature was maintained for 10 minutes, the pressure in the closed vessel was maintained at 2.6MPa (G) under gauge pressure by pressurizing with carbon dioxide, and the closed vessel was opened to release the content at atmospheric pressure, whereby the resin particles were foamed. As described above, the primary expanded particles having a multilayer structure including the core layer in a foamed state and the coating layer coating the core layer were obtained. Further, since the primary expanded beads immediately after release from the closed vessel contained moisture, curing was carried out at a temperature of 23℃for 24 hours.

Next, the secondary foaming step is performed as follows, and the primary expanded beads are further foamed. After the primary expanded particles are filled in the pressure-resistant container, air as an inorganic gas is injected into the pressure-resistant container, so that the inorganic gas is immersed in the air bubbles of the primary expanded particles. The pressure in the air bubbles in the primary expanded particles taken out from the pressure-resistant vessel is 0.45 to 0.5MPa (G) under gauge pressure. Then, steam is supplied to the primary expanded beads taken out of the pressure-resistant vessel, and the primary expanded beads are heated at atmospheric pressure. The pressure of the steam supplied during heating was 0.02 to 0.12MPa (G) under gauge pressure, and the heating time was 15 seconds. As described above, the primary expanded particles were further expanded to obtain expanded particles (secondary expanded particles).

Then, in-mold molding was performed as follows to prepare a foam molded particle. First, after the secondary expanded beads are sealed in a closed vessel, the inside of the closed vessel is pressurized by compressed air to raise the internal pressure of the cells of the secondary expanded beads to 0.09 to 0.13MPa (G) at the gauge pressure. The secondary expanded beads were filled into a mold of an EPP molding machine, and molded using steam. The steam forming pressure is 0.22-0.36 MPa (G) under the gauge pressure.

(examples 2 to 6 and comparative example 3)

The expanded beads of examples 2 to 6 and comparative example 3 have the same constitution as that of example 1 except that the constitution of the coating layer is changed as shown in Table 2, table 3 or Table 5.

Example 7 to example 9

The expanded beads of examples 7 to 9 have the same structure as the expanded beads of example 1, except that the carbon nanotubes (C) contained in the coating layer were changed as shown in table 4.

(comparative example 1, comparative example 2)

As shown in table 5, the expanded beads of comparative example 1 and comparative example 2 have the same constitution as the expanded beads of example 1 except that the polypropylene-based resin (D) is not contained in the base resin (II) in the coating layer.

Comparative examples 4 to 6

As shown in table 6, the expanded beads of comparative examples 4 to 6 were composed of only a core layer composed of the polypropylene-based resin (a) as a base resin, and had no coating layer. In addition, the base resin of comparative example 4 does not contain a conductive material. Carbon black was contained as the conductive material in the base resin of comparative example 5, and carbon nanotubes were contained as the conductive material in the base resin of comparative example 6.

In producing the expanded beads of comparative example 4, the polypropylene resin (a) was extruded from an extruder to produce a single-layer strand having the composition shown in table 6. The strand was cut to obtain resin pellets. Then, the resin particles were foamed by the same method as the foamed particles of example 1, thereby obtaining foamed particles of comparative example 4.

In producing the expanded beads of comparative examples 5 and 6, the conductive material and the polypropylene resin (a) were kneaded in an extruder, and then extruded from the extruder to produce a single-layer strand having the composition shown in table 6. The strand was cut to obtain resin pellets. Then, the resin particles were foamed by the same method as the foamed particles of example 1, thereby obtaining foamed particles of comparative examples 5 and 6.

Next, the evaluation methods of the characteristics shown in tables 2 to 6 will be described.

[ melting Point Tm of base resin (II) ] _(II) Heat of fusion Q _(II) ]

Melting Point Tm of base resin (II) _(II) Heat of fusion Q _(II) The measurement method of (2) was performed in the same manner as the measurement method of the melting point and heat of fusion of the polypropylene resin described above, except that the differential scanning calorimetry was performed using a test piece composed of the base resin (II) instead of a test piece composed of the polypropylene resin.

[ coating Property of coating layer ]

In examples 1 to 9 and comparative examples 1 to 3 each having a multilayer structure, round bar-shaped strands extruded from a coextrusion apparatus were visually observed, and the coating state of a coating layer was evaluated. In the column of "coating properties" in tables 2 to 5, the symbol "a" indicates that the entire side surface of the wire material is uniformly coated with the coating layer, the symbol "B" indicates that the thickness of the coating layer varies, but the entire side surface of the round bar-shaped wire material is coated with the coating layer, the symbol "C" indicates that the coating layer is difficult to coat the entire core layer, and a part of the core layer is exposed at the side surface of the wire material.

[ extrusion Property of coating layer ]

In examples 1 to 9 and comparative examples 1 to 3 having a multilayer structure, the extrudability of the clad layer was evaluated based on the load of the motor of the extruder for forming the clad layer when the core layer and the clad layer were co-extruded. The symbol "a" in the column of "extrudability of coating layer" in tables 2 to 5 indicates that the coating layer can be extruded without problems, and the symbol "B" indicates that the viscosity of the base resin (II) increases and that a motor of the extrusion molding machine for forming the coating layer is overloaded or the like is defective.

[ high temperature Peak Heat ]

The measurement sample used for measuring the high temperature peak heat amount may be primary expanded particles or expanded particles. In this example, differential scanning calorimetric measurement was performed using primary expanded beads as a measurement sample. Specifically, about 2mg of the primary expanded beads were used as test pieces, and the test pieces were heated and melted according to the method for measuring the transfer heat of plastics described in JIS K7122-1987, to obtain DSC curves at this time. The measurement temperature was in the range from 30℃to 30℃higher than the temperature at the end of the melting peak, and the heating rate at the time of heating was 10℃per minute.

The endothermic peak in the DSC curve thus obtained is divided into a natural peak Δh1 and a high temperature peak Δh2 by the above-described method (see fig. 1). Then, the total of the area of the intrinsic peak Δh1 and the area of the high temperature peak Δh2 was taken as the total heat value, and the area of the high temperature peak Wen Feng H2 was taken as the high temperature heat value.

[ bulk Density of expanded particles ]

More than 500 expanded beads were allowed to stand in an atmosphere having a relative humidity of 50% and a relative humidity of 1atm at a temperature of 23℃for 24 hours. The thus obtained foamed particle group was filled in a measuring cylinder in a natural pile manner, and the pile volume (unit: L) of the foamed particle group was read out from the scale of the measuring cylinder. Then, the bulk density (unit: g/L) of the expanded beads was obtained by dividing the mass (unit: g) of the expanded bead group in the measuring cylinder by the aforementioned bulk volume (unit: L).

[ moldability of foam-molded particles ]

The formability of the expanded granular molded article was evaluated based on the value of the welding ratio calculated by the following method. First, the expanded granular molded body is bent and broken so as to be substantially equally divided in the longitudinal direction. The fracture surface thus exposed was visually observed, and the number of expanded particles having peeled at the interface between the expanded particles and the number of expanded particles having broken inside were counted. Then, the ratio of the number of expanded particles broken inside the expanded particles to the total number of expanded particles exposed at the broken surface, that is, the total of the number of expanded particles peeled at the interface between the expanded particles and the number of expanded particles broken inside was calculated. The welding ratio was defined as a value expressed as a percentage (%).

The symbol "a+" in the column of "formability" in tables 2 to 6 indicates a welding ratio exceeding 95%, the symbol "a" indicates a welding ratio exceeding 80% and not more than 95%, and the symbol "B" indicates a welding ratio exceeding 70% and not more than 80%. In the evaluation of the formability, the welding ratio was judged to be acceptable because the formability was excellent when the symbol "a+" or "a" was 80% or more.

[ apparent Density of expanded molded particles ]

Dividing the mass (unit: kg) of the expanded granular molded body by the volume (unit: m) of the expanded granular molded body calculated based on the external dimension ³ ) The obtained value was used as the apparent density (unit: kg/m ³ )。

[ surface resistivity of expanded molded article)

By following JIS K6271-1:2015, the surface resistivity of the expanded granular molded article was measured. Specifically, first, test pieces having a rectangular parallelepiped shape of 100mm in length, 100mm in width and 20mm in thickness were collected from the expanded granular molded body. At this time, the test piece was collected so that at least one of two surfaces having dimensions of 100mm long by 100mm wide was a surface of the test piece, in other words, a surface that was in contact with the mold at the time of in-mold molding. After the electrodes were attached to the skin surface of the test piece, a voltage of 10V was applied between the electrodes in an atmosphere having a temperature of 23 ℃ and a relative humidity of 50%. Then, the surface resistivity (unit: Ω) at the time point when 30 seconds passed after the voltage was applied was used as the surface resistivity of the expanded granular molded product.

[ surface resistivity of base resin (II) ]

A test piece having a rectangular parallelepiped shape with a length of 100mm, a width of 100mm, and a thickness of 20mm was produced using the base resin (II). By following JIS K6271-1:2015, and the surface resistivity of the test piece was measured. Specifically, after electrodes were mounted on the surface of the test piece having a length of 100mm×a width of 100mm, a voltage of 10V was applied to the electrodes in an atmosphere having a temperature of 23 ℃ and a relative humidity of 50%. Then, the surface resistivity (unit: Ω) at the time point when 30 seconds passed after the voltage was applied was used as the surface resistivity of the expanded granular molded product.

TABLE 2

TABLE 3 Table 3

TABLE 4 Table 4

The data of the table is recorded in the table,

TABLE 6

As shown in tables 2 to 4, the core layer in the expanded beads of examples 1 to 9 was formed using the base resin (I), and the coating layer was formed using the base resin (II) having the specific composition. Therefore, the expanded beads of examples 1 to 9 were excellent in weldability and moldability. Further, by in-mold molding the expanded beads of examples 1 to 9, a molded article of expanded beads having electrostatic diffusibility can be easily obtained.

As shown in table 5, the coating layer of the resin particles of comparative example 1 did not contain the polypropylene-based resin (D). Therefore, in producing the resin particles of comparative example 1, it is difficult to form the clad layer and laminate the clad layer to the core layer.

More carbon nanotubes (C) than comparative example 1 were blended in the coating layer of the resin particles of comparative example 2. Thus, the formation of the coating layer becomes more difficult. In comparative example 2, the amount of carbon nanotubes (C) blended was large, and therefore, the formability was lowered.

The blending amount of the carbon nanotubes (C) in the expanded beads of comparative example 3 is smaller than the specific range. Therefore, although the expanded beads can be formed by coating the core layer with the coating layer, it is difficult to impart desired electrical characteristics to the expanded bead molded article even if the expanded beads of comparative example 3 are molded in a mold.

Comparative example 4 is an example of foamed particles composed of only a core layer and containing no conductive substance in the core layer. The expanded beads of comparative example 4 were poor in moldability.

Comparative example 5 and comparative example 6 are examples in which a conductive material was added to the expanded beads composed only of the core layer. In the above expanded beads, a large amount of conductive material needs to be added to reduce the resistance of the expanded beads. Therefore, the expanded beads of comparative examples 5 and 6 were inferior in moldability.

Claims

1. A process for producing polypropylene resin foam particles, which comprises foaming polypropylene resin particles having a core layer and a coating layer covering the core layer, wherein,

The base resin (I) of the core layer is a polypropylene resin (A),

2. The method for producing expanded polypropylene resin particles according to claim 1, wherein said polypropylene resin (B) has a heat of fusion Q _(B) [J/g]Heat of fusion Q with the base resin (II) _(II) [J/g]Ratio Q of _(B) /Q _(II) 1.2 to 3.5.

3. The method for producing polypropylene resin foam particles according to claim 1 or 2, wherein the polypropylene resin (D) has a weight average molecular weight of 4 to 10 ten thousand.

4. The method for producing polypropylene resin foam particles according to any one of claims 1 to 3, wherein the polypropylene resin (D) has a heat of fusion Q _(D) Is 0J/g to 50J/g.

5. The method for producing polypropylene-based resin foam particles according to any one of claims 1 to 4, wherein the base resin (II) has a melting point Tm _(II) Is 120-140 ℃ and the heat of fusion Q _(II) Is 5J/g to 50J/g.

6. The method for producing polypropylene-based resin foam particles according to any one of claims 1 to 5, wherein the base resin (II) has a melt flow rate MFR at a temperature of 230℃under a load of 2.16kg _(II) Is 1g/10 min to 30g/10 min, and the melt flow rate MFR of the base resin (II) _(II) And the institute are connected withMelt flow Rate MFR of the base resin (I) _(I) Specific MFR of _(II) /MFR _(I) Is 0.2 to 4.5 inclusive.

7. A polypropylene resin foam particle comprising a core layer in a foamed state and a coating layer coating the core layer, wherein,

the base resin (I) of the core layer is a polypropylene resin (A),

8. A polypropylene resin foam particle molded article obtained by molding the polypropylene resin foam particle of claim 7 in a mold, wherein,

surface resistivity of 1X 10 ⁵ Omega above 1×10 ⁹ Omega or less.