JP2019043975A - Core-shell type particle - Google Patents

Abstract

To provide a material that can be used as a cavity forming material for forming a cavity in a molding of desired shape.SOLUTION: A core-shell type particle has: a core part that is mainly composed of a polymer and has a 5% weight loss temperature of 100°C or more and 200°C or less; and a shell part that coats the core part, and contains a polymer having a higher melting point than the 5% weight loss temperature of the core part.SELECTED DRAWING: Figure 1

Description

  The present invention relates to core-shell particles.

  It has been studied to improve the on-ice performance of a tire by using a vulcanized rubber having air bubbles (see, for example, Patent Document 1).

Japanese Patent Application Laid-Open No. 11-60770

  It is believed that molded articles of desired shape having voids can be used in various applications. Then, an object of this invention is to provide the material which can be used as a space | gap formation material for forming a space | gap in the molded object of a desired shape.

  The present invention covers the core portion containing a polymer as a main component and having a 5% weight loss temperature of 100 ° C. or more and 200 ° C. or less, and covering the core portion and having a melting point higher than the 5% weight loss temperature of the core portion The present invention relates to a core-shell type particle comprising:

  In the core-shell type particle, the difference between the 5% weight loss temperature of the core portion and the melting point of the polymer having a melting point higher than the 5% weight loss temperature of the core portion may be 5 ° C. or more.

  In the core-shell type particle, the core portion may contain a polymer having a polycarbonate skeleton.

  In the core-shell type particle, the core portion may be columnar, and the shell portion may be provided on the outer peripheral surface of the core portion.

  The core-shell type particle of the present invention can be used as a void-forming material for forming a void in a molded product having a desired shape.

It is a figure showing typically one embodiment of core shell type particles of the present invention. It is the figure which observed the cross section of the molded object obtained in Example 1. FIG.

  Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

[Definition]
As used herein, the Y% weight loss temperature refers to the temperature calculated by the following method. The material to be measured is heated from room temperature (25 ° C.) at a heating rate of 10 ° C./min in a nitrogen atmosphere, and the weight change is measured. From the obtained thermogravimetric curve, the temperature at which the mass decreases by Y mass% with respect to that before heating is calculated as the Y% weight reduction temperature. Specifically, a thermogravimetric analyzer such as TA Instruments SDT2960 can be used.

  In the present specification, the melting point indicates a value measured by differential scanning calorimetry. Specifically, using a differential scanning calorimeter (for example, DSC Q100 V 9.9 Build 303 manufactured by TA Instruments), the melting point is about 1 mg of an aluminum pan in which a sample of about 5 mg is sealed, under a nitrogen atmosphere (1) 200 C. for 5 minutes, then (2) decrease temperature from 200.degree. C. to -50.degree. C. at a rate of 10.degree. C./minute, then (3) hold at -50.degree. It can measure from the differential scanning calorimetry curve obtained by the calorie measurement in process (4) on the conditions which heat up from -50 degreeC to 200 degreeC at the speed | rate of 1 / min.

[Core-shell type particle]
The core-shell type particle of the present embodiment contains a polymer as a main component, and covers a core portion having a 5% weight loss temperature of 100 ° C. or more and 200 ° C. or less, and a 5% weight loss temperature of the core portion And a shell portion comprising a polymer having a higher melting point.

  The 5% weight loss temperature of the core portion is preferably 180 ° C. or less, more preferably 175 ° C. or less, and still more preferably 170 ° C. or less from the viewpoint of void formation. The 5% weight loss temperature of the core portion is preferably 110 ° C. or higher, and more preferably 120 ° C. or higher, from the viewpoint of easiness of production of core-shell type particles.

  The 80% weight loss temperature of the core portion is preferably 200 ° C. or less, more preferably 180 ° C. or less, and still more preferably 175 ° C. or less from the viewpoint of void formation.

  In the core portion, the difference between the 5% weight loss temperature and the 80% weight loss temperature ((80% weight loss temperature)-(5% weight loss temperature)) is 80 ° C. or less from the viewpoint of void formation. Is more preferable, 60.degree. C. or less is more preferable, and 40.degree. C. or less is still more preferable.

  In the core-shell type particle of the present embodiment, the core portion may be columnar, and the shell portion may be provided, for example, in a tubular shape on the outer peripheral surface of the core portion.

  FIG. 1 is a view schematically showing an embodiment of the core-shell type particle of the present invention. The core-shell type particle 100 shown in FIG. 1 includes a cylindrical core portion 1 and a shell portion 2 provided on the outer peripheral surface of the core portion 1 so as to cover the core portion 1.

  The diameter of the core portion 1 when viewed from the axial direction is preferably 10 to 5000 μm, more preferably 30 to 1000 μm, and still more preferably 50 to 500 μm from the viewpoint of void formation.

  The length of the core-shell type particle 100 when viewed from a direction perpendicular to the axial direction of the core portion 1 is preferably 0.1 to 50 mm, preferably 0.3 to 30 mm, from the viewpoint of void formation. 20 mm is more preferable, and 1 to 10 mm is further preferable.

  As mentioned above, although the core-shell type particle concerning one embodiment was explained, core-shell type particles are not limited to the above-mentioned embodiment. The shape of the core-shell particles may be spherical, ellipsoidal, tetragonal or rectangular. The shell portion may cover the entire core portion.

  Hereinafter, the material which comprises a core part and a shell part is demonstrated.

(Core part)
The core portion contains a polymer as a main component. That is, the largest component of the core is a polymer. The polymer component contained in the core portion may be 50% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass, based on the total mass of the core portion. % Or more may be sufficient, and 100 mass% may be sufficient. Hereinafter, in some cases, the “polymer component in the core portion” is referred to as “polymer A”.

  Examples of the polymer A include polymers having a 5% weight loss temperature of 100 ° C. or more and 200 ° C. or less. The polymer A may include a polymer having a polycarbonate backbone. That is, the core may include a polymer having a polycarbonate backbone. When the core portion contains a polymer having a polycarbonate skeleton, the content of the polymer having a polycarbonate skeleton may be 50% by mass or more and 70% by mass or more based on the total mass of the polymer in the core portion. It may also be 80% by mass or more, 90% by mass or more, or 100% by mass.

  As a polymer which has polycarbonate frame | skeleton, the copolymer of an alkylene oxide and a carbon dioxide is mentioned, for example. The alkylene oxide may contain at least one selected from ethylene oxide and propylene oxide. The said alkylene oxide may be substituted by substituents, such as a carboxy group.

  The polymer having a polycarbonate skeleton preferably further has a carboxy group from the viewpoint of easily adjusting the 5% weight loss temperature of the polymer to 200 ° C. or less. That is, the core portion preferably contains a polymer having a polycarbonate skeleton and a carboxy group. As a polymer which has polycarbonate frame | skeleton and a carboxy group, the polycarbonate containing the structural unit represented by following formula (1) is mentioned, for example.

In formula (1), R ¹ , R ² and R ³ each independently represent a hydrogen atom, an alkyl group having 1 to 10 carbons which may be substituted, or a carbon number 6 to 20 which may be substituted. N represents an aryl group, n represents 1 or 2; R ¹ , R ² and R ³ may be bonded to each other to form a ring together with the atoms to which each is bonded.

R ^1, the alkyl group having 1 to 10 carbon atoms as ^{R 2} or ^{R 3,} for example, straight-chain alkyl groups, branched alkyl groups and carbon atoms of 3 to 10 carbon atoms having 1 to 10 carbon atoms Several 3-10 cyclic alkyl groups are mentioned. As a linear alkyl group, a methyl group, an ethyl group, n-propyl group, n-butyl group, n-pentyl group, n-hexyl group, n-heptyl group, and n-octyl group are mentioned, for example. As a branched alkyl group, an isopropyl group, an isobutyl group, t-butyl group, an isopentyl group, a neopentyl group, and 2-ethylhexyl group are mentioned, for example. Examples of the cyclic alkyl group include cyclopropyl group, cyclobutyl group, cyclopentyl group, cyclohexyl group, cycloheptyl group and cyclooctyl group. As said alkyl group, a linear alkyl group or a branched alkyl group is preferable. The alkyl group may be substituted by at least one substituent. Examples of the substituent include an alkoxy group, an acyloxy group, an alkoxycarbonyl group, a silyl group, a sulfanyl group, a cyano group, a nitro group, a sulfo group, a formyl group, an aryl group and a halogeno group.

Examples of the aryl group having 6 to 20 carbon atoms as R ^{1, R 2} or ^{R 3,} for example, a phenyl group, indenyl group, naphthyl group, and a tetrahydronaphthyl group. It is preferable that carbon number of the said aryl group is 6-14. The aryl group may be substituted by at least one substituent. As a substituent, for example, an alkyl group such as methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, sec-butyl group, tert-butyl group, aryl group, alkoxy group, acyloxy group, alkoxy Examples thereof include a carbonyl group, silyl group, sulfanyl group, cyano group, nitro group, sulfo group, formyl group and halogeno group.

R ¹ , R ² or R ³ is preferably a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, more preferably a hydrogen atom.

  The polycarbonate containing the structural unit represented by the said Formula (1) may further contain the structural unit represented by following formula (2). That is, specific examples of the polymer having a polycarbonate skeleton and a carboxy group include a polycarbonate including the structural unit represented by Formula (1) and the structural unit represented by Formula (2).

In formula (2), R ⁴ , R ⁵ and R ⁶ each independently represent a hydrogen atom, an alkyl group having 1 to 10 carbons which may be substituted, or a carbon number 6 to 20 which may be substituted. An aryl group is shown, and X is a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, a haloalkyl group having 1 to 10 carbon atoms, an ether bond containing group, an ester bond containing group, or an allyl group. R ⁴ , R ⁵ and R ⁶ may be bonded to each other to form a ring together with the atoms to which each is bonded.

Exemplary and preferred embodiments of the optionally substituted 1 to 10 alkyl group as R ⁴ , R ⁵ and R ⁶ are optionally substituted 1 to 10 as R ¹ , R ² and R ³ Are the same as the alkyl group of Exemplary and preferred embodiments of the optionally substituted aryl group having 6 to 20 carbon atoms as R ⁴ , R ⁵ and R ⁶ are optionally substituted carbons as R ¹ , R ² and R ³ It is the same as the aryl group of the formulas 6 to 20.

  As a C1-C10 alkyl group as X, a methyl group, an ethyl group, and n-propyl group are mentioned, for example. As for carbon number of the alkyl group as X, 1-4 are preferable.

  As a C1-C10 haloalkyl group as X, a fluoromethyl group, a chloromethyl group, a bromomethyl group, and an iodomethyl group are mentioned, for example. As for carbon number of the haloalkyl group as X, 1-4 are preferable.

  As an ether bond containing group, a C1-C4 alkyl group substituted by a C1-C4 alkoxy group, an allyloxy group etc. is mentioned. Specific examples of the ether bond-containing group include a methoxymethyl group, an ethoxymethyl group and an allyloxymethyl group.

  As ester bond containing group, a C1-C4 alkyl group substituted with a C1-C4 acyloxy group, a benzyloxycarbonyl group etc. is mentioned. Specific examples of the ester bond-containing group include an acetoxymethyl group and a butyryloxymethyl group.

  X is preferably a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and more preferably a hydrogen atom or a methyl group.

  An alkylene oxide capable of forming a constituent unit represented by the formula (1) as a method for producing a polycarbonate containing the constituent unit represented by the formula (1) and, if necessary, the constituent unit represented by the formula (2) The method etc. which carry out the polymerization reaction of the alkylene oxide which can form the structural | constituent unit represented by Formula (2), and a carbon dioxide as needed according to presence of a metal catalyst etc. are mentioned.

  When the core portion contains a polymer having a polycarbonate skeleton, the weight average molecular weight of the polymer is preferably 3000 or more, more preferably 10000 or more, from the viewpoint of handleability when producing core-shell type particles. . The weight average molecular weight of the polymer is preferably 1,000,000 or less, more preferably 500000 or less, and still more preferably 300000 or less, from the viewpoint of easily adjusting the 5% weight loss temperature to 200 ° C. or less.

  When the core portion contains a polymer having a polycarbonate skeleton, the glass transition temperature of the polymer is preferably 0 ° C. or higher, and preferably 10 ° C. or higher, from the viewpoint of handleability when producing core-shell type particles. It is more preferable that the temperature is 15 ° C. or more. The glass transition temperature of the polymer is preferably 50 ° C. or less, more preferably 40 ° C. or less, and still more preferably 30 ° C. or less from the viewpoint of the flexibility of the core-shell particles.

(Shell part)
The shell portion comprises a polymer having a melting point above the 5% weight loss temperature of the core portion. This makes it possible to thermally decompose the core portion while maintaining the shape of the shell portion, and it is considered that a void of a desired shape can be easily formed in the molded body. Hereinafter, in some cases, the “polymer having a melting point higher than the 5% weight loss temperature of the core portion” is referred to as “polymer B”.

  The melting point of the polymer B can be appropriately selected according to the core portion. The melting point of polymer B is greater than 100.degree. The melting point of the polymer B may exceed 160 ° C., may exceed 170 ° C., and may exceed 175 ° C., from the viewpoint of void formation. The melting point of the polymer B is preferably 300 ° C. or less from the viewpoint of handleability when producing core-shell type particles.

  Examples of the polymer B include olefin polymers, (meth) acrylate polymers, polyvinyl alcohol, polyesters, polycarbonates, polyacetals, polyphenylene ethers and polyphenylene sulfides; and mixtures and polymer alloys thereof. The polymer may be modified by known methods such as grafting and crosslinking. The shell part which concerns on this embodiment may contain only 1 type of these polymers, and may contain 2 or more types.

  As the olefin polymer, for example, high density polyethylene, low density polyethylene, polypropylene, ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, ethylene-acrylic acid ester copolymer, ethylene-methacrylic acid copolymer And ethylene-methacrylic acid ester copolymer, ethylene-glycidyl methacrylate copolymer and ethylene-vinyl alcohol copolymer.

  Examples of (meth) acrylate polymers include polymethyl (meth) acrylate, polyethyl (meth) acrylate, polypropyl (meth) acrylate, polybutyl (meth) acrylate, poly 2-ethylhexyl (meth) acrylate and polylauryl (meth) Acrylate, polyisobornyl (meth) acrylate, polybenzyl (meth) acrylate, polycyclohexyl (meth) acrylate, polyethylene glycol di (meth) acrylate, polyneopentyl glycol di (meth) acrylate, polyurethane (meth) acrylate, and polytrimethylolpropane Tri (meth) acrylate is mentioned.

  Examples of the polyvinyl alcohol include partially saponified polyvinyl alcohol, completely saponified polyvinyl alcohol, sulfonic acid-modified polyvinyl alcohol, carboxyl-modified polyvinyl alcohol, and silanol group-modified polyvinyl alcohol.

  Examples of polyesters include aromatic polyesters such as polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate; and polylactic acid, polycaprolactone, poly-3-hydroxybutyrate, poly (4-hydroxybutyrate), poly (3) Aliphatic polyesters such as -hydroxy propionate).

  Examples of polycarbonates include polybis (4-hydroxyphenyl) methane carbonate, poly 1,1-bis (4-hydroxyphenyl) ethane carbonate, poly 2,2-bis (4-hydroxyphenyl) propane carbonate, poly 2,2 Bis- (4-hydroxyphenyl) propane carbonate, and poly 2,2-bis (4-hydroxyphenyl) butane carbonate.

  Examples of polyacetals include polyformaldehyde (polyoxymethylene), polyacetaldehyde, polypropionaldehyde and polybutyraldehyde.

  As polyphenylene ether, for example, poly (2,6-dimethylphenylene-1,4-ether), poly (2,6-diethylphenylene-1,4-ether), poly (2,6-dibromophenylene-1) , 4-ether), poly (2-methyl-6-ethylphenylene-1,4-ether), poly (2-chloro-6-methylphenylene-1,4-ether), poly (2-methyl-6-) Isopropylphenylene-1,4-ether), poly (2,6-di-n-propylphenylene-1,4-ether), poly (2-chloro-6-bromophenylene-1,4-ether), poly ( 2-chloro-6-ethylphenylene-1,4-ether), poly (2-methylphenylene-1,4-ether), poly (2-chlorophenylene-1,4-ether), (2-phenylphenylene-1,4-ether), poly (2-methyl-6-phenylphenylene-1,4-ether), poly (2-bromo-6-phenylphenylene-1,4-ether) and Poly (2,4'-methylphenyl phenylene-1,4-ether) is mentioned.

  The difference between the 5% weight loss temperature of the core portion and the melting point of the polymer B ((melting point of polymer B) − (5% weight loss temperature of the core portion) is 5 ° C. or higher from the viewpoint of void formation. The temperature is preferably 10 ° C. or more, more preferably 15 ° C. or more.

  The melting point of the polymer B is preferably higher than the 80% weight loss temperature of the core portion. The difference between the 80% weight loss temperature of the core portion and the melting point of the polymer B ((melting point of polymer B) − (80% weight loss temperature of the core portion)) is 3 ° C. or higher from the viewpoint of void formation. It is preferable that it is 5 degreeC or more.

  The total mass of the material constituting the core portion is preferably 30 to 99% by mass, preferably 40 to 95% by mass, based on the total mass of the core-shell particles, from the viewpoint of easily forming sufficient voids. Is more preferable, and 50 to 90% by mass is more preferable.

  The total mass of the material constituting the shell portion is preferably 1 to 70 mass%, preferably 5 to 60 mass%, based on the total mass of the core-shell type particles, from the viewpoint of easily maintaining the formed void. Is more preferably 10 to 50% by mass.

  The core-shell type particle of the present embodiment can be obtained, for example, by applying a composition for forming a shell portion to a molded body obtained by molding a material that can be a core portion.

  Although there is no restriction | limiting in particular in the shaping | molding method of the material which can become a core part, For example, the method by melt extrusion is mentioned.

  There is no particular limitation on the method of applying the composition for forming the shell portion to the formed body, for example, a method of impregnating the formed body with a solution containing the constituent material of the shell portion and a solvent and drying to remove the solvent. Can be mentioned.

  The core-shell type particle of the present embodiment includes a step of forming a material that can be a core portion into a fiber, a step of forming a shell portion on the outer peripheral surface of the obtained fiber, and a fiber having a shell portion formed thereon. And cutting into lengths. According to such a method, it is possible to particularly easily manufacture core-shell type particles in which the core portion has a columnar shape and the shell portion is provided in a tubular shape on the outer peripheral surface of the core portion.

  The core-shell type particle of the present embodiment can be used as a void forming material for forming a void in a molded body having a desired shape. Hereinafter, the manufacturing method of the molded object which used the core-shell type particle of this embodiment as a space | gap formation material, and an example of the molded object obtained by the said manufacturing method are demonstrated.

[Method for producing molded body and molded body]
The molded body having a void is obtained by, for example, mixing the material for forming the molded body and the core-shell type particles (mixing step) and the mixture obtained by mixing, the 5% weight loss temperature of the core portion of the core-shell type particles It can manufacture by the method provided with the process (molding process) heated and shape | molded to the above temperature.

  In the molding step, for example, after the above mixture is filled in a mold, heating and pressurization may be performed so that the temperature in the mold becomes a temperature equal to or higher than the 5% weight loss temperature of the core portion.

  The material for forming the formed body is not particularly limited, but a material which can be kneaded (for example, melt-kneaded) at a temperature lower than the 5% weight loss temperature of the core portion of the core-shell type particles is preferable. Examples of the material for forming a molded body include a resin composition. The resin composition may contain a crosslinking agent, and the resin composition may be crosslinked in the molding step. The heating temperature in the molding step is preferably lower than the melting point of the polymer B contained in the shell portion.

  It is thought that voids corresponding to the shape of the core-shell particles can be formed in the molded body by the above method. Therefore, it is considered that by appropriately adjusting the shape of the core-shell type particles, it is possible to form a molded body having desired voids. The molded body according to the present embodiment may be a foamed rubber material.

  Hereinafter, the present invention will be more specifically described by way of examples. However, the present invention is not limited to these examples.

[Production of core-shell type particles]
Sumitomo Seika Co., Ltd. polypropylene carbonate (5% weight loss temperature: 152 ° C, 80% weight loss temperature: 173 ° C) 10 g diameter Capillary graph (barrel diameter 9.55 mm) manufactured by Toyo Seiki, capillary 1 mm in diameter and 40 mm in length And were pushed out of the capillary at 120 ° C. and a piston speed of 10 mm / min. The fibers of polypropylene carbonate extruded from the capillary were wound to a fiber diameter of 300 to 500 μm. The obtained fiber was dipped in a 30 wt% aqueous solution of Nippon Packard Bipobar JMR-20H (melting point 180 ° C.) and dried. The dried fiber was cut into a length of 5 to 8 mm to obtain core-shell type particles including polypropylene carbonate as a core portion and JMR-20H as a shell portion.

Example 1
10 parts by mass of the obtained core-shell particles, 90 parts by mass of ethylene-vinyl acetate copolymer, and 1.0 parts by mass of dicumyl peroxide (one-minute half-life temperature 175 ° C.) It knead | mixed by conditions of 110 degreeC, 80 rotations / min, and 5 minutes by Toyo Seiki Seisakusho make, brand name: 4 C150 type | mold, and the composition was obtained. The composition obtained is finely cut and then filled in a 40 mm × 40 mm × 5 mm mold, heated and pressed under conditions of a temperature of 170 ° C., a time of 40 minutes and a pressure of 20 MPa, and cooled to obtain a crosslinked molded article. Obtained. Although the volume of the molded body was the same size as the volume of the mold, a fibrous empty wall was formed inside. The figure which observed the cross section of the obtained molded object is shown in FIG.

(Comparative example 1)
1.0 part by mass of dicumyl peroxide, 1.4 parts by mass of Celmic C1 (compound name: azodicarbonamide) manufactured by Sankyo Kasei Co., Ltd., and 100 parts by mass of ethylene-vinyl acetate copolymer Add 1 part by mass of zinc oxide as an auxiliary agent, and knead it at 110 ° C., 80 rpm, and 5 minutes in a laboplast mill (trade name: 4C150 manufactured by Toyo Seiki Seisakusho Co., Ltd.) under the conditions of 5 minutes. The The composition obtained is finely cut and then filled in a 40 mm × 40 mm × 5 mm mold, heated and pressed under conditions of a temperature of 170 ° C., a time of 40 minutes and a pressure of 20 MPa, and cooled to obtain a crosslinked molded article. Obtained. Although there were innumerable fine holes in the inside of the molded body, the volume expanded more than the capacity of the mold, and the molded body itself did not have a desired shape in the first place.

(Comparative example 2)
1.0 parts by mass of dicumyl peroxide with respect to 80 parts by mass of ethylene-vinyl acetate copolymer, and polypropylene carbonate manufactured by Sumitomo Seika Chemicals Co., Ltd. (5% weight reduction temperature: 152 ° C., 80% weight reduction temperature: 20 parts by mass of 173 ° C.) was added, and the mixture was kneaded with a laboplast mill (manufactured by Toyo Seiki Seisakusho Co., Ltd., trade name: 4C150 type) under the conditions of 110 ° C., 80 rpm and 5 minutes to obtain a composition. The composition obtained is finely cut and then filled in a 40 mm × 40 mm × 5 mm mold, heated and pressed under conditions of a temperature of 170 ° C., a time of 40 minutes and a pressure of 20 MPa, and cooled to obtain a crosslinked molded article. Obtained. The resulting compact had a large volume of holes, and the compact did not have the desired shape.

  From the above, it was confirmed that the core-shell type particles of the examples can be used as a void-forming material for forming voids in a molded body having a desired shape.

  1 ... core part, 2 ... shell part, 100 ... core shell type particle.

Claims (4)

A core portion containing a polymer as a main component and having a 5% weight loss temperature of 100 ° C. or more and 200 ° C. or less;
Core-shell type particles, comprising: a shell part covering the core part and comprising a polymer having a melting point higher than the 5% weight loss temperature of the core part.   The core-shell type particle according to claim 1, wherein the difference between the 5% weight loss temperature of the core portion and the melting point of the polymer having a melting point higher than the 5% weight loss temperature of the core portion is 5 ° C or more.   The core-shell type particle according to claim 1 or 2, wherein the core part comprises a polymer having a polycarbonate skeleton.   The core-shell type particle according to any one of claims 1 to 3, wherein the core portion has a columnar shape, and the shell portion is provided on the outer peripheral surface of the core portion.