CN110970166A

CN110970166A - Multilayer insulated wire and method for manufacturing same

Info

Publication number: CN110970166A
Application number: CN201910730288.0A
Authority: CN
Inventors: 三浦刚; 木部有; 岩崎周; 中村孔亮
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 2018-09-28
Filing date: 2019-08-08
Publication date: 2020-04-07
Also published as: JP2020053316A

Abstract

The invention provides a multilayer insulated wire having excellent flame retardancy even when a metal hydroxide having a small BET specific surface area is used as a halogen-free flame retardant, and a method for producing the same. The multilayer insulated wire of the present invention comprises a conductor, an inner layer provided around the conductor, and an outer layer provided around the inner layer, wherein the inner layer comprises a resin composition containing a matrix polymer mainly composed of a polyolefin, the outer layer comprises a resin composition containing a matrix polymer mainly composed of a polyolefin and containing 80 to 250 parts by mass of a metal hydroxide per 100 parts by mass of the matrix polymer, and the outer layer has a degree of foaming at 350 ℃ of 10.6% or more.

Description

Multilayer insulated wire and method for manufacturing same

Technical Field

The present invention relates to a halogen-free flame-retardant multilayer insulated wire and a method for manufacturing the same.

Background

An insulated wire used for internal wiring of electronic equipment is required to have flame retardancy in order to prevent a fire from spreading along the wire in the event of a fire accident of the equipment. The standard for flame retardancy of the internal wiring material is specified by, for example, the UL758 standard in the united states.

On the other hand, in europe where railway vehicle networks are developed, a regional standard known as the EN standard (european standard) is widely adopted, and there is a demand for electric wires and cables in which a halogen-free material having heat resistance, flame retardancy, hydrolysis resistance, abrasion resistance, and low smoke emission and containing no halogen is used as a covering material.

Examples of the halogen-free material include polyolefin resins such as polyethylene and polypropylene. However, only polyolefin resins still lack flame retardancy, and thus a method of adding a halogen-free flame retardant is used. As the halogen-free flame retardant, there is a metal hydroxide such as magnesium hydroxide or aluminum hydroxide (patent document 1).

Further, it is known that a crosslinked material obtained by crosslinking a halogen-free crosslinkable resin composition has flame retardancy and mechanical properties, and is excellent in fuel resistance, cold resistance and room temperature storage properties, and thus can be suitably used for an insulating layer or a sheath of an insulated wire (patent document 2).

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2000-109638

Patent document 2: japanese patent laid-open publication No. 2015-21120

Disclosure of Invention

Problems to be solved by the invention

However, if an electric wire in which a resin composition filled with a metal hydroxide is used as a coating material is crosslinked, there is a problem that the Vertical Flame Test (VFT) fails although the processability is good, particularly when a metal hydroxide having a small BET specific surface area is used.

Accordingly, an object of the present invention is to provide a multilayer insulated wire having excellent flame retardancy even when a metal hydroxide having a small BET specific surface area is used as a halogen-free flame retardant.

Means for solving the problems

The present inventors have found that a multilayer insulated wire having excellent flame retardancy that is acceptable in the Vertical Flame Test (VFT) that is the flame retardancy test standard EN60332-1-2 can be obtained by using a resin composition containing polyolefin as a main component and 80 to 250 parts by mass of a metal hydroxide per 100 parts by mass of a matrix polymer and subjecting the outer layer to a crosslinking treatment so that the degree of foaming at 350 ℃.

Namely, the present invention provides the following multilayer insulated wire.

[1] A multilayer insulated wire having: the resin composition for the outer layer is characterized by comprising a conductor, an inner layer provided around the conductor, and an outer layer provided around the inner layer, wherein the inner layer comprises a resin composition containing a matrix polymer mainly composed of polyolefin, the outer layer comprises a resin composition containing a matrix polymer mainly composed of polyolefin and containing 80 to 250 parts by mass of a metal hydroxide per 100 parts by mass of the matrix polymer, and the outer layer has a degree of foaming at 350 ℃ of 10.6% or more.

[2]According to [1]The multilayer insulated wire has a BET specific surface area of 1m²10m above g²The ratio of the carbon atoms to the carbon atoms is less than g.

[3] The multilayer insulated wire according to item [1] or [2], wherein the blowing agent is contained in an amount of 1 part by mass or more and 10 parts by mass or less based on 100 parts by mass of the matrix polymer of the outer layer.

[4] The multilayer insulated wire according to [3], wherein the foaming agent is expanded graphite.

[5] The multilayer insulated wire according to any one of [1] to [3], wherein the gel fraction of the outer layer is 70% or more and less than 96%.

[6] The multilayer insulated wire according to [4], wherein the gel fraction of the outer layer is 96% or more and 99% or less.

[7] A method for manufacturing a multilayer insulated wire, comprising the steps of: molding an inner layer around the conductor, the inner layer comprising a resin composition containing a matrix polymer having a polyolefin as a main component; forming an outer layer around the inner layer, the outer layer comprising a resin composition containing a matrix polymer mainly composed of a polyolefin, containing 80 to 250 parts by mass of a metal hydroxide per 100 parts by mass of the matrix polymer, and containing 1 to 10 parts by mass of a blowing agent per 100 parts by mass of the matrix polymer; the outer layer is crosslinked so that the degree of foaming at 350 ℃ of the outer layer becomes 10.6% or more.

[8] The method of manufacturing a multilayer insulated wire according to [7], wherein in the step of crosslinking the outer layer, the outer layer is irradiated with an electron beam of 10KGy or more and less than 70 KGy.

[9] The method of producing a multilayer insulated wire according to [7], wherein the step of forming the outer layer further comprises a foaming agent in an amount of 1 to 10 parts by mass based on 100 parts by mass of the matrix polymer of the outer layer.

[10] The method of manufacturing a multilayer insulated wire according to [9], wherein the step of crosslinking the outer layer irradiates the outer layer with an electron beam of 70KGy to 150 KGy.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a halogen-free multilayer insulated wire excellent in flame retardancy even when a metal hydroxide having a small BET specific surface area is used as a halogen-free flame retardant, and a method for producing the same can be provided.

Drawings

Fig. 1 is a cross-sectional view showing an example of a multilayer insulated wire according to the present invention.

Fig. 2 is a schematic diagram of an apparatus for measuring the storage elastic modulus in the evaluation of the expansion start temperature.

FIG. 3 shows an example of the measurement result of the temperature dependence of the storage modulus of elasticity of the present invention.

Description of the symbols

10: multilayer insulated wire

1: conductor

2: inner layer

3: and (4) an outer layer.

Detailed Description

Hereinafter, a multilayer insulated wire according to an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a cross-sectional view of a multilayer insulated wire according to an embodiment of the present invention, the cross-sectional view being perpendicular to a longitudinal direction.

As shown in fig. 1, the multilayer insulated wire 10 according to the present embodiment includes a conductor 1, an inner layer 2 provided around the conductor 1, and an outer layer 3 provided around the inner layer 2. The thicknesses of the inner layer 2 and the outer layer 3 are not particularly limited, but the inner layer 2 is preferably 0.4mm or less and the outer layer 3 is preferably 5mm or less.

(conductor)

As the conductor 1, besides a metal wire such as a copper wire or a copper alloy wire which is generally used, an aluminum wire, a gold wire, a silver wire, or the like can be used. Further, a metal wire plated with a metal such as tin or nickel on the outer periphery of the metal wire may be used. Further, a twisted wire obtained by twisting a metal wire, such as a collective twisted conductor, may also be used. The cross-sectional area and the outer diameter of the conductor 1 can be appropriately changed in accordance with the electrical characteristics required for the multilayer insulated wire 10, and for example, the cross-sectional area is 1mm²Above 10mm²And a conductor having an outer diameter of 1.20mm to 2.30 mm.

(inner layer)

The inner layer 2 functions as an insulating layer of the multilayer insulated wire 10, and includes a resin composition containing a matrix polymer containing a polyolefin as a main component. The term "main component" as used herein means that the polyolefin content is 50% by mass or more based on 100% by mass of the matrix polymer. In the present invention, the matrix polymer of the inner layer means a polymer component contained in the inner layer.

The polyolefin used for the inner layer 2 may be, for example, polyethylene, polypropylene, an ethylene-propylene copolymer, an ethylene-propylene-diene copolymer, an ethylene α olefin copolymer, an ethylene-vinyl acetate copolymer, an ethylene acrylate copolymer, acid-modified products thereof, and the like, and these components may be used alone or in combination, and among these polyolefin resins, an ethylene-vinyl acetate copolymer, an ethylene α olefin copolymer, and the like are preferable.

In addition to these components, additives such as an antioxidant, a silane coupling agent, a flame retardant aid, a crosslinking agent, a crosslinking aid, a crosslinking accelerator, a surfactant, a softening agent, an inorganic filler, a compatibilizer, a stabilizer, an ultraviolet absorber, and a Hindered Amine Light Stabilizer (HALS) may be added as necessary. In fig. 1, the inner layer 2 of the multilayer insulated wire has been described as being formed of a single layer, but the present invention is not limited thereto, and a laminated structure of a plurality of inner layers may be employed.

(outer layer)

The outer layer 3 contains a resin composition containing a matrix polymer containing polyolefin as a main component and containing a metal hydroxide.

In the present invention, the matrix polymer of the outer layer means a polymer component contained in the outer layer 3.

The polyolefin used for the outer layer 3 is preferably polyethylene, polypropylene or the like, and among them, polyethylene, an ethylene vinyl acetate copolymer, an ethylene ethyl acrylate copolymer, an ethylene butene acrylate copolymer, an ethylene methyl acrylate copolymer or the like can be used alone or in combination because a large amount of a flame retardant can be added, and further, these polyolefins also include modified versions thereof (for example, maleic anhydride, acrylic acid grafted products or the like), and the blending ratio of each component in the matrix polymer is preferably 20 to 70 parts by mass of an ethylene vinyl acetate copolymer (EVA) having a vinyl acetate content (VA amount) of 60 mass% or more, 25 parts by mass or more of EVA having a VA amount of 30 to 50 mass%, and 1 to 30 parts by mass of a maleic anhydride-modified ethylene α olefin copolymer, when the total amount of the matrix polymer is 100 parts by mass.

Examples of the metal hydroxide include magnesium hydroxide, aluminum hydroxide, hydrotalcite, calcium aluminate hydrate, calcium hydroxide, and barium hydroxide. In view of dispersibility, the metal hydroxide may be subjected to surface treatment with a silane coupling agent, a titanate coupling agent, a fatty acid such as stearic acid, or the like.

In view of flame retardancy, the content of the metal hydroxide is preferably 80 parts by mass or more and 250 parts by mass or less, more preferably 150 parts by mass or more and 250 parts by mass or less, and further preferably 150 parts by mass or more and 200 parts by mass or less, with respect to 100 parts by mass of the matrix polymer constituting the resin composition.

The BET specific surface area of the metal hydroxide used in the present invention is, for example, 1m²10m above g²A ratio of 3m or less per gram²8m above the g²A ratio of the total amount of the compound to the total amount of the compound is 5m or less²7m above the g²BET specific surface area of,/g or less. Here, it is known that the smaller the BET specific surface area of the metal hydroxide, the better the processability such as MFI. The BET specific surface area of the metal hydroxide herein is based on the "carrier gas method" of JIS Z8830: 2013, and nitrogen gas is used as a referenceThe obtained value was measured for adsorbate. For example, the specific surface area/pore distribution can be measured using a "FlowSorb" (manufactured by shimadzu corporation).

The resin composition used for the outer layer 3 may be added with additives such as other flame retardants, flame retardant aids, fillers, crosslinking agents, crosslinking aids, plasticizers, metal chelating agents, softeners, reinforcing agents, surfactants, stabilizers, ultraviolet absorbers, light stabilizers, lubricants, antioxidants, colorants, processability improvers, inorganic fillers, compatibilizers, foaming agents, foam nucleating agents, and antistatic agents, as needed. In fig. 1, the outer layer 3 of the multilayer insulated wire is described as being formed of a single layer, but the present invention is not limited thereto, and a laminated structure of a plurality of outer layers may be employed.

(degree of crosslinking)

The present inventors have found that in the Vertical Flame Test (VFT), which is a flame retardancy test based on EN60332-1-2, the lower the degree of crosslinking of the resin composition used for the outer layer 3, the higher the yield tends to be. Further, it was found that it is preferable to control the degree of crosslinking so that the degree of foaming at 350 ℃ of the outer layer 3 becomes 10.6% or more as an index of flame retardancy.

Here, the crosslinking method is not particularly limited, and may be performed by a known method such as a crosslinking method by electron beam irradiation, a heat vulcanization method, a water crosslinking method, or the like. Among these, the crosslinking method by electron beam irradiation is preferable because the crosslinking speed is high and the crosslinking degree can be freely controlled by changing the irradiation dose.

In the case of using a crosslinking method by electron beam irradiation for a resin composition to which no blowing agent is added, the irradiation dose is usually 10KGy or more and less than 70KGy, preferably 20KGy or more and 65KGy or less. In this case, the gel fraction is preferably 70% or more and less than 96%.

(foaming agent)

As described above, although the flame retardancy of the multilayer insulated wire can be improved by reducing the degree of crosslinking of the resin composition used for the outer layer 3 to make the degree of foaming of the outer layer 3 at 350 ℃ 10.6% or more, it is sometimes advantageous to improve the flame retardancy of the multilayer insulated wire while maintaining a certain degree of crosslinking.

For example, it is generally known that the strength of a resin is improved by increasing the degree of crosslinking of a resin composition. Therefore, by increasing the crosslinking degree of the resin composition used for the outer layer 3, the strength of the outer layer 3 is increased, and further, the strength of the multilayer insulated wire such as heat resistance and abrasion resistance is increased.

The present inventors have found that by adding a foaming agent to the resin composition used for the outer layer 3, even if the degree of crosslinking is increased, the degree of foaming at 350 ℃ of the outer layer is maintained at 10.6% or more, and a specific effect that the Vertical Flame Test (VFT) of the flame retardancy test based on EN60332-1-2 is acceptable is obtained.

Examples of the blowing agent used in the present invention include azo-based blowing agents such as azodicarbonamide (a.d.c.a) and azobisisobutyronitrile (a.i.b.n), nitroso-based blowing agents such as dinitrosopentamethylenetetramine (d.p.t), N 'dinitroso-N, N' -dimethylterephthalamide (d.n.d.m.t.a), hydrazide-based blowing agents such as P-toluenesulfonyl hydrazide (t.s.h), P-oxybis-benzenesulfonyl hydrazide (o.b.s.h) and benzenesulfonyl hydrazide (b.s.h), trihydrazinotriazine (t.h.t) and acetone-P-sulfonylhydrazone, and these can be used alone or in combination of two or more.

Examples of the inorganic foaming agent include Sodium hydrogen carbonate, ammonium hydrogen carbonate, Sodium borohydride (Sodium borohydrate), siloxy hydride, and expanded graphite.

The content of the foaming agent is not particularly limited as long as it is an amount capable of providing a degree of foaming of the outer layer 3 at 350 ℃ of 10.6% or more, and examples thereof include 1 part by mass or more and 10 parts by mass or less, preferably 3 parts by mass or more and 8 parts by mass or less, and more preferably 4 parts by mass or more and 6 parts by mass or less, relative to 100 parts by mass of the matrix polymer.

The electron beam irradiation amount when the blowing agent is used is, for example, 70KGy to 150KGy, preferably 75KGy to 100 KGy. In this case, the gel fraction is preferably 96% to 99%.

(expansion onset temperature)

In the present invention, the expansion start temperature is a temperature at which the storage elastic modulus starts to increase when a predetermined contact load is applied to the outer layer 3, a predetermined strain is applied at a predetermined frequency, and the temperature is increased at a predetermined temperature increase rate.

Specifically, as shown in the following examples, the expansion start temperature refers to a temperature at which the storage elastic modulus of the outer layer 3 starts to increase when the frequency is 10Hz, the strain is 1%, and the temperature increase rate is 55 ℃/min under the contact load of 1N.

The upper limit of the expansion start temperature is preferably 344 ℃. For example, 342 ℃ or lower, preferably 340 ℃ or lower, and more preferably 337 ℃ or lower.

The lower limit of the expansion initiation temperature is not particularly limited, and examples thereof include 200 ℃ or more, 250 ℃ or more, 300 ℃ or more, and the like.

(foaming degree at 350 ℃ C.)

In the present invention, since the degree of foaming of the outer layer at 350 ℃ is 10.6% or more, a multilayer insulated wire having excellent flame retardancy can be obtained. The reason why 350 ℃ is used as a reference is that 350 ℃ is a temperature at which thermal decomposition of the outer layer starts to generate a combustible gas, and the degree of foaming at this temperature is focused from the viewpoint of flame retardancy.

The lower limit of the degree of foaming is 10.6%. In consideration of flame retardancy, for example, 11.1% or more, preferably 12.0% or more, and more preferably 12.9% or more can be given.

The upper limit of the degree of foaming is not particularly limited as long as it is a range in which the desired flame retardancy is obtained, and examples thereof include 100% or less, 150% or less, and 200% or less.

Further, an intermediate layer may be provided between the outer layer and the inner layer as necessary. As the intermediate layer, for example, an insulating resin layer formed of a generally used resin exemplified as a matrix polymer that can be used for the outer layer and the inner layer can be used. Further, an outermost layer may be provided outside the outer layer 1. The outermost layer may be the same as the intermediate layer.

Examples

The present invention will be described in further detail below based on examples and comparative examples, but the present invention is not limited thereto.

[ method for producing multilayer insulated wire ]

The inner layer 2 and the outer layer 3 were extrusion-molded around the inner layer 2 around a tin-plated conductor 1 (twisted wire conductor obtained by twisting 37 0.18mm wires) having an outer diameter of 1.23mm using a 40mm extruder. As the material of the inner layer 2, a compounded resin composition shown in table 1, which was previously roll kneaded, was used. As the material of the outer layer 3, a compounded resin composition shown in table 2, which was previously roll kneaded, was used. Extrusion was performed so that the thickness of each layer was 0.3mm for the inner layer and 0.4mm for the outer layer. Then, the multilayer insulated wire 10 was crosslinked by electron beam irradiation.

[ evaluation method of gel fraction ]

The degree of crosslinking of the outer layer was evaluated based on the gel fraction. The outer layer 3 was cut and separated with a knife. Weighed in advance and immersed in xylene heated to 110 ℃ for 24 hours. After the impregnation, the mixture was left at 20 ℃ under atmospheric pressure for 3 hours, and the mass ratio of the mass of the outer layer after vacuum drying at 80 ℃ for 4 hours to the mass of the outer layer before xylene impregnation (the latter is a percentage of the denominator) was determined as the gel fraction.

[ method for evaluating flame retardancy ]

Regarding the flame retardancy evaluation of the produced multilayer insulated wire 10, a Vertical Flame Test (VFT) was performed as a flame retardancy test according to EN60332-1-2, the multilayer insulated wire 10 having a length of 600mm was held vertically, a flame was brought close to the multilayer insulated wire for 1060 seconds, after the flame was removed, whether or not the flame was extinguished within 60 seconds was evaluated, the test was performed 3 times, and the case where the flame was extinguished within 60 seconds for 3 times was regarded as a pass (◎), and the case where the flame was not more than 2 times was regarded as a fail (x).

[ evaluation method of expansion initiation temperature ]

Next, a method of evaluating the expansion start temperature of the resin composition used for the outer layer 3 will be described. After kneading the compounded material rolls shown in Table 2, sheet samples press-molded to a thickness of 1mm were prepared. The temperature dependence of the storage elastic modulus at 70 ℃ to 530 ℃ was measured by using a rheometer MCR302 manufactured by Anton Paar. FIG. 2 shows a schematic view of the measuring apparatus. The measurement conditions were that the plate outer diameter was 25mm, the temperature rise rate was 55 ℃/min, the strain was 1%, the frequency was 10Hz, and the contact load was 1N. The plate used was an aluminum parallel plate.

Fig. 3 shows an example of the measurement result of the temperature dependence of the storage elastic modulus. The sheet sample highly filled with a filler such as a flame retardant has high rigidity and does not completely adhere to the plate. Therefore, it was found that in the test, the contact area between the sample and the plate increased due to the expansion of the sample, and the storage elastic modulus apparently increased. Therefore, as the expansion start temperature, the temperature at which the storage elastic modulus starts to increase was evaluated.

[ method for evaluating foaming degree at 350 ℃ ]

The conductor 1 was pulled out from the multi-layer insulated wire 10 cut into a length of about 15mm, and a tubular sample formed of the inner layer 2 and the outer layer 3 was prepared. The sectional area S of the outer layer 3 before heating was measured from the cross section of the tubular sample using a digital microscope VHX5000 (manufactured by Keyence)₀. The tubular sample is put into a furnace, raised to 350 ℃ at a heating rate of 50 ℃/min and immediately taken out. The tubular sample taken out was cut into a circular piece, and the heated cross-sectional area S of the outer layer 3 of the cross-section of the circular piece was measured in the same manner. The degree of foaming R at 350 ℃ was calculated from the formula (1)_f。

R_f＝(S-S₀)/S₀Formula (1)

[ Table 1]

[ Table 2]

[ evaluation results ]

As shown in Table 2, in examples 1 to 5, the Vertical Flame Test (VFT) was satisfactory and the flame retardance was good. Although 6m having a small BET specific surface area is used²Magnesium hydroxide per gram, but the degree of foaming at 350 ℃ of the outer layer is as high as 10.6% or more by setting the dose of electron beam irradiation to 65KGy or less. This is considered to be because the outer layer functions as a heat insulating layer, suppressing heat conduction into the insulated wire and suppressing combustion due to vaporization of the resin composition constituting the inner layer.

These results are not necessarily limited by the following theory and can be considered as follows. That is, it is considered that the crosslinking degree of the outer layer is suppressed and the foaming degree of the outer layer at 350 ℃ is improved by reducing the irradiation dose of the electron beam. Further, it is considered that since the degree of foaming of the outer layer at 350 ℃ is high, heat conduction into the inside of the multilayer insulated wire is suppressed, and burning due to vaporization of the resin composition constituting the inner layer made of polyolefin is suppressed, whereby the flame retardancy of the entire multilayer insulated wire is increased.

With respect to comparative examples 1 and 2, the vertical burning test (VFT) failed. This is considered to be because the resin composition to which no blowing agent is added has an excessively high irradiation dose of electron beam of 75KGy or more, and therefore the crosslinking reaction becomes excessive, and the gel fraction exceeds 97%, and therefore the degree of foaming of the outer layer at 350 ℃.

On the other hand, in example 6, the dose of the electron beam was set to 75KGy in the same manner as in comparative example 1, but the vertical burning test (VFT) was acceptable. It is considered that example 6 contains a foaming agent, and therefore, even when the dose of electron beam irradiation is 75KGy, the degree of foaming of the outer layer at 350 ℃ can be maintained at 10.6% or more, and high flame retardancy can be exhibited.

As described above, it was found that a multilayer insulated wire having high flame retardancy can be obtained by setting the degree of expansion of the outer layer of the multilayer insulated wire at 350 ℃ to 10.6% or more.

Claims

1. A multilayer insulated wire having:

a conductor,

An inner layer disposed around the conductor, and

an outer layer disposed about the inner layer,

the inner layer comprises a resin composition containing a matrix polymer having a polyolefin as a main component,

the outer layer comprises a resin composition containing a matrix polymer mainly composed of a polyolefin and containing 80 to 250 parts by mass of a metal hydroxide per 100 parts by mass of the matrix polymer,

the outer layer has a degree of foaming of 10.6% or more at 350 ℃.

2. The multilayer insulated wire according to claim 1, wherein the metal hydroxide has a BET specific surface area of 1m²10m above g²The ratio of the carbon atoms to the carbon atoms is less than g.

3. The multilayer insulated wire according to claim 1 or 2, comprising a blowing agent in an amount of 1 part by mass or more and 10 parts by mass or less based on 100 parts by mass of the matrix polymer of the outer layer.

4. The multilayer insulated wire of claim 3, said blowing agent being expanded graphite.

5. The multilayer insulated wire according to claim 1 to 3, wherein the gel fraction of the outer layer is 70% or more and less than 96%.

6. The multilayer insulated wire according to claim 4, wherein the gel fraction of the outer layer is 96% or more and 99% or less.

7. A method for manufacturing a multilayer insulated wire, comprising the steps of:

molding an inner layer around the conductor, the inner layer comprising a resin composition containing a matrix polymer having a polyolefin as a main component;

forming an outer layer around the inner layer, the outer layer comprising a resin composition containing a matrix polymer mainly composed of a polyolefin, containing 80 to 250 parts by mass of a metal hydroxide per 100 parts by mass of the matrix polymer, and containing 1 to 10 parts by mass of a blowing agent per 100 parts by mass of the matrix polymer;

the outer layer is crosslinked so that the degree of foaming of the outer layer at 350 ℃ is 10.6% or more.

8. The method of manufacturing a multilayer insulated wire according to claim 7,

in the step of crosslinking the outer layer, the outer layer is irradiated with an electron beam of 10KGy or more and less than 70 KGy.

9. The method of manufacturing a multilayer insulated wire according to claim 7,

in the step of forming the outer layer, the foaming agent is further contained in an amount of 1 to 10 parts by mass based on 100 parts by mass of the matrix polymer of the outer layer.

10. The method of manufacturing a multilayer insulated wire according to claim 9,

in the step of crosslinking the outer layer, the outer layer is irradiated with an electron beam of 70KGy to 150 KGy.