JP2024082649A - Crosslinked rubber composition, electric wire, cable, and method for producing crosslinked rubber composition - Google Patents

Abstract

[Problem] To provide a material having both a high degree of crosslinking and a high amount of crystallinity and excellent abrasion resistance, and an electric wire and cable using the same. [Solution] A crosslinked rubber composition containing a base polymer in which at least an ethylene-based copolymer resin is mixed with chlorinated polyethylene, the chlorinated polyethylene has a heat of fusion measured by differential scanning calorimetry (DSC) of more than 2 J/g, the ethylene-based copolymer resin has a melting point of 70°C or higher, the crosslinked rubber composition has a gel fraction of 70% or higher, and the residual heat of fusion, expressed as the ratio (percentage) of the heat of fusion of the crosslinked rubber composition measured by differential scanning calorimetry (DSC) after crosslinking to the heat of fusion of the crosslinked rubber composition measured by differential scanning calorimetry (DSC) before crosslinking, is 70% or higher, and the electric wire and cable using the crosslinked rubber composition. [Selected Figure] None

Description

The present invention relates to a crosslinked rubber composition, an electric wire, a cable, and a method for producing the crosslinked rubber composition.

In applications where flexibility or durability is required, rubber materials are primarily used as covering materials for electric wires and cables. Examples of general-purpose cables covered with such rubber materials include cab-tyre cables.

Cab-tyre cables are classified into fixed and movable types depending on the purpose of use. Of these, movable cab-tyre cables require resistance to repeated bending, as the cable itself moves, and resistance to friction in various environments.

On the other hand, a wide variety of rubber materials are used as covering materials for electric wires and cables, and among them, chlorine-based rubber is known as a highly functional material with excellent flame retardancy and oil resistance. We have been developing covering materials using chlorine-based rubber, particularly chlorinated polyethylene, which is highly economical, and have created various inventions to date. We have also found that alloying with ethylene-based copolymer resins and polyethylene, which are highly compatible with chlorinated polyethylene and have higher thermal fluidity than chlorinated polyethylene, is effective in improving processability.

In addition, in order to improve various properties, including heat resistance, the covering material for electric wires and cables is often subjected to a crosslinking process that chemically bonds the molecules of the polymer that is the covering material. In general, peroxide crosslinking, which is a type of crosslinking process that is widely used, is a process in which an organic peroxide is mixed into the covering material as a crosslinking agent in advance, and the peroxide is decomposed by applying heat after the cable is covered, and the resulting active species (radicals) extract hydrogen atoms from the polymer, causing crosslinking (see, for example, JP 2018-110130 A).

To improve the abrasion resistance required for cab-tyre cables, it is effective to take measures such as increasing the degree of cross-linking and the molecular weight and amount of crystallinity of the polymer. For example, crystals in a polymer are formed when some of the molecules adopt a strong crystal structure at a temperature below the melting point of the crystal. The more regular the structure of the polymer molecules, the easier it is to adopt a crystalline structure. The amount of crystallinity can be indicated by the heat of fusion measured by differential scanning calorimetry (DSC).

JP 2018-110130 A

Crosslinking using peroxides is preferable because it achieves a high degree of crosslinking, but it can reduce the amount of crystals in the polymer, which can lead to reduced abrasion resistance. This is because when crosslinking using peroxides, the polymer is heated to a high temperature to decompose the peroxide. This heat melts the crystals, and crosslinking occurs in the molten state, which reduces the regularity of the polymer structure.

SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a material which has both a high degree of crosslinking and a high amount of crystallinity and has excellent abrasion resistance, and an electric wire and cable using the same.
Other objects and novel features will become apparent from the description of this specification and the accompanying drawings.

The crosslinked rubber composition according to one embodiment of the present invention is a crosslinked rubber composition containing a base polymer in which at least an ethylene-based copolymer resin is mixed with a chlorinated polyethylene, the chlorinated polyethylene has a heat of fusion by differential scanning calorimetry (DSC) of more than 2 J/g, the ethylene-based copolymer resin has a melting point of 70°C or higher, the crosslinked rubber composition has a gel fraction of 70% or higher, and the crosslinked rubber composition has a residual heat of fusion expressed as the ratio (percentage) of the heat of fusion by differential scanning calorimetry (DSC) after crosslinking to the heat of fusion by differential scanning calorimetry (DSC) before crosslinking, of 70% or higher.

The electric wire according to one embodiment of the present invention is an electric wire having a conductor and an insulating layer covering the conductor, and the insulating layer is a crosslinked rubber composition containing a base polymer in which chlorinated polyethylene is mixed with at least an ethylene-based copolymer resin, the crosslinked rubber composition having a gel fraction of 70% or more, and a residual heat of fusion expressed as the ratio (percentage) of the heat of fusion measured by differential scanning calorimetry (DSC) after crosslinking to the heat of fusion measured by differential scanning calorimetry (DSC) before crosslinking of the crosslinked rubber composition, of 70% or more.

The cable according to one embodiment of the present invention is a cable having a conductor, an insulating layer covering the conductor, and a covering layer covering the insulating layer, and the covering layer is a crosslinked rubber composition containing a base polymer in which chlorinated polyethylene is mixed with at least an ethylene-based copolymer resin, the crosslinked rubber composition having a gel fraction of 70% or more, and a residual heat of fusion expressed as the ratio (percentage) of the heat of fusion measured by differential scanning calorimetry (DSC) after crosslinking to the heat of fusion measured by differential scanning calorimetry (DSC) before crosslinking of the crosslinked rubber composition, of 70% or more.

The method for producing a crosslinked rubber composition according to one embodiment of the present invention comprises preparing a rubber composition containing a base polymer in which at least an ethylene-based copolymer resin is mixed with chlorinated polyethylene, irradiating the rubber composition with an electron beam, and crosslinking the rubber composition to produce a crosslinked rubber composition, the crosslinked rubber composition having a gel fraction of 70% or more and a residual heat of fusion, expressed as the ratio (percentage) of the heat of fusion measured by differential scanning calorimetry (DSC) after crosslinking to the heat of fusion measured by differential scanning calorimetry (DSC) before crosslinking, of the crosslinked rubber composition being 70% or more.

The crosslinked rubber composition and manufacturing method thereof, which are an embodiment of the present invention, can provide a material that has both a high degree of crosslinking and a high amount of crystallinity, and has excellent abrasion resistance.

In addition, the electric wire and cable according to one embodiment of the present invention has both a high degree of cross-linking and a high amount of crystallinity, and has excellent abrasion resistance.

1 is a schematic cross-sectional view of a cable according to an embodiment of the present invention; FIG. 1 is a diagram showing a schematic configuration of an extruder used in the examples for carrying out a cable production (extrusion) process.

The following describes in detail an embodiment of the present invention with reference to the drawings. In all drawings used to explain the embodiment, the same reference numerals are used for components having the same functions, and repeated explanations will be omitted. In addition, in the following embodiment, explanations of the same or similar parts will not be repeated as a general rule, unless particularly necessary.

<Embodiment>
As described above, in order to obtain a crosslinked rubber composition having excellent abrasion resistance by having a high degree of crosslinking and by being able to retain a large amount of crystallinity in the polymer, the present inventors considered the application of crosslinking by electron beam irradiation, which does not require high temperatures during crosslinking, and investigated a crosslinked rubber composition containing a mixture containing chlorinated polyethylene and at least an ethylene-based copolymer resin as a base polymer.

The present inventors have discovered that by imparting specific properties to a crosslinked rubber composition having such a composition, it is possible to obtain a crosslinked rubber composition having excellent abrasion resistance, and have completed the present invention.
The crosslinked rubber composition, the electric wire, and the cable according to the present embodiment will be described in detail below.

[Crosslinked Rubber Composition]
The crosslinked rubber composition according to the present embodiment is constituted by containing a base polymer in which chlorinated polyethylene is mixed with at least an ethylene-based copolymer resin.

(Base polymer)
The base polymer used in the present embodiment is a base polymer in which at least an ethylene-based copolymer resin is mixed with chlorinated polyethylene. Each component will be described in detail below.

<Chlorinated polyethylene>
The chlorinated polyethylene used here is not particularly limited, and any known chlorinated polyethylene can be used.

From the viewpoint of improving abrasion resistance, it is preferable to use crystalline grade chlorinated polyethylene as this chlorinated polyethylene. Specifically, chlorinated polyethylene having a crystallinity generally considered to be crystalline grade is preferably chlorinated polyethylene having a heat of fusion of 2 J/g to 80 J/g as measured by differential scanning calorimetry (DSC). The heat of fusion of chlorinated polyethylene is more preferably 5 J/g to 30 J/g, and even more preferably 10 J/g to 20 J/g, taking into consideration the balance between abrasion resistance and flexibility.

In this specification, the heat of fusion is the heat of fusion measured by differential scanning calorimetry (DSC) and varies depending on the amount of crystals contained in the resin being measured. Crystals of chlorinated polyethylene melt at approximately 100 to 130°C. Measurements can be performed using an aluminum pan under conditions of a heating rate of 10°C/min, a cooling rate of 5°C/min, an upper limit temperature of 150°C, and a lower limit temperature of 25°C. To eliminate the influence of thermal history, the value of the heat of fusion at the second temperature increase was used. The amount of crystals in the resin can be evaluated from this heat of fusion. Therefore, hereinafter, this heat of fusion may also be described as the numerical value of the amount of crystals.

In addition, it is preferable that this chlorinated polyethylene has a chlorine content of 20 to 45% by mass, a Mooney viscosity of about 120 or less after 4 minutes of preheating at 121°C for 1 minute, and from the viewpoint of a balance between flame retardancy and flexibility, it is even more preferable that the chlorine content is 25 to 40% by mass, and a Mooney viscosity of 90 or less after 4 minutes of preheating at 121°C for 1 minute.

(Ethylene-based copolymer resin)
The ethylene-based copolymer resin used here is not particularly limited as long as it is a known ethylene-based copolymer resin. Specific examples of the ethylene-based copolymer resin include ethylene-vinyl acetate copolymer resin, ethylene-methyl acrylate copolymer resin, ethylene-ethyl acrylate copolymer resin, ethylene-propylene copolymer, ethylene-propylene-diene copolymer, and modified products thereof. Also, a mixed resin in which a plurality of types of these ethylene-based copolymer resins are mixed may be used.

Among these, it is particularly preferable to use ethylene-vinyl acetate copolymer resin, ethylene-ethyl acrylate copolymer resin, etc., in order to maintain a good balance between abrasion resistance, flexibility, and good moldability during cable extrusion coating.

As the ethylene-based copolymer resin, an ethylene-based copolymer resin having a melting point of 70°C or more is preferred, and from the viewpoint of the amount of crystals in the resin, an ethylene-based copolymer resin having a melting point of 80°C or more is more preferred, and an ethylene-based copolymer resin having a melting point of 85°C or more is even more preferred. A plurality of ethylene-based copolymer resins having a melting point of 70°C or more may be used in combination, and an ethylene-based copolymer resin having a melting point of 70°C or more may be mixed with an ethylene-based copolymer resin having a melting point of 70°C or more in the region where the characteristics are expressed. Specific examples of this ethylene-based copolymer resin include ethylene-α-olefin copolymers, and more specifically, ethylene butene copolymers.

The ethylene copolymer resin is not limited in terms of its physical properties, but may have a melt flow rate (MFR) of 6 g/10 min or less at 190°C and 2.16 kgf, for example, and from the viewpoint of improving abrasion resistance, it is preferable that the melt flow rate (MFR) is 1 g/10 min or less at 190°C and 2.16 kgf.

In the base polymer in which the above-mentioned chlorinated polyethylene is mixed with at least an ethylene-based copolymer resin, it is preferable that the content of chlorinated polyethylene is at least 50 parts by mass or more when the base polymer is taken as 100 parts by mass, from the viewpoints of abrasion resistance, processability, and flexibility.

The mixing ratio of the resins that make up this base polymer is preferably in the range of 90:10 to 50:50 by mass of chlorinated polyethylene and ethylene copolymer resin, more preferably in the range of 80:20 to 60:40, and a ratio range of around 70:30 is particularly preferred as this provides excellent properties in all respects, including abrasion resistance, processability, and flexibility.

In addition to the chlorinated polyethylene and ethylene copolymer resin, other rubbers and resins can be mixed into the base polymer. Specific examples include polyvinyl chloride, chloroprene rubber, ethylene propylene rubber, polyolefin resins, low-density polyethylene, high-density polyethylene, linear low-density polyethylene, polyurethane, and modified versions of these, as well as mixtures of multiple types of these.

(Additive)
The crosslinked rubber composition of the present embodiment may contain additives such as a plasticizer, a lubricant, a reinforcing agent, a filler, a flame retardant, a hydrogen chloride scavenger, an antioxidant, and a crosslinking aid.

Examples of plasticizers include mineral oils, phthalic acid-based agents such as bis(2-ethylhexyl) phthalate, diisononyl phthalate, diisodecyl phthalate, and diundecyl phthalate, adipic acid-based agents such as bis(2-ethylhexyl) adipate, diisononyl adipate, diisodecyl adipate, and bis(2-butoxyethyl) adipate, polyester-based agents, phosphoric acid-based agents, epoxy-based agents, and trimellitic acid-based agents. These can be used alone or in combination of two or more types.

Examples of the lubricant include fatty acid amides, zinc stearate, silicones, hydrocarbons, esters, alcohols, and metal soaps.
Examples of reinforcing agents include carbon black and silica.

Examples of fillers include diatomaceous earth, calcined diatomaceous earth, quartz, cristobalite, kaolinite, kaolin clay, calcined clay, talc, muscovite, wollastonite, serpentine, pyrophyllite, calcium carbonate, barium sulfate, titanium oxide, magnesium carbonate, dolomite, and aluminum oxide.

Examples of flame retardants include metal hydroxides, halogen-based, phosphorus-based, and antimony-based flame retardants.

Examples of hydrogen chloride scavengers include epoxy group-containing compounds, hydrotalcites, lead-containing compounds such as tribasic lead sulfate, tin-containing compounds, and metal soaps.

Examples of antioxidants include phenol-based antioxidants, sulfur-based antioxidants, phenol/thioester-based antioxidants, amine-based antioxidants, and phosphorous-based antioxidants.

Examples of cross-linking aids include trimethylolpropane trimethacrylate (TMPT), trimethylolpropane triacrylate, triallyl isocyanurate, triallyl cyanurate, N,N'-metaphenylene bismaleimide, ethylene glycol dimethacrylate, zinc acrylate, and zinc methacrylate.

(Degree of cross-linking and amount of crystallinity)
The above-mentioned materials are thoroughly kneaded to prepare a rubber composition before crosslinking, and then crosslinking treatment is performed to obtain the crosslinked rubber composition of the present embodiment. The materials can be kneaded using a known kneading method, and is not particularly limited. For example, a kneader is often used, but other than the kneader, a roll machine, an extruder, a mixer, an autoclave, or other commonly used kneading or reaction device is not particularly limited, and the kneading conditions are not limited to those described above.

Next, the crosslinking treatment can be carried out by irradiating with an electron beam (for example, an acceleration voltage of 2 MV and an electron beam irradiation dose of 100 kGy). The irradiation conditions at this time are not limited as long as the properties of the resulting crosslinked rubber composition satisfy the following conditions.

At an acceleration voltage of 2 MV, the electron beam irradiation dose is preferably 20 to 100 kGy, and more preferably 30 to 75 kGy in terms of the balance between abrasion resistance and degree of crosslinking.

The crosslinked rubber composition obtained by carrying out the crosslinking treatment in this manner preferably has a gel fraction of 70% or more. The gel fraction is an index for evaluating the degree of crosslinking of the crosslinked rubber composition, and can be calculated, for example, as follows:

When measuring the gel fraction, the material to be used is weighed in advance. The material is then immersed in xylene heated to 110°C for 24 hours. After immersion, the material is left at atmospheric pressure at 20°C for 3 hours and then vacuum dried at 80°C for 4 hours. The mass of the material after treatment is then weighed, and the gel fraction can be calculated as the ratio (percentage) of the mass after immersion (after treatment) to the mass before immersion in xylene (before treatment).

Furthermore, this crosslinked rubber composition has a heat of fusion residual rate, expressed as the ratio (percentage) of heat of fusion B measured by differential scanning calorimetry (DSC) after crosslinking to heat of fusion A measured by differential scanning calorimetry (DSC) before crosslinking, of 70% or more. That is, heat of fusion B is the heat of fusion of the crosslinked rubber composition and can be used to evaluate the amount of crystallization after crosslinking, while heat of fusion A is the heat of fusion of the rubber composition before crosslinking treatment and can be used to evaluate the amount of crystallization before crosslinking. From these heats of fusion, the heat of fusion residual rate can be calculated by the following formula.
Residual heat of fusion (%)={(heat of fusion B)/(heat of fusion A)}×100

By setting the residual heat of fusion at 70% or more in this way, the decrease in the amount of crystals before and after crosslinking can be suppressed and the amount of crystals after crosslinking can be secured, so that the resulting crosslinked rubber composition can have excellent abrasion resistance.

As described above, by having a specified gel fraction and a specified residual heat of fusion, it is possible to have both a high degree of crosslinking and a high amount of crystallinity, and the crosslinked rubber composition can have excellent abrasion resistance.

[Electric wires and cables]
The electric wire/cable in one embodiment of the present invention has a conductor and a covering layer that covers and protects the conductor, and the covering layer is made of the crosslinked rubber composition of the present embodiment described above. The covering layer can be formed by directly covering the conductor to form an electric wire, or can be formed by indirectly covering the conductor and an insulating layer that covers the conductor to form a cable.

A cross-sectional view of the cable according to this embodiment is shown in Figure 1. As shown in Figure 1, the cable 1 is composed of a conductor 2, an insulating layer 3, and a coating layer 4.

The conductor 2 may be any commonly used metal wire, such as copper wire, copper alloy wire, aluminum wire, gold wire, or silver wire. The conductor 2 may also be a metal wire plated with metal such as tin or nickel. Furthermore, the conductor 2 may be a twisted conductor made by twisting together metal wires.

The insulating layer 3 may be made of an insulating material that is normally used for electric wires, and is not particularly limited. Insulating materials for the insulating layer 3 include ethylene-propylene copolymer, polyvinyl chloride, fluororesin, polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl acrylate copolymer, ethylene-ethyl acrylate copolymer, polyolefin resin, natural rubber, and other synthetic rubbers. These may be used alone or in combination, with or without crosslinking.

The coating layer 4 is made of the crosslinked rubber composition of the present embodiment described above, and has a gel fraction of 70% or more and a residual heat of fusion of 70% or more.

The method for manufacturing this cable involves using an extruder to coat the outer circumference of the insulating layer 3 formed on the conductor 2 with the pre-crosslinked rubber composition described above, and then crosslinking the coating layer by electron beam irradiation, thereby producing the cable 1.

Figure 2 is a diagram showing the schematic configuration of an example of an extruder for producing a cable in this embodiment. As shown in Figure 2, the extruder 11 includes a cylinder 20, a screw 13 rotatably installed in the cylinder 20, a hopper 12 for supplying material into the cylinder 20, and a crosshead 16. The extruder 11 also includes a neck 15 between the crosshead 16 and the screw 13, and a breaker plate 14 between the neck 15 and the screw 13. The crosshead 16 has a die 17, and a cable core 18 formed by twisting together electric wires (conductors covered with an insulator) passing through the crosshead 16 is covered with a sheath in the crosshead 16, passes through the die 17, and is drawn out of the crosshead 16 as an uncrosslinked cable 19.

The outer circumference of the uncrosslinked cable 19 is covered with a rubber composition before crosslinking, and the rubber composition is crosslinked by irradiating it with an electron beam to produce a cable 1 covered with a crosslinked rubber composition. The crosslinking process may be carried out under the conditions described in the manufacturing method of the crosslinked rubber composition.

The cable obtained in this manner is, for example, a cable having the configuration shown in FIG. 1, and is particularly suitable for multi-core cables having electric wires with a conductor cross-sectional area of 38 mm2 or less as specified in the Electrical Appliance and Material Safety Law (Appendix ¹ ) and JIS C3327.

Although the configuration of a cable has been described here, it is also possible to form the insulator of an insulated electric wire having an insulator covering the conductor 2 using the crosslinked rubber composition of this embodiment.

Next, this embodiment will be described in detail with reference to examples and comparative examples.

[Examples 1 to 5, Comparative Examples 1 to 6]
The mixing of various additives into the base polymer, the manufacture of cables using each compound prepared, and the crosslinking treatment were carried out as follows. The following conditions are examples and are not intended to be limiting.

(Preparation of Rubber Composition Before Crosslinking)
Chlorinated polyethylene, ethylene copolymer resin, hydrogen chloride scavenger, plasticizer, lubricant, reinforcing agent, filler, flame retardant, crosslinking aid, and the like were charged into a 25 L pressure kneader (kneader tank temperature controlled at 100° C.) based on the formulations shown in Tables 3 and 4, and pressure kneaded at a rotor speed of 10 rpm for 10 minutes.

Here, by adding the ethylene copolymer resin at the end of the kneading process, it is possible to increase the viscosity of the material when kneading the additives, thereby improving the dispersibility of the additives. Note that these conditions are merely examples and are not limiting.

After the above kneading is completed, the material is quickly discharged into a single-screw extruder hopper, extruded into strands, cooled with water, and pelletized to produce pellets of the rubber composition. The granulation method is not limited to the above, and for example, pellets may be produced using hot-cut equipment without water cooling. A release agent can be used to prevent the pellets from sticking together. The release agent can be of any composition or form, such as powder, liquid, or mist, but it is effective to use talc, for example, in consideration of economics.

(Cable manufacturing and cross-linking)
In the present examples and comparative examples, an extruder 11 shown in FIG. 2 was used to prepare uncrosslinked cables as follows.

First, a conductor having a cross-sectional area of 38 ^mm2 (outer diameter 9.1 mm) was prepared by twisting together a number of tin-plated soft copper wires, and an ethylene-propylene rubber copolymer blend was extrusion-coated as an insulator to a thickness of 1.2 mm to obtain a cross-linked wire core. A cable core was prepared by twisting three of these wire cores together, and pellets of the rubber composition in each of the above examples were extrusion-coated to a thickness of 3.0 mm using a single-screw extruder with a screw diameter of 90 mm under the conditions shown in Table 1 or Table 2 to produce an uncross-linked cable (finished outer diameter: approximately 31 mm). The uncross-linked cable thus produced was once wound around a drum.

Here, Tables 1 and 2 show the extrusion conditions in the cable extrusion process. At this time, cylinders 1 to 5 are connected from the top in order from the hopper side to the head side to form cylinder 20. Also, the flanges in Tables 1 and 2 are the ends of cylinder 20, the ends on the neck 15 side.

Table 1 shows the extrusion conditions for uncrosslinked cables (Examples 1-5, Comparative Examples 1-3) for electron beam irradiation crosslinking, and Table 2 shows the extrusion conditions for uncrosslinked cables (Comparative Examples 4-6) for peroxide crosslinking.

In this case, peroxide crosslinking requires extrusion above the melting point of the crystals and below the decomposition temperature of the peroxide, which limits the extrusion temperature and reduces the degree of freedom in manufacturing. However, with electron beam crosslinking, there is no limit to the extrusion temperature as long as it is above the melting point of the crystals, which has the advantage of allowing greater freedom in manufacturing.

Next, in Examples 1 to 5 and Comparative Examples 1 to 3, the uncrosslinked cable was pulled out from the drum and irradiated with electron beams (accelerating voltage 2 MV, electron beam irradiation dose 100 kGy) in an electron beam irradiation device to produce a cable covered with a crosslinked rubber composition. The electron beam irradiation conditions at this time are not limited to these.

Note that the above cable manufacturing process is just one example, and the extruder, cable core, cable structure, and crosslinking conditions are not limited to those described above.

In Comparative Examples 4 to 6, the uncrosslinked cable was pulled out of the drum and heated to a temperature equal to or higher than the decomposition temperature of the peroxide to carry out the crosslinking treatment.

[Evaluation of characteristics]
The prepared compounds after kneading and the prepared cables after crosslinking treatment were evaluated as follows. The evaluation results are shown in Tables 3 and 4 together with the compositions.

(1) Amount of crystallization (heat of fusion)
Differential scanning calorimetry (DSC) was performed to measure the heat of fusion of the crosslinked rubber composition before and after crosslinking. The peak at approximately 100 to 130°C was attributed to chlorinated polyethylene, and the peak at approximately 60 to 100°C was attributed to ethylene-based copolymer resin, and the heat of fusion at each peak was summed up and evaluated as the amount of crystallinity. The measurement was performed using an aluminum pan under conditions of a heating rate of 10°C/min, a cooling rate of 5°C/min, an upper limit temperature of 150°C, and a lower limit temperature of 25°C. In order to eliminate the influence of thermal history, the value at the second temperature rise was used for the heat of fusion.

The amount of crystals in the rubber composition before crosslinking is shown as "amount of crystals (before crosslinking)" and the amount of crystals in the crosslinked rubber composition after crosslinking is shown as "amount of crystals (after crosslinking)".

(2) Crystal Proportion after Crosslinking In the above (1), the heat of fusion of the rubber composition before crosslinking was designated as heat of fusion A, and the heat of fusion of the crosslinked rubber composition after crosslinking was designated as heat of fusion B. The residual heat of fusion rate obtained by the following formula was used to evaluate the amount of crystals remaining after crosslinking treatment. Those with a residual heat of fusion rate of 70% or more were rated as good (symbol: ◯) as having a sufficient amount of crystals contributing to abrasion resistance, those with a residual heat of fusion rate of 77% or more were rated as excellent (symbol: ◎) as having a particularly sufficient amount of crystals, and those with a residual heat of fusion rate of less than 70% were rated as poor (symbol: ×).
Residual heat of fusion (%) = (heat of fusion B / heat of fusion A) x 100

(3) Degree of crosslinking (gel fraction)
A 0.5 g sample taken from the crosslinked rubber composition was placed in a 40 mesh brass wire net and extracted with xylene for 24 hours in a 110°C oil bath. The sample was naturally dried overnight and then vacuum dried at 80°C for 4 hours, after which the mass was weighed. From the obtained value, the gel fraction, which is an index showing the degree of crosslinking of the material, was calculated using the following formula. A gel fraction of 70% by mass or more was rated as good (symbol ◯) as having sufficient heat resistance for practical use and also contributing sufficiently to abrasion resistance, a gel fraction of 80% by mass or more was rated as excellent (symbol ◎) as having a particularly sufficient degree of crosslinking, and a gel fraction of less than 70% by mass was rated as poor (symbol ×).
Gel fraction = (b / a) x 100
a: Charge mass (g)
b: Mass after extraction and drying (g)

(4) Abrasion resistance (wear characteristics)
The pellets of the rubber composition were molded into a 2 m thick sheet piece at 180° C. using a heat press. In Comparative Examples 4 to 6, crosslinking was carried out simultaneously with this molding. In Examples 1 to 5 and Comparative Examples 1 to 3, the sheet piece was irradiated with electron beams (accelerating voltage: 2 MV, electron beam irradiation dose: as shown in Tables 3 and 4) using an electron beam irradiation device to produce a crosslinked sheet piece.

The crosslinked sheet pieces were attached to a jig simulating the shape of a cable, and an abrasion test was carried out in accordance with JIS C3005. The weight had a mass of 5 kg, the grinding wheel disc rotated 75 times, and the abrasion volume was calculated from the mass loss before and after the test and the specific gravity of each sample. Abrasion volumes of 0.6 mL or less were rated as good (symbolized as ◯), those of 0.5 mL or less were rated as excellent (symbolized as ◎) as being particularly excellent in abrasion resistance, and those of more than 0.6 mL were rated as poor (symbolized as ×).
Abrasion volume = (seat mass before test - seat mass after test) / specific gravity

(5) Overall Judgment Among the characteristics shown in (2) to (4) above, a sample that exhibited all of the characteristics being good or excellent was judged as "pass" (denoted with a symbol ○ or ◎, where ◎ indicates that all of the characteristics were excellent), and a sample that exhibited any one of the characteristics being poor was judged as "fail" (denoted with a symbol ×).

Of the products shown in Tables 3 and 4, *1: "Elaslen 252B" (crystal amount 20 J/g) is manufactured by Showa Denko K.K., *2: "VF-120T" (melting point: 85°C, MFR: 1 g/10 min, crystal amount 11 J/g) is manufactured by Ube Maruzen Polyethylene Co., Ltd., *3: "Carbon black" (arithmetic mean particle size: 68 nm) is HTC#S manufactured by Nippon Steel Carbon Co., Ltd., and *4: "DCP" is dicumyl peroxide manufactured by NOF Corporation.

From the above results, it was found that by applying electron beam crosslinking to a rubber composition containing a mixture of chlorinated polyethylene and ethylene copolymer resin, both the amount of crystallinity and the degree of crosslinking can be kept high even after crosslinking treatment, and excellent wear resistance can be obtained. In addition, Comparative Examples 4 to 6 revealed that in peroxide crosslinking, the degree of crosslinking increases as the amount of peroxide increases, but the amount of crystallinity decreases significantly. Therefore, by subjecting the rubber composition to electron beam crosslinking, in addition to oil resistance and good processability during cable extrusion coating, it is possible to obtain a well-balanced excellent wear resistance due to the combination of a high amount of crystallinity and a high degree of crosslinking, and the appearance is also good. Furthermore, it was found that excellent wear resistance can be obtained by setting the electron beam irradiation dose to 30 to 100 kGy at an acceleration voltage of 2 MV.

The invention made by the inventors has been specifically described above based on the embodiments, but it goes without saying that the invention is not limited to the above embodiments and can be modified in various ways without departing from the gist of the invention.

Reference Signs List 1 Cable 2 Conductor 3 Insulating layer 4 Covering layer 11 Extruder 12 Hopper 13 Screw 14 Breaker plate 15 Neck 16 Crosshead 17 Die 18 Cable core 19 Uncrosslinked cable 20 Cylinder

Claims (8)

A crosslinked rubber composition comprising a base polymer in which at least an ethylene-based copolymer resin is mixed with a chlorinated polyethylene,
The chlorinated polyethylene has a heat of fusion measured by differential scanning calorimetry (DSC) of more than 2 J/g;
The melting point of the ethylene copolymer resin is 70° C. or higher,
The crosslinked rubber composition has a gel fraction of 70% or more, and a residual heat of fusion, expressed as a ratio (percentage) of the heat of fusion of the crosslinked rubber composition after crosslinking as measured by differential scanning calorimetry (DSC) to the heat of fusion of the crosslinked rubber composition before crosslinking as measured by differential scanning calorimetry (DSC), of 70% or more. The crosslinked rubber composition according to claim 1,
The crosslinked rubber composition, wherein the crosslinking in the crosslinked rubber composition is effected by electron beam irradiation. The crosslinked rubber composition according to claim 1,
The crosslinked rubber composition contains the chlorinated polyethylene in an amount of at least 50 parts by mass per 100 parts by mass of the base polymer. The crosslinked rubber composition according to claim 1,
The crosslinked rubber composition, wherein the ethylene-based copolymer resin is an ethylene-vinyl acetate copolymer resin or an ethylene-ethyl acrylate copolymer resin. The crosslinked rubber composition according to claim 1,
The crosslinked rubber composition, wherein the ethylene copolymer resin has a melting point of 80° C. or higher. An electric wire having a conductor and an insulating layer covering the conductor,
The insulating layer is an electric wire composed of a crosslinked rubber composition containing a base polymer in which chlorinated polyethylene is mixed with at least an ethylene-based copolymer resin, the crosslinked rubber composition having a gel fraction of 70% or more and a residual heat of fusion, expressed as a ratio (percentage) of the heat of fusion of the crosslinked rubber composition after crosslinking measured by differential scanning calorimetry (DSC) to the heat of fusion of the crosslinked rubber composition before crosslinking measured by differential scanning calorimetry (DSC), of 70% or more. A cable having a conductor, an insulating layer covering the conductor, and a covering layer covering the insulating layer,
The cable is constituted of a crosslinked rubber composition, the coating layer of which contains a base polymer in which chlorinated polyethylene is mixed with at least an ethylene-based copolymer resin, the crosslinked rubber composition having a gel fraction of 70% or more and a residual heat of fusion, expressed as a ratio (percentage) of the heat of fusion of the crosslinked rubber composition after crosslinking as measured by differential scanning calorimetry (DSC) to the heat of fusion of the crosslinked rubber composition before crosslinking as measured by differential scanning calorimetry (DSC), of 70% or more. A rubber composition is prepared containing a base polymer in which at least an ethylene-based copolymer resin is mixed with a chlorinated polyethylene;
A method for producing a crosslinked rubber composition, comprising: irradiating the rubber composition with an electron beam to crosslink the rubber composition to produce a crosslinked rubber composition,
A method for producing a crosslinked rubber composition, wherein the crosslinked rubber composition has a gel fraction of 70% or more, and a residual heat of fusion, expressed as a ratio (percentage) of the heat of fusion of the crosslinked rubber composition after crosslinking as measured by differential scanning calorimetry (DSC) to the heat of fusion of the crosslinked rubber composition before crosslinking as measured by differential scanning calorimetry (DSC), is 70% or more.

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