Patent Description:
Polyethylene has been used as a base resin for forming an insulation layer of a power cable, for example (see Patent Literature <NUM>, Patent Literature <NUM>, Non-Patent Literature <NUM> and Non-Patent Literature <NUM> for example).

An object of the present disclosure is to improve insulating properties of an insulation layer.

According to the present disclosure, it is possible to improve insulating properties of an insulation layer.

With this configuration, it is possible to suppress local occurrence of electrical treeing.

Insulating properties of an insulation layer of a power cable are evaluated based on DC breakdown electric field strength as described in <CIT> or <CIT>, for example. Hitherto, flat electrodes (flat plate electrodes) have been used in the evaluation.

However, it was found that insulating properties of an insulation layer of a power cable may deteriorate at an electric field strength that is lower than the DC breakdown electric field strength. When insulating properties deteriorate, electrical treeing locally occurs in a portion of the insulation layer of the power cable. The electrical treeing is caused by foreign matter. The foreign matter is minute metal foreign matter or a minute void, for example.

The inventor(s) found that it is possible to observe the occurrence of electrical treeing described above by measuring the DC breakdown electric field strength of the insulation layer using a needle electrode. That is, it is possible to reproduce local occurrence of electrical treeing by creating a simulative situation using a needle electrode that has a sharp tip end.

As a result of the reproduction, it was found that a specific functional group (hydrocarbon group) in polyethylene affects insulating properties of the insulation layer. The inventor(s) found that polyethylene that includes the functional group at a specific proportion can suppress local occurrence of electrical treeing.

Next, an embodiment of the present disclosure will be described below with reference to the drawings. The present disclosure is not limited to these illustrations, but is intended to be indicated by the claims and encompass all the changes which fall within the meaning and scope equivalent to the claims.

A resin composition according to the present embodiment is a material that constitutes an insulation layer <NUM> of a power cable <NUM>, which will be described later. The resin composition contains at least a base resin, for example.

The base resin (base polymer) is a resin component that constitutes a main component of the resin composition. The base resin in the present embodiment includes polyethylene, for example.

In the resin composition according to the present embodiment, the polyethylene includes methine groups, methylene groups, and methyl groups, for example.

A "methine group" is a functional group represented by -CH. The methine group includes one carbon atom and one hydrogen atom bound to the carbon atom. In addition to the hydrogen atom, at least another carbon atom is bound to the carbon atom included in the methine group.

Specifically, as shown in <FIG>, for example, methine groups a constitute a portion of a main chain of the polyethylene. Branches are bound to carbon atoms included in the methine groups a. Examples of the branches include the following.

Note that carbon atoms included in the methine groups a to which branches are bound are tertiary carbon atoms. Since the polyethylene includes the above-described branches, the density and crystallinity of the polyethylene can be adjusted.

Note that in the polyethylene according to the present embodiment, methine groups are bound to branches at many positions. However, there may be a position at which a methine group is not bound to a branch but constitutes a portion of a vinyl group or the like.

A "methylene group" is a functional group represented by -CH<NUM>. The methylene group includes one carbon atom and two hydrogen atoms bound to the carbon atom.

Specifically, as shown in <FIG>, for example, methylene groups b1 constitute a portion of the main chain of the polyethylene. Carbon atoms that are included in branches are not bound to the carbon atoms included in the methylene groups b1. However, a methylene group b2 itself may constitute a portion of a branch including two or more carbon atoms.

A "methyl group" is a functional group represented by -CH<NUM>. The methyl group includes one carbon atom and three hydrogen atoms bound to the carbon atom.

Specifically, as shown in <FIG>, for example, the polyethylene includes a methyl group c1 at a terminal end of the main chain or a methyl group c2 at a terminal end of a branch.

Methine groups, methylene groups, and methyl groups included in the polyethylene can be analyzed using <NUM>C nuclear magnetic resonance (NMR, hereinafter abbreviated as "<NUM>C-NMR"), for example. In a <NUM>C-NMR spectrum of the polyethylene, multiple peaks attributed to methine groups, methylene groups, and methyl groups are detected according to their positions in the polyethylene.

The resin composition in the present embodiment has the following characteristics in the <NUM>C-NMR spectrum of the polyethylene, for example.

The polyethylene contained in the resin composition according to the present embodiment satisfies the following formula (<NUM>) regarding peaks attributed to methine groups in the <NUM>C-NMR spectrum.

Here, RX satisfies the following formula. <MAT> Hereinafter, RX will be also referred to as "proportion of methine groups".

X, Y, and Z respectively represent the following values in the <NUM>C-NMR spectrum of the polyethylene.

The integrated intensity of methine groups is a sum obtained by integrating intensities of peaks attributed to methine groups, which appear according to positions of methine groups in the polyethylene.

The integrated intensity of methylene groups is a sum obtained by integrating intensities of peaks attributed to methylene groups, which appear according to positions of methylene groups in the polyethylene.

The integrated intensity of methyl groups is a sum obtained by integrating intensities of peaks attributed to methyl groups, which appear according to positions of methyl groups in the polyethylene.

When the proportion RX of methine groups is more than <NUM>, the polyethylene easily decomposes. This is because the bond energy of a tertiary carbon atom included in a methine group is smaller than the bond energy of a methylene group or the bond energy of a methyl group. Accordingly, a methine group more easily decomposes when subjected to a high electric field, compared with a methylene group or a methyl group. Also, when the proportion RX of methine groups is more than <NUM>, the polyethylene includes many branches bound to the methine groups. The amount of amorphous portions of the polyethylene increases in proportion to the number of branches included in the polyethylene. Amorphous portions of the polyethylene easily transmit an electric current.

The proportion RX of methine groups in the polyethylene according to the present embodiment is <NUM> or less. Polyethylene having a proportion RX of <NUM> or less includes fewer methine groups than polyethylene having a proportion RX more than <NUM>. Therefore, even when a high electric field is locally applied to the polyethylene according to the present embodiment, decomposition of tertiary carbon atoms included in methine groups is less likely to occur. Also, polyethylene having a proportion RX of <NUM> or less includes fewer branches bound to methine groups than polyethylene having a proportion RX more than <NUM>. Accordingly, the polyethylene includes few amorphous portions generated due to the branches. Therefore, transmission of an electric current in the amorphous portions can be suppressed.

A crystal of polyethylene having a proportion RX less than <NUM> is larger than a crystal of polyethylene having a proportion RX of <NUM> or more. When crystals of polyethylene are large, a sink mark (a shrink mark, or a sunk spot) is likely to form between the crystals. The terms "sink mark" used here mean at least either a depression or a void formed through contraction of resin. Electrical treeing may start from such a sink mark when a high electric field is applied.

In the present embodiment, the proportion RX is <NUM> or more, and therefore, formation of a sink mark between crystals can be suppressed.

The polyethylene contained in the resin composition according to the present embodiment may satisfy the following formula (<NUM>)' regarding peaks attributed to methine groups in the <NUM>C-NMR spectrum.

In this case, it is possible to stably suppress formation of a sink mark between crystals.

Furthermore, in the resin composition according to the present embodiment, the content of methine groups and the content of methyl groups in the polyethylene satisfy the following formula (<NUM>). <MAT> Hereinafter, X/Z will also be referred to as a "ratio of methine groups to methyl groups".

When compared with polyethylene having a ratio X/Z of <NUM> or more, in polyethylene having a ratio X/Z less than <NUM>, the content of methine groups is smaller than the content of methyl groups. In this case, the polyethylene includes a portion in which two branches are generated from one quaternary carbon atom, for example. The quaternary carbon atom has a smaller bond energy than a tertiary carbon atom included in a methine group. Therefore, decomposition of the quaternary carbon atom is likely to occur when a high electric field is locally applied.

When compared with polyethylene having a ratio X/Z of <NUM> or less, in polyethylene having a ratio X/Z more than <NUM>, the content of methine groups is larger than the content of methyl groups. In this case, the polyethylene includes many methine groups that constitute vinyl groups, for example. Methine groups constituting vinyl groups do not have branches, and accordingly, the number of branches is small. Therefore, a minute sink mark may form between crystals.

In the present embodiment, the ratio X/Z is <NUM> or more. Therefore, it is possible to suppress generation of a portion in which two branches are generated from a quaternary carbon atom. Accordingly, it is possible to suppress decomposition that occurs due to the presence of a quaternary carbon atom even when a high electric field is locally applied.

In the present embodiment, the ratio X/Z is <NUM> or less. In this case, the polyethylene includes a predetermined number of branches. Therefore, it is possible to suppress formation of a minute sink mark between crystals.

The polyethylene in the present embodiment described above includes at least low density polyethylene (LDPE), for example. The density of LDPE is <NUM>/cm<NUM> or more and less than <NUM>/cm<NUM>. The polyethylene in the present embodiment includes LDPE as a main component, for example. The term "main component" used here means a component whose content is the largest.

The polyethylene in the present embodiment may also include a polyethylene other than LDPE. Examples of the polyethylene other than LDPE include linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), and high density polyethylene (HDPE).

Also, the base resin may further include a modified polyolefin obtained by grafting a polar group to a polyolefin, or a copolymer of an olefin and a polar monomer, for example. These are disclosed in <CIT> or <CIT>.

Two or more materials may also be used in combination out of the examples of polyethylene other than LDPE, the modified polyolefin obtained by grafting a polar group to a polyolefin, and the copolymer of an olefin and a polar monomer.

The resin composition may further contain at least any of an inorganic filler, a crosslinking agent, an antioxidant, and a lubricant as other additives. The inorganic filler, the crosslinking agent, the antioxidant, and the lubricant are disclosed in <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, and <CIT>, for example, and therefore descriptions thereof are omitted.

The resin composition according to the present embodiment containing the materials described above is loaded as pellets into an extruder when the insulation layer <NUM> of the power cable <NUM> is to be extrusion-molded.

The resin composition according to the present embodiment has the following properties.

In the present embodiment, a sheet made of the resin composition has a volume resistivity of <NUM>×<NUM><NUM> Ω·cm or more, for example. A method for measuring the volume resistivity is disclosed in <CIT> or <CIT>, for example, and therefore a description thereof is omitted.

In the present embodiment, the sheet made of the resin composition has a dielectric breakdown electric field strength of <NUM> kV/mm or more, for example, and the dielectric breakdown electric field strength may be <NUM> kV/mm or more. A method for measuring the dielectric breakdown electric field strength is disclosed in <CIT> or <CIT>, for example, and therefore a description thereof is omitted.

Furthermore, the resin composition according to the present embodiment has the following properties due to satisfying the above-described requirements for the <NUM>C-NMR spectrum of polyethylene.

In the present embodiment, local occurrence of electrical treeing can be suppressed due to the polyethylene contained in the resin composition satisfying the formula (<NUM>): RX≤<NUM>. Accordingly, a DC breakdown electric field strength of <NUM> kV/mm or more can be obtained when measured using a needle electrode having a tip end with a radius of curvature of <NUM>.

In the present embodiment, local occurrence of electrical treeing can be suppressed due to the polyethylene contained in the resin composition satisfying the formula (<NUM>): <NUM>≤RX. Accordingly, a DC breakdown electric field strength of <NUM> kV/mm or more can be obtained when measured using a needle electrode having a tip end with a radius of curvature of <NUM>.

In the present embodiment, local occurrence of electrical treeing can be suppressed due to the polyethylene contained in the resin composition satisfying the formula (<NUM>): <NUM>≤(X/Z)≤<NUM>. Accordingly, a DC breakdown electric field strength of <NUM> kV/mm or more can be obtained when measured using a needle electrode having a tip end with a radius of curvature of <NUM>.

In the present embodiment, the accumulation of space charge in the resin composition can be suppressed due to the resin composition further containing an inorganic filler. A method for measuring a field enhancement factor (FEF) is disclosed in <CIT> or <CIT>, for example, and therefore a description thereof is omitted.

Next, the power cable according to the present embodiment will be described with reference to <FIG>.

The power cable <NUM> according to the present embodiment includes a conductor <NUM>, an internal semiconductive layer <NUM>, an insulation layer <NUM>, an external semiconductive layer <NUM>, a shielding layer <NUM>, and a sheath <NUM>. The conductor <NUM>, the internal semiconductive layer <NUM>, the insulation layer <NUM>, the external semiconductive layer <NUM>, the shielding layer <NUM>, the sheath <NUM>, and dimensions are disclosed in <CIT> or <CIT>, and therefore descriptions thereof are omitted.

The insulation layer <NUM> is formed through extrusion molding using the resin composition of the present embodiment described above. Note that a sheet cut out from the extrusion-molded insulation layer <NUM> has the above-described properties of the resin composition according to the present embodiment.

Next, a method for manufacturing the power cable <NUM> according to the present embodiment will be described with reference to <FIG>.

The method for manufacturing the power cable <NUM> according to the present embodiment includes a resin composition preparation step S100, a conductor preparation step S200, a cable core formation step S300, a shielding layer formation step S400, and a sheath formation step S500, for example. The conductor preparation step S200, the cable core formation step S300, the shielding layer formation step S400, and the sheath formation step S500 are disclosed in <CIT> or <CIT>, for example, and therefore descriptions thereof are omitted. The following describes the resin composition preparation step S100.

First, the resin composition according to the present embodiment is prepared. The resin composition preparation step S100 includes a polyethylene preparation step S120 and a mixing step S140, for example.

In the present embodiment, polyethylene is produced by polymerizing a monomer using a high pressure method, for example. The monomer is ethylene, for example.

In the present embodiment, a polymerization peak temperature during polymerization of ethylene is mainly controlled, for example. As a result, polyethylene that satisfies the formulas (<NUM>), (<NUM>), and (<NUM>) in the <NUM>C-NMR spectrum described above is obtained.

Specifically, short chain branches (SCB) are generated through an intramolecular chain transfer reaction of polymer radicals during high-pressure radical polymerization. Furthermore, long chain branches (LCB) are generated as a result of chain transfer occurring between generated polymer molecules and polymer radicals. The above-described methine groups are formed at binding sites between the thus generated branches and the main chain.

For example, the polymerization peak temperature in a reaction tube of a polymerization apparatus is controlled to control the proportion of methine groups in the polyethylene. The higher the polymerization peak temperature is, the more likely thermal noise is to occur. It is possible to thus facilitate the generation of branches. That is, the proportion of methine groups in the polyethylene can be increased.

The proportion of methine groups in the polyethylene may also be controlled by controlling the position at which the temperature reaches the polymerization peak temperature in the reaction tube of the polymerization apparatus, for example. The temperature reaches the polymerization peak temperature at a predetermined position in the cooling section because the flow in the reaction tube of the polymerization apparatus is an extrusion flow. The polymerization peak temperature and the position at which the temperature reaches the polymerization peak temperature depend on operating conditions. At this time, a polymer may be deposited on a tube wall depending on the temperature distribution in the reaction tube. It is possible to facilitate the generation of branches by increasing the amount of the polymer deposited on the tube wall. That is, the proportion of methine groups in the polyethylene can be increased.

Furthermore, a polymerization pressure in the reaction tube of the polymerization apparatus may also be controlled. The higher the polymerization pressure is, the more the generation of branches can be facilitated. That is, the proportion of methine groups in the polyethylene can be increased.

Note that it is possible to reduce the proportion of methine groups in the polyethylene by performing control in a manner opposite to that described above.

The mixing step is performed subsequently to the polyethylene preparation step. A base resin including the polyethylene and additives are mixed, and pellets of the mixed material are formed.

On the other hand, the conductor <NUM> formed by twisting together a plurality of conductor core wires is prepared.

Next, a cable core is formed by applying the internal semiconductive layer <NUM>, the insulation layer <NUM>, and the external semiconductive layer <NUM> to the outer circumferential surface of the conductor <NUM>. Note that the internal semiconductive layer <NUM>, the insulation layer <NUM>, and the external semiconductive layer <NUM> are crosslinked.

Next, the shielding layer <NUM> constituted by a copper tape, for example, is formed on the outer surface of the external semiconductive layer <NUM>.

The sheath <NUM> is formed on the outer circumferential surface of the shielding layer <NUM>.

The power cable <NUM> is manufactured through the above steps.

One or more of the following effects can be achieved according to the present embodiment.

Although an embodiment of the present disclosure has been described specifically, the present disclosure is not limited to the above embodiment.

In the above embodiment, a case is described in which the resin composition contains an inorganic filler, but a configuration is also possible in which the resin composition does not contain an inorganic filler.

In the above embodiment, a case is described in which the polyethylene contained in the resin composition satisfies the formulas (<NUM>), (<NUM>), and (<NUM>). However, it is sufficient that the polyethylene contained in the resin composition satisfies at least the formula (<NUM>). That is, a configuration is also possible in which the <NUM>C-NMR spectrum of the polyethylene does not satisfy at least either the formula (<NUM>) or (<NUM>). In this case as well, it is possible to obtain the effect of suppressing electrical treeing due to the formula (<NUM>) being satisfied. However, the effect of suppressing electrical treeing can be obtained more stably in a case where the polyethylene further satisfies the formula (<NUM>) or (<NUM>).

Next, examples according to the present disclosure will be described. These examples are illustrations of the present disclosure.

Polyethylenes contained in resin compositions of samples <NUM>-<NUM> to <NUM>-<NUM> were produced under the following conditions.

The following evaluation was performed on each of the resin compositions of samples <NUM>-<NUM> to <NUM>-<NUM>.

The resin composition of each sample described above was dissolved in ortho chlorobenzene, and <NUM>C-NMR analysis was performed in an environment at a temperature of <NUM> using an NMR apparatus manufactured by JEOL Ltd. The proportion RX was determined regarding peaks attributed to methine groups in the obtained <NUM>C-NMR spectrum.

Here, RX satisfies RX={X/(X+Y+Z)}×<NUM>, as described above.

As shown in <FIG>, a DC breakdown electric field strength was measured using flat electrodes as described below.

As shown in <FIG> and <FIG>, a DC breakdown electric field strength was measured using a needle electrode A as described below.

A DC breakdown electric field strength was measured using a needle electrode B as described below. The radius of curvature of a tip end of the needle electrode B was <NUM>. Measurement conditions other than the radius of curvature of the tip end of the needle electrode B were the same as the measurement conditions under which the DC breakdown electric field strength was measured using the needle electrode A.

Table <NUM> below shows production conditions and evaluation results of the resin compositions of samples <NUM>-<NUM> to <NUM>-<NUM>.

Note that the ratio X/Z was <NUM> or more and <NUM> or less in samples <NUM>-<NUM> to <NUM>-<NUM>.

The proportion RX was <NUM> or more and <NUM> or less in samples <NUM>-<NUM> to <NUM>-<NUM>. Evaluation results of these samples in evaluation <NUM> were <NUM> kV/mm or more. Evaluation results of these samples in evaluation <NUM> and evaluation <NUM> were <NUM> kV/mm or more.

The proportion RX was <NUM> or less in these samples. Accordingly, the number of tertiary carbon atoms included in methine groups was small. Therefore, decomposition of tertiary carbon atoms included in methine groups was suppressed even when a high electric field was locally applied. Moreover, the number of branches bound to methine groups was small. Accordingly, the polyethylene included few amorphous portions generated due to the branches. Since the polyethylene included few amorphous portions, transmission of an electric current in the amorphous portions was suppressed. For the above reasons, local occurrence of electrical treeing was suppressed. It was confirmed that, consequently, the DC breakdown electric field strength measured using the needle electrode A having the tip end with a radius of curvature of <NUM> was high in samples <NUM>-<NUM> to <NUM>-<NUM>.

The proportion RX was <NUM> or more in these samples. Therefore, crystals of the polyethylene were small. Since the crystals were small, formation of a sink mark between the crystals was suppressed. Therefore, local occurrence of electrical treeing was stably suppressed. It was confirmed that, consequently, the DC breakdown electric field strength measured using the needle electrode B was high in samples <NUM>-<NUM> to <NUM>-<NUM>.

The proportion RX was <NUM> in sample <NUM>-<NUM>.

The DC breakdown electric field strengths respectively measured using the needle electrode A and the needle electrode B were less than <NUM> kV/mm in sample <NUM>-<NUM>.

The proportion RX was more than <NUM> in sample <NUM>-<NUM>, and accordingly, the number of tertiary carbon atoms included in methine groups was large. Therefore, decomposition of the tertiary carbon atoms was likely to occur when a high electric field was locally applied. Also, the number of branches bound to methine groups was large in sample <NUM>-<NUM>. Accordingly, the polyethylene included many amorphous portions. Since the polyethylene included many amorphous portions, an electric current was easily transmitted in the amorphous portions. It is thought that, consequently, the DC breakdown electric field strengths respectively measured using the needle electrode A and the needle electrode B were low in sample <NUM>-<NUM>.

The DC breakdown electric field strength measured using the flat electrodes was <NUM> kV/mm or more in sample <NUM>-<NUM> in which the proportion RX was <NUM>. The DC breakdown electric field strength measured using the needle electrode A was <NUM> kV/mm or more in sample <NUM>-<NUM>.

However, the DC breakdown electric field strength measured using the needle electrode B was less than <NUM> kV/mm in sample <NUM>-<NUM>.

The proportion RX was less than <NUM> in sample <NUM>-<NUM>. That is, the number of branches bound to methine groups was small, and accordingly, crystals of the polyethylene were large. Therefore, a sink mark was likely to form between the crystals. Electrical treeing started from such a sink mark when a high electric field was applied. It is thought that, consequently, the DC breakdown electric field strength measured using the needle electrode B was low in sample <NUM>-<NUM>.

Polyethylenes contained in resin compositions of samples <NUM>-<NUM> to <NUM>-<NUM> were polymerized using a high pressure method. Ethylene was used as a monomer as in Experiment <NUM>. The polymerization peak temperature and the polymerization pressure are shown in Table <NUM>.

At this time, samples <NUM>-<NUM> to <NUM>-<NUM> were produced by polymerizing the monomer at various polymerization pressures. Thus, the ratio (X/Z) of methine groups to methyl groups in polyethylene was controlled.

Note that sample <NUM>-<NUM> corresponds to sample <NUM>-<NUM> in Experiment <NUM>.

The following evaluation was performed on the resin compositions of samples <NUM>-<NUM> to <NUM>-<NUM>.

DC breakdown electric field strengths were respectively measured using the needle electrode A and the needle electrode B in the same manner as in experiment <NUM>.

A DC breakdown electric field strength was measured using a needle electrode C as described below. The radius of curvature of a tip end of the needle electrode C was <NUM>. Measurement conditions other than the radius of curvature of the tip end of the needle electrode C were the same as the measurement conditions under which the DC breakdown electric field strength was measured using the needle electrode A.

Table <NUM> below shows evaluation results of the resin compositions of samples <NUM>-<NUM> to <NUM>-<NUM> in experiment <NUM>.

The DC breakdown electric field strengths respectively measured using the needle electrode A and the needle electrode B were <NUM> kV/mm or more in sample <NUM>-<NUM> in which the ratio X/Z was <NUM>.

However, the DC breakdown electric field strength measured using the needle electrode C was less than <NUM> kV/mm in sample <NUM>-<NUM>.

In sample <NUM>-<NUM>, the ratio X/Z of methine groups to methyl groups was less than <NUM>. That is, the content of methine groups was smaller than the content of methyl groups. Accordingly, the polyethylene included portions in which two branches were generated from a quaternary carbon atom, for example. Therefore, decomposition of the quaternary carbon atom was likely to occur when a high electric field was locally applied. It is though that, consequently, the DC breakdown electric field strength measured using the needle electrode C was low in sample <NUM>-<NUM>.

The ratio X/Z was more than <NUM> in sample <NUM>-<NUM>. That is, the content of methine groups was larger than the content of methyl groups. Accordingly, many methine groups from which no branch was generated, such as a methine group constituting a vinyl group, were generated, and the number of branches was small. Therefore, a minute sink mark was formed between crystals, and electrical treeing may start from such the minute sink mark when a high electric field was applied. It is though that, consequently, the DC breakdown electric field strength measured using the needle electrode C was low in sample <NUM>-<NUM>.

The DC breakdown electric field strengths respectively measured using the needle electrode A and the needle electrode B were <NUM> kV/mm or more in samples <NUM>-<NUM> to <NUM>-<NUM> in which the ratio X/Z was <NUM> or more and <NUM> or less.

Furthermore, the DC breakdown electric field strength measured using the needle electrode C was <NUM> kV/mm or more in samples <NUM>-<NUM> to <NUM>-<NUM>.

The ratio X/Z was <NUM> or more in samples <NUM>-<NUM> to <NUM>-<NUM>. This suppressed generation of a portion in which two branches are generated from a quaternary carbon atom, for example. Therefore, decomposition that occurs due to the presence of a quaternary carbon atom was suppressed.

In samples <NUM>-<NUM> to <NUM>-<NUM>, the ratio X/Z of methine groups to methyl groups was <NUM> or less, and accordingly, excessive generation of methine groups from which no branch was generated was suppressed, and a predetermined number of branches was generated. Therefore, formation of a minute sink mark between crystals was suppressed.

It was confirmed that, consequently, the DC breakdown electric field strength measured using the needle electrode C was high in samples <NUM>-<NUM> to <NUM>-<NUM>.

Power cable samples were produced as described below.

Each of the power cables of samples <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> was immersed in silicone oil at a temperature of <NUM>, and a direct electric field of <NUM> kV/mm was applied in the thickness direction of the insulation layer for <NUM> hours. At this time, whether or not dielectric breakdown occurred in the insulation layer of each power cable was checked.

As a result of the above evaluation, it was confirmed that dielectric breakdown did not occur in the power cables of samples <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM>.

As prior art, the following references are disclosed.

Reference <NUM> discloses high pressure method as the method for manufacturing polyethylene, and discloses polyethylene having a very wide range of properties.

In contrast, the present disclosure can be considered as a so-called "selection invention", being the selection of individual elements, subsets, or sub-ranges, which have not been explicitly mentioned, within or overlapping with a known set or range in reference <NUM>.

In reference <NUM>, LDPE includes a low-density copolymer containing of ethylene with one or more comonomer(s) (paragraph <NUM> in reference <NUM>). Reference <NUM> describes that the upper limit of content of the comonomer in the LDPE copolymer is "<NUM> wt%" (paragraph <NUM> in reference <NUM>).

Here, in LDPE of reference <NUM>, as shown in <FIG>, assuming that the comonomer is propylene unit, the proportion RX of methine groups is estimated based on the content of the comonomer.

In <FIG>, when the content of propylene units as a comonomer is less than <NUM> wt%, by converting the unit wt% to mol % based on the molecular weight of each monomer unit, B is calculated as described below.

The proportion RX of methine groups in the above LDPE of reference <NUM> is obtained by the following equation (A).

When B<<NUM> is substituted into equation (A), RX is calculated as follows in reference <NUM>.

Thus, reference <NUM> describes LDPEs having the proportion RX of methine groups in a much wider range than the range of the present disclosure.

Reference <NUM> describes that the polymerization peak temperature is "up to <NUM>", which is wider than the range of polymerization peak temperatures (<NUM> to <NUM>) in samples <NUM>-<NUM> to <NUM>-<NUM> of the present disclosure (paragraph <NUM> in reference <NUM>).

Reference <NUM> describes that the polymerization pressure is "<NUM> bar (<NUM> MPa) or more", which is wider than the range of polymerization pressure (<NUM> MPa to <NUM> MPa) in samples <NUM>-<NUM> to <NUM>-<NUM> of the present disclosure (paragraph <NUM> in reference <NUM>).

Thus, reference <NUM> describes a much wider manufacturing conditions than those used in the Examples of the present disclosure.

Among inventive examples in reference <NUM>, the inventive example using manufacturing conditions closest to those of samples <NUM>-<NUM> to <NUM>-<NUM> and <NUM>-<NUM> to <NUM>-<NUM> of the present disclosure is "the inventive example <NUM>". In the inventive example <NUM> of reference <NUM>, the polymerization pressure is <NUM> MPa (<NUM> bar) and the polymerization peak temperature is <NUM>.

Regarding the inventive example <NUM> of reference <NUM>, the proportion RX of methine groups is estimated based on the tendency shown in the Examples of the present disclosure.

<FIG> shows the polymerization peak temperature dependence of the proportion RX of methine groups when the polymerization pressure is <NUM> MPa, based on Table <NUM> of the Examples of the present disclosure.

From the tendency shown in <FIG>, when the polymerization pressure is <NUM> MPa and the polymerization peak temperature is <NUM>, the proportion RX of methine groups is "<NUM>" (at the point a in <FIG>).

Next, as shown in <FIG>, based on the tendency of RX at the polymerization peak temperature of <NUM> in Table <NUM> of the present disclosure, it is assumed that even when the polymerization peak temperature is <NUM>, RX changes linearly with the same slope as that at the polymerization peak temperature of <NUM>.

In <FIG>, the straight line (solid line) of the tendency assumed at the above-mentioned polymerization peak temperature of <NUM>, is set to pass through "RX=<NUM>" obtained above at the polymerization pressure of <NUM> MPa.

In the straight line of <FIG>, when the polymerization peak temperature is <NUM> and the polymerization pressure is <NUM> MPa, as the manufacturing conditions of the inventive example <NUM> of reference <NUM>, the proportion RX of methine groups is estimated to be "<NUM>" (at the point b in <FIG>).

Furthermore, in the inventive example <NUM> of reference <NUM>, "<NUM>,<NUM>-octadiene" is added during the manufacturing process. Therefore, a lot of vinyl groups containing methine group are incorporated into LDPE (Table <NUM> of reference <NUM>). From this, in the inventive example <NUM> of reference <NUM>, the proportion RX of methine groups is even higher than the above estimated "<NUM>".

In each of the other inventive examples of reference <NUM>, the polymerization peak temperature is higher than that of the inventive example <NUM>. Therefore, the proportion RX of methine groups in each of the other inventive examples of reference <NUM> is even higher than that of the present disclosure.

From the above, the examples described in reference <NUM> are not included in the range of the proportion RX (<NUM> or less) of methine groups defined in the present disclosure.

As described above, the technical scope described in reference <NUM> is extremely wide and includes a range in which the effects of the present disclosure cannot be achieved. Furthermore, the specific examples in reference <NUM> are not included within the technical scope defined by the present disclosure and cannot obtain the effects of the present disclosure.

That is, reference <NUM> neither discloses nor suggests the range of the proportion RX of methine groups selected to solve new problems discovered by the present inventors. Since there is no disclosure nor suggestion in reference <NUM>, the skilled person in the art would not seriously contemplate the proportion RX of methine groups.

Accordingly, the present disclosure has novelty as selection invention compared to reference <NUM>.

Further, the present disclosure involves inventive step as selection invention as follows.

In Examples of reference <NUM>, tests for loss tan δ measurements are performed using cables (paragraphs <NUM> to <NUM> in reference <NUM>).

Reference <NUM> neither discloses nor suggests tests for measurements that can detect the influence of minute foreign matter in the insulation layer. Tests for measurements of Reference <NUM> cannot discover new problem of the present disclosure, which is the occurrence of electrical treeing caused due to minute foreign matter.

That is, reference <NUM> does not recognize the new problem of the present disclosure at all.

The effect of the invention in reference <NUM> is that loss tan δ is reduced in AC (paragraph <NUM> in reference <NUM>).

In contrast, the effect by satisfying the proportion RX of methine groups in the present disclosure is that the above new problem can be solved, that is, it is possible to suppress local occurrence of electrical treeing.

This effect of the present disclosure is neither disclosed nor suggested in reference <NUM> and is advantageous over reference <NUM>. Further, this effect of the present disclosure cannot be confirmed by tests for loss tan δ measurements, which is measured in the state of the cables. That is, this effect of the present disclosure is different from the effect described in reference <NUM>.

Since reference <NUM> does not recognize the new problem of the present disclosure as described above, the effect of the present disclosure cannot be expected by the skilled person in the art based on the state of the art at the time of the present application.

From the above, the present disclosure as selection invention is connected to the above technical effect. There is no hint in reference <NUM> that would lead the skilled person to the present disclosure as selection invention.

Accordingly, the present disclosure involves inventive step as selection invention compared to reference <NUM>.

Reference <NUM> describes the component (B) (modified polyethylene) of the resin composition (paragraph <NUM> in reference <NUM>). Reference <NUM> describes various additives (paragraph <NUM>).

However, Reference <NUM> neither discloses nor suggests the proportion RX of methine groups in the present disclosure, and the method for manufacturing polyethylene that satisfies the proportion.

That is, reference <NUM> neither discloses nor suggest the configuration of the present disclosure. The configuration of reference <NUM> cannot obtain the effect of the present disclosure.

Reference <NUM> describes a flooding compound including a polymeric filler and a branched olefinic fluid. Reference <NUM> describes that the branched olefinic fluid has at least <NUM> methine carbons per one-thousand total carbons (p16l15 to <NUM> in reference <NUM>).

However, reference <NUM> describes that the branched olefinic fluid is liquid at <NUM> and <NUM> atmosphere of pressure (p7 l8 to <NUM> in reference <NUM>). In other words, the branched olefinic fluid is no longer polyethylene.

Reference <NUM> describes a rubber composition including a rubber matrix containing a branched polyethylene, EPM and EPDM, and essential components containing a crosslinking agent and a reinforcing agent. Reference <NUM> describes that the degree of branching of the branched polyethylene is preferably <NUM> to <NUM> branches/<NUM> carbon atoms (paragraphs <NUM> and <NUM> in reference <NUM>).

However, in reference4, only the branched polyethylene whose degree of branching is <NUM> branches/<NUM> carbons has a possibility of overlapping with the range of the proportion RX of methine groups (<NUM> or less) of the present disclosure. In other words, other branched polyethylene in reference <NUM> do not satisfy the proportion RX of methine groups in the present disclosure.

Reference <NUM> neither discloses nor suggests the method for manufacturing polyethylene that satisfies the proportion RX of methine groups in the present disclosure.

In all examples of reference <NUM>, the degrees of branching are more than <NUM>. Therefore, all examples in reference <NUM> do not satisfy the proportion RX of methine groups (<NUM> or less) in the present disclosure.

In the first place, reference <NUM> neither discloses nor suggests the insulation properties of the rubber composition. That is, reference <NUM> does not recognize the new problem of the present disclosure.

The effect of the invention in reference <NUM> is that good heat resistance, compression set resistance and mechanical strength are simultaneously obtained (paragraph <NUM> in reference <NUM>).

In contrast, the effect of the present disclosure is neither disclosed nor suggested in reference <NUM> and is advantageous over reference <NUM>. Further, this effect of the present disclosure is clearly different from the effect described in reference <NUM>.

Accordingly, the present disclosure has novelty and involves inventive step compared to reference <NUM>.

Even if references <NUM> to <NUM> are combined, the present disclosure can obtain the unexpected effect that are different from that obtained by the combination of these references. The skilled person in the art cannot achieve the present disclosure based on references <NUM> to <NUM>.

Claim 1:
A resin composition for forming an insulation layer of a power cable, the resin composition comprising:
polyethylene,
wherein the polyethylene includes methine groups, and
the polyethylene satisfies the following formula (<NUM>) in a <NUM>C nuclear magnetic resonance spectrum, <MAT>
where RX satisfies RX = X/(X+Y+Z)} × <NUM>, and
X, Y, and Z respectively represent an integrated intensity of the methine groups, an integrated intensity of methylene groups, and an integrated intensity of methyl groups in the <NUM>C nuclear magnetic resonance spectrum of the polyethylene, measured as detailed in the description.