Crosslinkable resin composition

Abstract

The purpose of the present invention is to provide a crosslinkable resin composition that has both good water treeing resistance and good electrical characteristics (electrical insulation), and that can form an insulating layer for a power cable. The crosslinkable resin composition according to the present invention contains a resin composition including a low-density polyethylene (LDPE) and a polar ethylene copolymer (EAA, EBA, EEA, EVA, EVOH, and the like), and the content of polar monomers within the resin composition is 0.05-1.9 mass%, and particularly 0.1-1.8 mass%.

Description

Crosslinkable resin composition

[0001] The present invention relates to a crosslinkable resin composition, and more particularly to a crosslinkable resin composition that contains a resin mixture containing low-density polyethylene and a polar ethylene copolymer and is suitable for use in forming an insulating layer of a power cable that combines good water tree resistance with good electrical properties.

[0002] Cross-linked polyethylene is widely used for the insulation layer (insulation coating layer) of power cables because of its excellent electrical insulation properties and heat deformation resistance. However, when power cables are used in humid or water-filled environments, water trees are generated and propagated in the insulation layer, causing a significant deterioration in the insulation properties of the power cables.

[0003] In the past, in order to form an insulating layer with good water tree resistance, it has been proposed to use a crosslinkable resin composition obtained by adding polyethylene glycol to an ethylene-based polymer (see Patent Documents 1 and 2 listed below).

[0004] Special Publication No. 2013-515134 Japanese Patent Application Laid-open No. 56-28231

[0005] However, because an excessive amount of polyethylene glycol is added to such a resin composition, the insulating layer formed from the resin composition does not have good electrical properties (electrical insulation). In particular, the dielectric loss tangent (tan δ) under a high electric field becomes significantly high, which has adverse effects such as a decrease in power transmission efficiency and a local temperature rise.

[0006] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a crosslinkable resin composition capable of forming an insulating layer having both good water tree resistance and good electrical properties.

[0007] The crosslinkable resin composition of the present invention is characterized in that it contains a resin mixture (resin component) containing a low-density polyethylene and a polar ethylene copolymer, and the content (content) of the polar monomer in the resin mixture is 0.05 to 1.9 mass%.

[0008] The content of the polar monomer in the resin mixture is preferably 0.1 to 1.8% by mass.

[0009] The polar ethylene copolymer is preferably an ethylene-acrylate derivative copolymer, and the ethylene-acrylate derivative copolymer is preferably an ethylene-ethyl acrylate copolymer (EEA), an ethylene-butyl acrylate copolymer (EBA), or an ethylene-acrylic acid copolymer (EAA), particularly preferably EEA or EAA.

[0010] The polar ethylene copolymer may also be an ethylene-vinyl alcohol derivative copolymer, and the ethylene-vinyl alcohol derivative copolymer is preferably an ethylene-vinyl acetate copolymer (EVA) or an ethylene-vinyl alcohol copolymer (EVOH).

[0011] The composition preferably contains an organic peroxide or a silane compound as a crosslinking agent, and is preferably used to form an insulating layer for power cables and electric wires.

[0012] According to the crosslinkable resin composition of the present invention, by crosslinking it, it is possible to form an insulating material (insulating layer) that has both good water tree resistance and good electrical properties.

[0013] The crosslinkable resin composition of the present invention comprises a resin blend having low density polyethylene (LDPE) and a polar ethylene copolymer.

[0014] As the low-density polyethylene, any of those used to form the insulating layers of power cables and electric wires can be used. The density of low-density polyethylene is usually 0.91 to 0.94 g / cm 3 The melt flow rate is usually 0.01 to 10 g / 10 min (190° C., 2.16 kg).

[0015] The polar ethylene copolymer contained in the resin mixture is a copolymer of ethylene and a polar monomer. Specific examples of polar ethylene copolymers include ethylene-acrylate derivative copolymers such as ethylene-methyl methacrylate (EMMA), ethylene-ethyl acrylate copolymer (EEA), ethylene-butyl acrylate copolymer (EBA), and ethylene-acrylic acid copolymer (EAA); and ethylene-vinyl alcohol derivative copolymers such as ethylene-vinyl acetate copolymer (EVA) and ethylene-vinyl alcohol copolymer (EVOH). Of these, EEA, EAA, and EVOH are preferred, with EEA and EAA being particularly preferred.

[0016] The content of the polar monomer in the polar ethylene copolymer is preferably 0.1 to 90% by mass, more preferably 10 to 77% by mass.

[0017] The resin mixture (resin component) may consist of low-density polyethylene and a polar ethylene copolymer, but may also contain resins other than low-density polyethylene and polar ethylene copolymers, as long as the object of the present invention is not impaired. Examples of optional resins constituting the resin mixture include ethylene-based (co)polymers other than low-density polyethylene and polar ethylene copolymers, and propylene-based (co)polymers. Specific examples include high-density polyethylene, medium-density polyethylene, linear low-density polyethylene, ethylene-α-olefin copolymers, ethylene-non-conjugated diene copolymers, and polypropylene.

[0018] The content of the polar monomer in the resin mixture is usually 0.05 to 1.9% by mass, and preferably 0.1 to 1.8% by mass.

[0019] If the content of the polar monomer in the resin mixture is less than 0.05% by mass, the crosslinked resin body (insulating layer) formed from the resulting resin composition will not have good water tree resistance (see Comparative Example 3 described below). On the other hand, if the content of the polar monomer in the resin mixture exceeds 1.9% by mass, the crosslinked resin body (insulating layer) formed from the resulting resin composition will not exhibit good electrical properties (see Comparative Example 4 described below).

[0020] Therefore, when producing the crosslinkable resin composition of the present invention, the resin mixture is prepared by adjusting the mixing ratio of the low-density polyethylene, the polar ethylene copolymer, and any resin so that the content of the polar monomer in the resin mixture is 0.05 to 1.9 mass%.

[0021] The crosslinkable resin composition of the present invention may contain optional components, as required, that are used in ordinary resin compositions. Examples of such optional components include crosslinkers (organic peroxides, silane compounds), antioxidants (hindered phenol stabilizers, sulfur stabilizers, amine stabilizers), inorganic fillers (e.g., calcium carbonate, clay, diatomaceous earth, metal oxides, silica, carbon black, glass fiber, carbon fiber), plasticizers (phthalates, trimellitates, polyesters), softeners (mineral oils, process oils, fatty acids), ultraviolet absorbers (benzophenone-based UVA, benzotriazol-based UVA, salicylate ester-based UVA), antistatic agents (glycerin fatty acid esters, alkylsulfonic acids, tetraalkylammonium salts), flame retardants (metal hydroxides, phosphorus compounds, halogen compounds), lubricants (fatty acid amides, zinc stearate, silicone), foaming agents [azodicarbonamide (ADCA), sodium bicarbonate, 4,4'-oxybis(benzenesulfonylhydrazide) (OBSH)], colorants (azo compounds, anthraquinone compounds, phthalocyanine compounds, titanium oxide, carbon black), crosslinking aids [trimethylolpropane triacrylate (TMPTA), trimethylolpropane trimethacrylate (TMPTM), triallyl isocyanurate (TAIC), triallyl cyanurate (TAC)], light stabilizers (hindered amine type compounds, hindered piperidine type compounds), and the like.

[0022] The crosslinkable resin composition of the present invention is crosslinked to obtain a crosslinked resin that constitutes an insulating layer. The crosslinking method is not particularly limited, and examples thereof include: (1) a method in which a resin composition of the present invention containing an organic peroxide is heated at a predetermined temperature to perform chemical crosslinking; (2) a method in which a resin composition of the present invention containing a silane crosslinking agent is heated at a predetermined temperature to perform silane crosslinking; and (3) a method in which a resin composition of the present invention is irradiated with an electron beam to perform electron beam crosslinking.

[0023] As the heat treatment method, various methods can be used, such as a method of heating in an atmosphere of nitrogen, water vapor, silicone oil, molten salt, etc., or a method of heating during molding in a heat press or injection molding machine.

[0024] Examples of the present invention will be described below, but the present invention is not limited to these. In the following examples and comparative examples, the following compounds were used as low-density polyethylene, polar ethylene copolymer, or water tree inhibitor, antioxidant, and crosslinking agent constituting the resin composition.

[0025] Low density polyethylene (LDPE): Density = 0.92 g / cm 3 , low-density polyethylene having MFR (190°C, 2.16 kg) = 2.0 g / 10 min.

[0026] Polar ethylene copolymer (EAA): Density = 0.93 g / cm 3 , MFR (190°C, 2.16 kg) = 1.0 g / 10 min, AA content 10 mass% (4.1 mol%).

[0027] Polar ethylene copolymer (EEA-1): Density = 0.94 g / cm 3 , MFR (190°C, 2.16 kg) = 6.0 g / 10 min, EA content 18 mass% (5.8 mol%) Ethylene-ethyl acrylate copolymer.

[0028] Polar ethylene copolymer (EEA-2): Density = 0.93 g / cm 3 , MFR (190°C, 2.16 kg) = 250 g / 10 min, EA content 25 mass% (8.5 mol%) Ethylene-ethyl acrylate copolymer.

[0029] Polar ethylene copolymer (EVOH): Density = 1.19 g / cm 3 , MFR (190°C, 2.16 kg) = 3.8 g / 10 min, VOH content 77 mass% (68 mol%) Ethylene-vinyl alcohol copolymer.

[0030] Water tree inhibitor (PEG): Polyethylene glycol with a number average molecular weight of 20,000.

[0031] Antioxidant: Hindered phenol type stabilizer, 4,4'-thiobis-(3-methyl-6-t-butylphenol)

[0032] ・Crosslinking agent: dicumyl peroxide

[0033] <Production of Crosslinkable Resin Composition> [Example 1] According to the formulation shown in Table 1 below, 99 parts by mass of low-density polyethylene (LDPE), 1 part by mass of polar ethylene copolymer (EAA), and 0.15 parts by mass of an antioxidant were melt-kneaded, and 1.5 parts by mass of a crosslinking agent was added to the resulting composition. The mixture was heated to 60°C and mixed for 12 hours, and then cooled to room temperature to prepare a resin composition (crosslinkable resin composition of the present invention).

[0034] Example 2 A crosslinkable resin composition of the present invention was prepared in the same manner as in Example 1, except that 90 parts by mass of low-density polyethylene (LDPE), 10 parts by mass of polar ethylene copolymer (EAA), and 0.15 parts by mass of an antioxidant were melt-kneaded according to the formulation shown in Table 1 below.

[0035] Example 3 A crosslinkable resin composition of the present invention was prepared in the same manner as in Example 1, except that 95 parts by mass of low-density polyethylene (LDPE), 5 parts by mass of polar ethylene copolymer (EEA-1), and 0.15 parts by mass of an antioxidant were melt-kneaded according to the formulation shown in Table 1 below.

[0036] Example 4 A crosslinkable resin composition of the present invention was prepared in the same manner as in Example 1, except that 90 parts by mass of low-density polyethylene (LDPE), 10 parts by mass of polar ethylene copolymer (EEA-1), and 0.15 parts by mass of an antioxidant were melt-kneaded according to the formulation shown in Table 1 below.

[0037] Example 5 A crosslinkable resin composition of the present invention was prepared in the same manner as in Example 1, except that 99.5 parts by mass of low-density polyethylene (LDPE), 0.5 parts by mass of polar ethylene copolymer (EVOH), and 0.15 parts by mass of an antioxidant were melt-kneaded according to the formulation shown in Table 1 below.

[0038] Comparative Example 1 A comparative crosslinkable resin composition was prepared in the same manner as in Example 1, except that 100 parts by mass of low-density polyethylene (LDPE) and 0.15 parts by mass of an antioxidant were melt-kneaded according to the formulation shown in Table 1 below.

[0039] Comparative Example 2 A comparative crosslinkable resin composition was prepared in the same manner as in Example 1, except that 100 parts by mass of low-density polyethylene (LDPE), 0.6 parts by mass of a water tree inhibitor (PEG), and 0.15 parts by mass of an antioxidant were melt-kneaded according to the formulation shown in Table 1 below.

[0040] Comparative Example 3 A comparative crosslinkable resin composition was prepared in the same manner as in Example 1, except that 99.8 parts by mass of low-density polyethylene (LDPE), 0.2 parts by mass of polar ethylene copolymer (EAA), and 0.15 parts by mass of an antioxidant were melt-kneaded according to the formulation shown in Table 1 below.

[0041] Comparative Example 4 A comparative resin composition was prepared in the same manner as in Example 1, except that 91.5 parts by mass of low-density polyethylene (LDPE), 8.5 parts by mass of polar ethylene copolymer (EEA-2), and 0.15 parts by mass of an antioxidant were melt-kneaded according to the formulation shown in Table 1 below.

[0042] <Evaluation of Crosslinkable Resin Compositions> (1) Water Tree Acceleration Test (Water Tree Resistance): Each of the crosslinkable resin compositions obtained in the Examples and Comparative Examples was supplied to a mold and press-molded (crosslinked) at 180°C for 15 minutes using a heated press (Toho Machinery, TBD-50 model) to prepare a disc-shaped test specimen having a notch-shaped defect made of a crosslinked resin. According to ASTM D6097, the prepared disc-shaped test specimen was immersed in a 1.0 N aqueous sodium chloride solution, and a water tree acceleration test was performed under the conditions of an applied voltage of 5 kV, a frequency of 1,000 Hz, a temperature of 30°C, and a voltage application time of 500 hours. After voltage application, the test specimen was dyed, sliced, and then observed under a microscope. The probability of water tree occurrence and the maximum water tree length per test specimen were determined. The evaluation criteria were as follows: a water tree occurrence probability of 50% or less and a maximum water tree length of 600 μm or less was rated as "pass (◯)", and a failure to meet either of these criteria was rated as "fail (×)". The results are also shown in Table 1 below.

[0043] (2) Measurement of dielectric loss tangent (electrical properties): Each of the crosslinkable resin compositions obtained in the Examples and Comparative Examples was crosslinked by pressing at 180°C for 15 minutes using a hot press (Toho Machinery, TBD-50 model) to produce a sheet-like crosslinked resin product, and test pieces were prepared from each of the resulting crosslinked products. The dielectric loss tangent was measured at a temperature of 90°C and an electric field of 10 kV / mm in accordance with JIS C 2138.

[0044] (3) Measurement of volume resistivity (electrical properties): Each of the crosslinkable resin compositions obtained in the Examples and Comparative Examples was crosslinked by pressing at 180°C for 15 minutes using a heated press (Toho Machinery, TBD-50 model) to produce a sheet-like crosslinked resin product, and test pieces were prepared from each of the resulting crosslinked products. The volume resistivity was measured at a temperature of 90°C and an electric field of 10 kV / mm in accordance with JIS C 2139-3-1.

[0045] The evaluation criteria for electrical properties are that the dielectric loss tangent is 0.1% or less and the volume resistivity is 10 15 If the resistance was Ω·cm or more, it was rated as "pass (◯)", and if either of these was not satisfied, it was rated as "fail (×)". The results are also shown in Table 1 below.

[0046]

Claims

1. A crosslinkable resin composition comprising a resin mixture containing low-density polyethylene and a polar ethylene copolymer, wherein the content of the polar monomer in the resin mixture is 0.05 to 1.9 mass %.

2. The crosslinkable resin composition according to claim 1, wherein the content of the polar monomer in the resin mixture is 0.1 to 1.8% by mass.

3. The crosslinkable resin composition according to claim 2, wherein said polar ethylene copolymer is an ethylene-acrylate derivative copolymer.

4. The crosslinkable resin composition according to claim 3, wherein the ethylene-acrylate derivative copolymer is ethylene-ethyl acrylate copolymer (EEA), ethylene-butyl acrylate copolymer (EBA) or ethylene-acrylic acid copolymer (EAA).

5. The crosslinkable resin composition according to claim 3, wherein the ethylene-acrylate derivative copolymer is ethylene-ethyl acrylate copolymer (EEA) or ethylene-acrylic acid copolymer (EAA).

6. The crosslinkable resin composition according to claim 2, wherein said polar ethylene copolymer is an ethylene-vinyl alcohol derivative copolymer.

7. The crosslinkable resin composition according to claim 6, wherein the ethylene-vinyl alcohol derivative copolymer is an ethylene-vinyl acetate copolymer (EVA) or an ethylene-vinyl alcohol copolymer (EVOH).

8. The crosslinkable resin composition according to any one of claims 1 to 7, which contains an organic peroxide or a silane compound as a crosslinking agent.

9. The crosslinkable resin composition according to any one of claims 1 to 7, which is used to form an insulating layer for a power cable or electric wire.

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