Flame-retardant termite-repelling resin composition, power cable, and production method for same

Abstract

The present invention provides: a flame-retardant termite-repelling resin composition that can constitute, in a single layer, a sheath having excellent flame retardancy and termite repellency, that has excellent thermal stability, that has sufficient tensile strength and elongation characteristics which are mechanical properties necessary for a sheath of a power cable, that also has excellent cold resistance and oil resistance, and that is suitable for use in forming a sheath of a power cable; a power cable using said composition; and a production method for the same.　This flame-retardant termite-repelling resin composition comprises at least a polyvinyl chloride resin, an MBS resin, a polyester-based plasticizer, a stabilizer, and a flame retardancy agent, wherein, with respect to 100 parts by mass of the polyvinyl chloride resin, the content of the MBS resin is in the range of 1-20 parts by mass, the content of the polyester-based plasticizer is in the range of 20-40 parts by mass, the content of the stabilizer is in the range of 4-20 parts by mass, and the content of the flame retardancy agent is in the range of 3-20 parts by mass.

Description

Flame-retardant anti-termite resin composition, power cable and method of manufacturing the same

[0001] The present invention relates to a flame-retardant and anti-termite resin composition, a power cable, and a method for producing the same.

[0002] For cables such as power cables and communication cables, a sheath (protective outer coating) is widely used as the outermost layer around the conductor. The sheath is formed by extrusion coating a resin, such as polyethylene resin or polyvinyl chloride (PVC) resin, around the conductor.

[0003] The sheath is required to have various functions depending on the location where the cable is laid. For example, cables laid underground in areas with high termite activity are required to have anti-termite properties, which prevent damage by termites and other ants, as well as flame retardancy.

[0004] For this reason, it has been common to construct the sheath not as a single layer but as multiple layers, by overlapping a layer of a flame-retardant resin such as polyvinyl chloride resin, which has excellent flame-retardant properties, with a layer of an anti-termite resin such as nylon, which has excellent anti-termite properties.

[0005] For example, Patent Document 1 describes a sheath 13 composed of two layers, a flame-retardant vinyl layer 14 and an outermost layer 15, as shown in Figure 3, in which the polypropylene resin composition constituting the outermost layer 15 contains a polymer resin composed of a propylene homopolymer moiety and a resin component having a solubility parameter of 7.0 to 9.5, and a flame retardant, thereby achieving a Rockwell hardness of 85 or more and a flexural modulus of 1500 MPa or more. This sheath is imparted with flame retardancy by the flame-retardant vinyl layer 14 and the flame retardant contained in the outermost layer 15, and is imparted with termite resistance by setting the mass ratio of the propylene homopolymer moiety contained in the polymer resin forming the outermost layer 15 within a predetermined range and achieving high hardness and a high flexural modulus.

[0006] JP 2015-096583 A

[0007] The cable described in Patent Document 1 has a sheath that is made up of two layers: a flame-retardant vinyl layer and an outermost layer made of a polypropylene resin composition, which results in high manufacturing costs. Furthermore, since there has not yet existed a material that satisfies both high flame retardancy and termite resistance, it has not been possible to make the sheath out of a single layer.

[0008] Furthermore, when the cable is used as a power cable, it is necessary to satisfy the requirements for tensile strength, elongation, cold resistance, and oil resistance in addition to flame retardancy and termite resistance. Furthermore, since power cables are relatively large compared to other electric wire and cable products, it takes a long time to extrude a resin to form a sheath, and therefore there has been a demand for a resin composition that has excellent thermal stability and can withstand such long-term extrusion coating.

[0009] The object of the present invention is to provide a flame-retardant and anti-termite resin composition suitable for use in forming a power cable sheath, which can be used to form a single-layer sheath with excellent flame retardancy and anti-termite properties, has excellent thermal stability, fully satisfies the mechanical properties of tensile strength and elongation required for a power cable sheath, and also has excellent cold resistance and oil resistance, and is therefore suitable for use in forming a power cable sheath, as well as a power cable using the same and a method for producing the same.

[0010] The present inventors have discovered that a flame-retardant, anti-termite resin composition containing at least polyvinyl chloride resin, MBS resin, polyester plasticizer, stabilizer, and flame retardant, and limiting the contents of these to appropriate proportions, can provide a balanced performance that meets the basic requirements for a power cable, namely flame retardancy, anti-termite properties, tensile strength, elongation, cold resistance, oil resistance, and thermal stability, and have completed the present invention based on this finding.

[0011] That is, the gist of the present invention is as follows: (1) A flame-retardant, anti-termite resin composition containing at least polyvinyl chloride resin, MBS resin, polyester-based plasticizer, stabilizer, and flame retardant, wherein, relative to 100 parts by mass of the polyvinyl chloride resin, the content of the MBS resin is in the range of 1 part by mass to 20 parts by mass, the content of the polyester-based plasticizer is in the range of 20 parts by mass to 40 parts by mass, the content of the stabilizer is in the range of 4 parts by mass to 20 parts by mass, and the content of the flame retardant is in the range of 3 parts by mass to 20 parts by mass.

[0012] (2) The flame-retardant, anti-termite resin composition according to (1), wherein the content of the MBS resin is in the range of 3 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the polyvinyl chloride resin.

[0013] (3) The flame-retardant anti-termite resin composition according to (1) or (2), wherein the content of the polyester-based plasticizer is in the range of 20 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the polyvinyl chloride resin.

[0014] (4) The flame-retardant, anti-termite resin composition according to any one of (1) to (3) above, which has a hardness of 65 or more as measured using a type D durometer.

[0015] (5) The flame-retardant, anti-termite resin composition according to any one of (1) to (4) above, which is used to form a sheath that constitutes the outermost layer of a power cable.

[0016] (6) A power cable comprising at least a conductor and a sheath covering the conductor as an outermost layer, the sheath being made of the flame-retardant, anti-termite resin composition according to any one of (1) to (5) above.

[0017] (7) The power cable according to (6) above, further comprising, between the conductor and the sheath, in order from the conductor side, an inner semiconductive layer, an insulator layer, an outer semiconductive layer, and a shielding layer.

[0018] (8) The power cable according to (6) or (7) above, wherein the sheath is made of a single layer.

[0019] (9) A method for manufacturing a power cable having at least a conductor and a sheath covering the conductor as the outermost layer, the method including a sheath forming step of forming the sheath by extrusion coating the flame-retardant, anti-termite resin composition described in any one of (1) to (5) above onto the outer periphery of the conductor.

[0020] (10) The method for manufacturing a power cable described in (9) above, wherein the sheath forming step includes laminating an inner semiconductive layer, an insulator layer, an outer semiconductive layer, and a shielding layer in that order on the conductor, and then extrusion coating the flame-retardant, anti-termite resin composition on the shielding layer to form the sheath.

[0021] According to the present invention, it is possible to provide a flame-retardant and anti-termite resin composition that can be used to form a single-layer sheath having excellent flame retardancy and anti-termite properties, has excellent thermal stability, fully satisfies the mechanical properties of tensile strength and elongation required for a power cable sheath, and is also excellent in cold resistance and oil resistance, making it suitable for use in forming a power cable sheath, as well as a power cable using the same and a method for manufacturing the same.

[0022] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.

[0023] Hereinafter, embodiments of the present invention will be described in detail. Note that the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the present invention.

[0024] <Flame-retardant, anti-termite resin composition> The flame-retardant, anti-termite resin composition of the present invention contains at least a polyvinyl chloride resin, an MBS resin, a polyester-based plasticizer, a stabilizer, and a flame retardant, and relative to 100 parts by mass of the polyvinyl chloride resin, the content of the MBS resin is in the range of 1 part by mass to 20 parts by mass, the content of the polyester-based plasticizer is in the range of 20 parts by mass to 40 parts by mass, the content of the stabilizer is in the range of 4 parts by mass to 20 parts by mass, and the content of the flame retardant is in the range of 3 parts by mass to 20 parts by mass.

[0025] The flame-retardant, anti-termite resin composition of the present invention contains a polyvinyl chloride resin with excellent flame retardancy as a base resin, and can further enhance flame retardancy by further adding a flame retardant. Furthermore, by incorporating MBS resin and polyester-based plasticizer in a predetermined range, when the composition is used as a raw material for, for example, a sheath forming the outermost layer of a power cable, the resulting sheath has the excellent cold resistance and oil resistance desired for a power cable, as well as high hardness and resistance to ant damage, making it excellent in anti-termite properties. Furthermore, by incorporating a stabilizer in a predetermined range, the resulting sheath has sufficient tensile strength and elongation for a power cable sheath and excellent thermal stability capable of withstanding long-term extrusion coating. Therefore, the flame-retardant, anti-termite resin composition of the present invention can be used to form a single-layer sheath with excellent flame retardancy and anti-termite properties. The flame-retardant, anti-termite resin composition has excellent thermal stability, fully satisfies the mechanical properties required for a power cable sheath, such as tensile strength and elongation, and also has excellent cold resistance and oil resistance, making it suitable for use in forming a power cable sheath.

[0026] Here, the flame-retardant, anti-termite resin composition contains at least a polyvinyl chloride resin (A), an MBS resin (B), a polyester-based plasticizer (C), a stabilizer (D), and a flame retardant (E).

[0027] (Polyvinyl chloride resin (A)) Polyvinyl chloride resin (A) is a resin that has excellent flame retardancy among general-purpose plastics. Therefore, by using polyvinyl chloride resin as a base resin, the flame retardancy of the sheath can be improved even if the blending amount of a material that improves flame retardancy, such as a flame retardant, is small.

[0028] The average degree of polymerization of the polyvinyl chloride resin (A) is not particularly limited, but is preferably in the range of 900 to 5,000 from the viewpoint of suitable application to moldings for covering electric wires, cables, and the like.

[0029] The polyvinyl chloride resin (A) may be a polymer obtained by polymerizing a monomer containing vinyl chloride, and may contain a unit structure of another monomer component or a repeating unit structure. Among these, from the viewpoint of further improving the flame retardancy of the sheath, the polyvinyl chloride resin (A) is preferably a homopolymer of vinyl chloride.

[0030] As the resin component, other resins including the MBS resin (B) described below may be added. However, from the viewpoint of obtaining a sheath that is hard and less susceptible to damage by ants, the content of the polyvinyl chloride resin (A) is preferably 80 parts by mass or more, and more preferably 90 parts by mass or more, when the content of all resin components is 100 parts by mass, and it is also preferable that no other resins other than the MBS resin (B) are contained.

[0031] (MBS Resin (B)) When MBS resin (methyl methacrylate-butadiene-styrene copolymer resin) (B) is contained in the flame-retardant, anti-termite resin composition, it can improve impact strength in low-temperature environments and thereby enhance cold resistance. To achieve this effect, the MBS resin (B) is contained in an amount of 1 part by mass or more, preferably 3 parts by mass or more, per 100 parts by mass of polyvinyl chloride resin. On the other hand, if the content of MBS resin (B) exceeds 20 parts by mass, the hardness significantly decreases, resulting in a decrease in anti-termite properties, and the oil resistance also decreases, making it difficult to use the composition as a power cable sheath. For this reason, the content of MBS resin (B) is set to 20 parts by mass or less per 100 parts by mass of polyvinyl chloride resin. Therefore, the content of MBS resin (B) in the flame-retardant, anti-termite resin composition is in the range of 1 part by mass to 20 parts by mass, preferably 3 parts by mass to 20 parts by mass, per 100 parts by mass of polyvinyl chloride resin.

[0032] (Polyester-based plasticizer (C)) When the polyester-based plasticizer (C) is contained in the flame-retardant, anti-termite resin composition, it can improve tensile strength, elongation, and oil resistance because it has a higher molecular weight than phthalate-based plasticizers. To achieve this effect, the polyester-based plasticizer (C) is contained in an amount of 20 parts by mass or more per 100 parts by mass of polyvinyl chloride resin. On the other hand, if the content of the polyester-based plasticizer exceeds 40 parts by mass, the hardness significantly decreases, thereby reducing the anti-termite properties. For this reason, the content of the polyester-based plasticizer (C) is 40 parts by mass or less, preferably 30 parts by mass or less, per 100 parts by mass of polyvinyl chloride resin. Therefore, the content of the polyester-based plasticizer (C) in the flame-retardant, anti-termite resin composition is in the range of 20 parts by mass to 40 parts by mass, preferably 20 parts by mass to 30 parts by mass, per 100 parts by mass of polyvinyl chloride resin.

[0033] The polyester plasticizer (C) may be any plasticizer having a polyester structure formed by dehydration condensation of a polycarboxylic acid and a polyalcohol in the molecule. Among them, from the viewpoint of availability, one or both of adipic acid polyesters and sebacic acid polyesters, which are polyesters using adipic acid or sebacic acid as the polycarboxylic acid, are preferred.

[0034] (Stabilizer (D)) The stabilizer (D) is contained to suppress thermal degradation of the polyvinyl chloride resin (A). In particular, when used in the sheath of a power cable, the flame-retardant anti-termite resin composition must be extrusion coated for a long period of time due to the nature of the product, and therefore, particularly high thermal stability is required among electric wire and cable products, and therefore the stabilizer (D) must be contained. To exert this effect, the stabilizer (D) is contained in an amount of 4 parts by mass or more, preferably 10 parts by mass or more, per 100 parts by mass of polyvinyl chloride resin. On the other hand, if the content of the stabilizer (D) exceeds 20 parts by mass, the elongation when used in a sheath decreases. For this reason, the content of the stabilizer (D) is 20 parts by mass per 100 parts by mass of polyvinyl chloride resin. Therefore, the content of the stabilizer (D) in the flame-retardant anti-termite resin composition is in the range of 4 parts by mass to 20 parts by mass, preferably 10 parts by mass to 20 parts by mass, per 100 parts by mass of polyvinyl chloride resin.

[0035] While lead compounds have often been used as the stabilizer (D), from the viewpoints of safety, health, and the environment, it is preferable to incorporate a lead-free stabilizer, consisting of a compound containing no lead atoms, into the flame-retardant, anti-termite resin composition. Examples of the lead-free stabilizer include those commonly used in the coating of electric wires and cables, such as one or more stabilizers selected from the group consisting of Ca-Zn stabilizers, Ba-Zn stabilizers, polyol stabilizers, and organotin stabilizers. Among these, Ca-Zn stabilizers are preferred because of their excellent heat resistance and excellent dispersibility in the flame-retardant, anti-termite resin composition. The lead-free stabilizer can be a combination of multiple compounds that function as stabilizers. For example, a Ca-Zn stabilizer can be a combination of a Ca-containing compound, such as hydrotalcite, and a Zn-containing compound, such as zinc stearate.

[0036] (Flame retardant (E)) The flame retardant (E) is a compound having a flame retardant effect. To exert this effect, the flame retardant (E) is contained in an amount of 3 parts by mass or more, preferably 5 parts by mass or more, per 100 parts by mass of the polyvinyl chloride resin. On the other hand, the upper limit of the content of the flame retardant (E) is not particularly limited, but can be, for example, 20 parts by mass. Therefore, the content of the flame retardant (E) in the flame-retardant anti-termite resin composition is in the range of 3 parts by mass or more and 20 parts by mass or less, preferably 5 parts by mass or more and 20 parts by mass or less, per 100 parts by mass of the polyvinyl chloride resin.

[0037] Examples of the flame retardant (E) include antimony trioxide, zinc borate, and metal hydroxides such as magnesium hydroxide and calcium hydroxide. Among these, it is preferable to contain antimony trioxide, as it is widely used as a flame retardant for polyvinyl chloride resin (A).

[0038] (Other Components (F)) The flame-retardant, anti-termite resin composition according to this embodiment may contain other components as needed.

[0039] More specifically, the flame-retardant, anti-termite resin composition according to this embodiment may contain additives such as ultraviolet absorbers, light stabilizers, antioxidants, lubricants, crystal nucleating agents, softeners, antistatic agents, metal deactivators, antibacterial and antifungal agents, colorants, pigments, dyes, and fluorescent materials when it is necessary to further improve other properties.

[0040] On the other hand, the flame-retardant, anti-termite resin composition according to the present embodiment may contain a crosslinking agent, but preferably does not contain a crosslinking agent, particularly from the viewpoint of improving recyclability.

[0041] The content of the other component (F) in the flame-retardant, anti-termite resin composition according to this embodiment is not particularly limited, but from the viewpoint of preventing a decrease in flame retardancy or anti-termite properties due to the addition of the other component (F), the content is preferably 15 parts by mass or less, more preferably 10 parts by mass or less, even more preferably 7 parts by mass or less, even more preferably 5 parts by mass or less, and particularly preferably 3 parts by mass or less, relative to 100 parts by mass of polyvinyl chloride resin.

[0042] (Characteristics and Uses of Flame-Retardant, Anti-Termite Resin Composition) The flame-retardant, anti-termite resin composition according to this embodiment preferably has a hardness of 65 or more as measured using a Type D durometer. This makes it possible to make resins formed from the flame-retardant, anti-termite resin composition less susceptible to damage by ants. In particular, from the viewpoint of making resins formed from the flame-retardant, anti-termite resin composition even less susceptible to damage by ants, it is more preferable that the hardness of the flame-retardant, anti-termite resin composition be 68 or more as measured using a Type D durometer.

[0043] The flame-retardant, anti-termite resin composition according to the present embodiment is particularly suitable for use in forming the outermost sheath of a power cable. The flame-retardant, anti-termite resin composition according to the present embodiment has excellent flame retardancy and anti-termite properties, as well as excellent cold resistance and oil resistance, allowing the sheath to be formed as a single layer, thereby reducing the manufacturing cost of the power cable.

[0044] <Power cable> As conceptually shown in Fig. 1 , the power cable 1 according to this embodiment has at least a conductor 2 and a sheath 3 made of the above-mentioned flame-retardant and termite-resistant resin composition, which covers the conductor 2 as the outermost layer. Here, the power cable 1 is not particularly limited as long as it has a configuration in which an intermediate layer 4 is provided between the conductor 2 and the sheath 3. This power cable 1 can be obtained, for example, by the above-mentioned method.

[0045] The power cable 1 according to this embodiment has a sheath 3 made of the flame-retardant, anti-termite resin composition described above, and therefore can satisfy the cold resistance and oil resistance requirements for the power cable 1 and exhibit the desired flame retardancy and anti-termite properties. Therefore, the power cable 1 can be suitably installed in places where damage by ants may occur, or where fires caused by ant damage or damage due to surrounding fires may occur.

[0046] Here, the sheath 3 is made of insulating resin and is provided as the outermost layer of the power cable 1. It meets the requirements for cold resistance and oil resistance, exhibits the desired flame retardancy and termite resistance, and increases the mechanical strength of the outermost layer to provide protection from the outside.

[0047] Furthermore, it is preferable that the sheath 3 is made of a single layer. Since the power cable 1 can exhibit both the desired flame retardancy and termite resistance even when the sheath 3 is made of a single layer, the outer diameter of the power cable 1 can be made smaller than that of a power cable having a sheath made of two layers. Furthermore, since the above-mentioned flame-retardant and termite-resistant resin composition has high thermal stability and is suitable for extrusion processing, the material costs and manufacturing costs of the power cable 1 can be reduced, and the power cable 1 can be manufactured efficiently.

[0048] The thickness and shape of the sheath 3 vary depending on the voltage class and installation conditions of the power cable 1, and are not particularly limited. However, from the viewpoint of ensuring the thickness necessary for protection from the outside and not increasing the outer diameter of the power cable 1, the thickness is preferably in the range of 3.0 mm to 6.0 mm, and more preferably in the range of 3.0 mm to 5.5 mm.

[0049] An example of the power cable 1 is one in which an intermediate layer 4 is present between the conductor 2 and the sheath 3, and the intermediate layer 4 is composed of four layers, as in the power cable 1A shown in Fig. 2. In this case, the power cable 1 may further have, between the conductor 2 and the sheath 3, an inner semiconductive layer 5, an insulating layer 6, an outer semiconductive layer 7, and a shielding layer 8, in this order from the conductor 2 side, and may further have a sheath 3 laminated around the outer periphery as the outermost layer.

[0050] The conductor 2 is preferably made of copper, a copper alloy, aluminum or an aluminum alloy, and more preferably made of copper or a copper alloy. The conductor 2 may have a plating layer made of tin, silver or the like on its surface.

[0051] The internal semiconductive layer 5 is a semiconductive layer provided between the conductor 2 and the insulating layer 6 described below, and may be an internal semiconductive layer generally used in power cables. More specifically, a layer coated with a conductive fibrous (cloth) tape, an extruded polyethylene mixed with carbon, or a combination of these may be used. The internal semiconductive layer 5 may also be formed by crosslinking a resin composition for a semiconductive layer.

[0052] Here, the resin composition for the semiconductive layer contains, for example, a resin for the semiconductive layer, a conductive substance, a crosslinking agent, and an antioxidant.

[0053] Among these, the resin for the semiconductive layer is not particularly limited, but an ethylene-based polymer is usually used. The ethylene-based polymer is not particularly limited as long as it contains ethylene as a repeating unit, and examples thereof include polyethylene (low-density polyethylene, high-density polyethylene), ethylene-vinyl acetate copolymer (EVA), ethylene-ethyl acrylate copolymer (EEA), ethylene-methyl acrylate copolymer, ethylene-ethyl methacrylate copolymer, ethylene-1-butene copolymer, ethylene-α-olefin copolymer, and ethylene-propylene diene rubber (EPDM). These ethylene-based polymers can be used alone or in combination of two or more. Here, the polyethylene preferably includes crosslinked polyethylene. Crosslinked polyethylene is obtained by crosslinking a resin composition for the semiconductive layer containing polyethylene and a crosslinking agent.

[0054] The conductive material is not particularly limited, but typically conductive carbon is used. Examples of conductive carbon include carbon black, acetylene black, furnace black, ketjen black, thermal black, and graphite. These may be used alone or in combination of two or more.

[0055] The internal semiconductive layer 5 may be formed by extruding a resin for a semiconductive layer, or may be formed by a semiconductive tape.

[0056] The insulator layer 6 is an insulating layer that covers the internal semiconducting layer 5, and is generally formed by crosslinking a resin composition for the insulator layer. Here, the resin composition for the insulator layer contains, for example, a resin for the insulator layer, a crosslinking agent, and an antioxidant.

[0057] The resin for the insulating layer is preferably a polyolefin resin, which may be modified with an unsaturated organic acid or its derivative. The polyolefin resin is preferably polyethylene, polypropylene, or a copolymer of ethylene and propylene. The resin constituting the insulating layer is preferably a crosslinked resin.

[0058] The outer semiconductive layer 7 is a semiconductive layer that covers the insulator layer 6, and can be formed by using the above-mentioned resin composition for semiconductive layer and crosslinking it.

[0059] The shielding layer (metal shielding layer) 8 is a conductive layer that covers the outer semiconductive layer 7 and is made of, for example, a tape-shaped or extruded metal or alloy. More specifically, examples of the shielding layer include a layer made of copper tape or aluminum tape, and a layer formed by extruding aluminum, lead, or an iron alloy such as stainless steel onto the surface of the outer semiconductive layer 7.

[0060] In this embodiment, the power cable 1 has been described, but it goes without saying that the flame-retardant and anti-termite resin composition of the present invention can be applied to cables in general, since it can achieve the same effects as the power cable of the present invention if it is applied as a raw material for the sheath that constitutes the cable.

[0061] In addition, in the present embodiment, the power cable 1 has been described as being single-core (one core), in which the conductor 2 is made up of one core wire, but the power cable may be multi-core (e.g., three cores) in which the conductor 2 is made up of multiple core wires. Here, a multi-core power cable is made by bundling multiple conductors each having a surface coated with at least an inner semiconductive layer, an insulating layer, an outer semiconductive layer, and a shielding layer, in that order, and then pressing the surface of the bundle with tape or the like, and then covering the bundle with a sheath or the like.

[0062] <Method for manufacturing a power cable> The method for manufacturing a power cable according to this embodiment is a method for manufacturing a power cable 1 having at least a conductor 2 and a sheath 3 that covers the conductor 2 as an outermost layer. The method for manufacturing a power cable is not particularly limited, but an example thereof can include a method including a sheath formation step in which the above-mentioned flame-retardant and termite-resistant resin composition is extruded onto the outer periphery of the conductor 2 to form the sheath 3. This makes it possible to obtain a power cable 1 in which the sheath 3 is formed as an outermost layer around the outer periphery of the conductor 2.

[0063] Here, as a means for extruding the flame-retardant anti-termite resin composition in the sheath-forming step, a known extrusion molding means can be used. In this case, it is preferable to granulate the flame-retardant anti-termite resin composition by a molding means such as extrusion molding to form pellets, and then use these pellets to perform extrusion coating, in terms of ease of handling the flame-retardant anti-termite resin composition.

[0064] The temperature (molding temperature) at which the flame-retardant, anti-termite resin composition is extruded to form the sheath 3 in the sheath-forming step can be in the range of 150° C. or higher and 200° C. or lower. The time for extrusion coating the flame-retardant, anti-termite resin composition in the sheath-forming step can be in the range of 1 hour or higher and 100 hours or lower. By using the above-mentioned flame-retardant, anti-termite resin composition in the sheath-forming step, the thermal stability of the flame-retardant, anti-termite resin composition to be extrusion coated is improved, so that the sheath 3 can be extrusion coated stably even when the sheath 3 is extrusion coated for a long period of time, as in the case of forming the power cable 1.

[0065] In the sheath formation step, it is preferable to form the sheath 3 by sequentially laminating the inner semiconductive layer 5, the insulator layer 6, the outer semiconductive layer 7, and the shielding layer 8 on the conductor 2, and then extrusion-coating the flame-retardant, anti-termite resin composition on the shielding layer 8. This makes it possible to form a power cable, as shown in power cable 1A in Fig. 2 , which further has, in order from the conductor 2 side, the inner semiconductive layer 5, the insulator layer 6, the outer semiconductive layer 7, and the shielding layer 8 between the conductor 2 and the sheath 3. Here, when laminating the inner semiconductive layer 5, the insulator layer 6, the outer semiconductive layer 7, and the shielding layer 8 on the conductor 2, adjacent two or more layers of the inner semiconductive layer 5, the insulator layer 6, and the outer semiconductive layer 7 may be simultaneously extrusion-coated, for example.

[0066] Next, in order to further clarify the effects of the present invention, examples of the present invention and comparative examples will be described, but the present invention is not limited to these examples of the present invention.

[0067] <Performance Evaluation of Sheath-Forming Material (Sheet)> [Preparation of Sheath-Forming Material] A resin composition was obtained by blending polyvinyl chloride resin (A), MBS resin (B), polyester-based plasticizer (C), stabilizer (D), and flame retardant (E) in the proportions (parts by mass) shown in Table 1. The resin composition was then kneaded and molded using a rolling mill set at a kneading temperature of 150°C to prepare three sheet-shaped samples with different thicknesses. These three samples were heated to a molding temperature of 170°C and press-molded at a pressure of 18 MPa for 15 minutes to obtain smooth sheets with thicknesses of 1 mm, 2 mm, and 3 mm, respectively. Note that the obtained resin composition did not contain the other component (F).

[0068] [Evaluation of Sheath-Forming Material Properties] The following property evaluations were carried out using sheets made from the resin compositions of the present invention and comparative examples. The evaluation conditions for each property were as follows. The results are shown in Tables 1 and 2.

[0069] [1] Evaluation of Sacrifice Resistance (Hardness) Three small sample pieces measuring 50 mm in length, 25 mm in width, and 3 mm in thickness were cut out from the 3 mm thick sheets obtained in the present invention and comparative examples, and a Type D durometer hardness test was performed in accordance with JIS K 6253. The pressure plate of the Type D durometer was brought into contact with the surface (flat surface) of the sheet as the test piece, and the hardness reading was measured 5 seconds later. A hardness reading of 68 or more was evaluated as "◎" because the test piece was sufficiently resistant to ant damage, indicating excellent termite resistance. A hardness reading of 65 or more but less than 68 was evaluated as "◯" because the test piece was resistant to ant damage. A hardness reading of less than 65 was evaluated as "×" because the test piece was susceptible to ant damage and relatively poor in terms of termite resistance. The evaluation criteria for resistance to sacrifice (hardness) were determined based on the report by Yves et al. at the 7th International Conference on Power Insulated Cables in 2007, which showed that when the hardness (ShD) of the sheath of a power cable is 65 or higher, damage by ants is unlikely to occur, and when the sheath hardness (ShD) is 68 or higher, damage by ants is almost impossible.

[0070] [2] Evaluation of tensile strength and elongation Three No. 3 dumbbell specimen samples as defined in JIS K 6251 were prepared by cutting out 1 mm thick sheets obtained in the examples of the present invention and the comparative examples, and a tensile test was carried out in accordance with JIS C 3005.

[0071] When the tensile strength calculated from the results of the tensile test was 18 MPa or more, the specimen was evaluated as excellent in tensile strength and marked with "◎". When the tensile strength calculated from the results of the tensile test was less than 18 MPa, the specimen was evaluated as relatively poor in terms of tensile strength and marked with "×".

[0072] When the elongation calculated from the results of the tensile test was 200% or more, the specimen was evaluated as having excellent elongation properties, and when the elongation calculated from the results of the tensile test was less than 200%, the specimen was evaluated as having relatively poor elongation properties, and was evaluated as having poor elongation properties.

[0073] [3] Evaluation of Oil Resistance Three No. 3 dumbbell specimens, as defined in JIS K 6251, were cut out from the 1-mm-thick sheets obtained in the present invention and comparative examples, and subjected to an oil resistance test in accordance with JIS C 3005. The test oil temperature was 70°C, and the immersion time in the test oil was 4 hours. The oil elongation retention rate, which is the average value of the elongation after oil immersion relative to the average values ​​of the tensile strength and elongation before oil immersion, was calculated. When the oil elongation retention rate was 60% or higher, the specimen was evaluated as "Excellent" for excellent oil resistance and suitable for use in forming a power cable sheath. When the oil elongation retention rate was less than 60%, the specimen was evaluated as "Poor" for relatively poor oil resistance.

[0074] [4] Evaluation of Cold Resistance: The 2-mm-thick sheets obtained in the present invention and comparative examples were cut out to prepare small sample pieces measuring 38 mm in length, 6 mm in width, and 2 mm in thickness. These were subjected to cold resistance tests in accordance with JIS C 3005. When the lowest test temperature (cold resistance temperature) at which no damage occurred due to impact was −20°C or lower, the sample was evaluated as having excellent cold resistance and suitable for use in forming a power cable sheath, with a rating of “◎.” When the lowest test temperature at which no damage occurred due to impact was higher than −20°C and lower than −15°C, the sample was evaluated as having good cold resistance and suitable for use in forming a power cable sheath, with a rating of “◯.” When the lowest test temperature at which no damage occurred due to impact was higher than −15°C, the sample was evaluated as having relatively poor cold resistance, with a rating of “×.”

[0075] [5] Evaluation of Thermal Stability: The 1 mm thick sheets obtained in the present invention and comparative examples were shredded to prepare small sample pieces measuring 1 mm or less in length, width, and thickness. 2 g of the test piece sample was placed at the bottom of a test tube, and Congo Red test paper was placed on top of the test tube. A thermal stability test was performed in accordance with JIS K 6723. When the test tube containing the test piece sample was immersed in an oil bath, the time from when the tip of the Congo Red test paper placed on top of the test tube turned a bright blue (the time until the test piece thermally decomposed) was 12 hours or longer, the test piece was evaluated as "Excellent" in terms of excellent thermal stability and suitability for long-term extrusion coating. When the time from when the tip of the Congo Red test paper turned a bright blue was 10 hours or longer but less than 12 hours, the test piece was evaluated as "Good" in terms of good thermal stability and suitability for long-term extrusion coating. Furthermore, when the time required for the tip of the Congo Red test paper to turn a clear blue was less than 10 hours, the sample was evaluated as "x" as being relatively poor in terms of thermal stability.

[0076] [6] Evaluation of Flame Retardancy The 3 mm thick sheets obtained in the present invention and comparative examples were cut out to prepare small sample pieces measuring 130 mm length x 6.5 mm width x 3 mm thickness, and subjected to a flammability test in accordance with JIS K 7201-2. When the oxygen index obtained by this test was 26% or more, the sample was evaluated as having excellent flame retardancy and given a rating of "◎". When the oxygen index obtained by this test was less than 26%, the sample was evaluated as having relatively poor flame retardancy and given a rating of "×".

[0077] [7] Overall Evaluation: When all seven of the seven evaluation results for hardness, tensile strength, elongation, oil resistance, cold resistance, thermal stability, and flame retardancy were evaluated as "◎", these seven properties were deemed excellent and evaluated as "◎". When at least one of the seven evaluation results was evaluated as "◯" and the remaining results were evaluated as "◎", these seven properties were deemed at least good and evaluated as "◯". On the other hand, when the evaluation result for at least one of hardness, tensile strength, elongation, oil resistance, cold resistance, thermal stability, and flame retardancy was evaluated as "×", at least one of these seven properties was deemed unsatisfactory and evaluated as "×".

[0078]

[0079]

[0080] Details of each component used in preparing the resin compositions shown in Tables 1 and 2 are as follows.

[0081] [Polyvinyl chloride resin (A)] Polyvinyl chloride resin (manufactured by Shin-Dai-Ichi Vinyl Corporation, model number: ZEST1400Z, average degree of polymerization: 1450) [MBS resin (B)] MBS (methacrylate-butadiene-styrene copolymer) resin [manufactured by Kaneka Corporation, product name: Kane Ace M-511] [Polyester-based plasticizer (C)] Adipic acid-based polyester (manufactured by ADEKA Corporation, product name: Adeka Cizer PN-7650) [Phthalate-based plasticizer] Diundecyl phthalate (manufactured by New Japan Chemical Co., Ltd., product name: Sanso Cizer DUP) [Stabilizer (D)] Ca-Zn-based stabilizer (manufactured by Mizusawa Industrial Chemicals, Ltd., product name: Stavinex (registered trademark) NL-214) [Flame retardant (E)] Antimony trioxide

[0082] From the evaluation results in Tables 1 and 2, the resins produced from the resin compositions of Examples 1 to 8 of the present invention, which are flame-retardant, anti-termite resin compositions containing at least polyvinyl chloride resin (A), MBS resin (B), polyester plasticizer (C), stabilizer (D), and flame retardant (E), and whose contents are within the appropriate ranges of the present invention, were evaluated as "◎" or "◯" in all six evaluation results regarding hardness, elongation, oil resistance, cold resistance, thermal stability, and flame retardancy.

[0083] From the above results, it was confirmed that the resin compositions of Examples 1 to 8 of the present invention fully satisfy the mechanical properties of tensile strength and elongation required for power cable sheaths, and are also excellent in cold resistance and oil resistance.It is also possible to obtain sheaths that are excellent in both flame retardancy and termite resistance, and that they have excellent thermal stability.

[0084] In contrast, the resin composition of Comparative Example 1 had a polyester-based plasticizer (C) content of 15 parts by mass, which was less than the appropriate range of the present invention, and therefore the evaluation results for elongation and oil resistance were both "X" and were poor.

[0085] The resin composition of Comparative Example 2 had a polyester plasticizer (C) content of 45 parts by mass, which was greater than the appropriate range of the present invention, and therefore the evaluation result for hardness was "x" and was poor.

[0086] The resin composition of Comparative Example 3 had a stabilizer (D) content of 3 parts by mass, which was less than the appropriate range of the present invention, and therefore the evaluation result of the thermal stability was "x", indicating poor thermal stability.

[0087] The resin composition of Comparative Example 4 had a stabilizer (D) content of 30 parts by mass, which was greater than the appropriate range of the present invention, and therefore the evaluation result for elongation was "x" and was poor.

[0088] The resin composition of Comparative Example 5 had an MBS resin (B) content of 0.1 parts by mass, which was less than the appropriate range of the present invention, and therefore the evaluation result of cold resistance was "x", indicating poor cold resistance.

[0089] The resin composition of Comparative Example 6 had an MBS resin (B) content of 30 parts by mass, which was greater than the appropriate range of the present invention, and therefore the evaluation results for hardness and oil resistance were poor, being "x".

[0090] The resin composition of Comparative Example 7 contained a phthalate-based plasticizer but did not contain the polyester-based plasticizer (C), and therefore the evaluation result for oil resistance was "×" and was poor.

[0091] The resin composition of Comparative Example 8 did not contain the flame retardant (E), and therefore the evaluation result of the flame retardancy was "x", which was poor.

[0092] 1, 1A, 11 Power cable 2, 12 Conductor 3, 13 Sheath 14 Flame-retardant vinyl layer 15 Outermost layer 4, 16 Intermediate layer 5 Inner semiconductive layer 6 Insulator layer 7 Outer semiconductive layer 8 Shielding layer

Claims

1. A flame-retardant, anti-termite resin composition containing at least polyvinyl chloride resin, MBS resin, polyester-based plasticizer, stabilizer, and flame retardant, wherein, relative to 100 parts by mass of the polyvinyl chloride resin, the content of the MBS resin is in the range of 1 part by mass to 20 parts by mass, the content of the polyester-based plasticizer is in the range of 20 parts by mass to 40 parts by mass, the content of the stabilizer is in the range of 4 parts by mass to 20 parts by mass, and the content of the flame retardant is in the range of 3 parts by mass to 20 parts by mass.

2. A flame-retardant, anti-termite resin composition according to claim 1, wherein the content of the MBS resin is in the range of 3 to 20 parts by mass per 100 parts by mass of the polyvinyl chloride resin.

3. A flame-retardant, anti-termite resin composition as described in claim 1, wherein the content of the polyester-based plasticizer is in the range of 20 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the polyvinyl chloride resin.

4. The flame-retardant, anti-termite resin composition according to claim 1, which has a hardness of 65 or more as measured using a type D durometer.

5. The flame-retardant and anti-termite resin composition according to claim 1, which is used to form a sheath that constitutes the outermost layer of a power cable.

6. A power cable comprising at least a conductor and a sheath made of the flame-retardant and anti-termite resin composition according to any one of claims 1 to 5, which sheathes the conductor as the outermost layer.

7. The power cable according to claim 6, further comprising, between the conductor and the sheath, in this order from the conductor side, an inner semiconductive layer, an insulating layer, an outer semiconductive layer, and a shielding layer.

8. The power cable of claim 6, wherein the sheath consists of a single layer.

9. A method for manufacturing a power cable having at least a conductor and a sheath covering the conductor as the outermost layer, comprising a sheath forming step of forming the sheath by extruding a flame-retardant, anti-termite resin composition according to any one of claims 1 to 5 onto the outer periphery of the conductor.

10. A method for manufacturing a power cable as described in claim 9, wherein the sheath forming process comprises laminating an inner semiconductive layer, an insulator layer, an outer semiconductive layer and a shielding layer in that order on the conductor, and then extrusion coating the flame-retardant and anti-termite resin composition onto the shielding layer to form the sheath.

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