Flame-retardant and termite-resistant resin composition, power cable, and method for manufacturing the same.

Abstract

The present invention provides a flame-retardant and termite-resistant resin composition that allows for the construction of a single-layer sheath with excellent flame retardancy and termite resistance, possesses excellent thermal stability, fully satisfies the tensile strength and elongation requirements necessary for a power cable sheath, and also exhibits excellent cold resistance and oil resistance, making it suitable for use in forming power cable sheaths, as well as a power cable using the same and a method for manufacturing the same. [Solution] The flame-retardant and termite-resistant resin composition contains at least polyvinyl chloride resin, MBS resin, polyester plasticizer, stabilizer, and flame retardant, wherein the content of MBS resin is in the range of 1 to 20 parts by mass per 100 parts by mass of polyvinyl chloride resin, the content of polyester plasticizer is in the range of 20 to 40 parts by mass, the content of stabilizer is in the range of 4 to 20 parts by mass, and the content of flame retardant is in the range of 3 to 20 parts by mass.

Description

Technical Field

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

Background Art

[0002] As cables such as power cables and communication cables, those in which the outer peripheral side of a conductor is covered with a sheath (protective outer coating) as the outermost layer are widely used. The sheath is formed, for example, by extrusion-coating a resin such as polyethylene resin or polyvinyl chloride (PVC) resin onto the outer peripheral side of 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 active termite activity are required to have flame retardancy in addition to termite resistance, which is the property of preventing feeding damage by ants such as termites.

[0004] Therefore, it has been common practice to configure the sheath not as a single layer but as a plurality of layers, and to form the sheath by laminating a layer of a flame-retardant resin such as polyvinyl chloride resin having excellent flame retardancy and a layer of a termite-resistant resin such as nylon having excellent termite resistance.

[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. Among these, for the polypropylene resin composition constituting the outermost layer 15, a method is described that contains a polymer resin composed of a propylene homopolymerization part and a resin component with a solubility parameter of 7.0 or more and 9.5 or less, and a flame retardant, has a Rockwell hardness of 85 or more, and a flexural modulus of 1500 MPa or more. This sheath imparts flame retardancy by the flame retardant contained in the flame-retardant vinyl layer 14 and the outermost layer 15, and also imparts termite resistance by setting the mass ratio of the propylene homopolymerization part contained in the polymer resin forming the outermost layer 15 within a predetermined range, and achieving high hardness and high flexural modulus.

Prior Art Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2015-096583 [Overview of the project] [Problems that the invention aims to solve]

[0007] The cable described in Patent Document 1 has the problem of high manufacturing costs because the sheath is composed of two layers: a flame-retardant vinyl layer and an outermost layer formed of a polypropylene resin composition. Furthermore, since no material has existed that satisfies both flame retardancy and termite resistance to a high degree, it was not possible to construct the sheath with a single layer.

[0008] Furthermore, when using cables as power cables, they must meet requirements not only for flame retardancy and termite resistance, but also for tensile strength, elongation, cold resistance, and oil resistance. In addition, because power cables are relatively large compared to other wire and cable products, it takes a long time to form the sheath by extruding the resin coating. Therefore, there was a need for a resin composition with excellent thermal stability that could withstand such long-term extrusion coating.

[0009] The object of the present invention is to provide a flame-retardant and termite-resistant resin composition that allows for the construction of a single-layer sheath with excellent flame retardancy and termite resistance, possesses excellent thermal stability, fully satisfies the tensile strength and elongation requirements necessary for a power cable sheath, and also exhibits excellent cold resistance and oil resistance, making it suitable for use in forming power cable sheaths, as well as a power cable using the same and a method for manufacturing the same. [Means for solving the problem]

[0010] The inventors have found that by including at least polyvinyl chloride resin, MBS resin, polyester plasticizer, stabilizer, and flame retardant in a flame-retardant and termite-resistant resin composition, and by limiting the content of these to an appropriate ratio, the basic performance requirements for power cables, such as flame retardancy, termite resistance, tensile strength, elongation, cold resistance, oil resistance, and thermal stability, can be well-balanced. Based on this finding, the inventors have completed the present invention.

[0011] In other words, the gist of the present invention is as follows: (1) A flame-retardant and termite-resistant resin composition comprising at least polyvinyl chloride resin, MBS resin, polyester plasticizer, stabilizer, and flame retardant, wherein the content of the MBS resin is in the range of 1 to 20 parts by mass per 100 parts by mass of the polyvinyl chloride resin, the content of the polyester plasticizer is in the range of 20 to 40 parts by mass, the content of the stabilizer is in the range of 4 to 20 parts by mass, and the content of the flame retardant is in the range of 3 to 20 parts by mass.

[0012] (2) The flame-retardant and termite-resistant resin composition according to (1) above, 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 and termite-resistant resin composition according to (1) or (2) above, wherein the content of the polyester 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) A flame-retardant and termite-resistant resin composition according to any one of the above items (1) to (3), wherein the hardness measured using a Type D durometer is 65 or higher.

[0015] (5) A flame-retardant and termite-resistant resin composition according to any one of items (1) to (4) above, used for forming the sheath that constitutes the outermost layer of a power cable.

[0016] (6) A power cable comprising at least a conductor and a sheath made of the flame-retardant and termite-resistant resin composition described in any one of (1) to (5) above, which covers the conductor as the outermost layer.

[0017] (7) The power cable according to (6) above, further comprising an internal semiconducting layer, an insulating layer, an external semiconducting layer, and a shielding layer between the conductor and the sheath, in order from the conductor side.

[0018] (8) The power cable described in (6) or (7) above, wherein the sheath consists of a single layer.

[0019] (9) A method for manufacturing a power cable having at least a conductor and a sheath that covers the conductor as the outermost layer, comprising a sheath forming step of forming the sheath by extruding the flame-retardant and termite-resistant resin composition described in any one of the above items (1) to (5) onto the outer circumference of the conductor.

[0020] (10) The method for manufacturing a power cable according to (9) above, wherein the sheath forming step involves sequentially laminating an internal semiconducting layer, an insulating layer, an external semiconducting layer, and a shielding layer on the conductor, and then forming the sheath by extruding the flame-retardant and termite-resistant resin composition onto the shielding layer. [Effects of the Invention]

[0021] According to the present invention, it is possible to construct a sheath with excellent flame retardancy and termite resistance in a single layer, and it is possible to provide a flame-retardant and termite-resistant resin composition that has excellent thermal stability, satisfies the tensile strength and elongation which are mechanical properties required for a power cable sheath, and also has excellent 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. [Brief explanation of the drawing]

[0022] [Figure 1] This is a schematic cross-sectional view showing the structure of a power cable according to the present invention. [Figure 2]FIG. 2 is a cross-sectional view schematically showing an example of a specific structure of a power cable according to the present invention. [Figure 3] FIG. 3 is a cross-sectional view schematically showing the structure of a related-art cable. [Mode for Carrying Out the Invention]

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

[0024] <Flame-retardant and termite-resistant resin composition> The flame-retardant and termite-resistant 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. 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 or more and 20 parts by mass or less, the content of the polyester-based plasticizer is in the range of 20 parts by mass or more and 40 parts by mass or less, the content of the stabilizer is in the range of 4 parts by mass or more and 20 parts by mass or less, and the content of the flame retardant is in the range of 3 parts by mass or more and 20 parts by mass or less.

[0025] The flame-retardant and termite-resistant resin composition of the present invention contains polyvinyl chloride resin, which has excellent flame retardancy, as a base resin, and further flame retardancy can be enhanced by adding a flame retardant. Furthermore, by including MBS resin and polyester plasticizer in predetermined amounts, when applied as a raw material to form the outermost layer of a power cable, for example, a sheath can be obtained that has excellent cold resistance and oil resistance desired for power cables, as well as high hardness and resistance to termite damage, thus providing excellent termite resistance. In addition, by including a stabilizer in predetermined amounts, it is possible to obtain a sheath with sufficient tensile strength and elongation for a power cable sheath, and excellent thermal stability that can withstand long-term extrusion coating. Therefore, by using the flame-retardant and termite-resistant resin composition of the present invention, it is possible to construct a sheath with excellent flame retardancy and termite resistance in a single layer, has excellent thermal stability, sufficiently satisfies the tensile strength and elongation which are mechanical properties required for a power cable sheath, and also has excellent cold resistance and oil resistance, making it a flame-retardant and termite-resistant resin composition suitable for use in forming power cable sheaths.

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

[0027] (Polyvinyl chloride resin (A)) Polyvinyl chloride resin (A) is a resin with excellent flame retardancy among general-purpose plastics. Therefore, by using polyvinyl chloride resin as the base resin, the flame retardancy of the sheath can be improved even with a small amount of flame retardant or other flame-reducing materials added.

[0028] The average degree of polymerization of polyvinyl chloride resin (A) is not particularly limited, but from the viewpoint of suitably applying it to molded articles for covering electric wires and cables, it is preferably in the range of 900 to 5000.

[0029] Polyvinyl chloride resin (A) can broadly refer to polymers obtained by polymerizing monomers containing vinyl chloride, and may contain unit structures of other monomer components, as well as repeating unit structures. Among these, from the viewpoint of further improving the flame retardancy of the sheath, polyvinyl chloride resin (A) is preferably a homopolymer of vinyl chloride.

[0030] Other resins, including MBS resin (B) described later, may be added as resin components. However, from the viewpoint of obtaining a sheath with high hardness and resistance to ant damage, the content of polyvinyl chloride resin (A) is preferably 80 parts by mass or more, more preferably 90 parts by mass or more, when the total content of resin components is 100 parts by mass. Furthermore, it is preferable that no other resins other than MBS resin (B) are included.

[0031] (MBS resin (B)) MBS resin (methyl methacrylate-butadiene-styrene copolymer resin) (B) can improve cold resistance by increasing impact strength in low-temperature environments when included in a flame-retardant, termite-resistant resin composition. To achieve this effect, the MBS resin (B) should be 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 MBS resin (B) content exceeds 20 parts by mass, the hardness decreases significantly, reducing termite resistance, and the oil resistance also decreases, making it difficult to use as a sheath for power cables. For this reason, the MBS resin (B) content should be 20 parts by mass or less per 100 parts by mass of polyvinyl chloride resin. Therefore, the MBS resin (B) content in the flame-retardant, termite-resistant resin composition is in the range of 1 part by mass or more and 20 parts by mass or less, preferably 3 parts by mass or more and 20 parts by mass or less, per 100 parts by mass of polyvinyl chloride resin.

[0032] (Polyester-based plasticizer (C)) Polyester-based plasticizers (C), when included in a flame-retardant, termite-resistant resin composition, have a higher molecular weight than phthalate-based plasticizers, thus improving tensile strength, elongation, and oil resistance. To achieve this effect, the polyester-based plasticizer (C) should be included in amounts of 20 parts by mass or more per 100 parts by mass of polyvinyl chloride resin. On the other hand, if the polyester-based plasticizer content exceeds 40 parts by mass, the hardness decreases significantly, leading to a decrease in termite resistance. Therefore, the polyester-based plasticizer (C) content should be 40 parts by mass or less, preferably 30 parts by mass or less, per 100 parts by mass of polyvinyl chloride resin. Accordingly, the polyester-based plasticizer (C) content in a flame-retardant, termite-resistant resin composition should be in the range of 20 parts by mass or more and 40 parts by mass or less, preferably 20 parts by mass or more and 30 parts by mass or less, per 100 parts by mass of polyvinyl chloride resin.

[0033] Polyester-based plasticizers (C) can be broadly defined as plasticizers that have a polyester structure formed by the dehydration condensation of a polycarboxylic acid and a polyalcohol within their molecule. Among these, from the viewpoint of availability and other factors, it is preferable to use one or both of adipic acid-based polyesters and sebaciac acid-based polyesters, which are polyesters using adipic acid or sebaciac acid as the polycarboxylic acid.

[0034] (Stabilizer (D)) The stabilizer (D) is included to suppress the thermal degradation of the polyvinyl chloride resin (A). In particular, when used as a sheath for power cables, the flame-retardant and termite-resistant resin composition needs to be extruded and coated over a long period of time due to the nature of the product. Therefore, especially high thermal stability is required among wire and cable products, and thus the stabilizer (D) must be included. To achieve 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 stabilizer (D) exceeds 20 parts by mass, the elongation when used as a sheath decreases. For this reason, the content of stabilizer (D) is 20 parts by mass per 100 parts by mass of polyvinyl chloride resin. Accordingly, the content of stabilizer (D) in the flame-retardant and termite-resistant resin composition is in the range of 4 parts by mass or more and 20 parts by mass or less, preferably 10 parts by mass or more and 20 parts by mass or less, per 100 parts by mass of polyvinyl chloride resin.

[0035] While lead compounds have been widely used as stabilizers (D), from the viewpoint of safety, health, and the environment, it is preferable to include lead-free stabilizers, which are compounds that do not contain lead atoms, in the flame-retardant and termite-resistant resin composition. As lead-free stabilizers, those commonly used for coating electric wires and cables can be used, and for example, one or more selected from the group consisting of Ca-Zn-based stabilizers, Ba-Zn-based stabilizers, polyol-based stabilizers, and organotin-based stabilizers can be used. Among these, Ca-Zn-based stabilizers are preferred from the viewpoint of excellent heat resistance and excellent dispersibility in the flame-retardant and termite-resistant resin composition. As lead-free stabilizers, multiple compounds that act as stabilizers can be used in combination, and for example, as a Ca-Zn-based stabilizer, a compound containing Ca atoms, such as hydrotalcite, and a compound containing Zn atoms, such as zinc stearate, can be used in combination.

[0036] (Flame retardant (E)) The flame retardant (E) is a compound that has flame-retardant properties. To exhibit this property, 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 polyvinyl chloride resin. On the other hand, there is no particular upper limit to the amount of flame retardant (E) contained, but it can be, for example, 20 parts by mass. Therefore, the amount of flame retardant (E) in the flame-retardant termite-resistant 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 polyvinyl chloride resin.

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

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

[0039] More specifically, the flame-retardant and termite-resistant 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 phosphors if it is necessary to further improve other properties.

[0040] On the other hand, the flame-retardant and termite-resistant resin composition according to this embodiment may contain a crosslinking agent, but it is preferable that it does not contain a crosslinking agent, especially from the viewpoint of improving recyclability.

[0041] The content of other components (F) in the flame-retardant and termite-resistant resin composition according to this embodiment is not particularly limited, but from the viewpoint of making it less likely for the addition of other components (F) to reduce flame retardancy and termite resistance, it 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, per 100 parts by mass of polyvinyl chloride resin.

[0042] (Properties and applications of flame-retardant and termite-resistant resin compositions) The flame-retardant and termite-resistant resin composition according to this embodiment preferably has a hardness of 65 or higher when measured using a Type D durometer. This makes it difficult for termites to damage the resin formed from the flame-retardant and termite-resistant resin composition. In particular, from the viewpoint of further reducing the likelihood of termite damage to the resin formed from the flame-retardant and termite-resistant resin composition, it is more preferable that the hardness of the flame-retardant and termite-resistant resin composition when measured using a Type D durometer is 68 or higher.

[0043] The flame-retardant and termite-resistant resin composition according to this embodiment can be particularly suitably used to form the outermost sheath of a power cable. Because the flame-retardant and termite-resistant resin composition according to this embodiment has excellent flame retardancy and termite resistance, as well as excellent cold resistance and oil resistance, the sheath can be constructed as a single layer, and as a result, the manufacturing cost of the power cable can be reduced.

[0044] <Power Cable> The power cable 1 according to this embodiment, as conceptually shown in Figure 1, comprises 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 may have an intermediate layer 4 between the conductor 2 and the sheath 3, and there are no particular limitations on the intermediate layer 4. This power cable 1 can be obtained, for example, by the method described above.

[0045] In this embodiment, the power cable 1, since the sheath 3 is made of the above-mentioned flame-retardant and termite-resistant resin composition, can satisfy the requirements for cold resistance and oil resistance in the power cable 1, and can also exhibit desired flame retardancy and termite resistance. Therefore, it can be suitably laid in places where termite damage may occur, or in places where fires caused by termite damage or damage from 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 satisfies the requirements for cold resistance and oil resistance, exhibits desired flame retardancy and termite resistance, and enhances the mechanical strength of the outermost layer to provide protection from the outside.

[0047] Furthermore, it is preferable that the sheath 3 consists of a single layer. Even if the sheath 3 of the power cable 1 is made of a single layer, it can exhibit both the desired flame retardancy and termite resistance, so the outer diameter of the power cable 1 can be reduced compared to a power cable made of two layers of sheath. In addition, since the above-mentioned flame-retardant and termite-resistant resin composition has high thermal stability and is suitable for extrusion processing, the material cost and manufacturing cost 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, regarding the thickness in particular, from the viewpoint of ensuring sufficient thickness for protection from the outside and not increasing the outer diameter of the power cable 1, a range of 3.0 mm to 6.0 mm is preferred, and a range of 3.0 mm to 5.5 mm is more preferred.

[0049] One example of a power cable 1 is one in which an intermediate layer 4 exists between the conductor 2 and the sheath 3, and this intermediate layer 4 is composed of four layers, as shown in power cable 1A in Figure 2. In this case, the power cable 1 may further have an internal semiconducting layer 5, an insulating layer 6, an external semiconducting layer 7, and a shielding layer 8 between the conductor 2 and the sheath 3, in order from the conductor 2 side, and the sheath 3 may be laminated as the outermost layer on its outer circumference.

[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 also have a plating layer made of tin, silver, or the like on its surface.

[0051] The internal semiconducting layer 5 is a semiconducting layer provided between the conductor 2 and the insulating layer 6, which will be described later. Generally, an internal semiconducting layer used in power cables can be used. More specifically, a conductive material coated onto a fibrous (cloth) tape, an extruded type with carbon mixed into polyethylene, or a combination of these can be used. The internal semiconducting layer 5 may also be formed by crosslinking a resin composition for semiconducting layers.

[0052] Here, the resin composition for the semiconducting layer contains, for example, a resin for the semiconducting layer, a conductive substance, a crosslinking agent, and an anti-aging agent.

[0053] Of these, the resin for the semiconducting layer is not particularly limited, but usually an ethylene-based polymer is used. The ethylene-based polymer can be any polymer that contains ethylene as a repeating unit, and is not particularly limited, but examples 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, it is preferable that the polyethylene includes crosslinked polyethylene. Crosslinked polyethylene is obtained by crosslinking a resin composition for the semiconducting layer that contains polyethylene and a crosslinking agent.

[0054] Furthermore, while the conductive material is not particularly limited, conductive carbon is typically used. Examples of conductive carbon include carbon black, acetylene black, furnace black, Ketjen black, thermal black, and graphite. These can be used individually or in combination of two or more.

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

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

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

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

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

[0060] Although this embodiment has described the power cable 1, it goes without saying that the flame-retardant and termite-resistant resin composition of the present invention can be applied to cables in general, as it can achieve the same effects as the power cable of the present invention when applied as a raw material for the sheath constituting the cable.

[0061] Furthermore, although this embodiment has described a single-core (1-core) power cable 1 in which the conductor 2 is composed of one core wire, a multi-core (for example, 3-core) power cable may also be composed of multiple core wires. In this case, a multi-core power cable is made by bundling together multiple conductors, each coated in at least an internal semiconducting layer, an insulating layer, an external semiconducting layer, and a shielding layer in that order, and then securing the bundled surface with tape or the like before covering it with a sheath or the like.

[0062] <Manufacturing method for power cables> 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 the outermost layer. The method for manufacturing a power cable is not particularly limited, but for example, it is a method that includes a sheath forming step in which the above-mentioned flame-retardant and termite-resistant resin composition is extruded onto the outer circumference of the conductor 2 to form the sheath 3. This makes it possible to obtain a power cable 1 in which a sheath 3 is formed on the outer circumference of the conductor 2 as the outermost layer.

[0063] Here, known extrusion molding methods can be used as the means for extruding the flame-retardant and termite-resistant resin composition in the sheath formation process. In this case, it is preferable to granulate the flame-retardant and termite-resistant resin composition by molding means such as extrusion molding to form pellets, and then use these pellets for extrusion coating, as this allows for easy handling of the flame-retardant and termite-resistant resin composition.

[0064] Furthermore, the temperature (molding temperature) at which the flame-retardant and termite-resistant resin composition is extruded to form the sheath 3 during the sheath formation process can be in the range of 150°C to 200°C. Also, the time for extruding the flame-retardant and termite-resistant resin composition during the sheath formation process can be in the range of 1 hour to 100 hours. By using the above-mentioned flame-retardant and termite-resistant resin composition in the sheath formation process, the thermal stability of the extruded flame-retardant and termite-resistant resin composition is enhanced. Therefore, even when extruding the sheath 3 for extended periods, such as when forming the power cable 1, the sheath 3 can be stably extruded.

[0065] In the sheath formation process, it is preferable to form the sheath 3 by sequentially laminating an internal semiconducting layer 5, an insulating layer 6, an external semiconducting layer 7, and a shielding layer 8 on the conductor 2, and then extruding a flame-retardant and termite-resistant resin composition onto the shielding layer 8. This makes it possible to form a power cable, as shown in Figure 2, which further has an internal semiconducting layer 5, an insulating layer 6, an external semiconducting layer 7, and a shielding layer 8 between the conductor 2 and the sheath 3, in order from the conductor 2 side. Here, when laminating the internal semiconducting layer 5, insulating layer 6, external semiconducting layer 7, and shielding layer 8 on the conductor 2, two or more adjacent layers among the internal semiconducting layer 5, insulating layer 6, and external semiconducting layer 7 may be laminated by simultaneously extruding them. [Examples]

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

[0067] <Performance evaluation of sheath-forming material (sheet)> [Preparation of sheath-forming materials] A resin composition was obtained by blending polyvinyl chloride resin (A), MBS resin (B), polyester plasticizer (C), stabilizer (D), and flame retardant (E) in the proportions (parts by mass) shown in Table 1. This resin composition was then kneaded and molded using a roll mill set to a kneading temperature of 150°C to produce three sheet-like samples of 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. The obtained resin composition does not contain any other component (F).

[0068] [Characterization of sheath-forming materials] The following properties were evaluated using sheets made from the resin compositions described in the above-mentioned examples and comparative examples of the present invention. The evaluation conditions for each property are as follows. The results are shown in Tables 1 and 2.

[0069] [1] Evaluation of sacrificial resistance (hardness) Three small sample pieces measuring 50 mm in length, 25 mm in width, and 3 mm in thickness were prepared by cutting out the 3 mm thick sheets obtained in the present invention example and comparative example, and a Type D durometer hardness test was performed in accordance with JIS K 6253. Here, the pressure plate of the Type D durometer was brought into contact with the surface (flat surface) of the sheet, which was the test piece, and the hardness reading was measured after 5 seconds. When the hardness reading was 68 or higher, it was evaluated as "◎" indicating excellent termite resistance in that termite damage is unlikely to occur sufficiently. When the hardness reading was 65 or higher but less than 68, it was evaluated as "○" indicating good termite resistance in that termite damage is unlikely to occur. When the hardness reading was less than 65, it was evaluated as "×" indicating that termite damage is likely to occur and that termite resistance is relatively poor. The evaluation criteria for sacrificial resistance (hardness) were determined based on, for example, the report by Yves et al. at Jicable'07 7th International Conference on Power Insulated Cables, which found that when the hardness (ShD) of the power cable sheath is 65 or higher, termite damage becomes less likely, and when the hardness (ShD) of the sheath is 68 or higher, termite damage is almost eliminated.

[0070] [2] Evaluation of tensile strength and elongation Three dumbbell-shaped samples, size 3 as specified in JIS K 6251, were prepared by cutting out 1 mm thick sheets obtained in the present invention example and comparative example, and tensile tests were performed in accordance with JIS C 3005.

[0071] If the tensile strength calculated from the tensile test results was 18 MPa or higher, it was evaluated as having excellent tensile strength and given a "◎" rating. Conversely, if the tensile strength calculated from the tensile test results was less than 18 MPa, it was evaluated as having relatively poor tensile strength and given a "×" rating.

[0072] If the elongation calculated from the tensile test results was 200% or more, it was evaluated as having excellent elongation characteristics and given a "◎" rating. Conversely, if the elongation calculated from the tensile test results was less than 200%, it was evaluated as having relatively poor elongation characteristics and given a "×" rating.

[0073] [3] Evaluation of oil resistance Three dumbbell-shaped samples, size 3 as specified in JIS K 6251, were prepared by cutting out 1 mm thick sheets obtained in the present invention example and comparative example, and an oil resistance test was conducted in accordance with JIS C 3005. The temperature of the test oil was set to 70°C, and the immersion time in the test oil was 4 hours. The oil resistance elongation retention rate, which is the average elongation after oil immersion relative to the average tensile strength and elongation before oil immersion, was calculated. When the oil resistance elongation retention rate was 60% or higher, it was evaluated as "◎" indicating excellent oil resistance and suitability for use in forming power cable sheaths. Conversely, when the oil resistance elongation retention rate was less than 60%, it was evaluated as "×" indicating relatively poor oil resistance.

[0074] [4] Evaluation of cold tolerance Small sample pieces measuring 38 mm in length, 6 mm in width, and 2 mm in thickness were prepared by cutting out 2 mm thick sheets obtained in the present invention example and comparative example, and a cold resistance test was conducted in accordance with JIS C 3005. When the lowest test temperature at which no damage occurred due to impact (cold resistance temperature) was -20°C or lower, it was evaluated as "◎" indicating excellent cold resistance and suitability for use in forming power cable sheaths. When the lowest test temperature at which no damage occurred due to impact was higher than -20°C and below -15°C, it was evaluated as "○" indicating good cold resistance and suitability for use in forming power cable sheaths. When the lowest test temperature at which no damage occurred due to impact was higher than -15°C, it was evaluated as "×" indicating relatively poor cold resistance.

[0075] [5] Evaluation of thermal stability A 1 mm thick sheet obtained in the present invention example and comparative example was shredded to prepare small sample pieces with dimensions of 1 mm or less on each side (length, width, and thickness). Two g of these small sample pieces were placed at the bottom of a test tube, and Congo Red test paper was placed at the top of the test tube. A thermal stability test was then conducted in accordance with JIS K 6723. When the test tube containing the small sample pieces was immersed in the oil bath, the time until the tip of the Congo Red test paper placed at the top of the test tube turned bright blue (the time until thermal decomposition of the test piece) was 12 hours or more. This was evaluated as "◎" because it indicated excellent thermal stability and suitability for long-term extrusion coating. When the time until the tip of the Congo Red test paper turned bright blue was 10 hours or more but less than 12 hours, this was evaluated as "○" because it indicated good thermal stability and suitability for long-term extrusion coating. Furthermore, if the time it took for the tip of the Congo Red test paper to change to a clear blue was less than 10 hours, it was evaluated as "×" from the perspective of thermal stability, which is considered relatively poor.

[0076] [6] Evaluation of flame retardancy Small sample pieces measuring 130 mm (length) x 6.5 mm (width) x 3 mm (thickness) were prepared by cutting out 3 mm thick sheets obtained in the present invention example and comparative example, and a flammability test was conducted in accordance with JIS K 7201-2. If the oxygen index obtained in this test was 26% or higher, it was evaluated as having excellent flame retardancy, and was evaluated as "◎". If the oxygen index obtained in this test was less than 26%, it was evaluated as being relatively poor in terms of flame retardancy, and was evaluated as "×".

[0077] [7] Overall rating Of these evaluation results, if all seven evaluation results related to hardness, tensile strength, elongation, oil resistance, cold resistance, thermal stability, and flame retardancy were rated "◎", the product was evaluated as having excellent properties in all seven areas. Furthermore, if at least one of these seven evaluation results was rated "○" and the rest were rated "◎", the product was evaluated as having at least good properties in all seven areas. On the other hand, if at least one of the evaluation results for hardness, tensile strength, elongation, oil resistance, cold resistance, thermal stability, and flame retardancy was rated "×", the product was evaluated as having failed in at least one of these seven areas.

[0078] [Table 1]

[0079] [Table 2]

[0080] The details of each component used in the preparation of the resin compositions listed in Tables 1 and 2 are as follows.

[0081] [Polyvinyl chloride resin (A)] • Polyvinyl chloride resin (manufactured by Shin-Daiichi Vinyl Chloride Co., Ltd., model number: ZEST1400Z, average degree of polymerization: 1450) [MBS resin (B)] • MBS (methacrylate-butadiene-styrene copolymer) resin [manufactured by Kaneka Corporation, product name: KaneAce M-511] [Polyester-based plasticizer (C)] • Adipic acid-based polyester (manufactured by ADEKA Corporation, product name: ADEKA Sizer PN-7650) [Phthalate-based plasticizers] • Diundecyl phthalate (manufactured by Shin-Nippon Rika Co., Ltd., product name: Sanso-sizer DUP) [Stabilizer (D)] • Ca-Zn-based stabilizer (manufactured by Mizusawa Chemical Industries, Ltd., product name: Stabinex® 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 contain at least polyvinyl chloride resin (A), MBS resin (B), polyester plasticizer (C), stabilizer (D), and flame retardant (E) in the flame-retardant and termite-resistant resin compositions, and whose content is within the appropriate range for the present invention, were all evaluated as "◎" or "〇" for all six evaluation results related to 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 tensile strength and elongation, which are mechanical properties required for power cable sheaths, and also have excellent cold resistance and oil resistance, as well as excellent flame retardancy and termite resistance, and that they have excellent thermal stability.

[0084] In contrast, the resin composition of Comparative Example 1 contained 15 parts by mass of polyester plasticizer (C), which is less than the appropriate range for the present invention. As a result, both the elongation and oil resistance evaluation results were "×" and were inferior.

[0085] The resin composition of Comparative Example 2 contained 45 parts by mass of polyester plasticizer (C), which is more than the appropriate range for the present invention, and therefore the hardness evaluation result was "×" and was inferior.

[0086] The resin composition of Comparative Example 3 contained 3 parts by mass of stabilizer (D), which is less than the appropriate range for the present invention. Therefore, the thermal stability evaluation result was "×" and was inferior.

[0087] The resin composition of Comparative Example 4 contained 30 parts by mass of stabilizer (D), which is more than the appropriate range for the present invention, and therefore the elongation evaluation result was "×" and inferior.

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

[0089] The resin composition of Comparative Example 6 had a higher content of MBS resin (B) (30 parts by mass) than the appropriate range for the present invention, resulting in a "×" rating for hardness and oil resistance, indicating inferiority.

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

[0091] The resin composition of Comparative Example 8 did not contain the flame retardant (E), and therefore its flame retardancy evaluation result was "×", indicating inferiority. [Explanation of Symbols]

[0092] 1, 1A, 11 Power Cable 2, 12 conductors 3, 13 sheath 14. Flame-retardant vinyl layer 15 Outermost layer 4.16 Middle Class 5. Internal semiconducting layer 6. Insulator layer 7. Outer semiconducting layer 8 Shielding layer

Claims

1. It contains at least polyvinyl chloride resin, MBS resin, polyester plasticizer, stabilizer, and flame retardant. With respect 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 or more and 20 parts by mass or less. The content of the polyester-based plasticizer is in the range of 20 parts by mass or more and 40 parts by mass or less. The content of the aforementioned stabilizer is in the range of 4 parts by mass or more and 20 parts by mass or less, A flame-retardant and termite-resistant resin composition in which the flame retardant content is in the range of 3 parts by mass or more and 20 parts by mass or less.

2. The flame-retardant and termite-resistant resin composition according to claim 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.

3. The flame-retardant and termite-resistant resin composition according to 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 and termite-resistant resin composition according to claim 1, wherein the hardness measured using a Type D durometer is 65 or higher.

5. The flame-retardant and termite-resistant resin composition according to claim 1, used for forming the sheath that constitutes the outermost layer of a power cable.

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

7. The power cable according to claim 6, further comprising, between the conductor and the sheath, in order from the conductor side, an internal semiconducting layer, an insulating layer, an external semiconducting layer, and a shielding layer.

8. The power cable according to 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, A method for manufacturing a power cable, comprising a sheath forming step of forming the sheath by extruding the flame-retardant and termite-resistant resin composition according to any one of claims 1 to 5 onto the outer circumference of the conductor.

10. The method for manufacturing a power cable according to claim 9, wherein the sheath forming step involves sequentially laminating an internal semiconducting layer, an insulating layer, an external semiconducting layer, and a shielding layer on the conductor, and then forming the sheath by extruding the flame-retardant and termite-resistant resin composition onto the shielding layer.

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