Pellet

Abstract

To improve a pellet containing an insulating resin and a crosslinking agent.SOLUTION: A pellet contains a resin component containing unsaturated polyethylene, and a crosslinking agent, and has a central part and an outer peripheral part positioned on an outer periphery of the central part, wherein the central part and the outer peripheral part satisfy an expression (1): 0.32≤(C / A) / (D / B)≤0.91. When A is a content of the crosslinking agent per 100 mass% of the resin component contained in the central part, and the unit is mass%, B is a content of the crosslinking agent per 100 mass% of the resin component contained in the outer peripheral part, and the unit is mass%, C is a content of carbon-carbon double bonds per 1,000 carbons in the resin component contained in the central part, and D is a content of carbon-carbon double bonds per 1,000 carbons in the resin component contained in the outer peripheral part.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to pellets. [Background technology]

[0002] Cross-linked polyethylene is widely used as an insulating layer for power cables (for example, Patent Document 1).

[0003] The insulating layer is formed using pellets containing a resin composition including polyethylene and a crosslinking agent. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-132817 Summary of the Invention [Problem to be solved by the invention]

[0005] A power cable has an electrical insulation layer. Hereinafter, the electrical insulation layer will be referred to as the "insulation layer." The insulation layer is formed from pellets containing an insulating resin and a cross-linking agent. The insulating resin contains, for example, unsaturated polyethylene. The pellets are manufactured and stored before the power cable is manufactured. If the pellets are stored for a long period of time, they will be exposed to air for a long period of time. When the pellets are exposed to air for a long period of time, the cross-linking agent contained in the surface layer of the pellets may cross-link part of the insulating resin, forming cross-linked foreign matter. Furthermore, the carbon-carbon double bonds in the unsaturated polyethylene, which is a resin component of the pellets, are highly reactive. Therefore, when the pellets are exposed to air for a long period of time, the resin component may deteriorate in the surface layer of the pellets, starting from the carbon-carbon double bonds in the resin component. In this case, foreign matter may also be formed in the pellets. When the insulation layer of a power cable is manufactured, the pellets are heated. When the pellets are heated, the insulating resin contained in the pellets is cross-linked by the cross-linking agent. Therefore, the power cable has a cross-linked insulation layer. However, when a crosslinked insulating layer is formed from pellets containing foreign matter, the crosslinked insulating layer contains the foreign matter, and the inventors have found that when the pellets are stored for a long period of time, the foreign matter causes a deterioration in the electrical insulation properties of the insulating layer.

[0006] Furthermore, the inventors have found that the following problems arise due to the distribution of cross-linking agent and carbon-carbon double bonds in the pellets. When the amount of cross-linking agent is excessively small on the outside of the pellets, the cross-linking agent is not sufficiently dispersed in the extruder during the extrusion process in which the linear velocity of the cable core is increased, resulting in areas where the cross-linking agent is insufficient. In areas where the cross-linking agent is insufficient, the cross-linking reaction by the cross-linking agent is unlikely to occur during the cross-linking process. Alternatively, even when the amount of carbon-carbon double bonds is excessively small on the outside of the pellets, areas where the carbon-carbon double bonds are insufficient are generated in the extruder during the extrusion process in which the linear velocity is increased. In areas where the carbon-carbon double bonds are insufficient, the cross-linking reaction by the carbon-carbon double bonds is unlikely to occur during the cross-linking process. In the two cases described above, the degree of cross-linking is low in parts of the insulation layer of the power cable. As a result, excessive elongation may occur in the insulation layer when heated.

[0007] An object of the present disclosure is to provide an improved pellet containing an insulating resin and a cross-linking agent, particularly a pellet that can be stored for a long period of time and that can suppress a local decrease in the degree of cross-linking in the insulating layer. [Means for solving the problem]

[0008] According to one aspect of the present disclosure, there is provided a pellet including a resin component including unsaturated polyethylene and a crosslinking agent, the pellet comprising a central portion and an outer peripheral portion positioned on an outer periphery of the central portion, wherein the central portion and the outer peripheral portion satisfy formula (1): 0.32≦(C / A) / (D / B)≦0.91 ···(1) Here, A is the content of the crosslinking agent per 100% by mass of the resin component contained in the central portion, and is expressed in mass %, B is the content of the crosslinking agent per 100% by mass of the resin component contained in the peripheral portion, and is expressed in mass %, C is the content of carbon-carbon double bonds per 1000 carbon atoms in the resin component contained in the central portion, and D is the content of carbon-carbon double bonds per 1000 carbon atoms in the resin component contained in the peripheral portion. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to improve the long-term storage stability of the pellet and to suppress a local decrease in the degree of cross-linking in the insulating layer. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram of a pellet according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a diagram for explaining the evaluation method in the accelerated deterioration test of pellets. [Figure 3] FIG. 3 is a diagram showing predicted temperature transitions in the peripheral and central portions of Sample 16 when it is cooled. [Figure 4] FIG. 4 is a diagram showing predicted temperature transitions in the outer periphery and central portion of Sample 18 when it is cooled. [Figure 5] FIG. 5 is a diagram showing predicted temperature transitions in the outer periphery and central portion of sample 20 when it is cooled. [Figure 6] FIG. 6 is a diagram showing the change in the content of crosslinking agent and the content of carbon-carbon double bonds in the depth direction for the pellet of Sample 17. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Embodiments of the present disclosure] Next, embodiments of the present disclosure will be listed and described.

[0012] [1] The pellet according to one embodiment of the present disclosure comprises: A pellet comprising a resin component containing unsaturated polyethylene and a crosslinking agent, a central portion and an outer peripheral portion located on the outer periphery of the central portion, The central portion and the outer peripheral portion satisfy formula (1), 0.32≦(C / A) / (D / B)≦0.91 ···(1) where: A is the content of the crosslinking agent per 100% by mass of the resin component contained in the central portion, and is expressed in mass%, B is the content of the crosslinking agent per 100% by mass of the resin component contained in the outer peripheral portion, and is expressed in mass%, C is the content of carbon-carbon double bonds per 1000 carbon atoms in the resin component contained in the central portion, D is the content of carbon-carbon double bonds per 1000 carbon atoms in the resin component contained in the peripheral portion. This configuration improves the long-term storage stability of the pellet and also makes it possible to suppress a local decrease in the degree of cross-linking in the insulating layer.

[0013] [2] In the pellet according to the above [1], The central portion and the outer peripheral portion satisfy formula (2). 0.32≦B / A≦0.91 (2). This configuration stably improves the long-term storage stability of the pellet, and stably suppresses a local decrease in the degree of cross-linking in the insulating layer.

[0014] [3] In the pellet according to [1] or [2] above, The central portion and the outer peripheral portion satisfy formula (3). 0.66≦D / C≦1.89 (3). This configuration stably improves the long-term storage stability of the pellet, and stably suppresses a local decrease in the degree of cross-linking in the insulating layer.

[0015] [4] The pellet according to any one of [1] to [3] above, The outer peripheral portion satisfies the following formula: 1.45≦B≦2.86. This configuration stably improves the long-term storage stability of the pellet, and stably suppresses a local decrease in the degree of cross-linking in the insulating layer.

[0016] [5] The pellet according to any one of [1] to [4] above, The outer peripheral portion satisfies the following formula: 0.25≦D≦0.70. This configuration stably improves the long-term storage stability of the pellet, and stably suppresses a local decrease in the degree of cross-linking in the insulating layer.

[0017] [6] The pellet according to any one of [1] to [5] above, The thickness of the outer peripheral portion is not less than 0.1 mm and not more than 1 mm. This configuration stably improves the long-term storage stability of the pellet, and stably suppresses a local decrease in the degree of cross-linking in the insulating layer.

[0018] [7] The pellet according to any one of [1] to [6] above, A and B are calculated by measuring the cross section of the pellet by Fourier transform infrared spectroscopy or Raman scattering. According to this configuration, A and B can be determined stably.

[0019] [8] The pellet according to any one of [1] to [7] above, C and D are calculated by measuring the cross section of the pellet by Fourier transform infrared spectroscopy. According to this configuration, C and D can be determined stably.

[0020] [Details of the embodiments of the present disclosure] An embodiment of the present disclosure will be described below. However, the present disclosure is not limited to these examples, but is defined by the scope of the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0021] [Embodiment 1] FIG. 1 shows a pellet 1 of the first embodiment. The pellet 1 has a central portion 10 and an outer peripheral portion 11. The outer peripheral portion 11 is located outside the central portion 10.

[0022] Pellet 1 contains a resin component containing unsaturated polyethylene and a crosslinking agent. The unsaturated polyethylene as a resin component contains at least a carbon-carbon double bond (hereinafter sometimes referred to as a C=C bond). The unsaturated polyethylene is, for example, unsaturated low-density polyethylene (unsaturated LDPE). The crosslinking agent is, for example, an organic peroxide. Pellet 1 further contains an additive. The additive is, for example, an inorganic filler, an antioxidant, or a lubricant. The crosslinking agent and additive are disclosed in, for example, JP 2020-132819, US 2020 / 279672A, JP 2020-132818, US 2020 / 273598A, JP 2020-132817, US 2020 / 270426A, JP 2019-189842, and US 2021 / 032434A, and therefore, description thereof will be omitted.

[0023] The central portion 10 and the outer peripheral portion 11 satisfy the formula (1). 0.32≦(C / A) / (D / B)≦0.91 ···(1) where: A is the content of the crosslinking agent per 100% by mass of the resin component contained in the central portion 10, and the unit is % by mass. B is the content of the crosslinking agent per 100% by mass of the resin component contained in the outer peripheral portion 11, and the unit is % by mass. C is the content of carbon-carbon double bonds per 1000 carbon atoms in the resin component contained in the central portion 10 . D is the content of carbon-carbon double bonds per 1000 carbon atoms in the resin component contained in the peripheral portion 11.

[0024] In this way, when the central portion 10 and the outer peripheral portion 11 satisfy the formula (1), the long-term storage stability of the pellet 1 is improved, and a local decrease in the degree of cross-linking in the insulating layer of the power cable can be suppressed.

[0025] The pellets 1 are produced by a coating method, which includes the following steps: Resin preparation step First molding step Second molding step

[0026] (Resin preparation step) First, in the resin preparation step, a monomer is polymerized by a high-pressure method to prepare an unsaturated polyethylene (unsaturated LDPE) as a resin component. The monomer is, for example, ethylene. At this time, the content of carbon-carbon double bonds in the unsaturated polyethylene as a resin component is adjusted by using at least one of the following methods: using a chain transfer agent; adjusting the polymerization conditions (temperature, pressure); etc.

[0027] After preparing the resin components, a first composition and a second composition are prepared. The first composition is for forming the central portion 10. The second composition is for forming the outer peripheral portion 11. The first composition and the second composition contain the above-described unsaturated polyethylene and crosslinking agent. In the resin preparation step of this embodiment, the first composition and the second composition are previously configured so that the central portion 10 and the outer peripheral portion 11, respectively, satisfy the above-described formula (1).

[0028] (First molding step) Following the resin preparation step, a first molding step is performed. First, the first composition is heated, for example, at 80°C to produce a molten first composition. The molten first composition is extruded using an extruder to produce a linear first composition. Next, the linear first composition is cut to a predetermined length. This forms a granular resin molded body. Next, the granular resin molded body is naturally cooled in air at, for example, 25°C. This granular resin molded body corresponds to the central portion 10.

[0029] (Second molding step) Following the first molding step, a second molding step is performed. First, the second composition is heated, for example, at 80°C to prepare a molten second composition. Next, the molten second composition is applied to the periphery of the central portion 10 to form the outer peripheral portion 11 around the central portion 10. The outer peripheral portion 11 may be formed, for example, by coating or spraying the molten second composition onto the central portion 10. Alternatively, the second composition may be applied to the central portion 10 multiple times so that the outer peripheral portion 11 has a desired thickness. Next, the second composition is naturally cooled in air, for example, at 25°C. This provides the pellet 1 of embodiment 1.

[0030] Thus, the pellet of embodiment 1 has a central portion 10 containing the first composition and an outer peripheral portion 11 containing the second composition around it. The central portion 10 and outer peripheral portion 11 of the pellet 1 satisfy the above-mentioned formula (1).

[0031] [Embodiment 2] The pellet 1 of the second embodiment has a central portion 10 and an outer peripheral portion 11, similar to the pellet 1 of the first embodiment. The pellet 1 of the second embodiment is manufactured by a thermal diffusion method. The thermal diffusion method includes the following steps: Resin preparation step Molding step Cooling step

[0032] (Resin preparation step) First, unsaturated polyethylene is prepared as a resin component in the same manner as in the first embodiment.

[0033] Once the resin component is prepared, a first composition is prepared, which comprises an unsaturated polyethylene and a crosslinking agent.

[0034] (Molding step) The resin preparation step is followed by a molding step. In the molding step, the first composition is first heated to prepare a molten first composition. For example, the first composition is heated to 80°C to prepare a molten first composition. Next, the molten first composition is extruded to produce a linear first composition at a high temperature. Next, the linear first composition at a high temperature is cut to a desired length to form a hot pellet 1.

[0035] (Cooling step) The molding step is followed by a cooling step, which includes the following substeps: Primary cooling substep Secondary cooling substep

[0036] (Primary cooling substep) In the primary cooling sub-step, the surface of the high-temperature pellet 1 is rapidly cooled. Specifically, in the primary cooling sub-step, a fluid at the primary cooling temperature is supplied to the surface of the high-temperature pellet 1 for a predetermined time, thereby rapidly cooling the surface of the pellet 1. For example, in the primary cooling sub-step, CO2 gas at 5°C is sprayed onto the high-temperature pellet 1 for 30 seconds to rapidly cool the pellet 1. Rapid cooling generates a temperature difference between the central portion 10 and the outer peripheral portion 11 of the pellet 1. The temperature of the fluid and the time for spraying the fluid can be adjusted as appropriate.

[0037] (Secondary cooling substep) Immediately after the primary cooling substep, a secondary cooling substep is performed. In the secondary cooling substep, the pellet 1 is placed in air at a secondary cooling temperature. The secondary cooling temperature is, for example, 10°C to 40°C. This gradually cools the pellet 1. As the pellet is gradually cooled, a temperature difference between the central portion 10 and the outer peripheral portion 11 of the pellet 1 is maintained for a certain period of time. This temperature difference causes the crosslinking agent to thermally diffuse within the pellet 1. Specifically, the crosslinking agent diffuses from the cooled outer peripheral portion 11 to the high-temperature central portion 10. At this time, at least one of the secondary cooling temperature and the secondary cooling time is adjusted so that the central portion 10 and the outer peripheral portion 11 satisfy formula (2). 0.32≦B / A≦0.91 (2) On the other hand, since the carbon-carbon double bonds in the resin component do not thermally diffuse, the content of carbon-carbon double bonds in the resin component is the same in the central portion 10 and the peripheral portion 11, that is, C≈D. As a result, the central portion 10 and the outer peripheral portion 11 of the pellet 1 satisfy the above-mentioned formula (1).

[0038] The crosslinking agent contents (A and B) in the central portion 10 and the peripheral portion 11 can be measured by Fourier transform infrared spectroscopy (FT-IR) or Raman scattering. The carbon-carbon double bond contents (C and D) in the resin components of the central portion 10 and the peripheral portion 11 can be measured by FT-IR.

[0039] Moreover, the outer diameter and volume of the pellet 1 can be set appropriately.

[0040] [Summary of the embodiment] Since 0.32≦(C / A) / (D / B), the content of crosslinking agent in the peripheral portion 11 is not less than the content of crosslinking agent in the central portion 10, and the content of carbon-carbon double bonds in the resin component of the peripheral portion 11 is not more than the content of carbon-carbon double bonds in the resin component of the central portion 10.

[0041] Since the content of the cross-linking agent in the outer peripheral portion 11 is not too low, the cross-linking agent can be sufficiently dispersed in the extruder even in the extrusion process at a high linear speed, and the occurrence of areas where the cross-linking agent is insufficient can be suppressed. This allows the cross-linking reaction by the cross-linking agent to occur uniformly in the cross-linking process. As a result, it is possible to suppress a local decrease in the degree of cross-linking in the insulating layer.

[0042] Since the content of carbon-carbon double bonds in the resin component of the outer peripheral portion 11 is not excessive, deterioration of the resin component originating from the easily reactive carbon-carbon double bonds in the surface layer of the pellet 1 can be suppressed even if the pellet 1 is exposed to air for a long period of time. This can suppress the generation of foreign matter in the cross-linked insulating layer. In other words, the long-term storage stability of the pellet 1 can be improved.

[0043] On the other hand, since (C / A) / (D / B)≦0.91, the content of crosslinking agent in the peripheral portion 11 is less than the content of crosslinking agent in the central portion 10, and the content of carbon-carbon double bonds in the resin component of the peripheral portion 11 is not too small compared to the content of carbon-carbon double bonds in the resin component of the central portion 10.

[0044] By making the content of the cross-linking agent in the outer peripheral portion 11 lower than that in the central portion 10, cross-linking of a portion of the surface layer of the pellet 1 by the cross-linking agent can be suppressed even if the pellet 1 is exposed to air for a long period of time. This makes it possible to suppress the generation of foreign matter in the cross-linked insulating layer. In other words, the long-term storage stability of the pellet 1 can be improved.

[0045] Since the content of carbon-carbon double bonds in the resin component of the outer peripheral portion 11 is not too low, the carbon-carbon double bonds can be sufficiently dispersed in the extruder even during the extrusion process at a high linear speed, preventing the occurrence of areas with insufficient carbon-carbon double bonds. This allows the crosslinking reaction of the carbon-carbon double bonds to occur uniformly during the crosslinking process. As a result, it is possible to prevent a local decrease in the degree of crosslinking in the insulating layer. [Example]

[0046] Next, examples according to the present disclosure will be described. These examples are examples of the present disclosure, and the present disclosure is not limited to these examples.

[0047] (1) Pellet samples 1 to 8 Samples 1 to 8 were produced by the coating method described above. Table 1 shows the conditions for the resin preparation step, first molding step, and second molding step in the coating method. Samples 1 to 8 contain unsaturated low-density polyethylene (hereinafter simply referred to as LDPE) and the following crosslinking agents: Crosslinking agent CA1: Dicumyl peroxide (hereinafter referred to as DCP)

[0048] [Table 1]

[0049] (carbon-carbon double bond content) In Samples 1 to 8, the same LDPE was blended as a raw material for the resin component in the first resin composition and the second resin composition. The carbon-carbon double bond content per 1000 carbon atoms in the LDPE blended in Samples 1 to 8 was measured by FT-IR. 1 H nuclear magnetic resonance ( 1 The values ​​were determined based on a calibration curve prepared using LDPE, which is known by H-NMR analysis, as a standard sample.

[0050] (evaluation) Samples 1 to 8 were evaluated in the accelerated aging test and hot set tests 1 and 2 shown below.

[0051] (Evaluation method 1: Accelerated aging test) The pellets of Samples 1 to 8 were evaluated by an accelerated aging test. In the accelerated aging test, each pellet was stored in a thermostatic chamber under the following storage conditions. Temperature of the thermostatic chamber: 80℃ Temperature chamber atmosphere: air Storage time in the thermostatic chamber: 48 hours

[0052] Next, the pellets stored for a predetermined time were cut as shown in Figure 2 to obtain a cut surface 1a. The following observation areas of the cut surface 1a were observed using an optical microscope to confirm foreign matter. The observation area was one location 0.15 mm from the outer surface of the pellet (1b in Figure 2). In this example, foreign matter that was amber in color and 0.01 mm or larger in size in the observation area was considered to be foreign matter caused by crosslinking or resin deterioration. When no foreign matter was observed on the surface of the pellet, it was rated as "2A" for excellent long-term storage stability, and when foreign matter was observed, it was rated as "1A" for poor long-term storage stability. Samples rated as 1A were not subjected to the following hot set tests 1 and 2.

[0053] (Evaluation Method 2: Hot Set Test 1) Sheet samples (test pieces) were prepared using the pellets after the accelerated degradation test. The sheet samples were prepared by heating and melting the pellets at 120°C and extruding them into a sheet with a thickness of 1 mm. In evaluation method 2, the extrusion time for the sheet sample was 5 minutes. Next, the sheet was held at 180°C for 30 minutes to prepare a crosslinked sheet sample.

[0054] After crosslinking, hot set test 1 was carried out in accordance with JIS C3667:2008. The sheet sample was hung in an oven heated to 200°C and subjected to a load of 20 N / cm 2 A weight was attached to the bottom of the sheet sample to apply a load of 175% to the sheet sample. After the temperature in the oven recovered, the sheet sample was kept in the oven for 15 minutes. At this time, the elongation of the sheet sample in hot set test 1 was measured relative to the sheet sample before hot set test 1. In evaluation method 2, a sheet sample whose elongation was less than 175% was evaluated as a passing product with a grade of "2B," and a sheet sample whose elongation was 175% or more was evaluated as a failing product with a grade of "1B."

[0055] (Evaluation Method 3: Hot Set Test 2) In evaluation method 3, sheet samples were prepared in the same manner as in evaluation method 2, except that the extrusion time for the sheet sample was set to 30 seconds. Because evaluation method 3 has a short extrusion time, it corresponds to a situation in which the wire speed is increased in the extrusion process of the insulation layer of an actual power cable.

[0056] In Evaluation Method 3, Hot Set Test 2 was performed in the same manner as Hot Set Test 1. In Evaluation Method 3, sheet samples with an elongation of less than 175% were evaluated as passing and rated as "2C," and sheet samples with an elongation of 175% or more were evaluated as failing and rated as "1C."

[0057] The evaluation results of Samples 1 to 8 are shown in Table 2.

[0058] [Table 2]

[0059] (Samples 1 and 2) In Samples 1 and 2, the content of crosslinking agent in the outer peripheral portion 11 was less than the content of crosslinking agent in the central portion 10, i.e., (C / A) / (D / B)<0.32. As a result, in Samples 1 and 2, the elongation in Hot Set Test 2, in which the extrusion time was 30 seconds, was excessively large.

[0060] (Samples 3-6) In Samples 3 to 6, the crosslinking agent content and carbon-carbon double bond content in the outer peripheral portion 11 were appropriate, i.e., 0.32≦(C / A) / (D / B)≦0.91. As a result, in Samples 3 to 6, the outer peripheral portion 11 did not contain any foreign matter after the accelerated aging test. Furthermore, in Samples 3 to 6, even when the sheet sample extrusion time was 30 seconds, the sheet sample was sufficiently crosslinked in the subsequent crosslinking step. As a result, in Samples 3 to 6, excessive elongation in Hot Set Test 2 was suppressed.

[0061] (Samples 7 and 8) In Samples 7 and 8, the content of crosslinking agent in the outer periphery was equal to or greater than the content of crosslinking agent in the center, and (C / A) / (D / B) was greater than 0.91. Therefore, in Samples 7 and 8, the outer periphery 11 contained foreign matter after the accelerated aging test.

[0062] (2) Pellet samples 4-0 to 4-7 First, in the high-pressure method, several LDPEs with different carbon-carbon double bond contents were prepared by using at least one of the following methods: using a chain transfer agent, adjusting polymerization conditions (temperature, pressure), etc.

[0063] Sample 4-0 was a pellet prepared in the same manner as Sample 4. Samples 4-1 to 4-7 were prepared in the same manner as Sample 4-0, except that LDPE was blended in which the carbon-carbon double bond content of the peripheral portion 11 was different from that of Sample 4-0. Specifically, for Samples 4-0 to 4-7, B / A was set to 0.6, and D / C was varied within the range of 0.59 to 2.16. These samples were evaluated in the same manner as in (1). The evaluation results for Samples 4-0 to 4-7 are shown in Table 3.

[0064] [Table 3]

[0065] (Sample 4-1) In Sample 4-1, the content of carbon-carbon double bonds in the peripheral portion 11 was smaller than the content of carbon-carbon double bonds in the central portion 10, i.e., (C / A) / (D / B)>0.91. Therefore, in Sample 4-1, the elongation in Hot Set Test 2, in which the extrusion time was 30 seconds, was excessively large.

[0066] (Samples 4-0, 4-2 to 4-6) In Samples 4-0, 4-2, and 4-6, the crosslinker content and carbon-carbon double bond content in the outer peripheral portion 11 were appropriate, i.e., 0.32≦(C / A) / (D / B)≦0.91. Therefore, in Samples 4-0, 4-2, and 4-6, the outer peripheral portion 11 did not contain any foreign matter after the accelerated aging test. Furthermore, in Samples 4-0, 4-2, and 4-6, even when the sheet sample extrusion time was 30 seconds, the sheet sample was sufficiently crosslinked in the subsequent crosslinking step. As a result, in Samples 4-0, 4-2, and 4-6, excessive elongation in Hot Set Test 2 was suppressed.

[0067] (Sample 4-7) In Sample 4-7, the carbon-carbon double bond content in the peripheral portion was greater than that in the central portion, i.e., (C / A) / (D / B)<0.32, and therefore, in Sample 4-7, the peripheral portion 11 contained foreign matter after the accelerated aging test.

[0068] (3) Pellet samples 9-14 Samples 9 to 14 were produced using the coating method described above. Table 3 shows the conditions for the resin preparation step, first molding step, and second molding step in the coating method. Samples 9 to 14 contain the same LDPE as Sample 4 and the following crosslinkers: Crosslinker CA2 for Sample 9: t-butyldicumyl peroxide Crosslinker CA3 for Sample 10: Di(t-butyl peroxide) Sample 11 Crosslinker CA4: 2,5-dimethyl-2,5-di(t-butylperoxy)hexane Sample 12 Crosslinker CA5: 1,3-bis(t-butylperoxyisopropyl)benzene Sample 13 Crosslinker CA6: 4,4-bis[(t-butyl)peroxy]butylpentanoate Sample 14 Crosslinker CA7: 1,1-bis(1,1-dimethylethylperoxy)cyclohexane

[0069] [Table 4]

[0070] Samples 9 to 14 were evaluated in the same manner as in (1) and (2). Table 5 shows the evaluation results.

[0071] [Table 5]

[0072] (Samples 9-14) Although Samples 9 to 14 each contained a different type of crosslinking agent, the content of crosslinking agent and the content of carbon-carbon double bonds in the outer peripheral portion 11 were appropriate, i.e., 0.32≦(C / A) / (D / B)≦0.91. As a result, Samples 9 to 14 contained no foreign matter in the outer peripheral portion 11 after the accelerated aging test. Furthermore, Samples 9 to 14 were prevented from excessive elongation in Hot Set Test 2.

[0073] (4) Pellet samples 15-22 Samples 15 to 22 were manufactured by the thermal diffusion method described above. Table 6 shows the conditions for the resin preparation step, molding step, and cooling step in the thermal diffusion method.

[0074] [Table 6]

[0075] Samples 15 to 22 were evaluated in the same manner as in (1) to (3). Table 7 shows the evaluation results of Samples 15 to 22.

[0076] In addition, for Samples 15 to 22, the crosslinking agent content and carbon-carbon double bond content in the central portion 10 and the peripheral portion 11 were measured by FT-IR. Specifically, each pellet was cut as shown in FIG. 2, and the crosslinking agent content and carbon-carbon double bond content at each position were measured by FT-IR at a position 0.15 mm deep from the peripheral surface (1b in FIG. 2) and a position 0.90 mm deep from the peripheral surface (1c in FIG. 2) on the cut surface 1a, respectively. The crosslinking agent content and carbon-carbon double bond content at each position were determined. The content at the position 0.15 mm deep was defined as the content in the peripheral portion 11, and the content at the position 0.90 mm deep was defined as the content in the central portion 10. The carbon-carbon double bond content at each position was determined by the carbon-carbon double bond content. 1 The values ​​were determined based on a calibration curve prepared using known LDPE as a standard sample by H-NMR analysis.

[0077] [Table 7]

[0078] (Samples 15 and 16) As shown in Table 7, in Samples 15 and 16, the content of crosslinking agent in the outer peripheral portion 11 was the same as that in the central portion 10. This is thought to be due to the temperature changes in the central portion 10 and outer peripheral portion 11 as shown in Figure 3. Figure 3 shows the predicted temperature changes in the outer peripheral portion and central portion when cooling (primary cooling sub-step and secondary cooling sub-step) was performed on Sample 16. In the figure, the dashed line shows the predicted temperature change in the central portion 10, and the solid line shows the predicted temperature change in the outer peripheral portion 11.

[0079] Specifically, as shown in Figure 3, the time for the primary cooling sub-step was short, so the temperature of the outer peripheral portion 11 did not drop sufficiently in the primary cooling sub-step. Specifically, in the primary cooling sub-step, the temperatures of the central portion 10 and the outer peripheral portion 11 dropped rapidly up to 10 seconds after the start of cooling, but the temperatures of both the central portion 10 and the outer peripheral portion 11 remained relatively high. In the secondary cooling sub-step, the temperatures of both the central portion 10 and the outer peripheral portion 11 dropped gradually, and it is presumed that the temperature difference between the outer peripheral portion 11 and the central portion 10 was small, and thermal diffusion of the cross-linking agent from the outer peripheral portion 11 to the central portion 10 did not occur.

[0080] In Samples 15 and 16, B / A=1 and D / C=1, i.e., (C / A) / (D / B)>0.91. Therefore, in Samples 15 and 16, the outer peripheral portion 11 contained foreign matter after the accelerated aging test.

[0081] (Samples 17-19) For Sample 17, the changes in cross-linking agent content and carbon-carbon double bond content were measured using FT-IR from the pellet surface toward the depth direction. The results are shown in Figure 6. Figure 6 shows the cross-linking agent content and carbon-carbon double bond content measured by FT-IR at depths of 0.15 mm, 0.3 mm, 0.5 mm, 0.7 mm, 0.9 mm, 1.2 mm, and 1.5 mm from the outer surface of the pellet, measured on the cut surface 1a of the pellet as shown in Figure 2. The carbon-carbon double bond content at each position was determined based on the calibration curve described above. In Figure 6, the solid line indicates the change in cross-linking agent content, and the dashed line indicates the change in carbon-carbon double bond content. As shown in Figure 6, for Sample 17, the cross-linking agent content was lower near the pellet surface and increased toward the pellet center. On the other hand, the carbon-carbon double bond content did not change significantly toward the pellet depth. It was confirmed that the content of the cross-linking agent in Samples 18 and 19 changed in the depth direction, similar to Sample 17.

[0082] For Samples 17 to 19, the region where the crosslinker content was below the average value for the entire pellet was defined as the outer periphery, and the region where the crosslinker content exceeded the average value was defined as the central region. The "average value for the entire pellet" refers to the crosslinker content per 100% by mass of LDPE in the entire pellet when the entire pellet of each sample was melted at 80°C and allowed to cool naturally. For Samples 17 to 19, this was 3% by mass. For Samples 17 to 19, the region where the crosslinker content was 3% by mass or less was located within a depth of 0.3 mm from the surface of the pellet, so the 0.3 mm thick region was defined as the outer periphery. The region with a diameter of 2.4 mm, excluding the outer periphery, was defined as the central region.

[0083] As shown in Table 7 and Figure 6, in Samples 17 to 19, the content of cross-linking agent at a position 0.15 mm deep from the pellet surface in the outer peripheral portion 11 was lower than the content of cross-linking agent at a position 0.90 mm deep from the pellet surface in the central portion 10. This is thought to be due to the temperature changes in the central portion 10 and outer peripheral portion 11 as shown in Figure 4. Figure 4 is a diagram showing the predicted temperature changes in the outer peripheral portion 11 and central portion 10 when cooling (primary cooling sub-step and secondary cooling sub-step) was performed on Sample 18. In the figure, the dashed line shows the predicted temperature change in the central portion 10, and the solid line shows the predicted temperature change in the outer peripheral portion 11.

[0084] Specifically, as shown in Figure 4, the time of the primary cooling sub-step was appropriate, so the temperature of the peripheral portion 11 dropped significantly in the primary cooling sub-step. Specifically, in the primary cooling sub-step, the temperature of the peripheral portion 11 dropped to 5°C within 60 seconds after the start of cooling. On the other hand, the temperature of the central portion 10 did not drop as much as that of the peripheral portion 11, and the temperature difference between the peripheral portion 11 and the central portion 10 was large. Because the temperature difference between the peripheral portion 11 and the central portion 10 was large, it is presumed that thermal diffusion of the cross-linking agent from the peripheral portion 11 to the central portion 10 occurred in the primary cooling sub-step.

[0085] In Samples 17 to 19, 0.32≦B / A≦0.91 and D / C=1, i.e., 0.32≦(C / A) / (D / B)≦0.91. Therefore, in Samples 17 to 19, the outer peripheral portion 11 after the accelerated aging test contained no foreign matter. Furthermore, in Samples 17 to 19, excessive elongation in Hot Set Test 2 was suppressed.

[0086] (Samples 20-22) In Samples 20 to 22, as shown in Table 7, the content of crosslinking agent in the outer peripheral portion 11 was the same as that in the central portion 10. This is thought to be due to the temperature changes in the central portion 10 and outer peripheral portion 11 as shown in Figure 5. Figure 5 is a diagram showing the predicted temperature changes in the outer peripheral portion and central portion when cooling (primary cooling sub-step and secondary cooling sub-step) was performed on Sample 20. In the figure, the dashed line shows the predicted temperature change in the central portion 10, and the solid line shows the predicted temperature change in the outer peripheral portion 11.

[0087] Specifically, as shown in FIG. 5 , in the first cooling substep, the temperatures of the central portion 10 and the peripheral portion 11 dropped to 5°C within 300 seconds after the start of cooling. At this time, the temperature of the central portion 10 dropped more slowly than that of the peripheral portion 11. After the temperature of the peripheral portion 11 dropped to 5°C, the temperature of the central portion 10 also dropped to 5°C after a certain time. Therefore, there was a period of time when the temperature difference between the peripheral portion 11 and the central portion 10 became large, resulting in thermal diffusion of the crosslinking agent from the peripheral portion 11 to the central portion 10. Meanwhile, in the second cooling substep, the temperature of the peripheral portion 11 rose from 5°C to 25°C, and after a certain time, the temperature of the central portion 10 also rose to 25°C. During this process, the temperature of the peripheral portion 11 became higher than that of the central portion 10, and a certain temperature difference occurred, resulting in thermal diffusion of the crosslinking agent from the central portion 10 to the peripheral portion 11. As a result, it is presumed that thermal diffusion of the crosslinking agent does not occur during the cooling step.

[0088] In Samples 20 to 22, B / A=1 and D / C=1, i.e., (C / A) / (D / B)>0.91. Therefore, in Samples 20 to 22, the outer peripheral portion 11 contained foreign matter after the accelerated deterioration test.

[0089] From the above, the pellets disclosed in the present specification are A pellet comprising a resin component containing unsaturated polyethylene and a crosslinking agent, a central portion and an outer peripheral portion located on the outer periphery of the central portion, The central portion and the outer peripheral portion satisfy formula (1). 0.32≦(C / A) / (D / B)≦0.91 ···(1)

[0090] By satisfying formula (1) for the central portion 10 and the outer peripheral portion 11, the pellets are less likely to deteriorate even when stored for a long period of time. Furthermore, power cables manufactured using such pellets have excellent electrical insulation. Note that methods for manufacturing power cables are disclosed in, for example, JP 2020-132817 A and US 2020 / 0270426 A, and therefore will not be described here.

[0091] By satisfying formula (1) for the central portion 10 and the peripheral portion 11, the carbon-carbon double bonds of the cross-linking agent and the resin component can be sufficiently dispersed in the extruder, even during the extrusion process at a high linear velocity. This prevents a local decrease in the degree of cross-linking in the insulating layer. As a result, excessive elongation of the insulating layer can be prevented even when the insulating layer is heated.

[0092] In the pellet 1, the ratio B / A of the content A of the crosslinking agent in the central portion 10 to the content B of the crosslinking agent in the peripheral portion 11 is not limited to the values ​​in the examples. However, the central portion 10 and the peripheral portion 11 may satisfy formula (2). 0.32≦B / A≦0.91 (2)

[0093] In the pellet 1, the ratio D / C of the carbon-carbon double bond content D in the peripheral portion 11 to the carbon-carbon double bond content C in the central portion 10 is not limited to the values ​​in the examples, but the central portion 10 and the peripheral portion 11 may satisfy formula (3). 0.66≦D / C≦1.89 (3)

[0094] In the pellet 1, there is no particular limitation on the content B of the crosslinking agent in the outer peripheral portion 11. However, the outer peripheral portion 11 may satisfy 1.45≦B≦2.86.

[0095] In the pellet 1, there is no particular limitation on the content D of carbon-carbon double bonds in the outer peripheral portion 11. However, the outer peripheral portion 11 may satisfy 0.25≦D≦0.70.

[0096] As described above, it is possible to stably form the central portion 10 and the outer peripheral portion 11 that satisfy formula (1) by adjusting at least one of B / A, D / C, B, and D. As a result, it is possible to stably improve the long-term storage stability of the pellet 1 and stably suppress a local decrease in the degree of cross-linking in the insulating layer of the power cable.

[0097] In order to sufficiently crosslink the resin component, the content A of the crosslinking agent in the central portion 10 is preferably 1% by mass or more relative to 100% by mass of the resin component contained in the central portion. Furthermore, in order to prevent the resin component after crosslinking from containing a large amount of crosslinking by-products, the content A of the crosslinking agent in the central portion 10 is preferably 1% by mass or more and 10% by mass or less relative to 100% by mass of the resin component contained in the central portion.

[0098] The thickness of the outer peripheral portion 11 is not limited to 0.3 mm as disclosed in the examples. To prevent deterioration of the central portion 10, the outer peripheral portion 11 may have a thickness of 0.1 mm or more and 1 mm or less. The size (diameter) of the central portion 10 is not limited to 2.4 mm as disclosed in the examples. The diameter of the central portion 10 is preferably 1.8 mm or more and 6.0 mm or less. Furthermore, the diameter of the pellet 1 is not limited to 3 mm as disclosed in the examples. The diameter of the pellet 1 is preferably 2.0 mm or more and 8.0 mm or less.

[0099] In the examples, the crosslinking agent contents in the outer and central portions of pellets prepared by the thermal diffusion method were measured at depths of 0.15 mm and 0.9 mm from the pellet surface, respectively, but this is not limiting. The measurement position for the outer portion may be selected at the center of the thickness of the outer portion. The measurement position for the central portion may be selected at the center between the surface and center of the central portion. [Explanation of symbols]

[0100] 1 pellet 10 Center part 11 Outer area

Claims

1. A pellet comprising a resin component containing unsaturated polyethylene and a crosslinking agent, a central portion and an outer peripheral portion located on the outer periphery of the central portion, The central portion and the outer peripheral portion satisfy formula (1), 0.32≦(C / A) / (D / B)≦0.91...(1) where: A is the content of the crosslinking agent per 100% by mass of the resin component contained in the center portion, expressed in mass%, B is the content of the crosslinking agent per 100% by mass of the resin component contained in the outer peripheral portion, expressed in mass%, C is the content of carbon-carbon double bonds per 1000 carbon atoms in the resin component contained in the central portion, D is the content of carbon-carbon double bonds per 1000 carbon atoms in the resin component contained in the peripheral portion. pellet.

2. The central portion and the outer peripheral portion satisfy formula (2). 0.32≦B / A≦0.91 (2) The pellet of claim 1.

3. The central portion and the outer peripheral portion satisfy formula (3). 0.66≦D / C≦1.89 (3) The pellet according to claim 1 or claim 2.

4. The outer peripheral portion satisfies the following formula: 1.45≦B≦2.86 The pellet according to claim 1 or claim 2.

5. The outer peripheral portion satisfies the following formula: 0.25≦D≦0.70 The pellet according to claim 1 or claim 2.

6. The thickness of the outer peripheral portion is 0.1 mm or more and 1 mm or less. The pellet according to claim 1 or claim 2.

7. A and B are calculated by measuring the cross section of the pellet by Fourier transform infrared spectroscopy or Raman scattering. The pellet according to claim 1 or claim 2.

8. C and D are calculated by measuring the cross section of the pellet by Fourier transform infrared spectroscopy. The pellet according to claim 1 or claim 2.

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