Pellet

Abstract

A pellet including a resin component containing polyethylene, a cross-linking agent, and an antioxidant, the pellet including a central portion and an outer circumferential portion located on an outer circumference of the central portion, wherein a content of the cross-linking agent and a content of the antioxidant in each of the central portion and the outer circumferential portion satisfy the following expression (1):　(C / A) / (D / B) ≦ 0.97 ・・・ (1)　where　A: the content of the cross-linking agent per 100 wt% of the resin component constituting the central portion;　B: the content of the cross-linking agent per 100 wt% of the resin component constituting the outer circumferential portion;　C: the content of the antioxidant per 100 wt% of the resin component constituting the central portion;　D: the content of the antioxidant per 100 wt% of the resin component constituting the outer circumferential portion.

Description

PELLET

[0001] The present disclosure relates to a pellet.

[0002] Cross-linked polyethylene is widely used for an insulation layer of a power cable (e.g., PTL. 1).

[0003] The insulation layer is formed using a pellet including a resin composition which includes polyethylene and a cross-linking agent.

[0004] PTL. 1: Japanese Patent Laid-Open Publication No. 2020-132817

[0005] According to an aspect of the present disclosure, 　　　there is provided a pellet including a resin component containing polyethylene, a cross-linking agent, and an antioxidant, 　　　the pellet including a central portion and an outer circumferential portion located on an outer circumference of the central portion, 　　　wherein a content of the cross-linking agent and a content of the antioxidant in each of the central portion and the outer circumferential portion satisfy the following expression (1): 　　　(C / A) / (D / B) ≦ 0.97 ・・・ (1) 　　　where 　　　A: the content of the cross-linking agent per 100 wt% of the resin component constituting the central portion; 　　　B: the content of the cross-linking agent per 100 wt% of the resin component constituting the outer circumferential portion; 　　　C: the content of the antioxidant per 100 wt% of the resin component constituting the central portion; 　　　D: the content of the antioxidant per 100 wt% of the resin component constituting the outer circumferential portion.

[0006] FIG. 1 is a schematic configuration diagram of a pellet according to an embodiment of the present disclosure.FIG. 2 is a diagram for explaining an evaluation method in the accelerated degradation test of the pellet.FIG. 3 is a diagram illustrating the expected temperature changes in the outer circumferential portion and the central portion during cooling of Sample 16.FIG. 4 is a diagram illustrating the expected temperature changes in the outer circumferential portion and the central portion during cooling of Sample 18.FIG. 5 is a diagram illustrating the expected temperature changes in the outer circumferential portion and the central portion during cooling of Sample 20.FIG. 6 is a diagram illustrating changes in a content of a cross-linking agent and a content of an antioxidant in the depth direction for a pellet of Sample 17.Problem to be Solved by the Disclosure

[0007] A power cable has an electrical insulation layer. Hereinafter, the electrical insulation layer will be referred to as an "insulation layer". The insulation layer is formed from a pellet including an insulation resin, a cross-linking agent, and an antioxidant. The insulation resin is, for example, polyethylene. The pellet is produced and stored before the power cable is produced. When the pellet is stored for a long term, the pellet is exposed to air for a long term. When the pellet is exposed to air for a long term, the insulation resin may be partially cross-linked in a surface layer of the pellet by the cross-linking agent included in the surface layer of the pellet to form a cross-linked contaminant. Further, when the pellet is exposed to air, its resin component may degrade and discolor. The progress of cross-linking and the resin degradation in the pellet may lead to generation of contaminant in the pellet. The pellet is heated during the production of the insulation layer of the power cable. When the pellet is heated, the insulation resin included in the pellet is cross-linked by the cross-linking agent. Accordingly, the power cable has a cross-linked insulation layer. However, when a cross-linked insulation layer is formed from a pellet containing a contaminant, the cross-linked insulation layer contains the contaminant. The inventor has found that the contaminant causes a reduction in the electrical insulation of the insulation layer when the pellet is stored for a long term.

[0008] An object of the present disclosure is to provide an improved pellet including an insulation resin, a cross-linking agent, and an antioxidant. Particularly, an object of the present disclosure is to provide a pellet which can be stored for a long term.Advantageous Effect of the Disclosure

[0009] According to the present disclosure, degradation due to contact with air for a long term can be suppressed for the pellet including the insulation resin and the cross-linking agent.Embodiments of Disclosure

[0010] Next, embodiments of the present disclosure will be listed and described.

[0011] [1] An aspect of the present disclosure provides 　　　a pellet including a resin component containing polyethylene, a cross-linking agent, and an antioxidant, 　　　the pellet including a central portion and an outer circumferential portion located on an outer circumference of the central portion, 　　　wherein a content of the cross-linking agent and a content of the antioxidant in each of the central portion and the outer circumferential portion satisfy the following expression (1): 　　　 (C / A) / (D / B) ≦ 0.97・・・ (1) 　　　where 　　　A: the content of the cross-linking agent per 100 wt% of the resin component constituting the central portion; 　　　B: the content of the cross-linking agent per 100 wt% of the resin component constituting the outer circumferential portion; 　　　C: the content of the antioxidant per 100 wt% of the resin component constituting the central portion; 　　　D: the content of the antioxidant per 100 wt% of the resin component constituting the outer circumferential portion. 　　　This configuration can suppress degradation of the pellet due to contact with air and improve the long-term preservability of the pellet.

[0012] [2] In the pellet according to [1], the A and the B satisfy the relation: B ＜ A. 　　　This configuration can keep the long-term preservability of the pellet higher.

[0013] [3] In the pellet according to [1] or [2], the B is less than 10 wt%. 　　　This configuration can keep the long-term preservability of the pellet higher.

[0014] [4] In the pellet according to any one of [1] to [3], the A and the B satisfy the relation: B / A ≦ 0.9. 　　　This configuration can keep the long-term preservability of the pellet higher.

[0015] [5] In the pellet according to any one of [1] to [3], the outer circumferential portion does not include the cross-linking agent. 　　　This configuration can keep the long-term preservability of the pellet higher.

[0016] [6] In the pellet according to any one of [1] to [5], the A is 1 wt% or more and 10 wt% or less. 　　　This configuration allows the content of the cross-linking agent in the entire pellet to be adjusted.

[0017] [7] In the pellet according to any one of [1] to [6], the content of the cross-linking agent included in the pellet is 0.3 wt% or more and 5.0 wt% or less with respect to the resin component constituting the pellet being 100 wt%. 　　　This configuration can realize a predetermined degree of cross-linking upon cross-linking while keeping a long-term preservability of the pellet higher.

[0018] [8] In the pellet according to any one of [1] to [7], the D is 0.031 wt% or more and 0.0875 wt% or less. 　　　This configuration can keep the long-term preservability of the pellet higher.

[0019] [9] In the pellet according to any one of [1] to [8], a thickness of the outer circumferential portion is 0.1 mm or more and 1 mm or less. 　　　This configuration can keep the long-term preservability of the pellet higher.

[0020]

[0010] In the pellet according to any one of [1] to [9], each of the A, B, C, and D is calculated by measuring a cut surface, cut out from the pellet, by FT-IR or Raman scattering method. 　　　This configuration can keep the long-term preservability of the pellet higher.Details of the Embodiment of the Disclosure

[0021] Next, an embodiment of the present disclosure will be described below. The present disclosure is not limited to these exemplifications, but intended to be indicated by claims and encompass all the changes which fall within the meaning and scope equivalent to claims. Further, "wt%" indicates "mass%".

[0022] (Embodiment 1) 　　　FIG. 1 illustrates the pellet 1 of Embodiment 1. 　　　The pellet 1 includes a central portion 10 and an outer circumferential portion 11. The outer circumferential portion 11 is located outside the central portion 10.

[0023] The pellet 1 includes a resin containing polyethylene, a cross-linking agent, and an antioxidant. The cross-linking agent is, for example, an organic peroxide. The antioxidant is, for example, a phenol-based antioxidant, an amine-based antioxidant, a phosphorus-based antioxidant, or a sulfur-based antioxidant. The pellet 1 further includes other additives other than the cross-linking agent and the antioxidant. The additive is, for example, an inorganic filler, or a lubricant. Since the cross-linking agent, the antioxidant, and other additives are disclosed, for example, in Japanese Patent Laid-Open Publication No. 2020-132819, US2020 / 279672A, Japanese Patent Laid-Open Publication No. 2020-132818, US2020 / 273598A, Japanese Patent Laid-Open Publication No. 2020-132817, US2020 / 270426A, Japanese Patent Laid-Open Publication No. 2019-189842, and US2021 / 032434A, they are not explained here.

[0024] The central portion 10 and the outer circumferential portion 11 are configured so that the contents of the cross-linking agent and the antioxidant included in each of them satisfy the following expression (1): 　　　(C / A) / (D / B) ≦ 0.97 ・・・ (1) 　　　where A is the content (wt%) of the cross-linking agent per 100 wt% of the resin component constituting the central portion 10; B is the content (wt%) of the cross-linking agent per 100 wt% of the resin component constituting the outer circumferential portion 11; C is the content (wt%) of the antioxidant per 100 wt% of the resin component constituting the central portion 10; and D is the content (wt%) of the antioxidant per 100 wt% of the resin component constituting the outer circumferential portion 11.

[0025] The expression (1) is a parameter indicating the ratio of the cross-linking agent and the antioxidant included in each of the central portion 10 and the outer circumferential portion 11. When A to D satisfy the expression (1), the content B of the cross-linking agent in the outer circumferential portion 11 tends to be less than the content A of the cross-linking agent in the central portion 10, or the content D of the antioxidant in the outer circumferential portion 11 tends to be more than the content C of the antioxidant in the central portion 10. Therefore, in the outer circumferential portion 11, cross-linking due to contact between the cross-linking agent and air, and resin degradation due to contact with air are suppressed. On the other hand, since the central portion 10 is covered with the outer circumferential portion 11, cross-linking due to the contact between the cross-linking agent and air and resin degradation are suppressed. Therefore, the pellet 1 has excellent long-term preservability.

[0026] The content of the cross-linking agent in the outer circumferential portion 11 is preferably less than the content of the cross-linking agent in the central portion 10 from the viewpoint of satisfying the above expression (1) in the pellet 1. That is, it is preferable that B ＜ A. Further, the content of the antioxidant is not particularly limited as long as it satisfies the expression (1). For example, the content C of the antioxidant in the central portion 10 and the content D of the antioxidant in the outer circumferential portion 11 may be appropriately adjusted to satisfy the expression (1) depending on the values of A and B.

[0027] The pellet 1 is produced by a coating method. The coating method includes the following steps: ・a resin preparation step, ・a first molding step, and ・a second molding step.

[0028] (Resin preparation step) 　　　In the resin preparation step, a first composition and a second composition are firstly prepared. The first composition is for forming the central portion 10. The second composition is for forming the outer circumferential portion 11. The first composition includes polyethylene, a cross-linking agent, and an antioxidant. The second composition satisfies any one of the following conditions (a) to (c): (a) The second composition includes polyethylene. The second composition does not include a cross-linking agent, but includes an antioxidant. (b) The second composition includes polyethylene. The second composition includes a cross-linking agent and an antioxidant, and an amount of the cross-linking agent in the second composition is less than an amount of the cross-linking agent in the first composition, and an amount of the antioxidant is in such a range that the above expression (1) is satisfied. (c) The second composition includes polyethylene. The second composition includes a cross-linking agent and an antioxidant, and an amount of the antioxidant in the second composition is more than an amount of the antioxidant in the first composition, and an amount of the cross-linking agent in the second composition is in such a range that the above expression (1) is satisfied.

[0029] (First molding step) 　　　Subsequent to 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. Subsequently, the linear first composition is cut into a predetermined length. Accordingly, a granular molded resin is formed. Subsequently, the granular molded resin is allowed to cool naturally, for example, in an air at 25°C. The granular molded resin corresponds to the central portion 10.

[0030] (Second molding step) 　　　Subsequent to the first molding step, the second molding step is performed. First, the second composition is heated, for example, at 80°C to produce a molten second composition. Subsequently, the molten second composition is provided around the central portion 10 to form an outer circumferential portion 11 around the central portion 10. The outer circumferential portion 11 may be formed, for example, by applying or spraying the molten second composition onto the central portion 10. The second composition may be provided a plurality of times to the central portion 10 so that the outer circumferential portion 11 has a predetermined thickness. Subsequently, the second composition is allowed to cool naturally, for example, in an air at 25°C. Accordingly, the pellet 1 of Embodiment 1 is obtained.

[0031] Thus, the pellet of Embodiment 1 has the outer circumferential portion 11 including the second composition around the central portion 10 including the first composition. The pellet 1 is configured so that the contents of the cross-linking agent and the antioxidant included in each of the central portion 10 and the outer circumferential portion 11 satisfy the above expression (1).

[0032] (Embodiment 2) 　　　Similarly to the pellet 1 of Embodiment 1, the pellet 1 of Embodiment 2 has the central portion 10 and the outer circumferential portion 11. The pellet 1 of Embodiment 2 is produced by a thermal diffusion method. The thermal diffusion method includes the following steps: ・a resin preparation step, ・a molding step, and ・a cooling step.

[0033] (Resin preparation step) 　　　First, a first composition is prepared. The first composition includes polyethylene, a cross-linking agent, and an antioxidant.

[0034] (Molding step) 　　　Subsequent to the resin preparation step, a molding step is performed. In the molding step, the first composition is firstly heated to produce a molten first composition. For example, the first composition is heated to 80°C to produce a molten first composition. Subsequently, the molten first composition is extrusion molded to produce the first composition that is linear at high temperature. Subsequently, the first composition that is linear at high temperature is cut into a desired length to form the pellet 1 at high temperature.

[0035] (Cooling step) 　　　Subsequent to the molding step, a cooling step is performed. The cooling step includes the following substeps: ・a primary cooling substep, and ・a secondary cooling substep.

[0036] (Primary cooling substep) 　　　In the primary cooling substep, a surface of the pellet 1 at high temperature is rapidly cooled. Specifically, in the primary cooling substep, a fluid at a primary cooling temperature is supplied to the surface of the pellet 1 at high temperature for a predetermined time to rapidly cool the surface of the pellet 1. For example, in the primary cooling substep, CO2gas at 5°C is blown to the pellet 1 at high temperature for 30 seconds for rapid cooling. Rapid cooling generates a temperature difference between the central portion 10 and the outer circumferential portion 11 of the pellet 1. The temperature of the fluid and the time for blowing the fluid can be appropriately adjusted.

[0037] (Secondary cooling substep) 　　　The secondary cooling substep is performed immediately after the primary cooling substep. In the secondary cooling substep, the pellet 1 is placed in an air at the secondary cooling temperature. The secondary cooling temperature is, for example, 10°C to 40°C. Accordingly, the pellet 1 is gradually cooled. When the pellet 1 is gradually cooled, the temperature difference between the central portion 10 and the outer circumferential portion 11 of the pellet 1 is maintained for a certain period. Due to this temperature difference, the cross-linking agent thermally diffuses in the pellet 1. Specifically, the cross-linking agent diffuses from the cooled outer circumferential portion 11 to the central portion 10 at high temperature. Accordingly, the amount of the cross-linking agent in the central portion 10 is more than the amount of the cross-linking agent in the outer circumferential portion 11. On the other hand, the antioxidant is less likely to thermally diffuse, and the amount of the antioxidant in the central portion 10 is the same as that in the outer circumferential portion 11. That is, B ＜ A and C ≒ D, which means that the expression (1) is satisfied.

[0038] The amounts of the cross-linking agent and the antioxidant in the central portion 10 and the outer circumferential portion 11 can be measured by FT-IR or Raman scattering method.

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

[0040] (Summary of Embodiments) 　　　In the pellet 1, since the contents of the cross-linking agent and the antioxidant in the central portion 10 and the outer circumferential portion 11 satisfy the above expression (1), progress of cross-linking and resin degradation due to long-term contact with air can be suppressed. Accordingly , the pellet is excellent in long-term preservability. For example, when the accelerated degradation test described below is performed, less contaminant is formed. In addition, even when a sheet sample is formed using the pellets 1 after a long-term storage, less contaminant is formed.

[0041] Next, examples according to the present disclosure will be described. These examples are illustrative of the present disclosure, and the present disclosure is not limited by these examples.

[0042] (1) Samples 1 to 8 of pellets 　　　Samples 1 to 8 were produced using the above-mentioned coating method. Table 1 shows the conditions for the resin preparation step, the first molding step, and the second molding step in the coating method. Samples 1 to 8 include the following cross-linking agent and antioxidant: ・a cross-linking agent CA1: dicumyl peroxide (hereinafter also referred to as DCP) ・antioxidant AO1: 4,4’-thiobis(3-methyl-6-t-butylphenol)

[0043]

[0044] Samples 1 to 8 were evaluated by the accelerated degradation test and the sheet sample test described below.

[0045] (Evaluation method 1: Accelerated degradation test 1) 　　　The pellets of Samples 1 to 8 were evaluated by the accelerated degradation test 1. In the accelerated degradation test 1, each pellet was stored in a thermoregulated chamber. The storage conditions are as follows: ・temperature in thermoregulated chamber: 80°C ・atmosphere in thermoregulated chamber: air ・time for storage in thermoregulated chamber: 48 hours.

[0046] Subsequently, the pellet after being stored for a predetermined period was cut as illustrated in FIG. 2 to obtain a cut surface 1a. In the cut surface 1a, the following observation areas were observed with an optical microscope to check for contaminants. It was a single area (1b in FIG. 2) located 0.15 mm from the outer circumferential surface of the pellet that was to be observed. In this example, an amber-colored matter with a size of 0.01 mm or more in the observation area was regarded as a contaminant generated due to cross-linking or resin degradation. A case where no contaminant was observed on the surface of the pellet was evaluated as "2A", indicating excellent long-term preservability, and a case where a contaminant was observed was evaluated as "1A", indicating poor long-term preservability.

[0047] (Evaluation method 2: Accelerated degradation test 2) 　　　The pellets of each sample were evaluated by the accelerated degradation test 2. In the accelerated degradation test 2, evaluation was performed in the same manner as in the accelerated degradation test 1, except that the storage time in the thermoregulated chamber in the accelerated degradation test 1 was changed from 48 hours to 96 hours.

[0048] (Evaluation method 3: Sheet sample test) 　　　The sheet sample was produced using the pellet which had undergone the accelerated degradation test 2. The sheet sample was obtained by extrusion molding of the pellet, which had been molten by heating at 120°C, into a sheet having a thickness of 1 mm. Then, the sheet was held at 180°C to produce a cross-linked sheet sample. The surface of the sheet sample was observed with an optical microscope to check for contaminants. The contaminants were identified by the same criteria as those used in the accelerated degradation test. In this example, a case where no contaminant was observed on the surface of the sheet sample was evaluated as "2B", indicating acceptable, and a case where a contaminant was observed was evaluated as "1B", indicating unacceptable.

[0049] The evaluation results of Samples 1 to 8 are shown in Table 2.

[0050]

[0051] (Samples 1 to 6) 　　　In Samples 1 to 6, as shown in Table 2, the content C of the antioxidant in the central portion 10 was the same as the content D of the antioxidant in the outer circumferential portion 11. On the other hand, the content B of the cross-linking agent in the outer circumferential portion 11 was made less than the content A of the cross-linking agent in the central portion 10, and (C / A) / (D / B) was 0.97 or less. Therefore, the outer circumferential portions 11 after the accelerated degradation tests 1 and 2 contained no contaminant. The sheet sample also contained no contaminant. 　　　Further, Samples 1 to 6 show that the contents A and B of the cross-linking agents preferably satisfy 0 ≦ B / A ≦ 0.90.

[0052] (Samples 7 and 8) 　　　In Sample 7, the contents C and D of the antioxidants were the same, and the contents A and B of the cross-linking agent in the central portion 10 and the outer circumferential portion 11 were the same. In Sample 8, the contents C and D of the antioxidants were the same, and the content B of the cross-linking agent in the outer circumferential portion 11 was made more than the content A of the cross-linking agent in the central portion 10. Accordingly, (C / A) / (D / B) was 1 or more. As a result, in Samples 7 and 8, the outer circumferential portion 11 after the accelerated degradation test contained contaminants. The sheet sample also contained contaminants.

[0053] (2) Samples 4-1 to 4-7 of pellets 　　　As for Samples 4-1 to 4-7, the above-described Sample 4 was taken as Sample 4-0, and the content D of the antioxidant in the outer circumferential portion 11 was appropriately changed to produce pellets of Samples 4-1 to 4-7. Specifically, in Samples 4-0 to 4-7, the content C of the antioxidant in the central portion 10 was changed to 0.05wt% and the content D of the antioxidant in the outer circumferential portion 11 was changed within a range of 0.0125wt% to 0.0875wt%, while the content of the cross-linking agent in the outer circumferential portion 11 was made less than that in the central portion 10 (B ＜ A). These samples were evaluated as described above. The evaluation results of Samples 4-1 to 4-7 are shown in Table 3.

[0054]

[0055] In Samples 4-1 and 4-2, (C / A) / (D / B) is more than 0.97, and the outer circumferential portion 11 after the accelerated degradation test 2 contained contaminants. The sheet sample also contained contaminants. In these samples, by making the content B of the cross-linking agent in the outer circumferential portion 11 less than the content A in the central portion 10, the cross-linking reaction can be suppressed. However, since the content D of the antioxidant in the outer circumferential portion 11 is low, resin degradation is considered to progress in the outer circumferential portion 11. On the other hand, it is considered that resin degradation can be suppressed by making the contents D in Samples 4-0, and 4-3 to 4-7 more than that in Sample 4-1.

[0056] (3) Samples 9 to 14 of pellets 　　　Samples 9 to 14 were produced by the above-mentioned coating method. Table 4 shows conditions for the resin preparation step, the first molding step, and the second molding step in the coating method. Samples 9 to 14 include the following cross-linking agents: a cross-linking agent in Sample 9, CA2: t-butyldicumyl peroxide, a cross-linking agent in Sample 10, CA3: di(t-butyl peroxide), a cross-linking agent in Sample 11, CA4: 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, a cross-linking agent in Sample 12, CA5: 1,3-bis(t-butylperoxyisopropyl)benzene, a cross-linking agent in Sample 13, CA6: butyl 4,4-bis[(t-butyl)peroxy]pentanoate, and a cross-linking agent in Sample 14, CA7: 1,1-bis(1,1-dimethylethylperoxy)cyclohexane.

[0057]

[0058] Samples 9 to 14 were evaluated by the above-mentioned accelerated degradation test and sheet sample test. Table 5 shows the evaluation results.

[0059]

[0060] (Samples 9 to 14) 　　　As shown in Table 5, in Samples 9 to 14, the content B of the cross-linking agent in the outer circumferential portion 11 was made less than the content A of the cross-linking agent in the central portion 10, and (C / A) / (D / B) was 0.97 or less. Therefore, the outer circumferential portion 11 after the accelerated degradation test contained no contaminant. The sheet sample also contained no contaminant.

[0061] (4) Samples 15 to 22 of pellets 　　　Samples 15 to 22 were produced by the above-mentioned thermal diffusion method. Table 6 shows the conditions for the resin preparation step, the molding step, and the cooling step in the thermal diffusion method.

[0062]

[0063] Samples 15 to 22 were evaluated by the above-mentioned accelerated degradation test and sheet sample test. Table 7 shows the evaluation results of Samples 15 to 22.

[0064] In Samples 15 to 22, the contents of the cross-linking agent in the central portion 10 and the outer circumferential portion 11 were measured by FT-IR. Specifically, each pellet was cut as illustrated in FIG. 2. Then, FT-IR measurement was performed on the cut surface 1a at a position (1b in FIG. 2) at a depth of 0.15 mm from the outer circumferential surface and at a position (1c in FIG. 2) at a depth of 0.90 mm from the outer circumferential surface, to obtain the contents of the cross-linking agent at the positions. The content at the position at the depth of 0.15 mm was taken as the content in the outer circumferential portion 11, and the content at the position at the depth of 0.90 mm was taken as the content in the central portion 10. Further, the contents of the antioxidant in the central portion 10 and the outer circumferential portion 11 were also measured in the same manner as for the cross-linking agent.

[0065]

[0066] (Sample 15, 16) 　　　As shown in Table 7, in Samples 15 and 16, the content of the cross-linking agent in the outer circumferential portion 11 was the same as that in the central portion 10. This is probably because the temperatures in the central portion 10 and the outer circumferential portion 11 changed as illustrated in FIG. 3. FIG. 3 is a diagram illustrating the expected temperature changes in the outer circumferential portion and the central portion when cooling (primary cooling substep and secondary cooling substep) is applied to Sample 16. In the figure, a dashed line indicates the expected temperature change in the central portion 10, and a solid line indicates the expected temperature change in the outer circumferential portion 11.

[0067] Specifically, as illustrated in FIG. 3, since the primary cooling substep was short in time, the temperature in the outer circumferential portion 11 did not decrease sufficiently in the primary cooling substep. Specifically, in the primary cooling substep, the temperatures in the central portion 10 and the outer circumferential portion 11 decreased sharply until 10 seconds after the start of cooling, but the temperatures both in the central portion 10 and the outer circumferential portion 11 were relatively high. In the secondary cooling substep, the temperature decreases gradually both in the central portion 10 and the outer circumferential portion 11, the temperature difference between the outer circumferential portion 11 and the central portion 10 is small, and no thermal diffusion of the cross-linking agent from the outer circumferential portion 11 to the central portion 10 is presumed to occur.

[0068] In Samples 15 and 16, (C / A) / (D / B) was 1, and the outer circumferential portion 11 after the accelerated degradation test contained contaminants. The sheet sample also contained contaminants.

[0069] (Samples 17 to 19) 　　　For Sample 17, the changes in the contents of the cross-linking agent and the antioxidant were measured from the surface of the pellet toward the depth direction, and then the changes illustrated in FIG. 6 were observed. FIG. 6 illustrates the contents of the cross-linking agent and the antioxidant determined by measurement by FT-IR at positions 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 circumferential surface on the cut surface 1a obtained by cutting a pellet as illustrated in FIG. 2. In FIG. 6, a solid line indicates the change in the content of the cross-linking agent, and a dashed line indicates the change in the content of the antioxidant. As illustrated in FIG. 6, in Sample 17, it was observed that the content of the cross-linking agent was lower toward the surface of the pellet and the content of the cross-linking agent was increased toward the center of the pellet. On the other hand, it was observed that the antioxidant exhibited no significant change in the depth direction of the pellet. It was observed that the content of the cross-linking agent exhibited change in the depth direction in Samples 18 and 19 similarly to Sample 17.

[0070] In Samples 17 to 19, a region where the content of the cross-linking agent is not more than the average value for the entire pellet is taken as the outer circumferential portion, and a region where the content is more than the average value is taken as the central portion. The average value for the entire pellet means the content of the cross-linking agent per 100 wt% of low density polyethylene in the entire pellet when the entire pellet of each sample is molten at 80°C and then cooled naturally, which is 3 wt% for the pellets 17 to 19. In Samples 17 to 19, since a region where the content of the cross-linking agent was 3 wt% or less was a region from the surface of the pellet to a depth of 0.3 mm, the region with a thickness of 0.3 mm was taken as the outer circumferential portion, and the region with a diameter of 2.4 mm, excluding the outer circumferential portion, was taken as the central portion.

[0071] As shown in Table 7 and FIG. 6, in Samples 17 to 19, the content of the cross-linking agent at a depth of 0.15 mm from the pellet surface in the outer circumferential portion 11 was less than the content of the cross-linking agent at a depth of 0.90 mm from the pellet surface in the central portion 10. This is probably because the temperatures in the central portion 10 and the outer circumferential portion 11 changed as illustrated in FIG. 4. FIG. 4 is a diagram illustrating the expected temperature changes in the outer circumferential portion 11 and the central portion 10 when cooling (primary cooling substep and secondary cooling substep) is applied to Sample 18. In the figure, a dashed line indicates the expected temperature change in the central portion 10, and a solid line indicates the expected temperature change in the outer circumferential portion 11.

[0072] Specifically, as illustrated in FIG. 4, since the primary cooling substep had appropriate length of time, the temperature in the outer circumferential portion 11 decreased greatly in the primary cooling substep. Specifically, the temperature in the outer circumferential portion 11 decreased to 5°C until 60 seconds after the start of cooling in the primary cooling substep. On the other hand, the temperature in the central portion 10 did not decrease as much as that in the outer circumferential portion 11, and the temperature difference between the outer circumferential portion 11 and the central portion 10 was large. Due to the large temperature difference between the outer circumferential portion 11 and the central portion 10, thermal diffusion of the cross-linking agent from the outer circumferential portion 11 to the central portion 10 is presumed to occur in the primary cooling substep.

[0073] In Samples 17 to 19, the content B of the cross-linking agent in the outer circumferential portion 11 was less than the content A in the central portion 10, and the contents C and D of the antioxidant in the respective portions were the same, and (C / A) / (D / B) was 0.97 or less. Therefore, the outer circumferential portion 11 after the accelerated degradation test contained no contaminant resulting from cross-linking or resin degradation. The sheet sample also contained no contaminant.

[0074] (Samples 20 to 22) 　　　In Samples 20 to 22, the content B of the cross-linking agent in the outer circumferential portion 11 was the same as the content A in the central portion 10, as shown in Table 7. This is probably because the temperatures in the central portion 10 and the outer circumferential portion 11 changed as illustrated in FIG. 5. FIG. 5 is a diagram illustrating the expected temperature changes in the outer circumferential portion and the central portion when cooling (primary cooling substep and secondary cooling substep) is applied to Sample 20. In the figure, a dashed line indicates the expected temperature change in the central portion 10, and a solid line indicates the expected temperature change in the outer circumferential portion 11.

[0075] Specifically, in the primary cooling substep, the temperatures in the central portion 10 and the outer circumferential portion 11 decreased to 5°C until 300 seconds after the start of cooling, as illustrated in FIG. 5. In this event, the temperature in the central portion 10 decreased more slowly than that in the outer circumferential portion 11: the temperature in the outer circumferential portion 11 decreased to 5°C, and after some time elapsed, the temperature in the central portion 10 also decreased to 5°C. Therefore, there was a period in which the temperature difference between the outer circumferential portion 11 and the central portion 10 became large, which caused thermal diffusion of the cross-linking agent from the outer circumferential portion 11 to the central portion 10. On the other hand, in the secondary cooling substep, the temperature in the outer circumferential portion 11 rose from 5°C to 25°C, and after some time elapsed, the temperature in the central portion 10 also rose to 25°C. During this process, the temperature in the outer circumferential portion 11 became higher than the temperature in the central portion 10, generating a predetermined temperature difference. Accordingly, thermal diffusion of the cross-linking agent occurs from the central portion 10 to the outer circumferential portion 11. Consequently, during the cooling step, no thermal diffusion of the cross-linking agent is presumed to occur.

[0076] In Samples 20 to 22, (C / A) / (D / B) was more than 0.97, and the outer circumferential portion 11 after the accelerated degradation test contained contaminants. The sheet sample also contained contaminants.

[0077] As described above, the pellet disclosed in the present specification is a pellet including a resin component containing polyethylene, and a cross-linking agent, the pellet including a central portion and an outer circumferential portion located on an outer circumference of the central portion, wherein a content of the cross-linking agent and a content of the antioxidant in each of the central portion and the outer circumferential portion satisfy the relational expression: (C / A) / (D / B) ≦ 0.97.

[0078] Thus, the pellet is less likely to degrade upon a long term storage. The power cable produced using the pellet has a good electrical insulation. Since the method of producing the power cable is disclosed, for example, in Japanese Patent Laid-Open Publication No. 2020-132817 and US2020 / 0270426, it is not explained here.

[0079] The content of the cross-linking agent in the outer circumferential portion 11 is not restricted to 2.85 wt% disclosed for Sample 6. It will be understood that the pellet in which the content B of the cross-linking agent in the outer circumferential portion 11 is less than the content A in the central portion 10 is less likely to degrade upon a long-term storage, compared to the pellet in which the amount of the cross-linking agent in the outer circumferential portion 11 is the same as that in the central portion 10.

[0080] However, the pellet 1 in which the outer circumferential portion 11 includes a large amount of the cross-linking agent may degrade in a short period even when the content of the cross-linking agent in the outer circumferential portion 11 is less than that in the central portion 10. Accordingly, the content B of the cross-linking agent in the outer circumferential portion 11 is desirably 0 wt% or more and less than 10 wt%, or 0 wt% or more and 9 wt% or less, with respect to the resin component constituting the outer circumferential portion 11 being 100 wt%.

[0081] In order to sufficiently cross-link the resin component, the content A of the cross-linking agent in the central portion 10 may be 1 wt% or more with respect to the resin component constituting the central portion being 100 wt%. In order to prevent the resin component after cross-linking from containing a large amount of by-products of the cross-linking, the content A of the cross-linking agent in the central portion 10 may be 1 wt% or more and 10 wt% or less with respect to the resin component constituting the central portion being 100 wt%.

[0082] In the pellet 1, the content B of the cross-linking agent in the outer circumferential portion 11 is preferably less than the content A in the central portion 10 from the viewpoint of satisfying the above expression (1) and more reliably suppressing cross-linking in the outer circumferential portion 11. Alternatively, the content D of the antioxidant in the outer circumferential portion 11 is preferably more than the content C in the central portion 10.

[0083] In the pellet 1, the ratio B / A between the content A of the cross-linking agent in the central portion and the content B of the cross-linking agent in the outer circumferential portion is not limited to the values in Examples. When the pellet 1 is prepared by the coating method, each of the content A and the content B can be adjusted, and therefore the ratio B / A may be adjusted, for example, to satisfy 0 ≦ B / A ≦ 0.9. On the other hand, when the pellet 1 is produced by the thermal diffusion method, the content A and the content B may be adjusted by thermal diffusion so that B / A ≦ 0.9. The ratio B / A may be adjusted, for example, to satisfy 0.6≦B / A≦0.9, although the lower limit is not particularly limited.

[0084] The content of the cross-linking agent included in the pellet 1, that is, the total amount of the cross-linking agent included in the central portion 10 and the outer circumferential portion 11 may be 0.3 wt% or more and 5.0 wt% or less with respect to the resin component constituting the pellet 1 being 100 wt%. With excessively small amount of the cross-linking agent, a desired degree of cross-linking cannot be achieved when the insulation layer is formed from the pellet 1, and a predetermined insulation properties cannot be obtained in some cases. By setting each content within the above-described range, an insulation layer having excellent insulation properties can be formed while improving the long-term preservability of the pellet 1.

[0085] In the pellet 1, the content D of the antioxidant included in the outer circumferential portion 11 may be 0.031 wt% or more and 0.0875 wt% or less from the viewpoint of more reliably suppressing resin degradation in the outer circumferential portion 11. The content C of the antioxidant included in the central portion 10 is not particularly limited so long as the above expression (1) is satisfied. On the other hand, the content C may be 0.031 wt% or more and 0.0875 wt% or less from the viewpoint of satisfying the expression (1).

[0086] A thickness of the outer circumferential portion 11 is not restricted to 0.3 mm disclosed in Examples. In order to prevent the central portion 10 from degrading, it is enough for the outer circumferential portion 11 to have a thickness of 0.1 mm or more and 1 mm or less. A size (diameter) of the central portion 10 is not restricted to 2.4 mm disclosed in Examples. The diameter of the central portion 10 is preferably 1.8 mm or more and 6.0 mm or less. The diameter of the pellet 1 is not restricted to 3 mm disclosed in Examples. The diameter of the pellet 1 is preferably 2.0 mm or more and 8.0 mm or less.

[0087] For the contents of the cross-linking agent in each of the outer circumferential portion and the central portion in the pellet produced by the thermal diffusion method, positions at depths of 0.15 mm and 0.9 mm from the pellet surface were respectively selected in Examples, but not limited thereto. As the measurement position of the outer circumferential portion, the middle of the thickness of the outer circumferential portion may be selected. As the measurement position of the central portion, the middle between the surface of the central portion and the center may be selected.

[0088] 1　　　Pellet 10　　　Central portion 11　　　Outer circumferential portion

Claims

1. A pellet including a resin component containing polyethylene, a cross-linking agent, and an antioxidant, 　　　the pellet comprising a central portion and an outer circumferential portion located on an outer circumference of the central portion, 　　　wherein a content of the cross-linking agent and a content of the antioxidant in each of the central portion and the outer circumferential portion satisfy the following expression (1): 　　　(C / A) / (D / B) ≦ 0.97 ・・・ (1) 　　　where 　　　A: the content of the cross-linking agent per 100 wt% of the resin component constituting the central portion; 　　　B: the content of the cross-linking agent per 100 wt% of the resin component constituting the outer circumferential portion; 　　　C: the content of the antioxidant per 100 wt% of the resin component constituting the central portion; 　　　D: the content of the antioxidant per 100 wt% of the resin component constituting the outer circumferential portion.

2. The pellet according to claim 1, 　　　wherein the A and the B satisfy the relation: B ＜ A.

3. The pellet according to claim 1 or 2, 　　　wherein the B is less than 10 wt%.

4. The pellet according to any one of claims 1 to 3, 　　　wherein the A and the B satisfy the relation: B / A ≦ 0.9.

5. The pellet according to any one of claims 1 to 3, 　　　wherein the outer circumferential portion does not include the cross-linking agent.

6. The pellet according to any one of claims 1 to 5, 　　　wherein the A is 1 wt% or more and 10 wt% or less.

7. The pellet according to any one of claims 1 to 6, 　　　wherein the content of the cross-linking agent included in the pellet is 0.3 wt% or more and 5.0 wt% or less with respect to the resin component constituting the pellet being 100 wt%.

8. The pellet according to any one of claims 1 to 7, 　　　wherein the D is 0.031 wt% or more and 0.0875 wt% or less.

9. The pellet according to any one of claims 1 to 8, 　　　wherein a thickness of the outer circumferential portion is 0.1 mm or more and 1 mm or less.

10. The pellet according to any one of claims 1 to 9, 　　　wherein each of the A, B, C, and D is calculated by measuring a cut surface, cut out from the pellet, by FT-IR or Raman scattering method.

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