Pellet

Abstract

To improve a pellet containing an insulating resin, a crosslinking agent and an antioxidant.SOLUTION: A pellet contains a resin component containing polyethylene, a crosslinking agent and an antioxidant, and has a central part and an outer peripheral part positioned on an outer periphery of the central part, wherein when a content of the crosslinking agent per 100 wt.% of the resin component constituting the central part is represented by A, a content of the crosslinking agent per 100 wt.% of the resin component constituting the outer peripheral part is represented by B, a content of the antioxidant per 100 wt.% of the resin component constituting the central part is represented by C, and a content of the antioxidant per 100 wt.% of the resin component constituting the outer peripheral part is represented by D, (C / A) / (D / B)≤0.97 is satisfied.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, a cross-linking agent, and an antioxidant. The insulating resin is, for example, 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 cross-links part of the insulating resin, forming cross-linked foreign matter. Furthermore, when the pellets are exposed to air, their resin components may deteriorate and discolor. The progression of cross-linking in the pellets or resin deterioration may result in the formation of foreign matter 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, if the cross-linked insulation layer is formed from pellets containing foreign matter, the cross-linked insulation layer contains foreign matter. The inventors have found that when the pellet is stored for a long period of time, the foreign matter causes a decrease in the electrical insulating properties of the insulating layer.

[0006] An object of the present disclosure is to provide an improved pellet containing an insulating resin, a cross-linking agent, and an antioxidant, and in particular to provide a pellet that can be stored for a long period of time.

[0007] According to one aspect of the present disclosure, A pellet containing a resin component containing polyethylene, a crosslinking agent, and an antioxidant, a central portion and an outer peripheral portion located on the outer periphery of the central portion, the content of the crosslinking agent in each of the central portion and the peripheral portion and the content of the antioxidant in each of the central portion and the peripheral portion satisfy the following formula (1): pellet. (C / A) / (D / B)≦0.97 (1) where: A: The content of the crosslinking agent per 100 wt% of the resin component constituting the central portion B: Content of the crosslinking agent per 100 wt% of the resin component constituting the outer periphery 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 periphery is. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to suppress deterioration of pellets containing an insulating resin and a cross-linking agent due to contact with air over a long period of time. [Brief explanation of the drawings]

[0009] [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 the crosslinking agent and the antioxidant in the pellet of Sample 17 in the depth direction. DETAILED DESCRIPTION OF THE INVENTION

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

[0011] [1] One aspect of the present disclosure is A pellet containing a resin component containing polyethylene, a crosslinking agent, and an antioxidant, It includes a central portion and an outer peripheral portion located on the outer periphery of the central portion, where the content of the crosslinking agent and the content of the antioxidant in each of the central portion and the outer peripheral portion satisfy the following formula (1): Pellets. (C / A) / (D / B)≦0.97···(1) Here, A: The content of the crosslinking agent per 100 wt% of the resin component constituting the central portion B: The content of the crosslinking agent per 100 wt% of the resin component constituting the outer peripheral 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 peripheral portion is. According to this configuration, deterioration of the pellets due to contact with air can be suppressed, and the long-term storage stability of the pellets can be improved.

[0012] [2] In the pellets according to [1] above, A and B satisfy the relationship B < A. According to this configuration, the long-term storage stability of the pellets can be maintained at a higher level.

[0013] [3] In the pellets according to [1] or [2] above, B is less than 10 wt%. According to this configuration, the long-term storage stability of the pellets can be maintained at a higher level.

[0014] [4] In the pellets according to any one of [1] to [3] above, A and B satisfy the relationship B / A≦0.9, According to this configuration, the long-term storage stability of the pellets can be maintained at a higher level.

[0015] [5] In the pellets according to any one of [1] to [3] above, the outer peripheral portion does not contain the crosslinking agent, This configuration allows the pellets to maintain a high level of long-term storage stability.

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

[0017] [7] The pellet according to any one of [1] to [6] above, The content of the crosslinking agent contained in the pellet is 0.3 wt % or more and 5.0 wt % or less with respect to 100 wt % of the resin component constituting the pellet. This configuration allows the pellets to maintain high long-term storage stability while achieving a predetermined degree of crosslinking when crosslinked.

[0018] [8] The pellet according to any one of [1] to [7] above, The D is 0.031 wt% or more and 0.0875 wt% or less. This configuration allows the pellets to maintain a high level of long-term storage stability.

[0019] [9] The pellet according to any one of [1] to [8] above, The thickness of the outer peripheral portion is 0.1 mm or more and 1 mm or less. This configuration allows the pellets to maintain a high level of long-term storage stability.

[0020]

[10] The pellet according to any one of [1] to [9] above, Each of A, B, C, and D is calculated by measuring a cross section of the pellet by FT-IR or Raman scattering. This configuration allows the pellets to maintain a high level of long-term storage stability.

[0021] [Details of the embodiments of the present disclosure] An embodiment of the present disclosure will be described below. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. Furthermore, wt% indicates mass %.

[0022] [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.

[0023] Pellet 1 includes a resin containing polyethylene, a crosslinking agent, and an antioxidant. The crosslinking 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. Pellet 1 further includes additives other than the crosslinking agent and the antioxidant. The additives are, for example, inorganic fillers and lubricants. The crosslinking agent, antioxidant, and other additives 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 will not be described here.

[0024] The central portion 10 and the peripheral portion 11 are configured so that the amounts of the cross-linking agent and antioxidant contained therein satisfy the following formula (1). (C / A) / (D / B)≦0.97 (1) Here, A is the content (wt%) of crosslinking agent per 100 wt% of the resin component that constitutes the central portion 10. B is the content (wt%) of crosslinking agent per 100 wt% of the resin component that constitutes the outer peripheral portion 11. C is the content (wt%) of antioxidant per 100 wt% of the resin component that constitutes the central portion 10. D is the content (wt%) of the antioxidant per 100 wt% of the resin component that constitutes the outer peripheral portion 11.

[0025] Formula (1) is a parameter indicating the ratio of the crosslinking agent and antioxidant contained in each of the central portion 10 and the outer peripheral portion 11. When A to D satisfy formula (1), the content B of the crosslinking agent in the outer peripheral portion 11 is less than the content A of the crosslinking agent in the central portion 10, or the content D of the antioxidant in the outer peripheral portion 11 tends to be more than the content C of the antioxidant in the central portion 10. Therefore, the outer peripheral portion 11 is suppressed from crosslinking due to contact between the crosslinking agent and air and resin deterioration due to contact with air. On the other hand, since the central portion 10 is covered with the outer peripheral portion 11, crosslinking and resin deterioration due to contact between the crosslinking agent and air are suppressed. Therefore, the pellet 1 has excellent long-term storage stability.

[0026] From the viewpoint of satisfying the above formula (1) in the pellet 1, it is preferable that the content of the crosslinking agent in the outer peripheral portion 11 is less than the content of the crosslinking agent in the central portion 10. That is, it is preferable that B < A. Further, the content of the antioxidant is not particularly limited as long as formula (1) is satisfied. For example, the content C of the antioxidant in the central portion 10 and the content D of the antioxidant in the outer peripheral portion 11 may be appropriately adjusted so as to satisfy formula (1) depending on the numerical values of A and B.

[0027] This pellet 1 is manufactured by a coating method. The coating method has the following steps. · Resin preparation step · First molding step · Second molding step

[0028] (Resin preparation step) In the resin preparation step, first, 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 contains polyethylene, a crosslinking agent, and an antioxidant. The second composition satisfies any one of the following conditions (a) to (c). [[ID=…]] (a) The second composition contains polyethylene. The second composition contains an antioxidant but does not contain a crosslinking agent. (b) The second composition includes polyethylene. The second composition includes a crosslinker and an antioxidant, the amount of the crosslinker in the second composition being less than the amount of the crosslinker in the first composition, and the amount of the antioxidant being in a range that satisfies the above formula (1). (c) The second composition includes polyethylene. The second composition includes a crosslinker and an antioxidant, the amount of the antioxidant in the second composition being greater than the amount of the antioxidant in the first composition, and the amount of the crosslinker in the second composition being within a range that satisfies the above formula (1).

[0029] (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.

[0030] (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.

[0031] As described above, the pellet of embodiment 1 has a central portion 10 containing the first composition and an outer portion 11 containing the second composition surrounding the central portion 10. The pellet 1 is configured so that the contents of the crosslinking agent and the antioxidant in each of the central portion 10 and the outer portion 11 satisfy the above formula (1).

[0032] [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

[0033] (Resin preparation step) First, a first composition is prepared, which includes polyethylene, a crosslinking agent, and an antioxidant.

[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 sub-step) Immediately after the primary cooling sub-step, a secondary cooling sub-step is performed. In the secondary cooling sub-step, pellet 1 is placed in the air at the secondary cooling temperature. The secondary cooling temperature is, for example, from 10°C to 40°C. Thereby, pellet 1 is gradually cooled. When pellet 1 is gradually cooled, the temperature difference between the central portion 10 and the outer peripheral portion 11 of pellet 1 is maintained for a certain period. Due to this temperature difference, the crosslinking agent thermally diffuses within pellet 1. Specifically, the crosslinking agent diffuses from the cooled outer peripheral portion 11 to the high-temperature central portion 10. As a result, the amount of the crosslinking agent in the central portion 10 becomes larger than the amount of the crosslinking agent in the outer peripheral portion 11. On the other hand, the antioxidant is difficult to thermally diffuse, and the amount of the antioxidant is the same between the central portion 10 and the outer peripheral portion 11. That is, B < A and C ≈ D, and the formula (1) is satisfied.

[0038] Note that the amounts of the crosslinking agent and the antioxidant in the central portion 10 and the outer peripheral portion 11 can be measured by FT-IR or Raman scattering method.

[0039] [[ID=H11]] Also, the outer diameter, volume of pellet 1 can be appropriately set.

[0040] [Summary of the embodiment] According to pellet 1, since the contents of the crosslinking agent and the antioxidant in the central portion 10 and the outer peripheral portion 11 satisfy the above formula (1), the progress of crosslinking due to long-term contact with air and resin deterioration can be suppressed, and the long-term storage property is excellent. For example, when the following accelerated deterioration test is performed, the formation of foreign matters is less, and even when a sheet sample is formed using pellet 1 after long-term storage, the formation of foreign matters is less. [Examples]

[0041] Next, examples according to the present disclosure will be described. These examples are an example 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 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 the following crosslinkers and antioxidants. Crosslinking agent CA1: Dicumyl peroxide (hereinafter referred to as DCP) Antioxidant AO1: 4,4′-thiobis(3-methyl-6-t-butylphenol)

[0043] [Table 1]

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

[0045] [Evaluation method 1: Accelerated aging test 1] The pellets of Samples 1 to 8 were evaluated by accelerated aging test 1. In accelerated aging test 1, each pellet was stored in a constant temperature bath under the following storage conditions. Temperature of the thermostatic chamber: 80℃ Temperature chamber atmosphere: air Storage time in the incubator: 48 hours

[0046] Next, the pellets after storage for a predetermined time were cut as shown in Figure 2 to obtain a cut surface 1a. The following observation portions of the cut surface 1a were observed with an optical microscope to confirm foreign matter. The observation portion was one location 0.15 mm from the outer periphery 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 portion 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.

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

[0048] [Evaluation method 3: Sheet sample test] Sheet samples were prepared from the pellets after the accelerated aging test 2. The pellets were heated to 120°C to melt them, and extruded into a sheet with a thickness of 1 mm. The sheet was then held at 180°C to prepare a crosslinked sheet sample. The surface of the sheet sample was observed with an optical microscope to check for foreign matter. The foreign matter was identified using the same criteria as in the accelerated aging test. In this example, a sheet sample with no foreign matter observed on its surface was rated as a passing product with a grade of "2B," and a sheet sample with foreign matter observed was rated as a failing product with a grade of "1B."

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

[0050] [Table 2]

[0051] [Samples 1-6] In Samples 1 to 6, as shown in Table 2, the antioxidant content C in the central portion 10 and the antioxidant content D in the peripheral portion 11 were the same, while the crosslinking agent content B in the peripheral portion 11 was less than the crosslinking agent content A in the central portion 10, so that (C / A) / (D / B) was 0.97 or less. Therefore, the peripheral portion 11 did not contain any foreign matter after the accelerated aging tests 1 and 2. The sheet samples also did not contain any foreign matter. Furthermore, in Samples 1 to 6, it is preferable that the crosslinking agent contents A and B satisfy the relationship 0≦B / A≦0.90.

[0052] [Samples 7 and 8] Sample 7 had the same antioxidant contents C and D, and the same crosslinking agent contents A and B in the central portion 10 and the outer peripheral portion 11. Sample 8 had the same antioxidant contents C and D, and the crosslinking agent content B in the outer peripheral portion 11 was made higher than the crosslinking agent content A in the central portion 10. As a result, (C / A) / (D / B) became 1 or more. As a result, in Samples 7 and 8, the outer peripheral portion 11 after the accelerated degradation test contained foreign substances. The sheet sample also contained foreign substances.

[0053] (2) Pellet Samples 4-1 to 4-7 For Samples 4-1 to 4-7, with the above Sample 4 as Sample 4-0, the antioxidant content D in the outer peripheral portion 11 was appropriately changed to prepare pellets of Samples 4-1 to 4-7. Specifically, for Samples 4-0 to 4-7, while making the crosslinking agent content in the outer peripheral portion 11 less than that in the central portion 10 (B < A), the antioxidant content C in the central portion 10 was 0.05 wt%, and the antioxidant content D in the outer peripheral portion 11 was changed within the range of 0.0125 wt% to 0.0875 wt%. The above evaluations were performed on these samples. The evaluation results of Samples 4-1 to 4-7 are shown in Table 3.

[0054]

Table 3

[0055] In Samples 4-1 and 4-2, (C / A) / (D / B) exceeded 0.97, and the outer peripheral portion 11 after the accelerated degradation test 2 contained foreign substances. The sheet sample also contained foreign substances. In these samples, although the crosslinking reaction could be suppressed by making the crosslinking agent content B in the outer peripheral portion 11 less than the content A in the central portion 10, it is considered that resin degradation progressed in the outer peripheral portion 11 due to the low antioxidant content D in the outer peripheral portion 11. On the other hand, in Samples 4-0, 4-3 to 4-7, it is considered that resin degradation could be suppressed by making the content D higher than that in Sample 4-1 and the like.

[0056] (3) Pellet Samples 9 to 14 Samples 9 to 14 were produced by the coating method described above. Table 4 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 following crosslinking agents: 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

[0057] [Table 4]

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

[0059] [Table 5]

[0060] [Samples 9-14] As shown in Table 5, in samples 9 to 14, the content B of crosslinking agent in the outer peripheral portion 11 was less than the content A of crosslinking agent in the central portion 10, and (C / A) / (D / B) was 0.97 or less. Therefore, the outer peripheral portion 11 after the accelerated aging test did not contain any foreign matter. Furthermore, the sheet sample also did not contain any foreign matter.

[0061] (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.

[0062] [Table 6]

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

[0064] In addition, for Samples 15 to 22, the content of the crosslinking agent in the central portion 10 and the peripheral portion 11 was measured by FT-IR. Specifically, each pellet was cut as shown in FIG. 2, and the crosslinking agent content at each position was determined 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. 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 antioxidant content in the central portion 10 and the peripheral portion 11 was also measured in the same manner as the crosslinking agent.

[0065] [Table 7]

[0066] [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.

[0067] 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.

[0068] In Samples 15 and 16, the (C / A) / (D / B) ratio was 1, and the outer peripheral portion 11 contained foreign matter after the accelerated deterioration test. The sheet sample also contained foreign matter.

[0069] [Samples 17-19] For Sample 17, we measured the changes in the crosslinker and antioxidant content from the pellet surface toward the depth direction, and the results were as shown in Figure 6. Figure 6 shows the crosslinker and antioxidant 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, after cutting the pellet as shown in Figure 2. In Figure 6, the solid line indicates the change in crosslinker content, and the dashed line indicates the change in antioxidant content. As shown in Figure 6, for Sample 17, the crosslinker content was lower near the pellet surface and increased toward the center. On the other hand, the antioxidant content did not change significantly toward the pellet depth. Similar changes in the crosslinker content toward the depth direction were also observed for Samples 18 and 19.

[0070] In Samples 17 to 19, the region where the crosslinking agent content was below the average value for the entire pellet was defined as the outer periphery, and the region where the crosslinking agent content exceeded the average value was defined as the central region. The average value for the entire pellet refers to the crosslinking agent content per 100 wt% of low-density polyethylene in the entire pellet when the entire pellet of each sample was melted at 80°C and allowed to cool naturally, and for Pellets 17 to 19, it was 3 wt%. In Samples 17 to 19, the region where the crosslinking agent content was 3 wt% or less was 0.3 mm from the surface, so the region with a thickness of 0.3 mm 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.

[0071] 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.

[0072] 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.

[0073] In samples 17 to 19, the crosslinking agent content B in the outer peripheral portion 11 was lower than the content A in the central portion 10, and the antioxidant contents C and D in each portion were the same, with (C / A) / (D / B) being 0.97 or less. Therefore, the outer peripheral portion 11 after the accelerated aging test did not contain any foreign matter associated with crosslinking or resin degradation. The sheet sample also did not contain any foreign matter.

[0074] [Samples 20-22] In Samples 20 to 22, as shown in Table 7, the content B of crosslinking agent in the outer peripheral portion 11 was the same as the content A 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.

[0075] 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.

[0076] In Samples 20 to 22, the (C / A) / (D / B) ratio exceeded 0.97, and the outer peripheral portion 11 contained foreign matter after the accelerated deterioration test. The sheet sample also contained foreign matter.

[0077] From the above, the pellets disclosed in the present specification are A pellet containing a resin component containing polyethylene and a crosslinking agent, a central portion and an outer peripheral portion located on the outer periphery of the central portion, the content of the crosslinking agent and the content of the antioxidant in each of the central portion and the peripheral portion satisfy the relational expression (C / A) / (D / B)≦0.97; pellet.

[0078] Therefore, the pellets are less likely to deteriorate even when stored for a long period of time. Furthermore, power cables manufactured using such pellets have good 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.

[0079] The content of crosslinking agent in the outer portion 11 is not limited to 2.85 wt% or less as disclosed in Sample 6. It will be understood that a pellet in which the content B of crosslinking agent in the outer portion 11 is less than the content A in the central portion 10 is less likely to deteriorate when stored for a long period of time than a pellet in which the amount of crosslinking agent in the outer portion 11 and the central portion 10 is the same.

[0080] However, if the outer peripheral portion 11 contains a large amount of cross-linking agent, the pellet 1 may deteriorate in a short time even if the content of cross-linking agent in the outer peripheral portion 11 is less than that in the central portion 10. Therefore, the content B of cross-linking agent in the outer peripheral portion 11 is preferably 0 wt% or more and less than 10 wt%, or 0 wt% or more and 9 wt% or less, relative to 100 wt% of the resin component constituting the outer peripheral portion 11.

[0081] In order to sufficiently crosslink the resin component, the content A of the crosslinking agent in the central portion 10 is preferably 1 wt% or more relative to 100 wt% of the resin component constituting the central portion. Also, 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 wt% or more and 10 wt% or less relative to 100 wt% of the resin component constituting the central portion.

[0082] In the pellet 1, from the viewpoint of satisfying the above formula (1) and more reliably suppressing crosslinking in the outer peripheral portion 11, it is preferable that the content B of the crosslinking agent in the outer peripheral portion 11 is less than the content A in the central portion 10. Alternatively, it is preferable that the content D of the antioxidant in the outer peripheral portion 11 is greater than the content C in the central portion 10.

[0083] In pellet 1, the ratio B / A of the content A of crosslinking agent in the central portion to the content B of crosslinking agent in the peripheral portion is not limited to the numerical values ​​in the examples. When pellet 1 is produced by a coating method, the content A and the content B can be adjusted independently, and the ratio B / A may be adjusted to satisfy, for example, 0≦B / A≦0.9. On the other hand, when pellet 1 is produced by a thermal diffusion method, the content A and the content B may be adjusted by thermal diffusion so that B / A≦0.9. Although the lower limit is not particularly limited, the ratio B / A may be adjusted to satisfy, for example, 0.6≦B / A≦0.9.

[0084] The content of cross-linking agent contained in pellet 1, that is, the total amount of cross-linking agent contained in central portion 10 and peripheral portion 11, is preferably 0.3 wt% or more and 5.0 wt% or less, relative to 100 wt% of the resin components constituting pellet 1. If the amount of cross-linking agent is too small, the desired degree of cross-linking may not be achieved when forming an insulating layer from pellet 1, and the desired insulating properties may not be obtained. By setting the content of each component within the above range, it is possible to form an insulating layer with excellent insulating properties while improving the long-term storage stability of pellet 1.

[0085] In the pellet 1, from the viewpoint of more reliably suppressing resin deterioration in the outer peripheral portion 11, the content D of the antioxidant contained in the outer peripheral portion 11 is preferably 0.031 wt% or more and 0.0875 wt% or less. On the other hand, the content C of the antioxidant contained in the central portion 10 is not particularly limited as long as it satisfies the above formula (1). From the viewpoint of satisfying formula (1), the content C is preferably 0.031 wt% or more and 0.0875 wt% or less.

[0086] 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.

[0087] 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]

[0088] 1 pellet 10 Center part 11 Outer area

Claims

1. A pellet containing a resin component containing polyethylene, a crosslinking agent, and an antioxidant, a central portion and an outer peripheral portion located on the outer periphery of the central portion, the content of the crosslinking agent in each of the central portion and the peripheral portion and the content of the antioxidant in each of the central portion and the peripheral portion satisfy the following formula (1): pellet. (C / A) / (D / B)≦0.97...(1) where: A: The content of the crosslinking agent per 100 wt % of the resin component constituting the central portion B: Content of the crosslinking agent per 100 wt % of the resin component constituting the outer periphery 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 is.

2. A and B satisfy the relationship B<A, The pellet of claim 1.

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

4. A and B satisfy the relationship of B / A≦0.

9. The pellet according to claim 1 or claim 2.

5. The outer peripheral portion does not contain the crosslinking agent. The pellet according to claim 1 or claim 2.

6. The A is 1 wt% or more and 10 wt% or less, The pellet according to claim 1 or claim 2.

7. the content of the crosslinking agent contained in the pellet is 0.3 wt % or more and 5.0 wt % or less with respect to 100 wt % of the resin component constituting the pellet; The pellet according to claim 1 or claim 2.

8. The D is 0.031 wt% or more and 0.0875 wt% or less, The pellet according to claim 1 or claim 2.

9. 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.

10. Each of A, B, C, and D is calculated by measuring a cut surface of the pellet by FT-IR or Raman scattering. The pellet according to claim 1 or claim 2.

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