Pellet

Abstract

To improve a pellet containing an insulation resin and a crosslinking agent.SOLUTION: A pellet contains a resin component containing polyethylene and a crosslinking agent, and includes a central part and an outer peripheral part positioned on the outer periphery of the central part, wherein the content of the crosslinking agent in the outer peripheral part is smaller than the content of the crosslinking agent in the central 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] JP 2020-132817 A Summary of the Invention [Problem to be solved by the invention]

[0005] The power cable has an electrical insulating layer. Hereinafter, the electrical insulating layer is referred to as an "insulating layer". The insulating layer is formed from pellets containing an insulating resin and a cross-linking agent. The insulating resin is, for example, polyethylene. The pellets are manufactured and stored before manufacturing the power cable. When the pellets are stored for a long period of time, the pellets are exposed to air for a long period of time. When the pellets are exposed to air for a long period of time, a part of the insulating resin in the surface layer of the pellet is cross-linked by the cross-linking agent contained in the surface layer of the pellet, and a cross-linked foreign matter is formed. Hereinafter, the cross-linked foreign matter is simply referred to as a foreign matter. In addition, when the insulating layer of the power cable is manufactured, the pellets are heated. When the pellets are heated, the insulating resin contained in the pellet is cross-linked by the cross-linking agent. Therefore, the power cable has a cross-linked insulating layer. However, when the cross-linked insulating layer is formed from pellets containing foreign matter, the cross-linked insulating layer contains foreign matter. The inventor has found that when the pellets are stored for a long period of time, the foreign matter causes a decrease in the electrical insulation of the insulating layer.

[0006] It is an object of the present disclosure to provide an improved pellet comprising an insulating resin and a cross-linking agent, 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 comprising a resin component including polyethylene and a crosslinking agent, A central portion and an outer peripheral portion located on an outer periphery of the central portion, The content of the crosslinking agent in the outer peripheral portion is less than the content of the crosslinking agent in the central portion. Pellets are provided. Effect of the Invention

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

[0009] [Figure 1] FIG. 1 is a schematic diagram of a pellet according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a diagram for explaining an evaluation method in an accelerated deterioration test of pellets. [Diagram 3] FIG. 3 is a diagram showing predicted temperature transitions in the peripheral and central parts of Sample 16 when it is cooled. [Figure 4] FIG. 4 is a diagram showing predicted temperature transitions in the peripheral and central parts of Sample 18 when it is cooled. [Diagram 5] FIG. 5 is a diagram showing predicted temperature transitions in the peripheral and central parts of sample 20 when the sample 20 is cooled. [Figure 6] FIG. 6 is a diagram showing the change in the content of the cross-linking agent in the pellet of Sample 17 in the depth direction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 comprising a resin component including polyethylene and a crosslinking agent, A central portion and an outer peripheral portion located on an outer periphery of the central portion, The content of the crosslinking agent in the outer peripheral portion is less than the content of the crosslinking agent in the central portion. It is a pellet. 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 pellet according to the above [1], The content of the crosslinking agent in the outer peripheral portion is less than 10 wt % relative to 100 wt % of the resin component constituting the outer peripheral portion. This configuration allows the long-term storage stability of the pellets to be maintained at a higher level.

[0013] [3] In the pellet according to the above [1] or [2], When the content of the crosslinking agent in the central portion is A and the content of the crosslinking agent in the peripheral portion is B, B / A≦0.9. This configuration allows the long-term storage stability of the pellets to be maintained at a higher level.

[0014] [4] In the pellet according to the above [1], The outer peripheral portion does not contain the crosslinking agent. This configuration allows the long-term storage stability of the pellets to be maintained at a higher level.

[0015] [5] The pellet according to any one of [1] to [4] above, The content of the crosslinking agent in the central portion is 1 wt % or more and 10 wt % or less with respect to 100 wt % of the resin component constituting the central portion. According to this configuration, the content of the crosslinking agent in the entire pellet can be adjusted.

[0016] [6] The pellet according to any one of [1] to [5] above, The content of the crosslinking agent contained in the pellet is 1 wt % or more and 10 wt % or less with respect to 100 wt % of the resin component constituting the pellet. According to this configuration, it is possible to achieve a predetermined degree of crosslinking when crosslinked while maintaining the long-term storage stability of the pellets at a high level.

[0017] [7] The pellet according to any one of [1] to [6] above, The thickness of the outer circumferential portion is not less than 0.1 mm and not more than 1 mm. This configuration allows the long-term storage stability of the pellets to be maintained at a higher level.

[0018] [8] The pellet according to any one of [1] to [7] above, The content of the crosslinking agent in each of the central portion and the peripheral portion is calculated by measuring a cut surface of the pellet by FT-IR or Raman scattering method. This configuration allows the long-term storage stability of the pellets to be maintained at a higher level.

[0019] [Details of the embodiment of the present disclosure] Hereinafter, one embodiment of the present disclosure will be described. Note that the present disclosure is not limited to these examples, but is indicated by the claims, and is intended to include all modifications within the meaning and scope of the claims. In addition, wt% indicates mass%.

[0020] [Embodiment 1] FIG. 1 shows a pellet 1 according to a 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.

[0021] The pellet 1 contains a resin containing polyethylene and a crosslinking agent. The crosslinking agent is, for example, an organic peroxide. The pellet 1 further contains an additive. The additive is, for example, an inorganic filler, a crosslinking agent, an antioxidant, and a lubricant. The crosslinking agent and the additive are disclosed in, for example, JP 2020-132819, US2020 / 279672A, JP 2020-132818, US2020 / 273598A, JP 2020-132817, US2020 / 270426A, JP 2019-189842, and US2021 / 032434A, so the description will be omitted.

[0022] Outer peripheral portion 11 is formed so that the content of cross-linking agent is less than the content of cross-linking agent in central portion 10. Therefore, in outer peripheral portion 11, the cross-linking agent is less likely to react with air and is less likely to deteriorate. In addition, central portion 10 is covered by outer peripheral portion 11 and is less likely to come into contact with air. Therefore, in central portion 10, the cross-linking agent is less likely to react with air and is less likely to deteriorate.

[0023] The pellets 1 are produced by a coating method, which comprises the following steps: Resin preparation step First molding step Second molding step

[0024] (Resin preparation step) In the resin preparation step, first, a first composition and a second composition are prepared. The first composition is for forming a central portion 10. The second composition is for forming an outer peripheral portion 11. The first composition contains polyethylene and a crosslinking agent. The second composition satisfies the following condition (a) or (b). (a) The second composition includes polyethylene. The second composition does not include a crosslinker. (b) the second composition comprises polyethylene, the amount of crosslinker in the second composition being less than the amount of crosslinker in the first composition; Thus, the amount of cross-linking agent contained in the second composition is less than the amount of cross-linking agent contained in the first composition.

[0025] (First molding step) Following the resin preparation step, the first molding step is performed. First, the first composition is heated, for example, at 80° C. to prepare a molten first composition. The molten first composition is extruded by an extruder to prepare 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.

[0026] (Second molding step) Following the first molding step, the second molding step is performed. First, the second composition is heated, for example, at 80° C. to prepare a molten second composition. Then, 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 applying or spraying the molten second composition to the central portion 10. The second composition may also be applied to the central portion 10 multiple times so that the outer peripheral portion 11 has a desired thickness. Then, the second composition is naturally cooled in air at, for example, 25° C. This results in the pellet 1 of the first embodiment.

[0027] Thus, the pellet of embodiment 1 has an outer peripheral portion 11 containing the second composition around a central portion 10 containing the first composition. In this pellet 1, the amount of crosslinking agent in the outer peripheral portion 11 is less than the amount of crosslinking agent in the central portion 10.

[0028] [Embodiment 2] The pellet 1 of the embodiment 2 has a central portion 10 and an outer peripheral portion 11, similarly to the pellet 1 of the embodiment 1. The pellet 1 of the embodiment 2 is manufactured by a thermal diffusion method. The thermal diffusion method includes the following steps. Resin preparation step Molding step Cooling step

[0029] (Resin preparation step) First, a first composition is prepared. The first composition includes polyethylene and a crosslinking agent.

[0030] (Molding step) Following the resin preparation step, a molding step is performed. 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. The molten first composition is then extrusion molded to produce a linear first composition at a high temperature. The linear first composition at a high temperature is then cut to a desired length to form a pellet 1 at a high temperature.

[0031] (Cooling step) The molding step is followed by a cooling step, which comprises the following sub-steps: Primary cooling sub-step Secondary cooling sub-step

[0032] (First cooling substep) In the primary cooling sub-step, the surface of the high-temperature pellets 1 is quenched. Specifically, in the primary cooling sub-step, a fluid at the primary cooling temperature is supplied to the surface of the high-temperature pellets 1 for a predetermined period of time, thereby quenching the surface of the pellets 1. For example, in the primary cooling sub-step, CO 2 The high-temperature pellets 1 are quenched by blowing gas onto them for 30 seconds. The quenching creates a temperature difference between the central portion 10 and the peripheral portion 11 of the pellets 1. The temperature of the fluid and the time for blowing the fluid can be adjusted as appropriate.

[0033] (Secondary cooling sub-step) Immediately after the primary cooling sub-step, a secondary cooling sub-step is performed. In the secondary cooling sub-step, 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 causes the pellet 1 to be gradually cooled. When 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. Due to this temperature difference, the crosslinking agent is thermally diffused within the 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 crosslinking agent in the central portion 10 becomes greater than the amount of crosslinking agent in the outer peripheral portion 11.

[0034] The amounts of the crosslinking agent in the central portion 10 and the peripheral portion 11 can be measured by FT-IR or Raman scattering.

[0035] Moreover, the outer diameter and volume of the pellet 1 can be set appropriately.

[0036] [Summary of the embodiment] According to the pellet 1, the content of the crosslinking agent in the outer peripheral portion 11 is less than that in the central portion 10, so that the formation of foreign matter due to contact with air over a long period of time can be suppressed, and the pellet 1 has excellent long-term storage properties. For example, when the following accelerated deterioration test was performed, little foreign matter was formed, and even when a sheet sample was formed using the pellet 1 after long-term storage, little foreign matter was formed. EXAMPLES

[0037] Next, examples according to the present disclosure will be described. These examples are merely examples of the present disclosure, and the present disclosure is not limited to these examples.

[0038] (1) Pellet samples 1 to 8 Samples 1 to 8 were produced by the coating method described above. Table 1 shows the conditions of the resin preparation step, the first molding step, and the second molding step in the coating method. Samples 1 to 8 contain the following crosslinking agents. · Crosslinking agent CA1: Dicumyl peroxide (hereinafter referred to as DCP)

[0039] [Table 1]

[0040] Samples 1 to 8 were evaluated by the accelerated aging test and sheet sample test described below.

[0041] [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. The storage conditions were as follows. - Temperature of thermostatic chamber: 80℃ Temperature chamber atmosphere: Air Storage time in thermostatic chamber: 48 hours

[0042] Next, the pellets after storage for a predetermined time were cut as shown in FIG. 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 portion 0.15 mm from the outer peripheral surface of the pellet (1b in FIG. 2). In this example, foreign matter was defined as anything in the observation portion that was amber in color and had a size of 0.01 mm or more. 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.

[0043] [Evaluation method 2: Sheet sample test] A sheet sample was prepared from the pellets after the accelerated aging test. The pellets were heated to 120°C to melt, 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 confirm the presence of foreign matter. The foreign matter was identified using the same criteria as in the accelerated aging test. In this example, the sheet sample was evaluated as passing with a grade of "2B" when no foreign matter was observed on the surface, and as failing with a grade of "1B" when foreign matter was observed.

[0044] The evaluation results of Samples 1 to 8 are shown in Table 2.

[0045] [Table 2]

[0046] [Sample 1-6] In Samples 1 to 6, as shown in Table 2, the content of the crosslinking agent in the outer peripheral portion 11 was less than that in the central portion 10. Therefore, the outer peripheral portion 11 did not contain any foreign matter after the accelerated aging test. The sheet samples also did not contain any foreign matter. Furthermore, when the amount of cross-linking agent in the central portion 10 and the amount of cross-linking agent in the outer peripheral portion 11 satisfied the following conditions, the outer peripheral portion 11 did not contain any foreign matter after the accelerated deterioration test. conditions: Amount of crosslinker in the central portion 10: A Amount of crosslinking agent in the outer peripheral portion 11: B 0≦B / A≦0.90

[0047] [Sample 7, 8] In sample 7, the content of the crosslinking agent in the outer peripheral portion 11 was the same as that in the central portion 10. In sample 8, the content of the crosslinking agent in the outer peripheral portion 11 was greater than that in the central portion 10. In samples 7 and 8, the outer peripheral portion 11 contained foreign matter after the accelerated aging test. The sheet sample also contained foreign matter.

[0048] (2) Pellet samples 9-14 Samples 9 to 14 were produced by the coating method described above. Table 3 shows the conditions of the resin preparation step, the first molding step, and the second molding step in the coating method. Samples 9 to 14 contain the following crosslinking agents. Crosslinker CA2 for sample 9: t-butyl dicumyl 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

[0049] [Table 3]

[0050] Samples 9 to 14 were evaluated by the accelerated aging test and the sheet sample test described above. Table 4 shows the evaluation results.

[0051] [Table 4]

[0052] [Sample 9-14] As shown in Table 1, in Samples 9 to 14, the content of the crosslinking agent in the outer peripheral portion 11 was less than that in the central portion 10. Therefore, the outer peripheral portion 11 did not contain any foreign matter after the accelerated aging test. The sheet samples also did not contain any foreign matter.

[0053] (3) Pellet samples 15-22 Samples 15 to 22 were manufactured by the thermal diffusion method described above. Table 5 shows the conditions of the resin preparation step, molding step, and cooling step in the thermal diffusion method.

[0054] [Table 5]

[0055] Samples 15 to 22 were evaluated by the accelerated aging test and the sheet sample test described above. Table 6 shows the evaluation results of Samples 15 to 22.

[0056] In addition, in 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 determined as the content at the peripheral portion 11, and the content at the position 0.90 mm deep was determined as the content at the central portion 10.

[0057] [Table 6]

[0058] [Samples 15 and 16] As shown in Table 6, 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 believed to be due to the fact that the temperatures in the central portion 10 and outer peripheral portion 11 changed as shown in FIG. 3. FIG. 3 is a diagram showing the predicted temperature changes in the outer peripheral portion and the 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.

[0059] Specifically, as shown in Fig. 3, the time of 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 were 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.

[0060] In samples 15 and 16, the outer peripheral portion 11 contained foreign matter after the accelerated aging test. The sheet sample also contained foreign matter.

[0061] [Sample 17-19] When the change in the crosslinking agent content of Sample 17 was measured from the pellet surface toward the depth direction, the change was confirmed as shown in Figure 6. Figure 6 shows the crosslinking agent content obtained by cutting the pellet as shown in Figure 2, and measuring the crosslinking agent content at each position 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 peripheral surface on the cut surface 1a by FT-IR. As shown in Figure 6, it was confirmed that the crosslinking agent content of Sample 17 was lower toward the pellet surface and increased toward the center of the pellet. It was also confirmed that the crosslinking agent content of Sample 18 and Sample 19 changed in the depth direction in the same way.

[0062] In samples 17 to 19, the region where the crosslinking agent content is equal to or less than the average value in the entire pellet was defined as the outer periphery, and the region where the content exceeds the average value was defined as the central region. The average value in the entire pellet refers to the content of the crosslinking agent per 100 wt% of the low-density polyethylene forming the pellet, and is 3 wt% in pellets 17 to 19. In samples 17 to 19, the region where the crosslinking agent content is equal to or less than 3 wt% 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.

[0063] As shown in Table 6 and FIG. 6, in Samples 17 to 19, the content of the crosslinking agent at a position 0.15 mm deep from the pellet surface in the outer peripheral portion 11 was less than the content of the crosslinking agent at a position 0.90 mm deep from the pellet surface in the central portion 10. This is believed to be due to the temperature transition in the central portion 10 and the outer peripheral portion 11 as shown in FIG. 4. FIG. 4 is a diagram showing the predicted temperature transition in the outer peripheral portion 11 and the central portion 10 when cooling (primary cooling sub-step and secondary cooling sub-step) is performed in Sample 18. In the figure, the dashed line indicates the predicted temperature transition in the central portion 10, and the solid line indicates the predicted temperature transition in the outer peripheral portion 11.

[0064] Specifically, as shown in Fig. 4, the time of the first cooling sub-step was appropriate, so that the temperature of the outer peripheral portion 11 dropped significantly in the first cooling sub-step. Specifically, in the first cooling sub-step, the temperature of the outer 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 outer peripheral portion 11, and the temperature difference between the outer peripheral portion 11 and the central portion 10 was large. Since the temperature difference between the outer peripheral portion 11 and the central portion 10 was large, it is presumed that thermal diffusion of the cross-linking agent from the outer peripheral portion 11 to the central portion 10 occurred in the first cooling sub-step.

[0065] In Samples 17 to 19, the outer peripheral portion 11 did not contain any foreign matter after the accelerated deterioration test. Also, the sheet sample did not contain any foreign matter.

[0066] [Sample 20-22] In Samples 20 to 22, as shown in Table 6, the content of crosslinking agent in the outer peripheral portion 11 was the same as that in the central portion 10. This is believed to be due to the fact that the temperatures in the central portion 10 and outer peripheral portion 11 changed as shown in FIG. 5. FIG. 5 is a diagram showing the predicted temperature changes in the outer peripheral portion and the 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.

[0067] Specifically, as shown in FIG. 5, in the first cooling sub-step, the temperatures of the central portion 10 and the outer peripheral portion 11 dropped to 5° C. in 300 seconds after the start of cooling. At this time, the temperature of the central portion 10 dropped more slowly than that of the outer peripheral portion 11, and after the temperature of the outer peripheral portion 11 dropped to 5° C., the temperature of the central portion 10 also dropped to 5° C. after a period of time. Therefore, there was a time period in which the temperature difference between the outer peripheral portion 11 and the central portion 10 became large, and thermal diffusion of the crosslinking agent occurred from the outer peripheral portion 11 to the central portion 10. On the other hand, in the second cooling sub-step, after the temperature of the outer peripheral portion 11 rose from 5° C. to 25° C., the temperature of the central portion 10 also rose to 25° C. after a period of time. In this process, the temperature of the outer peripheral portion 11 became higher than the temperature of the central portion 10, and a predetermined temperature difference occurred, so that thermal diffusion of the crosslinking agent occurred from the central portion 10 to the outer peripheral portion 11. As a result, it is presumed that thermal diffusion of the crosslinking agent does not occur in the cooling step.

[0068] In Samples 20 to 22, the outer peripheral portion 11 contained foreign matter after the accelerated deterioration test. The sheet sample also contained foreign matter.

[0069] From the above, the pellets disclosed in the present specification are A pellet comprising a resin component including polyethylene and a crosslinking agent, A central portion and an outer peripheral portion located on an outer periphery of the central portion, The content of the crosslinking agent in the outer periphery is less than the content of the crosslinking agent in the central portion.

[0070] Therefore, the pellets are not easily deteriorated even if stored for a long time. In addition, a power cable manufactured using such pellets has good electrical insulation. Note that a method for manufacturing a power cable is disclosed in, for example, JP 2020-132817 A and US 2020 / 0270426 A, and therefore the description will be omitted.

[0071] The content of the crosslinking agent in the outer peripheral portion 11 is not limited to 2.85 wt % or less as disclosed in Sample 6. It will be understood that a pellet having a lower content of crosslinking agent in the outer peripheral portion 11 than in the central portion 10 is less likely to deteriorate when stored for a long period of time than a pellet having the same amount of crosslinking agent in the outer peripheral portion 11 and the central portion 10.

[0072] However, when 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 the cross-linking agent in the outer peripheral portion 11 is less than that in the central portion 10. Therefore, the content of the cross-linking agent in the outer peripheral portion 11 should be 0 wt% or more and less than 10 wt%, or 9 wt% or less, relative to 100 wt% of the resin component constituting the outer peripheral portion 11.

[0073] In order to sufficiently crosslink the resin components, the content of the crosslinking agent in the central portion 10 is preferably 1 wt% or more relative to 100 wt% of the resin components constituting the central portion. Also, in order to prevent the resin components after crosslinking from containing a large amount of crosslinking by-products, the content 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 components constituting the central portion.

[0074] In the pellet 1, the ratio B / A between the content A of the crosslinking agent in the central portion and the content B of the crosslinking agent in the peripheral portion is not limited to the numerical value in the embodiment. When the pellet 1 is produced by a coating method, the content A and the content B can be adjusted separately, so that the ratio B / A may be adjusted to satisfy, for example, 0≦B / A≦0.9. On the other hand, when the 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. The lower limit is not particularly limited, but the ratio B / A may be adjusted to satisfy, for example, 0.6≦B / A≦0.9.

[0075] The thickness of the outer peripheral portion 11 is not limited to 0.3 mm as disclosed in the examples. In order 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.

[0076] In the embodiment, the contents of the crosslinking agent in the outer peripheral portion and the central portion of the 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 the present invention is not limited thereto. The measurement position of the outer peripheral portion may be selected at the middle of the thickness of the outer peripheral portion. The measurement position of the central portion may be selected at the middle between the surface and the center of the central portion. [Explanation of symbols]

[0077] 1 pellet 10 Center part 11 Outer area

Claims

1. A pellet comprising a resin component including polyethylene and a crosslinking agent, A central portion and an outer peripheral portion located on an outer periphery of the central portion, The content of the crosslinking agent in the outer peripheral portion is less than the content of the crosslinking agent in the central portion. pellet.

2. The content of the crosslinking agent in the outer peripheral portion is less than 10 wt % relative to 100 wt % of a resin component constituting the outer peripheral portion.

2. The pellet of claim 1.

3. When the content of the crosslinking agent in the central portion is A and the content of the crosslinking agent in the outer peripheral portion is B, B / A≦0.

9. The pellet according to claim 1 or 2.

4. The outer peripheral portion does not contain the crosslinking agent.

2. The pellet of claim 1.

5. The content of the crosslinking agent in the central portion is 1 wt % or more and 10 wt % or less with respect to 100 wt % of a resin component constituting the central portion. The pellet according to claim 1 or 2.

6. The content of the crosslinking agent contained in the pellet is 1 wt % or more and 10 wt % or less with respect to 100 wt % of a resin component constituting the pellet. The pellet according to claim 1 or 2.

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

8. The content of the crosslinking agent in each of the central portion and the peripheral portion is calculated by measuring a cut surface of the pellet by an FT-IR or Raman scattering method. The pellet according to claim 1 or 2.

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