A dense waterproof cross-linked polyethylene cable material, cable and preparation method

Abstract

The invention relates to the technical field of polymer cable materials, and discloses a dense waterproof cross-linked polyethylene cable material, a cable and a preparation method. The cable material is composed of the following raw materials by weight: 80-85 parts of low-fluidity low-density polyethylene, 15-20 parts of high-fluidity low-density polyethylene, 3-5 parts of ethylene-vinyl acetate copolymer, 1.5-2 parts of composite nucleating agent, 2-3 parts of cross-linking agent, 0.3-0.5 parts of auxiliary cross-linking agent, and 0.5-1 parts of antioxidant; the composite nucleating agent is formed by carrying ultra-high molecular weight polyethylene through nano-silica aerogel, and the ultra-high molecular weight polyethylene is dispersed in low-density polyethylene. The uniformly dispersed ultra-high molecular weight polyethylene first forms fine crystal nuclei at a higher temperature, which prompts the cross-linked low-density polyethylene to form dense crystals, reduces grain boundary defects, and the dense structure effectively prevents the penetration of water vapor, inhibits the formation of water dendrites, and improves the breakdown field strength of the cable material.

Description

Technical Field

[0001] The invention relates to the technical field of polymer cable materials, and in particular to a dense waterproof cross-linked polyethylene cable material, a cable and a preparation method thereof. Background Art

[0002] High-voltage power transmission technology is currently the main indicator of power transmission technology. With the gradual increase in power transmission voltage levels, the technical requirements for cables are getting higher and higher. As cables are key components of power transmission, the characteristics of the insulation materials in them directly determine the technical level of the cables.

[0003] At present, from 35kV medium and low voltage cables to 110kV and above high voltage cables, cables with cross-linked polyethylene as the main insulation material are gradually replaced. Cross-linked polyethylene (XLPE) has become the preferred insulation material for cables due to its good electrical properties, high insulation resistance, low dielectric loss tangent value, and high breakdown strength. Cross-linked polyethylene (XLPE) is a linear polyethylene molecular structure converted into a three-dimensional network structure, thereby realizing the transition from thermoplastic material to thermosetting material, raising the working temperature of the material from 70°C to 90°C, and the cross-linked three-dimensional network macromolecular structure greatly improves the mechanical properties and environmental stress cracking resistance of the material.

[0004] Peroxide cross-linked polyethylene is a product in which the peroxide cross-linking agent is uniformly pre-dispersed in polyethylene. After molding, the peroxide is thermally decomposed under high temperature conditions to form free radicals with high chemical activity. The free radicals capture hydrogen atoms in the polyethylene molecules, and the polyethylene that loses hydrogen atoms becomes a macromolecular free radical. The macromolecular free radicals react with each other to form carbon-carbon crosslinks to obtain network cross-linked polyethylene. Peroxide cross-linked polyethylene has a high degree of cross-linking, and it has electrical properties of low dielectric loss and high voltage resistance, making it the preferred material for high-voltage cables. At present, cross-linked polyethylene is gradually overcoming technical barriers and improving technical standards to promote its use in higher voltage transmission lines.

[0005] However, cross-linked polyethylene (XLPE) cables have always been prone to water dendrites. According to relevant technical analysis, the main reason is that in the early stage of cross-linked polyethylene hot processing, the early cross-linking of polyethylene forms gel particles, which affects the cleanliness and uniformity of polyethylene; oxidation reaction and degradation reaction occur during heating and extrusion, forming internal defects; uneven crystal distribution and crystal structure defects after cross-linking. These defects will lead to defects and micro channels inside the cross-linked polyethylene (XLPE). When the cable insulation material is in a humid environment, external moisture can easily penetrate into the insulation medium. At the defect, under the influence of water and electric field, a dendritic discharge channel is formed, which will cause more water to penetrate into the insulation layer, and the water dendrites will continue to grow, eventually leading to breakdown. The existence of water dendrites is the main reason why the insulation of high-voltage cross-linked polyethylene cables is easily broken down.

[0006] At present, medium and high voltage cables are gradually using cross-linked polyethylene insulation. The damp and water environment of cables such as ground cables and submarine cables is inevitable. When the cables are subjected to long-term moisture erosion, moisture penetrates the insulation layer to form water trees, which affects the reliability of cross-linked polyethylene insulation materials in 110KV and above cables. In order to achieve the reliability of peroxide cross-linked polyethylene insulation materials in high voltage cables and prevent the occurrence of internal defects in cross-linked polyethylene, blocking water vapor from penetrating into the cable is the key to preventing the formation of water trees. Summary of the invention

[0007] In order to effectively inhibit the generation of internal defects of cross-linked polyethylene, prevent water vapor from penetrating to form water trees, and enhance the breakdown field strength of cross-linked polyethylene, the present invention discloses a dense waterproof cross-linked polyethylene cable material, which promotes the formation of uniformly distributed and finely sized crystals of cross-linked polyethylene, reduces grain boundary defects, and forms a dense structure. Its significant effect is that it can effectively prevent water vapor from penetrating into the grain boundary defects to form water trees, thereby improving the breakdown field strength of the material. Further, a cable prepared using the dense waterproof cross-linked polyethylene cable material and a method for preparing the cable are disclosed.

[0008] In order to achieve the above object, the present invention adopts the following technical solution:

[0009] First, the present invention provides a dense waterproof cross-linked polyethylene cable material, which is characterized by being composed of the following raw materials by weight: 80-85 parts of low-fluidity low-density polyethylene, 15-20 parts of high-fluidity low-density polyethylene, 3-5 parts of ethylene-vinyl acetate copolymer, 1.5-2 parts of composite nucleating agent, 2-3 parts of cross-linking agent, 0.3-0.5 parts of auxiliary cross-linking agent, and 0.8-1 parts of antioxidant; wherein:

[0010] The melt index of the low-fluidity low-density polyethylene is 1.5-2.5 g / 10 min (200° C., 5 kg);

[0011] The high fluidity low density polyethylene has a melt index of 15-30 g / 10 min (190° C., 2.16 kg);

[0012] The composite nucleating agent is a composite of nano-silicon dioxide aerogel and ultra-high molecular weight polyethylene;

[0013] The dense waterproof cross-linked polyethylene cable material is prepared by the following method:

[0014] (1) Grinding and pre-compounding nano-silica aerogel, ultra-high molecular weight polyethylene powder, maleic anhydride grafted polyethylene, white oil, and silane coupling agent in a grinder at a mass ratio of 100:30-50:3-5:1-2:0.3-0.5, and then hot-melt blending and extruding in a co-rotating twin-screw extruder, crushing and grinding until the D80 particle size is less than 20 μm, to obtain a composite nucleating agent;

[0015] (2) Low-fluidity low-density polyethylene, ethylene-vinyl acetate copolymer, and composite nucleating agent are mixed uniformly according to weight and stored in silo #1; high-fluidity low-density polyethylene, cross-linking agent, auxiliary cross-linking agent, and antioxidant are mixed uniformly and stored in silo #2; the material in silo #1 is fed into the first stage co-rotating twin-screw mixer of the two-stage mixing extruder unit for mixing and extrusion by using a loss-in-weight scale, and the feed section temperature is set to 150°C, the compression section temperature is set to 170°C, the homogenization section temperature is set to 180°C, and the discharge section temperature is set to 160°C; the main engine speed is set to 280-350rpm; the extruded material enters the second stage single screw extruder of the two-stage mixing extruder unit, and at the same time, the material in silo #2 is weighed according to the weight of the formula, added to the second stage single screw extruder, and the temperature of the single screw extruder is set to 115-120°C, and the speed is set to 50-80rpm to extrude into strips, which are then chain conveyed, air-cooled, and pelletized to obtain a dense and waterproof cross-linked polyethylene cable material.

[0016] Preferably, the viscosity average molecular weight of the ultra-high molecular weight polyethylene powder is 1 million to 3 million; and the D50 particle size passes through a 100 mesh sieve.

[0017] Preferably, the silane coupling agent is at least one of KH-570, KH-560, KH-550, KH-540, and KH-858.

[0018] Preferably, the equipment used for the grinding pre-compounding includes but is not limited to a ball mill and a jet mill; the powdery nano-silica aerogel and the ultra-high molecular weight polyethylene powder are uniformly pre-compounded through mechanical compounding.

[0019] Preferably, the content of vinyl acetate (VA) in the ethylene-vinyl acetate copolymer is 10-20wt%; particularly preferably, the content of vinyl acetate (VA) in the ethylene-vinyl acetate copolymer is 15-20wt%; vinyl acetate (VA) in the ethylene-vinyl acetate copolymer is a polar group, which is dispersed in polyethylene to introduce deep traps, and can effectively improve the electrical properties of the cable material.

[0020] Preferably, the cross-linking agent is at least one of dicumyl peroxide (DCP) and benzoyl peroxide (BPO); particularly preferably, the cross-linking agent is dicumyl peroxide (DCP).

[0021] Preferably, the auxiliary cross-linking agent is at least one of diallyl phthalate, triallyl cyanurate, and 1,2-polybutadiene.

[0022] Preferably, the antioxidant is a synergistic composition of a hindered phenol antioxidant and a phosphite antioxidant in a mass ratio of 3:1; further preferably, the antioxidant is a synergistic composition of one of antioxidant 1010 or antioxidant 300 and antioxidant 918 in a mass ratio of 3:1.

[0023] Preferably, the first stage co-rotating twin-screw mixer of the two-stage mixing extruder unit has a screw length-diameter ratio greater than 44 / 1. A higher screw length-diameter ratio can extend the mixing time of the material, enhance the mixing effect, and better ensure the uniform mixing and dispersion of low-fluidity low-density polyethylene, ethylene-vinyl acetate copolymer, and composite nucleating agent, so that the particle size of the dispersed phase of the composite nucleating agent is reduced and the distribution is uniform; in equipment that cannot meet the large length-diameter ratio, it is preferred to increase the reverse thread element to ensure the material mixing time in the screw. For example, a conventional co-rotating twin-screw mixer with a length-diameter ratio of 40:1 is provided with at least 3 groups of reverse thread elements to ensure that the material mixing time in the screw exceeds 5 minutes.

[0024] The first stage of the two-stage compounding extruder unit has a co-rotating twin-screw compounder with high shear, which makes the composite nucleating agent more evenly dispersed in the low-fluidity low-density polyethylene at a higher temperature. The better technical effect is reflected in the uniform crystal distribution of the cross-linked cable material and the smaller crystal nucleus size. The second stage of the two-stage compounding extruder unit adopts a single screw with low shear force. By assisting the high-fluidity low-density polyethylene, the cross-linking agent is dispersed in the low-density polyethylene, avoiding the premature decomposition of the cross-linking agent.

[0025] The composite nucleating agent is a composite of nano-silica aerogel carrying ultra-high molecular weight polyethylene. The nano-silica aerogel is conducive to promoting heterogeneous nucleation, so that the grain growth size of low-density polyethylene is small and evenly distributed; in particular, the nano-silica aerogel has micropores, and is formed by composite carrying with ultra-high molecular weight polyethylene. The composite nucleating agent is evenly dispersed in polyethylene. When used for cable extrusion and high-temperature cross-linking, the crystallization of ultra-high molecular weight polyethylene occurs at a higher temperature. The evenly dispersed ultra-high molecular weight polyethylene first forms a uniform micro-nucleus at a higher temperature, and further promotes the cross-linked low-density polyethylene grain size to reduce, forming a dense crystalline structure. The present invention significantly promotes the regularity of the low-density polyethylene crystalline structure through the composite nucleating agent, reduces grain boundary defects, and forms a dense structure. Its dense structure can effectively prevent the penetration and diffusion of water vapor and effectively inhibit the formation of water dendrites. Furthermore, when used for cable insulation materials, the cable is in a humid and water environment for a long time, and the cable still maintains a high breakdown field strength.

[0026] Second, the present invention further provides a cable prepared by using the above-mentioned dense waterproof cross-linked polyethylene cable material.

[0027] Third, the present invention provides a method for preparing the cable, which is characterized in that: the specific preparation method is:

[0028] (1) Grinding and dispersing conductive carbon black, silicone powder, and oxidized polyethylene wax in a mass ratio of 100:1-2:2-3 to obtain a conductive dispersion; mixing the conductive dispersion, a crosslinking agent, and ethylene-vinyl acetate copolymer in a mass ratio of 15-20:1-2:70-80 in an internal mixer at 100°C for 6-8 minutes, and then opening the mixer to obtain a shielding material;

[0029] (2) The shielding material of step (1) is used as the inner and outer shielding material, and the dense waterproof cross-linked polyethylene cable material is used as the insulating layer material. The copper wire is twisted to form a conductor core, and the inner shielding layer, the insulating layer, and the outer shielding layer are successively wrapped on the conductor core to complete the extrusion; the conductor core is cross-linked at a pressure of 1.0-1.2MPa and 260-320°C through a high-temperature nitrogen pipeline to obtain a cross-linked cable core;

[0030] (3) According to the cable requirements, a corrugated aluminum sheath is wrapped around the cross-linked cable core and a high-density polyethylene sheath is extruded and coated to obtain a cable.

[0031] The inner shielding layer, insulating layer and outer shielding layer of the three-layer co-extrusion extruder are equipped with 65-type, 150-type and 90-type single-screw extruders respectively; the processing temperature for extrusion of the inner and outer shielding layers is set at 100-110°C; the processing temperature for extrusion of the insulating layer is set as: first section: 90°C, second section 100°C, third section 105°C, fourth section 105°C, fifth section 110°C, sixth section 110°C, seventh section 110°C, eighth section 115°C, ninth section 115°C and tenth section 118°C.

[0032] In summary, the dense waterproof cross-linked polyethylene cable material, cable and preparation method of the present invention have the following beneficial effects compared with the prior art:

[0033] (1) The cable material of the present invention uses a composite nucleating agent composed of nano-silica aerogel and ultra-high molecular weight to provide fine crystal nuclei and promote heterogeneous nucleation, so that the grains are evenly distributed and the size becomes smaller.

[0034] (2) The nano-silica aerogel used in the cable material of the present invention has micropores. By being composited with ultra-high molecular weight polyethylene to form a composite nucleating agent, the ultra-high molecular weight polyethylene is promoted to be uniformly dispersed in low-density polyethylene. When used for cable extrusion and high-temperature cross-linking, the crystallization of the ultra-high molecular weight polyethylene occurs at a higher temperature. The uniformly dispersed ultra-high molecular weight polyethylene first forms fine crystal nuclei at a higher temperature, which promotes the crystallization of the cross-linked low-density polyethylene to form dense grains and reduce grain boundary defects. Its dense structure can effectively prevent the penetration and diffusion of water vapor and effectively inhibit the formation of water dendrites.

[0035] (3) The cable material of the present invention is suitable for preparing cables that are in a humid or water environment for a long time, and the cables maintain a relatively high breakdown field strength.

[0036] (4) The cable material and cable manufacturing process of the present invention are easy to control and suitable for large-scale production. DETAILED DESCRIPTION

[0037] In order to enable those skilled in the art to further understand the technical means, technical objectives and technical effects achieved by the invention, the present invention is described in detail below in conjunction with the embodiments. However, the present invention is not limited in any form. It should be pointed out that for those of ordinary skill in the art, several adjustments and improvements can be made without departing from the concept of the present invention. These all belong to the scope of protection of the present invention. In view of the fact that the basic raw materials and process control have an influence on the breakdown field strength of the final specimen, the same basic raw materials and control process are used for sample preparation and specimen cross-linking preparation.

[0038] Basic raw material grades and performance indicators used in the implementation plan:

[0039] Nano-silica aerogel: particle size is less than 200nm.

[0040] Ultra-high molecular weight polyethylene: Model 4012, D50 particle size exceeds 135 mesh, particle size viscosity average molecular weight 1.7 million, produced by Celanese.

[0041] Low-fluidity low-density polyethylene: Model 2420H, melt index is 2g / 10min (200℃, 5kg), produced by Huizhou CNOOC Shell.

[0042] High fluidity low-density polyethylene: Model XJ710, melt index 24g / 10min (190℃, 2.16kg), produced by Lotte, South Korea.

[0043] Ethylene-vinyl acetate copolymer: Model VS430, VA content 19wt%, produced by Lotte, South Korea.

[0044] Example 1

[0045] The formula composition of the dense waterproof cross-linked polyethylene cable material in parts by weight is: 80 parts of low-fluidity low-density polyethylene, 20 parts of high-fluidity low-density polyethylene, 5 parts of ethylene-vinyl acetate copolymer, 2 parts of composite nucleating agent, 2.5 parts of cross-linking agent DCP, 0.5 parts of auxiliary cross-linking agent triallyl cyanurate, and 1 part of antioxidant; the antioxidant is a composition of antioxidant 1010 and antioxidant 918 in a mass ratio of 3:1; the cable material is prepared according to the following process:

[0046] (1) Pre-compounding nano-silica aerogel, ultra-high molecular weight polyethylene powder, maleic anhydride grafted polyethylene, white oil, and silane coupling agent KH-560 in a jet mill at a mass ratio of 100:40:3:1:0.5 by high-speed airflow impact grinding, and then hot-melt blending and extruding into blocks in a Φ35 co-rotating twin-screw extruder at a temperature of 185°C, and then crushing, pulverizing, and grinding to a D80 particle size of less than 20 μm to obtain a composite nucleating agent;

[0047] (2) Low-fluidity low-density polyethylene, ethylene-vinyl acetate copolymer, and composite nucleating agent are mixed evenly according to the weight of the formula and stored in silo #1; high-fluidity low-density polyethylene, cross-linking agent, auxiliary cross-linking agent, and antioxidant are mixed evenly and stored in silo #2; the materials in silo #1 are sent to the first-stage co-rotating twin-screw mixer of the two-stage mixing extruder unit for mixing and extrusion by using a loss-in-weight scale. The first-stage co-rotating twin-screw mixer is a Φ75 twin-screw extruder with a length-to-diameter ratio of 48:1 and a screw with one set of reverse threads. The materials are mixed and extruded. The residence time in the screw is 350s; the temperature of the feed section is set at 150°C, the compression section at 170°C, the homogenization section at 180°C, and the discharge section at 160°C; the main engine speed is 320rpm; the extruded material enters the second-stage single-screw extruder of the two-stage mixing extruder unit, and at the same time, the material in the 2# silo is weighed according to the weight loss of the formula, added to the second-stage single-screw extruder, and the temperature of the single-screw extruder is set at 115°C and the speed is 50rpm to extrude into strips, which are then chain conveyed, air-cooled, and pelletized to obtain a dense and waterproof cross-linked polyethylene cable material.

[0048] Example 2

[0049] The formula of dense waterproof cross-linked polyethylene cable material is as follows: 85 parts of low-fluidity low-density polyethylene, 15 parts of high-fluidity low-density polyethylene, 5 parts of ethylene-vinyl acetate copolymer, 2 parts of composite nucleating agent, 2.5 parts of cross-linking agent DCP, 0.5 parts of auxiliary cross-linking agent 1,2-polybutadiene, and 1 part of 1010 antioxidant. The cable material is prepared according to the following process:

[0050] (1) Pre-compounding nano-silica aerogel, ultra-high molecular weight polyethylene powder, maleic anhydride grafted polyethylene, white oil, and silane coupling agent KH-570 in a jet mill at a mass ratio of 100:50:3:1:0.5 by high-speed airflow impact grinding, and then hot-melt blending and extruding into blocks in a Φ35 co-rotating twin-screw extruder at a temperature of 185°C, and then crushing, pulverizing, and grinding to a D80 particle size of less than 20 μm to obtain a composite nucleating agent;

[0051] (2) Low-fluidity low-density polyethylene, ethylene-vinyl acetate copolymer, and composite nucleating agent are mixed evenly according to the weight of the formula and stored in silo #1; high-fluidity low-density polyethylene, cross-linking agent, auxiliary cross-linking agent, and antioxidant are mixed evenly and stored in silo #2; the materials in silo #1 are sent to the first-stage co-rotating twin-screw mixer of the two-stage mixing extruder unit for mixing and extrusion by using a loss-in-weight scale. The first-stage co-rotating twin-screw mixer is a Φ75 twin-screw extruder with a length-to-diameter ratio of 48:1 and a screw with one set of reverse threads. The materials are mixed and extruded. The residence time in the screw is 383s; the temperature of the feed section is set at 150°C, the compression section at 170°C, the homogenization section at 180°C, and the discharge section at 160°C; the main engine speed is 300rpm; the extruded material enters the second-stage single-screw extruder of the two-stage mixing extruder unit, and at the same time, the material in the 2# silo is weighed according to the weight loss of the formula, added to the second-stage single-screw extruder, and the temperature of the single-screw extruder is set at 115°C and the speed is 50rpm to extrude into strips, which are then chain conveyed, air-cooled, and pelletized to obtain a dense and waterproof cross-linked polyethylene cable material.

[0052] Example 3

[0053] The formula composition of the dense waterproof cross-linked polyethylene cable material in parts by weight is: 82 parts of low-fluidity low-density polyethylene, 18 parts of high-fluidity low-density polyethylene, 5 parts of ethylene-vinyl acetate copolymer, 2 parts of composite nucleating agent, 3 parts of cross-linking agent DCP, 0.4 parts of auxiliary cross-linking agent diallyl phthalate, and 0.8 parts of antioxidant; the antioxidant is a composition of antioxidant 1010 and antioxidant 918 in a mass ratio of 3:1; the cable material is prepared according to the following process:

[0054] (1) Pre-compounding nano-silica aerogel, ultra-high molecular weight polyethylene powder, maleic anhydride grafted polyethylene, white oil, and silane coupling agent KH-560 in a jet mill at a mass ratio of 100:30:3:1:0.5 by high-speed airflow impact grinding, and then hot-melt blending and extruding into blocks in a Φ35 co-rotating twin-screw extruder at a temperature of 185°C, and then crushing, pulverizing, and grinding to a D80 particle size of less than 20 μm to obtain a composite nucleating agent;

[0055] (2) Low-fluidity low-density polyethylene, ethylene-vinyl acetate copolymer, and composite nucleating agent are mixed evenly according to the weight of the formula and stored in silo #1; high-fluidity low-density polyethylene, cross-linking agent, auxiliary cross-linking agent, and antioxidant are mixed evenly and stored in silo #2; the materials in silo #1 are sent to the first-stage co-rotating twin-screw mixer of the two-stage mixing extruder unit for mixing and extrusion by using a loss-in-weight scale. The first-stage co-rotating twin-screw mixer is a Φ75 twin-screw extruder with a length-to-diameter ratio of 40:1. The screw reverse threads are adjusted to 3 groups. The materials are mixed and extruded. The residence time in the screw is 326s; the temperature of the feed section is set at 150℃, the compression section at 170℃, the homogenization section at 180℃, and the discharge section at 160℃; the main engine speed is 310rpm; the extruded material enters the second-stage single-screw extruder of the two-stage mixing extruder unit, and at the same time, the material in the 2# silo is weighed according to the weight loss of the formula, added to the second-stage single-screw extruder, and the temperature of the single-screw extruder is set at 115℃ and the speed is 80rpm to extrude into strips, which are then chain conveyed, air-cooled, and pelletized to obtain a dense and waterproof cross-linked polyethylene cable material.

[0056] Example 4

[0057] The formula composition of the dense waterproof cross-linked polyethylene cable material in parts by weight is: 85 parts of low-fluidity low-density polyethylene, 15 parts of high-fluidity low-density polyethylene, 3 parts of ethylene-vinyl acetate copolymer, 1.5 parts of composite nucleating agent, 2.5 parts of cross-linking agent DCP, 0.5 parts of auxiliary cross-linking agent triallyl cyanurate, and 1 part of antioxidant; the antioxidant is a composition of antioxidant 1010 and antioxidant 918 in a mass ratio of 3:1; the cable material is prepared according to the following process:

[0058] (1) Pre-compounding nano-silica aerogel, ultra-high molecular weight polyethylene powder, maleic anhydride grafted polyethylene, white oil, and silane coupling agent KH-570 in a jet mill at a mass ratio of 100:30:3:1:0.5 by high-speed airflow impact grinding, and then hot-melt blending and extruding into blocks in a Φ35 co-rotating twin-screw extruder at a temperature of 185°C, and then crushing, pulverizing, and grinding to a D80 particle size of less than 20 μm to obtain a composite nucleating agent;

[0059] (2) Low-fluidity low-density polyethylene, ethylene-vinyl acetate copolymer, and composite nucleating agent are mixed evenly according to the weight of the formula and stored in silo #1; high-fluidity low-density polyethylene, cross-linking agent, auxiliary cross-linking agent, and antioxidant are mixed evenly and stored in silo #2; the materials in silo #1 are fed into the first-stage co-rotating twin-screw mixer of the two-stage mixing extruder unit by loss-in-weight weighing for mixing and extrusion. The first-stage co-rotating twin-screw mixer is a Φ75 twin-screw extruder with a length-to-diameter ratio of 40:1. The screw reverse thread is adjusted to 3 groups. When the material stays in the screw The material was extruded into strips at 150°C, 170°C for compression, 180°C for homogenization and 160°C for discharge. The main engine speed was 300 rpm. The extruded material entered the second-stage single-screw extruder of the two-stage mixing extruder unit. At the same time, the material in the 2# silo was weighed according to the weight loss of the formula and added to the second-stage single-screw extruder. The temperature of the single-screw extruder was set to 115°C and the speed was 80 rpm. The strips were extruded into strips after chain conveying, air cooling and pelletizing to obtain a dense and waterproof cross-linked polyethylene cable material.

[0060] Example 5

[0061] (1) Grinding and dispersing conductive carbon black, silicone powder, and oxidized polyethylene wax in a ball mill at a mass ratio of 100:2:3 for 30 minutes to obtain a conductive dispersion; mixing the conductive dispersion, crosslinking agent DCP, and ethylene-vinyl acetate copolymer in a mass ratio of 20:1:80 in an internal mixer at 100°C for 6 minutes, and then opening the mixer to obtain a shielding material;

[0062] (2) The shielding material of step (1) is used as the inner and outer shielding material, and the dense waterproof cross-linked polyethylene cable material obtained in Example 1 is used as the insulating layer material and fed into a three-layer co-extruder. The inner shielding layer, the insulating layer, and the outer shielding layer of the three-layer co-extruder are equipped with 65-type, 150-type, and 90-type single-screw extruders respectively; the processing temperature for extrusion of the inner and outer shielding layers is set at 100°C; the processing temperature for extrusion of the insulating layer is set as follows: first stage: 90°C, second stage 100°C, third stage 105°C, fourth stage 105°C, The fifth section is 110℃, the sixth section is 110℃, the seventh section is 110℃, the eighth section is 115℃, the ninth section is 115℃, and the tenth section is 118℃. The copper wires are twisted to form a conductor core, and the inner shielding layer, the insulating layer, and the outer shielding layer are successively wrapped on the conductor core to complete the extrusion. The cross-linking treatment is carried out by a high-temperature nitrogen pipeline to obtain a cross-linked cable core. Among them, the pipeline pressure is 1MPa, and the temperature distribution of the pipeline 1-6 zones is: 320℃-310℃-300℃-290℃-280℃-260℃;

[0063] (3) According to the cable requirements, a corrugated aluminum sheath is wrapped around the cross-linked cable core and a high-density polyethylene sheath is extruded and coated to obtain a cable.

[0064] Comparative Example 1

[0065] The solution of Example 1 was implemented without adding a composite nucleating agent. The breakdown field strength of the cross-linked polyethylene cable material was significantly reduced after cross-linking, and the breakdown field strength decreased by more than 12% after aging in a wet environment. This was mainly due to the lack of uniform nucleation crystallization, the presence of many defects and gaps inside the material, and the water vapor easily penetrated into the material to form water trees.

[0066] Comparative Example 2

[0067] According to the scheme of Example 1, nano-silica aerogel is directly used to replace the composite nucleating agent. The breakdown field strength of the cross-linked polyethylene cable material decreases to a certain extent after cross-linking, and decreases after aging in a wet environment, mainly because the nucleation crystallization uniformity and fine size of the single nano-silica aerogel nucleating agent are limited.

[0068] Comparative Example 3

[0069] According to the scheme of Example 1, only ultra-high molecular weight polyethylene was added as a nucleating agent. The breakdown field strength of the cross-linked polyethylene cable material decreased after cross-linking, and the breakdown field strength decreased after aging in a wet environment. This is mainly because the ultra-high molecular weight polyethylene does not carry silica aerogel, the gas dispersion uniformity is limited, there is a certain nucleation crystallization interface inside the material, and water vapor penetrates into the material.

[0070] Comparative Example 4

[0071] The formula composition of the dense waterproof cross-linked polyethylene cable material in parts by weight is: 80 parts of low-fluidity low-density polyethylene, 20 parts of high-fluidity low-density polyethylene, 5 parts of ethylene-vinyl acetate copolymer, 2 parts of composite nucleating agent, 2.5 parts of cross-linking agent DCP, 0.5 parts of auxiliary cross-linking agent triallyl cyanurate, and 1 part of antioxidant; the antioxidant is a composition of antioxidant 1010 and antioxidant 918 in a mass ratio of 3:1;

[0072] (1) Pre-compounding nano-silica aerogel, ultra-high molecular weight polyethylene powder, maleic anhydride grafted polyethylene, white oil, and silane coupling agent KH-560 in a jet mill at a mass ratio of 100:40:3:1:0.5 by high-speed airflow impact grinding, and then hot-melt blending and extruding into blocks in a Φ35 co-rotating twin-screw extruder at a temperature of 185°C, and then crushing, pulverizing, and grinding to a D80 particle size of less than 20 μm to obtain a composite nucleating agent;

[0073] (2) Low-fluidity low-density polyethylene, ethylene-vinyl acetate copolymer, composite nucleating agent, high-fluidity low-density polyethylene, cross-linking agent, co-cross-linking agent and antioxidant are mixed uniformly according to the weight proportion of the formula, and the mixture is fed into a single-screw extruder for extrusion. The temperature of the single-screw extruder is set at 115° C. and the speed is set at 50 rpm to extrude into strips. The strips are then chain-conveyed, air-cooled and pelletized to obtain a dense and waterproof cross-linked polyethylene cable material.

[0074] Comparative Example 4 was implemented according to the scheme of Example 1, but the composite nucleating agent was not pre-dispersed by a two-stage mixing extruder. Through testing, the breakdown field strength of the cross-linked polyethylene cable material was greatly reduced after cross-linking, and the breakdown field strength decreased more after aging in a wet environment. This is mainly because it is difficult to effectively disperse the composite nucleating agent by relying on a single screw, the crystal nucleus particle size is large and uneven, the material density is reduced, and water vapor easily penetrates into the material to form water trees.

[0075] Comparative Example 5

[0076] According to the scheme of Example 1, the use of ethylene-vinyl acetate copolymer is eliminated. The breakdown field strength of the cross-linked polyethylene cable material decreases after cross-linking, indicating that ethylene-vinyl acetate copolymer can effectively inhibit the accumulation of space charge.

[0077] Evaluation test of the water tree resistance of the above cable materials:

[0078] Each group of samples was prepared into a cross-linked plate specimen to simulate the cable operation environment and perform electrothermal aging in a high humidity environment. The changes in the breakdown field strength before and after electrothermal aging in a humid environment were compared to evaluate the ability of the cable material to resist water branches in a humid environment. The smaller the decrease in the breakdown field strength after electrothermal aging in a high humidity environment, the better the cable material's resistance to water branches, reflecting that the cable material has fewer internal defects, a dense structure, and excellent waterproof penetration performance. The specific tests are as follows:

[0079] (1) Preparation of test specimens:

[0080] The cable materials of Examples 1-4 and Comparative Examples 1-5 were hot-melt extruded in a 20-type single-screw extruder at an extrusion temperature of 110°C, and the hot-melt materials were fed into a mold of a flat vulcanizer while hot, and a pressure of 10 MPa was applied at a constant temperature of 110°C for shaping, and the pressure was maintained for 10 minutes; the pressure was increased and maintained at 15 MPa for 10 minutes; the temperature was raised to 180°C, and thermally cross-linked at 15 MPa for 25 minutes, naturally cooled and cut; before the breakdown strength test, the test piece was treated in a vacuum oven at 80°C for 48 hours to obtain a sheet test piece with a thickness of 0.5 mm.

[0081] (2) Specimens subjected to heat aging treatment in a wet environment:

[0082] The specimen was subjected to an electrothermal aging electrode with a frequency of 50 Hz and a field strength of 7 kV / mm, and aged in an environment of 80% relative humidity and 70°C for 25 days to obtain a specimen aged in a wet heat environment.

[0083] (3) Breakdown strength test:

[0084] The breakdown field strength of each group of specimens before and after electrothermal aging in a wet environment was tested with reference to IEC 60243-1:2013 (Part 1 of the test method for electrical strength of insulating materials). The breakdown field strength test uses a hemispherical electrode, with the high-voltage electrode as the upper electrode and the ground electrode as the lower electrode. The diameter of the electrode ball is 20 mm. Before the experiment begins, the specimen is placed between the two electrodes to ensure that the insulating oil completely immerses the specimen. The voltage is increased at a rate of 1 kv / s until the specimen is broken down. The power frequency AC breakdown field strength is obtained according to E=U / d, where U is the voltage when the specimen is broken down and d is the thickness of the specimen. The test environment is 23°C and the humidity is 50%. Ten samples are tested in each group of samples. The breakdown data is analyzed using a two-parameter Weibull distribution. The breakdown field strength corresponding to the cumulative failure probability of 63.2% is used as the final data of the power frequency breakdown field strength. The test results are shown in Table 1.

[0085] Table 1 Breakdown field strength data table

[0086]

[0087] According to the above test, the technical solution of the present invention can effectively prevent the penetration and diffusion of water vapor by promoting the cross-linked polyethylene to form dense crystals and reduce the internal defect structure of the material, effectively inhibit the formation and expansion of water dendrites, and has a higher breakdown field strength. Moreover, when the cable material is aged in a wet environment and corroded by water vapor for a long time, the breakdown field strength will not show a significant decrease.

[0088] It should be understood that any technical solution that can be obtained by technicians in this technical field through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the existing technology should be within the protection scope determined by the claims.

Claims

1. A dense waterproof cross-linked polyethylene cable material, characterized in that The invention is composed of the following raw materials in parts by weight: 80-85 parts of low-fluidity low-density polyethylene, 15-20 parts of high-fluidity low-density polyethylene, 3-5 parts of ethylene-vinyl acetate copolymer, 1.5-2 parts of composite nucleating agent, 2-3 parts of crosslinking agent, 0.3-0.5 parts of auxiliary crosslinking agent, and 0.5-1 parts of antioxidant; wherein: The low-fluidity low-density polyethylene has a melt index of 1.5-2.5 g / 10 min at 200° C. and 5 kg; The high fluidity low density polyethylene has a melt index of 15-30 g / 10 min at 190° C. and 2.16 kg; The composite nucleating agent is a composite of nano-silica aerogel and ultra-high molecular weight polyethylene, and is prepared by the following method: grinding and pre-compounding nano-silica aerogel, ultra-high molecular weight polyethylene powder, maleic anhydride grafted polyethylene, white oil, and silane coupling agent in a grinder at a mass ratio of 100:30-50:3-5:1-2:0.3-0.5, and then hot-melt blending and extruding in a co-rotating twin-screw extruder, crushing and grinding until the D80 particle size is less than 20 μm, to obtain the composite nucleating agent.

2. A dense waterproof cross-linked polyethylene cable material as claimed in claim 1, characterized in that: The viscosity average molecular weight of the ultra-high molecular weight polyethylene powder is 1 million to 3 million; and the D50 particle size passes through a 100-mesh sieve.

3. A dense waterproof cross-linked polyethylene cable material as claimed in claim 1, characterized in that: The content of vinyl acetate in the ethylene-vinyl acetate copolymer is 10-20 wt %.

4. A dense waterproof cross-linked polyethylene cable material as claimed in claim 1, characterized in that: The cross-linking agent is at least one of dicumyl peroxide and benzoyl peroxide.

5. A dense waterproof cross-linked polyethylene cable material as claimed in claim 1, characterized in that: The auxiliary cross-linking agent is selected from at least one of diallyl phthalate, triallyl cyanurate, and 1,2-polybutadiene.

6. A dense waterproof cross-linked polyethylene cable material as claimed in claim 1, characterized in that: The antioxidant is a synergistic composition of a hindered phenol antioxidant and a phosphite antioxidant in a mass ratio of 3:

1.

7. A method for preparing a dense waterproof cross-linked polyethylene cable material as claimed in any one of claims 1 to 6, the specific method being as follows: (1) grinding and pre-compounding nano-silica aerogel, ultra-high molecular weight polyethylene powder, maleic anhydride grafted polyethylene, white oil, and silane coupling agent in a grinder at a mass ratio of 100:30-50:3-5:1-2:0.3-0.5, and then hot-melt blending and extruding in a co-rotating twin-screw extruder, crushing and grinding until the D80 particle size is less than 20 μm, to obtain a composite nucleating agent; (2) low-fluidity low-density polyethylene, ethylene-vinyl acetate copolymer, and composite nucleating agent are mixed uniformly according to weight and stored in silo #1; high-fluidity low-density polyethylene, cross-linking agent, auxiliary cross-linking agent, and antioxidant are mixed uniformly and stored in silo #2; the materials in silo #1 are fed into the first-stage co-rotating twin-screw mixer of the two-stage mixing extruder unit for mixing and extrusion by using a loss-in-weight scale, and the feed section temperature is set to 150°C, the compression section temperature to 170°C, the homogenization section temperature to 180°C, and the discharge section temperature to 160°C; the main engine is turned on and off. The extruded material enters the second-stage single-screw extruder of the two-stage mixing extruder unit. At the same time, the material in the 2# silo is weighed according to the weight loss of the formula, and added to the second-stage single-screw extruder. The temperature of the single-screw extruder is set to 115-120°C, and the speed is 50-80rpm to extrude into strips. After chain conveying, air cooling, and pelletizing, a dense waterproof cross-linked polyethylene cable material is obtained; the first-stage co-rotating twin-screw mixer of the two-stage mixing extruder unit has a screw length-diameter ratio greater than 44 / 1.

8. A method for preparing a dense waterproof cross-linked polyethylene cable material as claimed in claim 7, characterized in that: The silane coupling agent is selected from at least one of KH-570, KH-560, KH-550, KH-540, and KH-858.

9. A cable prepared from the dense waterproof cross-linked polyethylene cable material according to claim 1.

10. A method for preparing the cable according to claim 9, characterized in that: The specific preparation method is: (1) Grinding and dispersing conductive carbon black, silicone powder, and oxidized polyethylene wax in a mass ratio of 100:1-2:2-3 to obtain a conductive dispersion; mixing the conductive dispersion, a crosslinking agent, and ethylene-vinyl acetate copolymer in a mass ratio of 15-20:1-2:70-80 in an internal mixer at 100° C. for 6-8 minutes, and then opening the mixer to obtain a shielding material; (2) The shielding material of step (1) is used as the inner and outer shielding material, and the dense waterproof cross-linked polyethylene cable material described in claim 1 is used as the insulating layer material. The copper wire is twisted to form a conductor core, and the inner shielding layer, the insulating layer, and the outer shielding layer are successively wrapped on the conductor core to complete the extrusion; cross-linking is performed through a high-temperature nitrogen pipeline at a pressure of 1-1.2 MPa and 260-320° C. to obtain a cross-linked cable core; (3) According to the cable requirements, a corrugated aluminum sheath is wrapped around the cross-linked cable core, and a high-density polyethylene sheath is extruded and coated to obtain a cable.

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