Flame-retardant polyethylene sheath material for ultrahigh-voltage cable and preparation method of flame-retardant polyethylene sheath material

Abstract

The invention relates to the technical field of cable materials, and particularly discloses a flame-retardant polyethylene sheath material for an ultrahigh-voltage cable and a preparation method thereof.The flame-retardant polyethylene sheath material for the ultrahigh-voltage cable provided by the invention takes low-density polyethylene as a main base material, and the molecular structure of the low-density polyethylene has more long-chain branches; and meanwhile, by adding the composite filler and the auxiliary agent, the prepared polyethylene sheath material has excellent mechanical properties, high and low temperature resistance and flame retardance.

Description

Technical Field

[0001] The present invention relates to the technical field of cable materials, and in particular to a flame-retardant polyethylene sheath material for ultra-high voltage cables and a preparation method thereof. Background Art

[0002] Ultra-high voltage cable refers to a power cable used to transmit power above 110kV. It is generally used as a hub in large-scale power transmission systems. It is a high-tech high-voltage cable mainly used for long-distance power transmission. Due to the special working conditions of ultra-high voltage cables, the performance requirements of the sheath material are relatively high, such as elongation at break, breaking strength, flame retardancy, etc.

[0003] The sheath material used for ultra-high voltage cables is basically polyethylene sheath material, but polyethylene material still has significant defects in key performance. On the one hand, polyethylene's high temperature resistance is insufficient. When the temperature rises due to current load or external heat source during cable operation, the sheath is prone to softening, deformation or even melting, exposing the internal conductor, significantly increasing the risk of short circuit and electrical fire; on the other hand, polyethylene material has poor inherent flame retardant properties. When a fire occurs, it is difficult to effectively prevent the flame from spreading along the cable, and it is impossible to buy precious time for personnel evacuation and fire rescue, posing a serious threat to the safety of life and property.

[0004] For example, Chinese patent document CN202310413407.6 discloses a halogen-free, low-smoke flame-retardant polyolefin sheathing material and its preparation process, comprising the following steps: Step 1: Preparation of a halogen-free flame retardant; Step 2: Preparation of a flame retardant synergist: dispersing montmorillonite in N,N-dimethylformamide and modifying it with octadecyl isocyanate to obtain a flame retardant synergist; Step 3: Ethylene-octene copolymer elastomer, high-density polyethylene, ethylene-octene copolymer grafted maleic anhydride copolymer, halogen-free flame retardant, flame retardant synergist, lubricant, and organic peroxide, stirring, mixing, kneading, extruding and granulating to obtain a polyolefin sheathing material. Although the mechanical properties and flame retardant properties of the prepared sheathing material have been improved to a certain extent, further improvement is still needed. Summary of the Invention

[0005] In view of the deficiencies in the prior art, the present invention aims to provide a flame-retardant polyethylene sheath material for ultra-high voltage cables and a preparation method thereof.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A flame-retardant polyethylene sheath material for an ultra-high voltage cable comprises the following components in parts by weight: 40-60 parts of low-density polyethylene, 8-12 parts of EPDM rubber, 10-15 parts of PE grafted maleic anhydride, 15-20 parts of a composite filler, 2-4 parts of a lubricant, 1-3 parts of an antioxidant, 1-2 parts of an ultraviolet absorber, and 0.2-0.5 parts of a heat stabilizer.

[0008] In the technical solution disclosed in the present invention, the preparation method of the composite filler is as follows:

[0009] S1. Preparation of composite mica material

[0010] The carboxymethyl chitosan is added into deionized water and stirred to dissolve, and then mica powder is added thereto and uniformly dispersed by ultrasonication. The composite mica material is obtained by filtering, drying, calcining and grinding.

[0011] In this step, the mass ratio of mica powder to carboxymethyl chitosan is 10-15:3-6. In some embodiments of the present invention, for example, 10:3, 10:4, 10:5, 10:6, 12:3, 12:5, 12:6, 15:3, 15:5, and 15:6 can be selected, but the ratio is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0012] In this step, calcination is carried out in a nitrogen atmosphere at a temperature of 400-500° C. for 2-3 hours.

[0013] In this step, carboxymethyl chitosan is first dissolved in deionized water. Carboxymethyl chitosan molecules contain a large number of hydroxyl groups. Through hydrogen bonding, the carboxymethyl chitosan is loaded on the surface and interlayers of the mica powder. The mica powder is then calcined under a nitrogen atmosphere to form a dense carbon layer that isolates oxygen and prevents heat transfer, thereby improving the flame retardant properties of the mica powder.

[0014] S2. Preparation of double bond modified composite materials

[0015] The composite mica material is dispersed in an ethanol aqueous solution, and then vinyltriethoxysilane is added thereto, stirred, filtered, washed, and dried to obtain a double-bond modified composite material.

[0016] In this step, the mass ratio of the composite mica material to vinyltriethoxysilane is 10-15:2-4. In some embodiments of the present invention, for example, 10:2, 10:3, 10:4, 12:2, 12:3, 12:4, 15:2, 15:3, 15:4 can be selected, but it is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0017] In this step, double bonds are introduced into the composite mica material, which facilitates the subsequent reactions.

[0018] S3. Preparation of composite fillers

[0019] The double-bond modified composite material is dispersed in an organic solvent, and then 1,4-butenediol, phosphonic acid-B-styryl ester and benzoyl peroxide are added thereto, and the mixture is heated and stirred to react. Subsequently, 1,3-bis(4'-glycidyl ether phenyl)adamantane and triethylamine are added thereto, and the reaction is continued. After the reaction is completed, the composite filler is obtained by filtering, washing and drying.

[0020] In this step, the mass ratio of the double bond modified composite material, 1,4-butenediol, phosphonic acid-B-styryl ester, benzoyl peroxide, 1,3-bis(4'-glycidyl ether phenyl)adamantane and triethylamine is 10-15:3-6:3-6:0.05-0.1:4-8:2-4.

[0021] In this step, the temperature of the heating and stirring reaction is 60-80°C, for example, 60°C, 65°C, 70°C, 75°C, or 80°C can be selected, and the heating and stirring reaction time is 3-5h, for example, 3h, 3.5h, 4h, 4.5h, or 5h can be selected, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0022] In this step, 1,4-butenediol and phosphonic acid-B-styryl ester are copolymerized to form a composite copolymer through an addition reaction between double bonds. The molecular structure of phosphonic acid-B-styryl ester contains a benzene ring structure and a phosphonic acid group. The benzene ring structure can improve the thermal stability of the molecular chain, making the material less likely to undergo thermal decomposition at high temperatures, thereby improving the heat resistance of the material. The phosphonic acid group has good flame retardancy, thereby improving the flame retardancy of the material. The flexible long chain structure of the alkyl group in 1,4-butenediol improves the low-temperature impact resistance of the material. Subsequently, 1,4-butenediol is used to The hydroxyl groups in enediol react with the epoxy groups in 1,3-bis(4'-glycidyl ether phenyl)adamantane. By introducing the adamantyl structure, the thermal stability of the material is further improved. The adamantane structure can also disperse stress and reduce crack propagation, thereby improving the impact resistance of the polyethylene sheath material. In addition, the unreacted epoxy groups in 1,3-bis(4'-glycidyl ether phenyl)adamantane can also form a network structure through a cross-linking reaction with the anhydride in the PE-grafted maleic anhydride, further improving the mechanical properties of the polyethylene sheath material.

[0023] In the technical solution disclosed in the present invention, the lubricant is selected from any one of calcium stearate, polyethylene wax, stearic acid, and stearic acid monoglyceride.

[0024] In the technical solution disclosed in the present invention, the antioxidant is selected from antioxidant 1010 or antioxidant 1076.

[0025] In the technical solution disclosed in the present invention, the ultraviolet absorber is selected from one or more of titanium dioxide, UV328, UV327, and UV326.

[0026] In the technical solution disclosed in the present invention, the heat stabilizer is selected from calcium zinc heat stabilizer CZ-113 or calcium zinc heat stabilizer CZ-116.

[0027] The present invention also provides a method for preparing the flame-retardant polyethylene sheath material for ultra-high voltage cables, comprising the following steps: adding low-density polyethylene, EPDM rubber, PE grafted maleic anhydride, composite filler, lubricant, antioxidant, ultraviolet absorber and heat stabilizer according to the formula amount into a high-speed mixer, mixing for 30-60 minutes, and then adding the mixture into a twin-screw extruder, setting the temperature to 160-180°C and the screw speed to 150-300 r / min, and melt-extruding and granulating to obtain the flame-retardant polyethylene sheath material for ultra-high voltage cables.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) The flame-retardant polyethylene sheath material for ultra-high voltage cables provided by the present invention uses low-density polyethylene as the main base material. The molecular structure of low-density polyethylene has a large number of long chain branches, which gives the material excellent flexibility and elasticity. At the same time, by adding composite fillers and additives, the polyethylene sheath material prepared has excellent mechanical properties, high and low temperature resistance and flame retardant properties.

[0030] (2) The present invention loads carboxymethyl chitosan on the surface and interlayer of mica powder and then calcines it under a nitrogen atmosphere. The carboxymethyl chitosan forms a dense carbon layer that can isolate oxygen and prevent heat transfer, thereby improving the flame retardant properties of the mica powder.

[0031] (3) Through the addition reaction between double bonds, 1,4-butenediol and phosphonic acid-B-phenylenediol copolymerize to form a composite copolymer. The molecular structure of phosphonic acid-B-phenylenediol contains a benzene ring structure and a phosphonic acid group. The benzene ring structure can improve the thermal stability of the molecular chain, making the material less likely to undergo thermal decomposition at high temperatures, thereby improving the heat resistance of the material. The phosphonic acid group has good flame retardancy, thereby improving the flame retardancy of the material. The flexible long chain structure of the alkyl group in 1,4-butenediol improves the low-temperature impact resistance of the material. Subsequently, 1,4-butene is used to The hydroxyl groups in the diol react with the epoxy groups in 1,3-bis(4'-glycidyl ether phenyl)adamantane. By introducing the adamantyl structure, the thermal stability of the material is further improved. The adamantane structure can also disperse stress and reduce crack propagation, thereby improving the impact resistance of the polyethylene sheath material. In addition, the unreacted epoxy groups in 1,3-bis(4'-glycidyl ether phenyl)adamantane can also form a network structure through a cross-linking reaction with the anhydride in the PE-grafted maleic anhydride, further improving the mechanical properties of the polyethylene sheath material. DETAILED DESCRIPTION

[0032] The present invention is further described in detail below through specific preferred embodiments, but the present invention is not limited to the following embodiments.

[0033] It should be noted that, unless otherwise specified, all chemical reagents involved in the present invention were purchased through commercial channels.

[0034] The low-density polyethylene used in the embodiments of the present invention is VL8005; the EPDM rubber is Dow 4725P; the PE grafted maleic anhydride is DuPont 40E529; the CAS number of phosphonic acid-B-phenylenediol is 1707-08-0; and the mesh size of the mica powder is 400 mesh.

[0035] Example 1

[0036] A method for preparing a flame-retardant polyethylene sheath material for an ultra-high voltage cable comprises the following steps:

[0037] 50 parts of low-density polyethylene, 10 parts of EPDM rubber, 12 parts of PE grafted maleic anhydride, 18 parts of composite filler, 3 parts of lubricant stearic acid, 2 parts of antioxidant 1010, 1 part of ultraviolet absorber UV328 and 0.3 parts of calcium zinc heat stabilizer CZ-113 were added to a high-speed mixer and mixed for 45 minutes. The mixture was then added to a twin-screw extruder, the temperature was set to 170°C, the screw speed was set to 200 r / min, and melt-extruded and granulated to obtain a flame-retardant polyethylene sheath material for ultra-high voltage cable.

[0038] Wherein, the preparation method of the composite filler is as follows:

[0039] S1. Add 3 g of carboxymethyl chitosan to 100 mL of deionized water and stir to dissolve. Then, add 10 g of mica powder and disperse evenly by ultrasonication. After suction filtration and drying, calcinate at 400 ° C for 3 h under a nitrogen atmosphere and grind through a 400 mesh sieve to obtain a composite mica material;

[0040] S2, 10g of the composite mica material was dispersed in 100mL of ethanol aqueous solution (the volume ratio of ethanol to water was 3:1), and then 2g of vinyltriethoxysilane was added thereto, and the mixture was stirred for 2h. After filtering, washing, and drying, a double-bond modified composite material was obtained;

[0041] S3. Disperse 10 g of the double-bond modified composite material in 100 mL of the organic solvent DMF, then add 3 g of 1,4-butenediol, 3 g of phosphonic acid-B-phenylyl ester and 0.05 g of benzoyl peroxide, heat and stir to react at 80 ° C for 3 h, then add 4 g of 1,3-bis(4'-glycidyl ether phenyl)adamantane and 2 g of triethylamine, and continue to react at 80 ° C for 2 h. After the reaction is completed, filter, wash and dry to obtain a composite filler.

[0042] Example 2

[0043] A method for preparing a flame-retardant polyethylene sheath material for an ultra-high voltage cable comprises the following steps:

[0044] 40 parts of low-density polyethylene, 8 parts of EPDM rubber, 10 parts of PE grafted maleic anhydride, 15 parts of composite filler, 2 parts of lubricant stearic acid, 1 part of antioxidant 1076, 1 part of ultraviolet absorber UV328 and 0.2 parts of calcium zinc heat stabilizer CZ-113 were added to a high-speed mixer and mixed for 45 minutes. The mixture was then added to a twin-screw extruder, the temperature was set to 170°C, the screw speed was set to 200 r / min, and melt-extruded and granulated to obtain a flame-retardant polyethylene sheath material for ultra-high voltage cable.

[0045] Wherein, the preparation method of the composite filler is as follows:

[0046] S1. Add 6 g of carboxymethyl chitosan to 100 mL of deionized water and stir to dissolve. Then, add 15 g of mica powder and disperse evenly by ultrasonication. After suction filtration and drying, calcinate at 400 ° C for 3 h under a nitrogen atmosphere and grind through a 400 mesh sieve to obtain a composite mica material;

[0047] S2, 15g of the composite mica material was dispersed in 100mL of ethanol aqueous solution (the volume ratio of ethanol to water was 3:1), and then 4g of vinyltriethoxysilane was added thereto, and stirred for 2h. After filtering, washing, and drying, a double-bond modified composite material was obtained;

[0048] S3. Disperse 15 g of the double-bond modified composite material in 100 mL of the organic solvent DMF, then add 6 g of 1,4-butenediol, 6 g of phosphonic acid-B-phenylyl ester and 0.1 g of benzoyl peroxide, heat and stir to react at 80 ° C for 3 h, then add 8 g of 1,3-bis(4'-glycidyl ether phenyl)adamantane and 4 g of triethylamine, and continue to react at 80 ° C for 2 h. After the reaction is completed, filter, wash and dry to obtain a composite filler.

[0049] Example 3

[0050] A method for preparing a flame-retardant polyethylene sheath material for an ultra-high voltage cable comprises the following steps:

[0051] 60 parts of low-density polyethylene, 12 parts of EPDM rubber, 15 parts of PE grafted maleic anhydride, 20 parts of composite filler, 4 parts of lubricant stearic acid, 3 parts of antioxidant 1076, 2 parts of ultraviolet absorber UV328 and 0.5 parts of calcium zinc heat stabilizer CZ-113 were added to a high-speed mixer and mixed for 45 minutes. The mixture was then added to a twin-screw extruder, the temperature was set to 170°C, the screw speed was set to 200 r / min, and melt-extruded and granulated to obtain a flame-retardant polyethylene sheath material for ultra-high voltage cable.

[0052] Wherein, the preparation method of the composite filler is as follows:

[0053] S1. Add 5 g of carboxymethyl chitosan to 100 mL of deionized water and stir to dissolve. Then, add 12 g of mica powder and disperse evenly by ultrasonication. After suction filtration and drying, calcinate at 400 ° C for 3 h under a nitrogen atmosphere and grind through a 400 mesh sieve to obtain a composite mica material;

[0054] S2, 12g of the composite mica material was dispersed in 100mL of ethanol aqueous solution (the volume ratio of ethanol to water was 3:1), and then 3g of vinyltriethoxysilane was added thereto, and stirred for 2h. After filtering, washing, and drying, a double-bond modified composite material was obtained;

[0055] S3. Disperse 12 g of the double-bond modified composite material in 100 mL of the organic solvent DMF, then add 4 g of 1,4-butenediol, 6 g of phosphonic acid-B-phenylyl ester and 0.1 g of benzoyl peroxide, heat and stir to react at 80 ° C for 3 h, then add 6 g of 1,3-bis(4'-glycidyl ether phenyl)adamantane and 3 g of triethylamine, and continue to react at 80 ° C for 2 h. After the reaction is completed, filter, wash and dry to obtain a composite filler.

[0056] Comparative Example 1

[0057] A method for preparing a flame-retardant polyethylene sheath material for an ultra-high voltage cable comprises the following steps:

[0058] 50 parts of low-density polyethylene, 10 parts of EPDM rubber, 12 parts of PE grafted maleic anhydride, 18 parts of composite filler, 3 parts of lubricant stearic acid, 2 parts of antioxidant 1010, 1 part of ultraviolet absorber UV328 and 0.3 parts of calcium zinc heat stabilizer CZ-113 were added to a high-speed mixer and mixed for 45 minutes. The mixture was then added to a twin-screw extruder, the temperature was set to 170°C, the screw speed was set to 200 r / min, and melt-extruded and granulated to obtain a flame-retardant polyethylene sheath material for ultra-high voltage cable.

[0059] Wherein, the preparation method of the composite filler is as follows:

[0060] S1, 10g of mica powder was dispersed in 100mL of ethanol aqueous solution (the volume ratio of ethanol to water was 3:1), and then 2g of vinyltriethoxysilane was added thereto, and stirred for 2h. After filtering, washing, and drying, double-bond modified mica powder was obtained;

[0061] S2. Disperse 10 g of double-bond modified mica powder in 100 mL of organic solvent DMF, then add 3 g of 1,4-butenediol, 3 g of phosphonic acid-B-phenylyl ester and 0.05 g of benzoyl peroxide, heat and stir to react at 80 ° C for 3 h, then add 4 g of 1,3-bis(4'-glycidyl ether phenyl)adamantane and 2 g of triethylamine, and continue to react at 80 ° C for 2 h. After the reaction is completed, filter, wash and dry to obtain a composite filler.

[0062] Compared with Example 1, Comparative Example 1 does not treat the mica powder with carboxymethyl chitosan loading.

[0063] Comparative Example 2

[0064] A method for preparing a flame-retardant polyethylene sheath material for an ultra-high voltage cable comprises the following steps:

[0065] 50 parts of low-density polyethylene, 10 parts of EPDM rubber, 12 parts of PE grafted maleic anhydride, 18 parts of composite filler, 3 parts of lubricant stearic acid, 2 parts of antioxidant 1010, 1 part of ultraviolet absorber UV328 and 0.3 parts of calcium zinc heat stabilizer CZ-113 were added to a high-speed mixer and mixed for 45 minutes. The mixture was then added to a twin-screw extruder, the temperature was set to 170°C, the screw speed was set to 200 r / min, and melt-extruded and granulated to obtain a flame-retardant polyethylene sheath material for ultra-high voltage cable.

[0066] Wherein, the preparation method of the composite filler is as follows:

[0067] S1. Add 3 g of carboxymethyl chitosan to 100 mL of deionized water and stir to dissolve. Then, add 10 g of mica powder and disperse evenly by ultrasonication. After suction filtration and drying, calcinate at 400 ° C for 3 h under a nitrogen atmosphere and grind through a 400 mesh sieve to obtain a composite mica material;

[0068] S2, 10g of the composite mica material was dispersed in 100mL of ethanol aqueous solution (the volume ratio of ethanol to water was 3:1), and then 2g of vinyltriethoxysilane was added thereto, and the mixture was stirred for 2h. After filtering, washing, and drying, a double-bond modified composite material was obtained;

[0069] S3. Disperse 10 g of the double-bond modified composite material in 100 mL of the organic solvent DMF, then add 3 g of 1,4-butenediol and 0.05 g of benzoyl peroxide, heat and stir to react at 80 ° C for 3 h, then add 4 g of 1,3-bis(4'-glycidyl ether phenyl)adamantane and 2 g of triethylamine, and continue to react at 80 ° C for 2 h. After the reaction is completed, filter, wash and dry to obtain a composite filler.

[0070] Comparative Example 2 is compared with Example 1, except that B-phenylphenylphosphonate is not added.

[0071] Comparative Example 3

[0072] A method for preparing a flame-retardant polyethylene sheath material for an ultra-high voltage cable comprises the following steps:

[0073] 50 parts of low-density polyethylene, 10 parts of EPDM rubber, 12 parts of PE grafted maleic anhydride, 18 parts of composite filler, 3 parts of lubricant stearic acid, 2 parts of antioxidant 1010, 1 part of ultraviolet absorber UV328 and 0.3 parts of calcium zinc heat stabilizer CZ-113 were added to a high-speed mixer and mixed for 45 minutes. The mixture was then added to a twin-screw extruder, the temperature was set to 170°C, the screw speed was set to 200 r / min, and melt-extruded and granulated to obtain a flame-retardant polyethylene sheath material for ultra-high voltage cable.

[0074] Wherein, the preparation method of the composite filler is as follows:

[0075] S1. Add 3 g of carboxymethyl chitosan to 100 mL of deionized water and stir to dissolve. Then, add 10 g of mica powder and disperse evenly by ultrasonication. After suction filtration and drying, calcinate at 400 ° C for 3 h under a nitrogen atmosphere and grind through a 400 mesh sieve to obtain a composite mica material;

[0076] S2, 10g of the composite mica material was dispersed in 100mL of ethanol aqueous solution (the volume ratio of ethanol to water was 3:1), and then 2g of vinyltriethoxysilane was added thereto, and the mixture was stirred for 2h. After filtering, washing, and drying, a double-bond modified composite material was obtained;

[0077] S3. Disperse 10 g of the double-bond modified composite material in 100 mL of the organic solvent DMF, then add 3 g of 1,4-butenediol, 3 g of phosphonic acid-B-phenylphenyl ester and 0.05 g of benzoyl peroxide, heat and stir at 80°C for 3 h, and after the reaction is completed, filter, wash and dry to obtain a composite filler.

[0078] Compared with Example 1, in Comparative Example 3, no 1,3-bis(4'-glycidyl ether phenyl)adamantane was added.

[0079] The polyethylene sheath materials prepared in Examples 1-3 and Comparative Examples 1-3 were prepared into samples that met the test standards, and then performance tests were performed, as follows:

[0080] Tensile strength test: tested according to GB / T 1040.1-2018 standard, with a tensile speed of 100 mm / min;

[0081] Oxygen index test: tested according to GB / T 2406.2-2009 standard;

[0082] Heat aging resistance test: Heat aging was performed in a heat aging oven according to the standard GBT 7141-2008 at 135°C for 168 hours, and then the tensile strength retention was tested.

[0083] Unnotched impact strength test: tested in accordance with GB / T 1843-2008 standard, temperature is -40℃;

[0084] The test results are shown in Table 1.

[0085] Table 1 Performance test results of different groups

[0086]

[0087] Finally, it should be noted that the above embodiments do not limit the present invention in any form. Those skilled in the art will appreciate that modifications and improvements can be made based on the present invention. Therefore, any modifications or improvements made without departing from the spirit of the present invention are intended to fall within the scope of protection claimed in the present invention.

Claims

1. A flame-retardant polyethylene sheath material for ultra-high voltage cables, characterized in that: The invention comprises the following components in parts by weight: 40-60 parts of low-density polyethylene, 8-12 parts of EPDM rubber, 10-15 parts of PE grafted maleic anhydride, 15-20 parts of composite filler, 2-4 parts of lubricant, 1-3 parts of antioxidant, 1-2 parts of ultraviolet absorber and 0.2-0.5 parts of heat stabilizer.

2. The flame-retardant polyethylene sheath material for ultra-high voltage cables according to claim 1, characterized in that: The preparation method of the composite filler is as follows: S1, adding carboxymethyl chitosan to deionized water, stirring to dissolve, then adding mica powder thereto, ultrasonically dispersing it evenly, filtering, drying, calcining, and grinding to obtain a composite mica material; S2, dispersing the composite mica material in an ethanol aqueous solution, then adding vinyltriethoxysilane thereto, stirring, filtering, washing, and drying to obtain a double-bond modified composite material; S3. Disperse the double-bond modified composite material in an organic solvent, then add 1,4-butenediol, phosphonic acid-B-phenylyl ester and benzoyl peroxide thereto, heat and stir to react, then add 1,3-bis(4'-glycidyl ether phenyl)adamantane and triethylamine thereto, continue to react, and after the reaction is completed, filter, wash and dry to obtain a composite filler.

3. The flame-retardant polyethylene sheath material for ultra-high voltage cables according to claim 2, characterized in that: In step S1, the mass ratio of mica powder to carboxymethyl chitosan is 10-15:3-6.

4. The flame-retardant polyethylene sheath material for ultra-high voltage cables according to claim 2, characterized in that: In step S2, the mass ratio of the composite mica material to vinyltriethoxysilane is 10-15:2-4.

5. The flame-retardant polyethylene sheath material for ultra-high voltage cables according to claim 2, characterized in that: In step S3, the mass ratio of the double bond modified composite material, 1,4-butenediol, phosphonic acid-B-styryl ester, benzoyl peroxide, 1,3-bis(4'-glycidyl ether phenyl)adamantane and triethylamine is 10-15:3-6:3-6:0.05-0.1:4-8:2-4.

6. The flame-retardant polyethylene sheath material for ultra-high voltage cables according to claim 1, characterized in that: The lubricant is selected from any one of calcium stearate, polyethylene wax, stearic acid, and stearic acid monoglyceride.

7. The flame-retardant polyethylene sheath material for ultra-high voltage cables according to claim 1, characterized in that: The antioxidant is selected from antioxidant 1010 or antioxidant 1076.

8. The flame-retardant polyethylene sheath material for ultra-high voltage cables according to claim 1, characterized in that: The ultraviolet absorber is selected from one or more of titanium dioxide, UV328, UV327, and UV326.

9. The flame-retardant polyethylene sheath material for ultra-high voltage cables according to claim 1, characterized in that: The heat stabilizer is selected from calcium zinc heat stabilizer CZ-113 or calcium zinc heat stabilizer CZ-116.

10. The method for preparing the flame-retardant polyethylene sheath material for ultra-high voltage cables according to any one of claims 1 to 9, characterized in that: The method comprises the following steps: adding low-density polyethylene, EPDM rubber, PE grafted maleic anhydride, composite filler, lubricant, antioxidant, ultraviolet absorber and heat stabilizer into a high-speed mixer according to the formula amount, mixing for 30-60 minutes, adding the mixture into a twin-screw extruder, setting the temperature to 160-180°C and the screw speed to 150-300 r / min, and melt-extruding and granulating to obtain a flame-retardant polyethylene sheath material for ultra-high voltage cables.

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