Thermoplastic low-smoke halogen-free flame-retardant sheath material for nuclear cable and preparation method of thermoplastic low-smoke halogen-free flame-retardant sheath material

Abstract

The invention belongs to the technical field of power cables, and particularly relates to a thermoplastic low-smoke halogen-free flame-retardant sheath material for nuclear power and a preparation method thereof. The sheath material is prepared from the following components: ultra-high molecular weight polyethylene, ethylene-vinyl acetate copolymer, a compatilizer, polyethylene carbon black master batch, magnesium hydroxide, an antioxidant, a lubricant and a synergist. The low-smoke halogen-free flame-retardant sheath material for the nuclear cable provided by the invention has excellent comprehensive properties (radiation resistance, low temperature resistance, flame retardance and the like). And the cable has good low temperature resistance (-35 DEG C), and can still pass a cable bunching combustion test after being subjected to 80-year long-term aging and being subjected to irradiation aging by being superposed with a cobalt 60 gamma source. The prepared low-smoke halogen-free cable material is thermoplastic, irradiation and vulcanization are not needed after forming, the production process is simple, and the economical efficiency is good.

Description

Technical Field

[0001] The invention belongs to the technical field of power cables, and in particular relates to a thermoplastic low-smoke halogen-free flame-retardant sheath material for nuclear power and a preparation method thereof. Background Art

[0002] Nuclear energy, a low-carbon, safe, and clean energy source, is experiencing rapid development. Nuclear cables, as supporting components of nuclear power plants, are responsible for transmitting electrical energy, making their quality paramount. According to technical research, the design of sheath materials used in current nuclear cables must address three key challenges: 1. Low-temperature resistance to -35°C (primarily for northern China); 2. Long service life (the design service life of fourth-generation nuclear power plants is 80 years); and 3. The ability to pass the cable bundle combustion test after an equivalent 80-year long-term aging (aging temperature and time calculated based on activation energy) followed by irradiation with a cobalt-60 gamma source (cumulative dose of 375 kGy, dose rate of 1±0.5 kGy / h).

[0003] Patent application number 201710459205.X discloses a radiation-crosslinked cable sheathing material for nuclear power plant cables. Through radiation crosslinking, the cable material achieves properties such as oil resistance, abrasion resistance, torque resistance, and flame retardancy. The patent does not mention its low-temperature resistance to -35°C or its flame retardancy after accelerated aging and irradiation to 375kGy. On the one hand, its low-temperature applicability is questionable, and nuclear power plants located in high-latitude (low-temperature) areas may not be able to use it, limiting its application. On the other hand, this solution relies on radiation crosslinking, requiring an additional electron beam irradiation step after extrusion compared to thermoplastic materials. This complex process results in high costs.

[0004] Patent application number 201210592102.8 discloses a peroxide-cured crosslinking method for preparing AP1000 nuclear power cable sheathing material, also a crosslinked sheathing material. This technical solution also fails to address the -35°C low-temperature issue. Furthermore, peroxide-cured sheathing materials require an additional vulcanization step after extrusion, compared to thermoplastic materials. This process is also complex and uneconomical.

[0005] Therefore, it is necessary to develop a new sheath material and a matching preparation process to solve the problems existing in the prior art. Summary of the Invention

[0006] In order to solve the problems existing in the prior art, the present invention provides a low-smoke halogen-free flame-retardant sheath material for thermoplastic nuclear cables, and a preparation method and application thereof; the cable material of the present invention has the characteristics of good flame retardancy, good low-temperature resistance, and can still pass the cable bundle combustion test after equivalent to 80 years of long-term aging (aging temperature and time are calculated based on activation energy) and superimposed cobalt-60γ source irradiation (cumulative dose of 375kGy, dose rate of 1±0.5kGy / h).

[0007] The first aspect of the present invention provides a low-smoke, halogen-free, flame-retardant sheath material for a thermoplastic nuclear cable, comprising the following blended components: 20-40 parts of ultra-high molecular weight polyethylene; 10-45 parts of ethylene-vinyl acetate copolymer; 8-15 parts of compatibilizer; 5-8 parts of polyethylene carbon black masterbatch; 150-180 parts of magnesium hydroxide; 5-10 parts of antioxidant; 0.5-1.5 parts of lubricant; 10-15 parts of synergist.

[0008] Furthermore, the ultra-high molecular weight polyethylene has a molecular weight of 1.5-3 million and a melting point of 130-136°C. Due to its long molecular chain and large molecular weight, ultra-high molecular weight polyethylene can significantly improve the heat-oxidative aging resistance, radiation resistance and low-temperature resistance of low-smoke halogen-free cable materials. However, due to its large molecular weight, poor melt fluidity and poor mixing effect with inorganic flame retardants, it cannot be directly added to low-smoke halogen-free cable materials for application. The present invention pre-crushes the ultra-high molecular weight polyethylene into powder with a grinder and then mixes and granulates it with other components, thus solving the shortcoming of its difficulty in application in low-smoke halogen-free cable materials and successfully preparing low-smoke halogen-free cable materials for nuclear cables with long service life, good low-temperature resistance and radiation resistance.

[0009] In some embodiments of the present invention, the ultra-high molecular weight polyethylene is crushed to a particle size of 100 mesh to 500 mesh.

[0010] The present invention provides a thermoplastic low-smoke, halogen-free, flame-retardant sheathing material for nuclear cables. A further technical solution is that the vinyl acetate content of the ethylene-vinyl acetate copolymer is 15%-70%, and the melt flow rate (under test conditions of 190°C and 2.16 kg) is 0.2-5 g / 10 min. A higher vinyl acetate monomer content improves radiation resistance, resulting in improved flame retardancy in the irradiated cable material.

[0011] Furthermore, the ethylene vinyl acetate has a vinyl acetate content of 33%-70% and a melt flow rate of 0.2-0.5 g / 10 min. A lower melt flow rate indicates a higher molecular weight, which macroscopically translates to better thermal and oxygen aging resistance and a longer service life.

[0012] In some embodiments of the present invention, the compatibilizer is a mixture of maleic anhydride grafted polyethylene wax and maleic anhydride grafted EVA wax in a mass ratio of 1:2 to 2:1. In a preferred embodiment, the ratio of the two is 1:1. Maleic anhydride grafts act as a "bridge" between non-polar and polar materials, which can improve the mixing effect between the components in the molten state and achieve the purpose of improving mechanical properties. Compared with traditional maleic anhydride grafted polyethylene / EVA, grafting maleic anhydride onto the molecular chain of small molecule wax can further improve its dispersibility in the mixture, thereby achieving a better mixing effect and further improving the mechanical properties of the cable material.

[0013] Furthermore, the polyethylene carbon black masterbatch (the carbon black masterbatch carrier is polyethylene) has a carbon black content of 40%-60%. In a preferred embodiment, the polyethylene carbon black masterbatch has a carbon black content of 50%.

[0014] In some embodiments, the magnesium hydroxide is doubly modified with vinyl silane and stearic acid, and has a D50 particle size of 80-120 nm.

[0015] In some embodiments, the magnesium hydroxide is commercially available silane-modified magnesium hydroxide, which is then modified with stearic acid. Preferably, in the stearic acid modification step, the amount of stearic acid is 0.3-0.8 parts by weight (based on 100 parts of magnesium hydroxide), for example, 0.3 parts, 0.4 parts, 0.5 parts, 0.6 parts, 0.7 parts, 0.8 parts, etc.

[0016] In a preferred embodiment, the magnesium hydroxide is graded using a grading wheel to a D50 particle size of 100 nm. The surface is dually modified with vinyl silane and stearic acid. Vinyl silane modification enhances the bonding between the magnesium hydroxide and the polymer material, thereby improving its dispersibility. Stearic acid can further improve the elongation at break of low-smoke, halogen-free cable materials, further enhancing the effectiveness of the combination. The flame retardancy of magnesium hydroxide is related to its particle size. The inventors have discovered that when the particle size of magnesium hydroxide reaches the nanoscale, a nanosize effect occurs, further enhancing barrier properties. This significantly improves the flame retardancy of low-smoke, halogen-free cable materials compared to magnesium hydroxide with a particle size of approximately 1 micron. However, currently available magnesium hydroxide particle sizes are difficult to achieve in the nanoscale due to limitations in production processes and powder agglomeration. This invention innovatively applies the grading wheel to the cable material industry, using it to grade commercially available magnesium hydroxide. The powerful centrifugal force generated by the high-speed rotation of the grading wheel allows nanoparticles meeting the particle size requirements to pass through the grading wheel and be collected in a powder bin. The remaining particles of other sizes are screened and used in ordinary cable materials, and the added value of the materials is improved through classification and utilization.

[0017] Furthermore, the antioxidant is a mixture of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] 1010, phosphites, thiophenols, and thioesters. The weight ratio of each component in the antioxidant is ≥ 10%, and preferably, the weight ratio of each antioxidant type is ≥ 15%. By combining the main and auxiliary antioxidants, the service life of the cable material is significantly extended.

[0018] In some embodiments, the thiophenol antioxidant includes but is not limited to antioxidant 300# and antioxidant 1035.

[0019] In some embodiments, the thioester antioxidant includes but is not limited to antioxidants DLTP and DSTP.

[0020] In a preferred embodiment, the mass ratio of the components in the antioxidant is 1:1:1:1.

[0021] In some embodiments, the lubricant is a mixture of silicone masterbatch, silicone oil and polyethylene wax; the mass ratio of silicone masterbatch, silicone oil and polyethylene wax in the lubricant is (3-5): (1-3):1.

[0022] In a preferred embodiment, the lubricant is a 4:2:1 mixture of silicone masterbatch, silicone oil and polyethylene wax.

[0023] In some embodiments, the synergist is a mixture of nano-montmorillonite and melamine cyanurate in a mass ratio of 1:1-2:3. The melamine cyanurate is added in an amount ≤ 1.5% of the total (total components). Nano-montmorillonite, due to its unique lamellar structure, can block electron beams, thereby reducing their impact on polymer materials in high-radiation environments. Melamine cyanurate combustion produces non-combustible gases, which, when combined with montmorillonite and magnesium hydroxide condensed phase flame retardant, can produce a synergistic flame retardant effect. However, the amount of melamine cyanurate added should not exceed 1.5% of the total, otherwise the cable material toxicity index will not meet the low toxicity requirement (toxicity index ≤ 5).

[0024] A second aspect of the present invention provides a method for preparing the low-smoke, halogen-free, flame-retardant sheath material for the thermoplastic core cable, comprising the following steps: S1. Grind the ultra-high molecular weight polyethylene into a powder for use, with a particle size of 100 to 500 mesh; The commercially available magnesium hydroxide was graded by a classifying wheel, and the powder with a D50 particle size of 80-120 nm was screened out for use; S2. The magnesium hydroxide obtained in step S1 is mixed with ethylene-vinyl acetate copolymer and an antioxidant and prepared as a masterbatch by a twin-screw extruder; S3. The ultra-high molecular weight polyethylene powder obtained in step S1, the masterbatch obtained in step S2 and other components are mixed and granulated by a reciprocating single-screw extruder to obtain the cable material.

[0025] Furthermore, in step S1, the frequency of the classifying wheel is set to 80-120 Hz, and the nozzle flow rate of the compressed air is 5-8m 3 / min.

[0026] In a preferred embodiment, in step S1, the ultra-high molecular weight polyethylene is crushed to 200-300 mesh, 300-400 mesh; In a preferred embodiment, the frequency of the classifying wheel is set to 105 Hz (i.e., the rotational speed), and the nozzle flow rate of the compressed air is 6m 3 / min.

[0027] Beneficial effects: (1) The cable material prepared by the present invention is a sheath material for thermoplastic nuclear cables, which has excellent comprehensive performance, including excellent flame retardancy, radiation resistance and low temperature resistance. It does not require irradiation and vulcanization after extrusion molding, and the production process is simple and economical (low process cost). (2) The preparation method of the present invention comprises the following steps: crushing ultra-high molecular weight polyethylene into powder and then adding the powder to a low-smoke halogen-free cable material; and combining the powder with an ethylene-vinyl acetate copolymer having a low melt index and a high vinyl acetate content and five different types of primary / auxiliary antioxidants to prepare a sheath material for a nuclear cable having excellent comprehensive performance; (3) Magnesium hydroxide with a D50 particle size of about 100 nm was prepared by grading commercially available magnesium hydroxide using a grading wheel; and nano-montmorillonite and melamine cyanurate were compounded as synergists to significantly improve the flame retardant properties of the cable. The cable can still pass the cable bundle combustion test after equivalent to 80 years of long-term aging and superimposed cobalt-60γ source irradiation. DETAILED DESCRIPTION

[0028] In order to make the above-mentioned objects, features and advantages of the present invention more clearly understood, the specific embodiments of the present invention are described in detail below in conjunction with the embodiments of the specification. In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention can also be implemented in other ways different from those described herein, and those skilled in the art can make similar generalizations without violating the connotation of the present invention.

[0029] The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials are commercially available unless otherwise specified.

[0030] Performance testing method: The radiation aging simulation conditions in the embodiment are: irradiation with a cobalt-60γ source (cumulative dose of 375 kGy, dose rate of 1±0.5 kGy / h); (1) Tensile strength: Tested in accordance with GB / T 2951-2008 "General test methods for insulation and sheath materials of electric and optical cables"; (2) Elongation at break: Tested in accordance with GB / T 2951-2008 "General test methods for insulation and sheath materials of electric and optical cables"; (3) Toxicity index: tested in accordance with Ministry of Public Safety Standard 02-713; (4) Bundle combustion: Tested in accordance with GB / T 19666-2019 "General Rules for Flame Retardant and Fire Resistant Wires, Cables or Optical Cables"; (5) Long-term thermal oxidation aging and radiation aging: refer to enterprise standards.

[0031] (6) Thermal aging life assessment: Refer to the Arrhenius equation, and the end of life is when the elongation at break retention rate is 50%.

[0032] (7) -35℃ low temperature tensile test: Test in accordance with GB / T 2951-2008 "General test methods for insulation and sheath materials of electric and optical cables".

[0033] The materials used in the examples and comparative examples of the present invention are as follows: Ultra-high molecular weight polyethylene: Ticona 5113; Ethylene vinyl acetate copolymer: Taiho UE33002; Compatibilizer: Clariant MA4221 and homemade EVA wax-g-MAH mixture, EVA wax is Honeywell AC400A; Carbon black masterbatch: Cabot 2762; Magnesium hydroxide: Aifur F71 (D50 particle size 100 nm after classification treatment); Lubricant: a mixture of Wujiang silicone masterbatch PMAF-15, Wacker silicone oil AK350, and Sino polyethylene wax 119; in an embodiment of the present invention, the mass ratio of the three is 4:2:1; Synergists: Nanocor I.44P, Green Melamine Cyanurate; Metallocene-catalyzed linear low-density polyethylene: ExxonMobil 3518CB; The antioxidant is a mixture of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] 1010, phosphite 168, thiophenol 300#, and thioester 412S antioxidant; the ratio of each component is 1:1:1:1.

[0034] Table 1 Raw material ratios of various examples and comparative examples (specific details are subject to the examples) Example 1 This embodiment provides a thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable. The thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable includes, in parts by weight, 20 parts of ultra-high molecular weight polyethylene, 45 parts of ethylene-vinyl acetate copolymer, 15 parts of a compatibilizer, 5 parts of a carbon black masterbatch, 180 parts of magnesium hydroxide (graded, D50=100 nm), 5 parts of an antioxidant, 1 part of a lubricant, 5 parts of nano-montmorillonite, and 5 parts of melamine cyanurate.

[0035] This embodiment provides a method for preparing a thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable, specifically comprising: First, ultra-high molecular weight polyethylene was crushed into powder with a particle size of 100-500 mesh by a pulverizer, and magnesium hydroxide was screened out with a D50 particle size of 100 nm by a classifying wheel (the frequency of the classifying wheel was set to 105 Hz, and the nozzle flow rate of the compressed air was 6m 3 / min), then magnesium hydroxide, ethylene-vinyl acetate copolymer and antioxidant are prepared into masterbatch through a twin-screw extruder (the twin-screw temperature is set at 100-120°C, the twin-screw speed is 180r / min, and the measured material temperature is 170°C). Finally, the ultra-high molecular weight polyethylene powder, masterbatch and other components are mixed and granulated through a reciprocating single-screw extruder (the reciprocating single-screw temperature is set at 140-160°C, the screw speed is 200r / min, and the measured material temperature is 180°C) to obtain the cable material.

[0036] Example 2 This embodiment provides a thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable. The thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable includes, in parts by weight, 20 parts of ultra-high molecular weight polyethylene, 45 parts of ethylene-vinyl acetate copolymer, 15 parts of a compatibilizer, 5 parts of a carbon black masterbatch, 180 parts of magnesium hydroxide (graded, D50=100 nm), 5 parts of an antioxidant, 1 part of a lubricant, 5 parts of nano-montmorillonite, and 5 parts of melamine cyanurate.

[0037] This embodiment provides a method for preparing a thermoplastic low-smoke halogen-free flame-retardant sheath material for nuclear cables. Ultra-high molecular weight polyethylene is added in a granular state, and the remaining specific steps are the same as those in Example 1.

[0038] Example 3 This embodiment provides a thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable. The thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable includes, in parts by weight, 30 parts of ultra-high molecular weight polyethylene, 35 parts of ethylene-vinyl acetate copolymer, 15 parts of a compatibilizer, 5 parts of a carbon black masterbatch, 180 parts of magnesium hydroxide (graded, D50=100 nm), 5 parts of an antioxidant, 1 part of a lubricant, 5 parts of nano-montmorillonite, and 5 parts of melamine cyanurate.

[0039] This embodiment provides a method for preparing a thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable, and the specific steps are the same as those in Example 1.

[0040] Example 4 This embodiment provides a thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable. The thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable includes, in parts by weight, 20 parts of ultra-high molecular weight polyethylene, 45 parts of ethylene-vinyl acetate copolymer, 15 parts of a compatibilizer, 5 parts of a carbon black masterbatch, 180 parts of magnesium hydroxide (graded, D50=100nm), 5 parts of an antioxidant, 1 part of a lubricant, 5 parts of nano-montmorillonite, and 10 parts of melamine cyanurate.

[0041] This embodiment provides a method for preparing a thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable, and the specific steps are the same as those in Example 1.

[0042] Comparative Example 1 This embodiment provides a thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable. The thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable includes, in parts by weight, 20 parts of ultra-high molecular weight polyethylene, 45 parts of ethylene-vinyl acetate copolymer, 15 parts of a compatibilizer, 5 parts of a carbon black masterbatch, 180 parts of magnesium hydroxide (ungraded, D50=1 micron), 5 parts of an antioxidant, 1 part of a lubricant, 5 parts of nano-montmorillonite, and 5 parts of melamine cyanurate.

[0043] This embodiment provides a method for preparing a thermoplastic low-smoke halogen-free flame-retardant sheath material for nuclear cables. The magnesium hydroxide is not pre-classified using a grading wheel, and the remaining specific steps are the same as those in Example 1.

[0044] Comparative Example 2 This embodiment provides a thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable. The thermoplastic low-smoke halogen-free flame-retardant sheath material for a nuclear cable includes, in parts by weight, 30 parts of metallocene-catalyzed linear low-density polyethylene, 45 parts of ethylene-vinyl acetate copolymer, 15 parts of a compatibilizer, 5 parts of a carbon black masterbatch, 180 parts of magnesium hydroxide (unfractionated, D50=1 micron), 5 parts of an antioxidant, 1 part of a lubricant, 5 parts of nano-montmorillonite, and 5 parts of melamine cyanurate.

[0045] This embodiment provides a method for preparing a thermoplastic low-smoke halogen-free flame-retardant sheath material for nuclear cables. The magnesium hydroxide is not pre-classified using a grading wheel, and the remaining specific steps are the same as those in Example 1.

[0046] Application Example: The sheath materials prepared in each embodiment and comparative example are used to prepare nuclear cables. A nuclear cable, with a model specification of WDZ-HLYJY-0.6 / 1kV 1×630, is insulated with cross-linked polyethylene (XLPE) using radiation cross-linking and a thermal elongation of 50%. The insulation is wrapped with two layers of low-smoke, halogen-free, flame-retardant fabric tape. The outer sheath is produced by extrusion and has a nominal thickness of 3.7 mm. The cable outer diameter is 46 mm.

[0047] The specific test results are shown in Table 2: Table 2 The test results in Table 2 show that, by comparing Examples 1 and 2, pre-crushing ultra-high molecular weight polyethylene into a powder before mixing it with other materials can effectively improve the tensile strength and elongation at break of the cable material. A comparison of Examples 1 and 4 shows that when melamine cyanurate is added at a level of 10 parts per million (3.5%), the toxicity index fails the standard.

[0048] In the bundled combustion test, by comparing the experimental data of Examples 1-4 and Comparative Example 1, it is fully demonstrated that the flame retardant properties of the cable material are improved by the grading wheel grading treatment of magnesium hydroxide; although the bundled combustion performance of the sheath material prepared in Examples 1-4 is slightly deteriorated (compared with the original) after 80 years of long-term aging and irradiation aging, it still maintains excellent flame retardancy, still far exceeding the national standard requirements, and is suitable for harsh scenarios such as nuclear power and chemical industry; while Comparative Example 1 (not graded by the grading wheel) cannot meet the national standard requirements when the same amount is added.

[0049] By comparing Examples 1-4, Comparative Example 1 and Comparative Example 2, it can be seen that the addition of ultra-high molecular weight polyethylene can effectively extend the service life of the cable material to more than 80 years; when metallocene-catalyzed linear low-density polyethylene is added, the service life is only 57.5 years.

[0050] In summary, the present invention provides a thermoplastic low-smoke, halogen-free, flame-retardant sheathing material for nuclear power plants and its preparation method. This sheathing material exhibits excellent comprehensive properties (radiation resistance, low-temperature resistance, long service life, and flame retardancy). By crushing ultra-high molecular weight polyethylene and adding it to the low-smoke, halogen-free cable material, the material is combined with an ethylene-vinyl acetate copolymer with a low melt index and high vinyl acetate content and five different types of primary and secondary antioxidant compounds. This significantly extends the service life of the low-smoke, halogen-free sheathing material for nuclear cables and improves its radiation resistance and low-temperature resistance. Commercially available magnesium hydroxide is innovatively graded using a grading wheel. Magnesium hydroxide with a D50 particle size of approximately 100 nm is used as the primary flame retardant, and nano-montmorillonite and melamine cyanurate are compounded as synergists to significantly enhance the flame retardancy of the cable. The material can withstand the equivalent of 80 years of long-term aging and combined aging with a cobalt-60 gamma irradiation source in a bundled cable combustion test. Furthermore, the low-smoke, halogen-free cable material prepared by the present invention is thermoplastic and does not require irradiation or vulcanization after molding, resulting in a simple and cost-effective production process.

[0051] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A thermoplastic low-smoke halogen-free flame-retardant sheath material for nuclear cables, characterized in that: Contains the following components blended: 20-40 parts of ultra-high molecular weight polyethylene; 10-45 parts of ethylene-vinyl acetate copolymer; 8-15 parts of compatibilizer; 5-8 parts of polyethylene carbon black masterbatch; 150-180 parts of magnesium hydroxide; 5-10 parts of antioxidant; 0.5-1.5 parts of lubricant; 10-15 parts of synergist; The D50 particle size of the magnesium hydroxide is 80-120 nm; The ultra-high molecular weight polyethylene has a molecular weight of 1.5-3 million and a melting point of 130-136° C.; and the particle size of the ultra-high molecular weight polyethylene is 100 mesh to 500 mesh; The content of vinyl acetate in the ethylene-vinyl acetate copolymer is 15%-70%, and the melt flow rate is 0.2-5g / 10min.

2. The low-smoke, halogen-free, flame-retardant sheath material for thermoplastic nuclear cables according to claim 1, characterized in that: The vinyl acetate content of the ethylene-vinyl acetate is 33%-70%, and the melt flow rate is 0.2-0.5g / 10min.

3. The low-smoke, zero-halogen, flame-retardant sheath material for thermoplastic nuclear cables according to claim 1, characterized in that: The magnesium hydroxide is doubly modified with vinylsilane and stearic acid; the amount of the modifier stearic acid is 0.3%-0.8% of the mass of the magnesium hydroxide.

4. The low-smoke, halogen-free, flame-retardant sheath material for thermoplastic nuclear cables according to claim 1, characterized in that: The compatibilizer is a mixture of maleic anhydride grafted polyethylene wax and maleic anhydride grafted EVA wax; the mass ratio is 1:2-2:

1.

5. The low-smoke, zero-halogen, flame-retardant sheath material for thermoplastic nuclear cables according to claim 1, characterized in that: The carbon black content in the polyethylene carbon black masterbatch is 40%-60%.

6. The low-smoke, zero-halogen, flame-retardant sheath material for thermoplastic nuclear cables according to claim 1, characterized in that: The antioxidant is a mixture of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], phosphite, thiophenol and thioester antioxidants; The mass proportion of each component in the antioxidant is ≥10%.

7. The low-smoke, halogen-free, flame-retardant sheath material for thermoplastic nuclear cables according to claim 1, characterized in that: The lubricant is a mixture of silicone masterbatch, silicone oil and polyethylene wax; The mass ratio of silicone masterbatch, silicone oil and polyethylene wax in the lubricant is (3-5): (1-3):

1.

8. The low-smoke, zero-halogen, flame-retardant sheath material for thermoplastic nuclear cables according to claim 1, characterized in that: The synergist is a mixture of nano-montmorillonite and melamine cyanurate; the mass ratio of the two is 1:1-2:3; and the added amount of the melamine cyanurate is ≤1.5% of the total amount.

9. The method for preparing the low-smoke halogen-free flame-retardant sheath material for thermoplastic nuclear cables according to any one of claims 1 to 8, characterized in that: The following steps are involved: S1. Grind the ultra-high molecular weight polyethylene into a powder for use, with a particle size of 100 to 500 mesh; The commercially available magnesium hydroxide was modified and then classified by a classifying wheel to screen out powder with a D50 particle size of 80-120 nm for use; S2. The magnesium hydroxide obtained in step S1 is mixed with ethylene-vinyl acetate copolymer and an antioxidant and prepared as a masterbatch by a twin-screw extruder; S3. The ultra-high molecular weight polyethylene powder obtained in step S1, the masterbatch obtained in step S2 and other components are mixed and granulated by a reciprocating single-screw extruder to obtain the sheath material.

10. The preparation method according to claim 9, characterized in that In step S1, the frequency of the classifying wheel is set to 80-120 Hz, and the nozzle flow rate of the compressed air is 5-8 m 3 / min.

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