A method for preparing a fire-resistant cable insulation

Abstract

The present application relates to cable material technical field, specifically to a kind of preparation method of fire-resistant cable insulation layer.The insulation layer is made of the following raw materials by weight parts:ethylene-vinyl acetate copolymer 40-60 parts, polyolefin elastomer 15-25 parts, flame retardant 20-35 parts, boron nitride hybrid carbon nanotube grafting agent 3-10 parts, modified hollow glass microsphere agent 5-12 parts, crosslinking agent 1-3 parts, antioxidant 0.5-2 parts.The present application is through the synergistic effect of boron nitride hybrid carbon nanotube grafting agent and modified hollow glass microsphere agent, boron nitride hybrid carbon nanotube forms three-dimensional heat conduction network, improves the heat conduction and heat dissipation performance and mechanical strength of insulation layer;The surface of modified hollow glass microsphere is loaded with rare earth lanthanum and nano ceramic particles, and forms a dense ceramic protective layer when burning, significantly improves the fire resistance limit, and the present application is suitable for medium and high voltage fire-resistant cable, mining cable, marine cable and special cable for fire-fighting line.

Description

Technical Field

[0001] This invention relates to the field of cable technology, and more specifically to a method for preparing a fire-resistant cable insulation layer. Background Technology

[0002] With the increasing demands for fire safety in modern buildings and industrial facilities, the demand for fire-resistant cables is growing. Fire-resistant cables can maintain circuit integrity for a certain period under fire conditions, providing power for personnel evacuation and fire rescue. The insulation layer, as a core component of the cable, directly determines the overall fire resistance rating of the cable.

[0003] Traditional fire-resistant cable insulation primarily employs a structure of mica tape wrapped with cross-linked polyethylene (XLPE) or ethylene propylene rubber (EPR). This structure presents several technical problems: First, poor thermal conductivity and heat dissipation. Cables generate Joule heat during normal operation; poor thermal conductivity of the insulation layer leads to heat accumulation, accelerating insulation material aging and shortening cable lifespan. Studies show that for every 10°C increase in cable operating temperature, insulation lifespan is reduced by approximately 50%. Second, insufficient fire resistance. Traditional insulation layers are prone to carbonization and cracking under high-temperature flames, forming conductive paths and causing short-circuit failure. Ordinary XLPE insulated cables typically maintain circuit integrity for only 15-30 minutes under a 750°C flame, failing to meet the 90-minute fire resistance time requirement of GB / T 19216. Third, unsatisfactory ceramization effect. Existing ceramized polyolefin materials form ceramic layers with poor density, low strength, and easy peeling at high temperatures, failing to effectively isolate flame and heat transfer. Fourth, difficulty in balancing mechanical strength and flexibility. Adding a large amount of inorganic flame retardant will reduce the elongation at break and flexibility of the material, affecting the laying and installation of cables.

[0004] Carbon nanotubes possess ultra-high strength and excellent thermal and electrical conductivity, making them ideal fillers for improving the performance of polymer-based composites. Boron nitride exhibits high thermal conductivity (approximately 300 W / m·K), good electrical insulation, and thermal stability, making it an ideal material for improving the thermal conductivity of insulation layers. However, the key to improving the fire resistance of cable insulation layers lies in how to hybridize and modify boron nitride and carbon nanotubes to achieve uniform dispersion in the polymer matrix and form a three-dimensional thermally conductive network, while simultaneously synergistically enhancing their effects with ceramic fillers such as hollow glass microspheres.

[0005] Hollow glass microspheres possess a hollow structure, low density, and good flowability, and can react with polymer decomposition products at high temperatures to form a ceramicized protective layer. However, ordinary hollow glass microspheres have poor interfacial bonding strength with the polymer matrix and are prone to cracking in flames, resulting in limited ceramicization effects.

[0006] To address the aforementioned technical problems, this invention develops a cable insulation material with high thermal conductivity, high fire resistance, and high mechanical strength by designing a synergistic system of boron nitride hybrid carbon nanotube grafting agent and modified hollow glass microsphere agent. Summary of the Invention

[0007] In view of the deficiencies of the prior art, the purpose of this invention is to provide a method for preparing a fire-resistant cable insulation layer to solve the problems mentioned in the background art.

[0008] The present invention solves the technical problem by adopting the following technical solution:

[0009] This invention provides a method for preparing a fire-resistant cable insulation layer, comprising the following steps:

[0010] The process includes wire drawing and annealing, wire twisting, insulation extrusion, mica tape wrapping, testing, cabling, sheath extrusion, finished product inspection and packaging. The preparation of the insulation layer includes the following steps:

[0011] Step 1: Weigh out the following components by weight: ethylene-vinyl acetate copolymer, polyolefin elastomer, flame retardant, boron nitride hybrid carbon nanotube grafting agent, modified hollow glass microsphere agent, crosslinking agent, and antioxidant.

[0012] Step 2: Put the ethylene-vinyl acetate copolymer and polyolefin elastomer into a mixer and plasticize them at 110-130℃ for 3-5 minutes to obtain the plasticized base material;

[0013] Step 3: Add flame retardant, boron nitride hybrid carbon nanotube grafting agent, modified hollow glass microsphere agent and antioxidant to the plasticized base material, and continue to mix for 8-15 minutes to obtain the compound.

[0014] Step 4: The compound is melt-blended and extruded through a twin-screw extruder at an extrusion temperature of 120-150℃ and a screw speed of 100-300 r / min to obtain insulating material granules.

[0015] Step 5: Add insulating material granules into the extruder hopper and extrude the insulating layer onto the surface of the stranded conductor. The extrusion temperature is 130-160℃ and the extrusion speed is 10-30m / min to obtain the insulated wire core.

[0016] Step 6: Wrap mica tape around the surface of the insulated wire core, with an overlap rate of 15-25% and 1-3 wrapping layers.

[0017] Step 7: Cross-link the insulated wire core wrapped with mica tape by electron beam irradiation or silane irradiation with an irradiation dose of 80-150 kGy to obtain the fire-resistant cable insulation layer.

[0018] Preferably, the insulating layer is made from the following raw materials in parts by weight: 40-60 parts of ethylene-vinyl acetate copolymer, 15-25 parts of polyolefin elastomer, 20-35 parts of flame retardant, 3-10 parts of boron nitride hybridized carbon nanotube grafting agent, 5-12 parts of modified hollow glass microsphere agent, 1-3 parts of crosslinking agent, and 0.5-2 parts of antioxidant.

[0019] Preferably, the method for preparing the boron nitride hybridized carbon nanotube grafting agent is as follows:

[0020] S101: By weight, add 5-10 parts boron nitride and 3-8 parts multi-walled carbon nanotubes to 40-60 parts anhydrous ethanol, and ultrasonically disperse for 30-60 min to obtain a mixed dispersion.

[0021] S102: Add 2-5 parts of silane coupling agent KH550 and 1-3 parts of polyvinylpyrrolidone to the mixed dispersion, heat to 60-80℃, and stir at 400-600r / min for 2-4h to obtain the hybrid modified liquid.

[0022] S103: Add 4-8 parts of grafting solution and 0.5-1.5 parts of benzoyl peroxide to the hybrid modification solution. The grafting solution is glycidyl methacrylate or hydroxyethyl acrylate. The polymerization grafting reaction is carried out at 70-85℃ for 3-5 hours. After the reaction is completed, the solution is filtered, washed and dried to obtain boron nitride hybrid carbon nanotube grafting agent.

[0023] In boron nitride-hybridized carbon nanotube grafting agents, boron nitride and carbon nanotubes are co-hybridized. Hexagonal boron nitride (h-BN) has a graphite-like layered structure, a thermal conductivity as high as 300 W / m·K, and is an excellent electrical insulator. Carbon nanotubes possess a one-dimensional nanostructure and ultra-high strength. After ultrasonic dispersion, the carbon nanotubes interweave between the boron nitride sheets, forming a three-dimensional hybrid network structure of "sheet-wire". This structure can simultaneously leverage the thermal conductivity and insulation properties of boron nitride and the mechanical reinforcing properties of carbon nanotubes. Surface modification with a silane coupling agent, KH550, allows the amino groups on the surface of boron nitride to react with the hydroxyl groups on the surface of boron nitride and the carboxyl groups on the surface of carbon nanotubes, introducing organic functional groups onto the surface of the hybrid and providing reaction sites for subsequent graft polymerization. Polyvinylpyrrolidone (PVP), as a nonionic surfactant, adsorbs onto the surface of the hybrid, preventing nanoparticle aggregation through steric hindrance.

[0024] Glycidyl methacrylate (GMA) or hydroxyethyl acrylate (HEA) undergoes free radical polymerization with double bonds on the surface of the hybrid under the action of initiator BPO, grafting polymer chains onto the surface of the boron nitride-carbon nanotube hybrid. The epoxy groups in GMA can react with the carboxyl or hydroxyl groups in EVA to enhance the interfacial bonding with the matrix; the hydroxyl groups in HEA can form hydrogen bonds with the polymer.

[0025] The triple function of grafted polymer chains: improving the dispersion stability of hybrids in EVA matrix; polymer chains acting as "molecular bridges" to enhance the interfacial bonding strength between hybrids and matrix; epoxy groups or hydroxyl groups participating in cross-linking reactions to increase cross-linking density and mechanical strength.

[0026] Preferably, the boron nitride is hexagonal boron nitride with a particle size of 50-200 nm; the multi-walled carbon nanotubes have a diameter of 10-30 nm and a length of 5-20 μm; and the polyvinylpyrrolidone has a molecular weight of 10,000-50,000.

[0027] Preferably, the method for preparing the modified hollow glass microsphere agent is as follows:

[0028] S201: By weight, mix 2-5 parts of 2-5% lanthanum nitrate solution, 1-3 parts of nanocellulose, 4-7 parts of silicon dioxide and 2-3 parts of silicon carbide powder, and stir at 300-500 r / min for 30-60 min at 40-60℃ to obtain an inert liquid.

[0029] S202: Add 10-15 parts of hollow glass microspheres to 3-5 times their weight of the active liquid, sonicate for 20-40 minutes, stir at 200-300 r / min, and treat at 40-60℃ to obtain a modified microsphere suspension.

[0030] S203: Filter the modified microsphere suspension and dry it at 80-100℃ for 4-6 hours to obtain the modified hollow glass microsphere agent.

[0031] Preferably, the hollow glass microspheres have a particle size of 10-100 μm, a wall thickness of 1-2 μm, and a true density of 0.2-0.5 g / cm³.

[0032] Preferably, the length of the nanocellulose is 100-300 nm and the diameter is 5-15 nm; the particle size of the silica is 20-50 nm; the particle size of the silicon carbide is 30-80 nm; and the mass fraction of the lanthanum nitrate solution is 2-5%.

[0033] The composition of the impregnation solution for modified hollow glass microspheres: Lanthanum nitrate provides rare earth lanthanum ions, nanocellulose provides nanoscale bio-based reinforcing fibers, and silica and silicon carbide provide nano-ceramic particles. These four components are combined to form a "rare earth-nanofiber-ceramic particle" composite impregnation system.

[0034] Ultrasonic modification: Under ultrasonic conditions, lanthanum ions, nanocellulose, nanosilica, and nanosilicon carbide in the ultrasonic solution penetrate into the micropores and defects on the surface of hollow glass microspheres, forming a uniform coating layer. Function of lanthanum ions: Rare earth lanthanum can promote the formation of a glassy phase on the surface of glass microspheres at high temperatures, lowering the ceramization temperature; simultaneously, lanthanum ions can react with polymer decomposition products to generate stable lanthanum oxides, enhancing the density of the ceramic layer. Function of nanocellulose: Nanocellulose acts as a bio-based template, guiding the orderly arrangement of nano-ceramic particles on the surface of hollow glass microspheres; carbonization at high temperatures forms a carbon skeleton, enhancing the structural strength of the ceramic layer. Function of nanosilica and silicon carbide: As ceramization precursors, sintering at high temperatures forms a dense SiO2-SiC composite ceramic layer, effectively isolating flames and heat; the high hardness of silicon carbide improves the wear resistance and erosion resistance of the ceramic layer.

[0035] Preferably, the flame retardant is a compound of at least two of aluminum hydroxide, magnesium hydroxide, ammonium polyphosphate, and melamine cyanurate; the crosslinking agent is at least one of dicumyl peroxide and triallyl isocyanurate; and the antioxidant is at least one of antioxidant 1010 and antioxidant 168.

[0036] Preferably, the length-to-diameter ratio of the twin-screw extruder in step S304 is 36:1-48:1; and the thickness of the insulation layer in step S305 is 0.8-2.5 mm.

[0037] Preferably, in step S307, the accelerating voltage for electron beam irradiation crosslinking is 1.0-2.5 MeV, and the beam current is 10-30 mA.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] This invention utilizes a boron nitride-hybridized carbon nanotube grafting agent to blend high thermal conductivity boron nitride and high-strength carbon nanotubes, forming a "lamellar-linear" three-dimensional network structure. Polymer chains are then introduced onto the surface of the hybrid through graft polymerization. This structure forms a three-dimensional thermally conductive network within the EVA matrix, increasing the thermal conductivity of the insulation layer and effectively reducing the cable's operating temperature. Simultaneously, the grafted polymer chains enhance interfacial bonding, improving tensile strength. The modified hollow glass microspheres of this invention undergo ultrasonic modification treatment using an aqueous solution containing rare-earth lanthanum, nanocellulose, silica, and silicon carbide. Lanthanum ions lower the ceramization temperature, nanocellulose provides carbon skeleton reinforcement, and silica and silicon carbide form a dense ceramic layer. Under high flame temperatures, the modified hollow glass microspheres rapidly form a continuous and dense ceramization protective layer, effectively isolating the flame and heat, achieving a fire resistance limit of over 950℃ × 90 min. The boron nitride-hybridized carbon nanotube grafting agent and the modified hollow glass microspheres exhibit a synergistic effect. The high thermal conductivity network formed by carbon nanotubes and boron nitride can rapidly conduct and diffuse heat, delaying the temperature rise of the insulation layer; modified hollow glass microspheres form a ceramic layer at high temperatures, and the combination of the two achieves a dual refractory mechanism of "thermal conduction-ceramic barrier". The preparation process of this invention adopts a combination of intensive mixing-twin-screw extrusion-irradiation crosslinking process to ensure uniform dispersion of nanofillers in the matrix and formation of crosslinked networks, resulting in stable and reliable product performance. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to specific examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] Example 1

[0042] A fire-resistant cable insulation layer is made from the following raw materials in parts by weight: 50 parts of ethylene-vinyl acetate copolymer (VA content 22%), 20 parts of polyolefin elastomer, 28 parts of flame retardant (aluminum hydroxide: ammonium polyphosphate = 2:1), 6 parts of boron nitride hybridized carbon nanotube grafting agent, 8 parts of modified hollow glass microsphere agent, 2 parts of dicumyl peroxide, and 1 part of antioxidant 1010.

[0043] (I) Preparation of boron nitride hybrid carbon nanotube grafting agent: S101: Add 8 parts of hexagonal boron nitride (particle size 100nm) and 5 parts of multi-walled carbon nanotubes (tube diameter 20nm, length 10μm) to 50 parts of anhydrous ethanol, and ultrasonically disperse for 45min (power 400W) to obtain a mixed dispersion; S102: Add 3.5 parts of silane coupling agent KH550 and 2 parts of polyvinylpyrrolidone (molecular weight 30000) to the mixed dispersion, heat to 70℃, and stir at 500r / min for 3h to obtain a hybrid modified liquid; S103: Add 6 parts of glycidyl methacrylate and 1 part of benzoyl peroxide to the hybrid modified liquid, and polymerize and graft at 78℃ for 4h. Filter, wash and dry to obtain boron nitride hybrid carbon nanotube grafting agent.

[0044] (II) Preparation of modified hollow glass microspheres; S201: Mix 3.5 parts of lanthanum nitrate solution (3.5% by mass), 2 parts of nanocellulose (200 nm in length and 10 nm in diameter), 5.5 parts of silica (35 nm in particle size) and 2.5 parts of silicon carbide (50 nm in particle size), and stir at 400 r / min for 45 min at 50 °C to obtain an aqueous solution; S202: Add 12 parts of hollow glass microspheres (50 μm in particle size, 1.5 μm in wall thickness, and 0.35 g / cm³ in true density) to 4 times the mass of the aqueous solution, and sonicate for 30 min (300 W power), stirring at 250 r / min, and at 50 °C to obtain a modified microsphere suspension; S203: Filter the modified microsphere suspension and dry it at 90 °C for 5 h to obtain the modified hollow glass microspheres.

[0045] (III) Preparation of the insulating layer: S301: Weigh each component according to the weight parts; S302: Put EVA and POE into a mixer and plasticize at 120℃ for 4 min to obtain a plasticized base material; S303: Add flame retardant, boron nitride hybrid carbon nanotube grafting agent, modified hollow glass microsphere agent and antioxidant to the plasticized base material, and continue to mix for 12 min to obtain a compound; S304: Melt-blend extruder the compound through a twin-screw extruder at an extrusion temperature of 130℃ and a screw speed of 200 r / min to obtain... Insulating material granules; S305: Add the insulating material granules to an extruder and extrude the insulation layer onto the surface of the stranded conductor. The extrusion temperature is 145℃, the extrusion speed is 20m / min, and the insulation layer thickness is 1.5mm to obtain the insulated wire core; S306: Wrap mica tape around the surface of the insulated wire core with an overlap rate of 20% and wrap 2 layers; S307: Perform electron beam irradiation crosslinking on the insulated wire core wrapped with mica tape. The accelerating voltage is 1.5MeV, the beam current is 20mA, and the irradiation dose is 120kGy to obtain the fire-resistant cable insulation layer.

[0046] Example 2

[0047] Raw material ratio: EVA 40 parts, POE 15 parts, flame retardant 20 parts (magnesium hydroxide: melamine cyanurate = 1:1), boron nitride hybridized carbon nanotube grafting agent 3 parts, modified hollow glass microspheres 5 parts, dicumyl peroxide 1 part, antioxidant 168 0.5 parts. Boron nitride hybridized carbon nanotube grafting agent: boron nitride 5 parts, carbon nanotubes 3 parts, grafting solution hydroxyethyl acrylate 4 parts. Modified hollow glass microspheres: lanthanum nitrate solution 2 parts (2%), nanocellulose 1 part, silicon dioxide 4 parts, silicon carbide 2 parts. Preparation process is as described in Example 1.

[0048] Example 3

[0049] Raw material ratio: EVA 60 parts, POE 25 parts, flame retardant 35 parts (aluminum hydroxide: ammonium polyphosphate: melamine cyanurate = 1:1:1), boron nitride hybridized carbon nanotube grafting agent 10 parts, modified hollow glass microsphere agent 12 parts, triallyl isocyanurate 3 parts, antioxidant 1010 2 parts. Boron nitride hybridized carbon nanotube grafting agent: boron nitride 10 parts, carbon nanotubes 8 parts, grafting solution glycidyl methacrylate 8 parts. Modified hollow glass microsphere agent: lanthanum nitrate solution 5 parts (5%), nanocellulose 3 parts, silicon dioxide 7 parts, silicon carbide 3 parts. Preparation process is as described in Example 1.

[0050] Scale settings

[0051] Comparative Example 1: No boron nitride hybridized carbon nanotube grafting agent was added; otherwise, it was the same as in Example 1.

[0052] Comparative Example 2: No modified hollow glass microspheres were added; otherwise, it was the same as in Example 1.

[0053] Comparative Example 3: No grafting polymerization was performed in the boron nitride hybrid carbon nanotube grafting agent (only KH550 was modified), and the rest was the same as in Example 1.

[0054] Comparative Example 4: The modified hollow glass microsphere agent used unmodified hollow glass microspheres (directly added), and the rest was the same as in Example 1.

[0055] Comparative Example 5: Lanthanum nitrate was not added to the solution, and the rest was the same as in Example 1.

[0056] Comparative Example 6: No irradiation crosslinking (thermoplastic), otherwise the same as Example 1.

[0057] Comparative Example 7: Commercially available ordinary fire-resistant cable insulation material (ceramized polyolefin).

[0058] Performance testing

[0059] Table 1. Test results of basic performance of insulating layer materials

[0060]

[0061] Table 2 Fire resistance test results

[0062]

[0063] Table 3 Results of thermal aging and flame retardant performance tests

[0064]

[0065] Results Analysis

[0066] (1) Mechanical properties: The tensile strength of Examples 1-3 was 13.2-15.2 MPa and the elongation at break was 305-335%, which was better than that of Comparative Examples 1-7. The strength of Comparative Example 1 (without grafting agent) was only 11.8 MPa, which proved the reinforcing effect of boron nitride hybrid carbon nanotube grafting agent; the strength of Comparative Example 3 (without grafting polymerization) was 12.5 MPa, which proved the strengthening effect of grafting polymerization on interfacial bonding.

[0067] (2) Thermal conductivity: The thermal conductivity of Examples 1-3 is 0.52-0.62 W / m·K, while that of Comparative Example 1 is only 0.35 W / m·K and that of Comparative Example 7 is only 0.30 W / m·K. This demonstrates that the three-dimensional thermally conductive network of boron nitride-carbon nanotubes significantly improves the thermal conductivity of the insulating layer.

[0068] (3) Refractory performance: Examples 1-3 all passed the refractory test at 950℃ for 90 minutes, while Comparative Examples 1-7 all broke down within 75-85 minutes. This proves that the dense ceramic layer formed by the modified hollow glass microspheres in the flame is the key to the refractory performance. Comparative Example 4 (unmodified microspheres) had a refractory time of only 78 minutes, proving the necessity of liquid modification; Comparative Example 5 (without lanthanum nitrate) had a refractory time of 80 minutes, proving the role of rare earth lanthanum in lowering the ceramicization temperature and promoting the formation of a dense ceramic layer.

[0069] (4) Thermal aging performance: After thermal aging, the tensile strength retention rate of Examples 1-3 was 90-93%, and the elongation at break retention rate was 86-90%, which was better than the comparative examples. The thermal aging performance of the irradiated crosslinked (comparative example 6) was better than that of the uncrosslinked, which proved that the crosslinked network improved the thermal aging resistance.

[0070] (5) Synergistic effect: Comparing Example 1 with Comparative Examples 1-2, it can be seen that when boron nitride hybrid carbon nanotube grafting agent and modified hollow glass microsphere agent are used together, the fire resistance performance (passing through 950℃×90min) is better than that of using them alone (<85min), proving that there is a synergistic effect between the two: the former forms a heat-conducting network to delay the temperature rise, and the latter forms a ceramic layer to block the flame, realizing a dual fire resistance mechanism of "heat conduction-ceramic barrier".

[0071] In summary, the fire-resistant cable insulation layer prepared by this invention through the synergistic effect of boron nitride hybrid carbon nanotube grafting agent and modified hollow glass microsphere agent exhibits excellent mechanical properties, thermal conductivity, fire resistance, and thermal aging performance, meeting the fire resistance requirement of 950℃×90min in GB / T 19216-2008. It is suitable for medium and high voltage fire-resistant cables, mining cables, marine cables, and fire protection circuits.

[0072] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0073] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing a fire-resistant cable insulation layer, characterized in that, Includes the following steps: The process includes wire drawing and annealing, wire twisting, insulation extrusion, mica tape wrapping, testing, cabling, sheath extrusion, finished product inspection and packaging. The preparation of the insulation layer includes the following steps: Step 1: Weigh out the following components by weight: ethylene-vinyl acetate copolymer, polyolefin elastomer, flame retardant, boron nitride hybrid carbon nanotube grafting agent, modified hollow glass microsphere agent, crosslinking agent, and antioxidant. Step 2: Put the ethylene-vinyl acetate copolymer and polyolefin elastomer into a mixer and plasticize them at 110-130℃ for 3-5 minutes to obtain the plasticized base material; Step 3: Add flame retardant, boron nitride hybrid carbon nanotube grafting agent, modified hollow glass microsphere agent and antioxidant to the plasticized base material, and continue to mix for 8-15 minutes to obtain the compound. Step 4: The compound is melt-blended and extruded through a twin-screw extruder at an extrusion temperature of 120-150℃ and a screw speed of 100-300 r / min to obtain insulating material granules. Step 5: Add insulating material granules into the extruder hopper and extrude the insulating layer onto the surface of the stranded conductor. The extrusion temperature is 130-160℃ and the extrusion speed is 10-30m / min to obtain the insulated wire core. Step 6: Wrap mica tape around the surface of the insulated wire core, with an overlap rate of 15-25% and 1-3 wrapping layers. Step 7: Cross-link the insulated wire core wrapped with mica tape by electron beam irradiation or silane irradiation with an irradiation dose of 80-150 kGy to obtain the fire-resistant cable insulation layer.

2. The method for preparing the fire-resistant cable insulation layer according to claim 1, characterized in that, The insulating layer is made from the following raw materials in parts by weight: 40-60 parts of ethylene-vinyl acetate copolymer, 15-25 parts of polyolefin elastomer, 20-35 parts of flame retardant, 3-10 parts of boron nitride hybridized carbon nanotube grafting agent, 5-12 parts of modified hollow glass microsphere agent, 1-3 parts of crosslinking agent, and 0.5-2 parts of antioxidant.

3. The method for preparing the fire-resistant cable insulation layer according to claim 2, characterized in that, The preparation method of the boron nitride hybridized carbon nanotube grafting agent is as follows: S101: By weight, add 5-10 parts boron nitride and 3-8 parts multi-walled carbon nanotubes to 40-60 parts anhydrous ethanol, and ultrasonically disperse for 30-60 min to obtain a mixed dispersion. S102: Add 2-5 parts of silane coupling agent KH550 and 1-3 parts of polyvinylpyrrolidone to the mixed dispersion, heat to 60-80℃, and stir at 400-600r / min for 2-4h to obtain the hybrid modified liquid. S103: Add 4-8 parts of grafting solution and 0.5-1.5 parts of benzoyl peroxide to the hybrid modification solution. The grafting solution is glycidyl methacrylate or hydroxyethyl acrylate. The polymerization grafting reaction is carried out at 70-85℃ for 3-5 hours. After the reaction is completed, the solution is filtered, washed and dried to obtain boron nitride hybrid carbon nanotube grafting agent.

4. The method for preparing the fire-resistant cable insulation layer according to claim 3, characterized in that, The boron nitride is hexagonal boron nitride with a particle size of 50-200 nm; the multi-walled carbon nanotubes have a diameter of 10-30 nm and a length of 5-20 μm; the polyvinylpyrrolidone has a molecular weight of 10,000-50,000.

5. The method for preparing the fire-resistant cable insulation layer according to claim 1, characterized in that, The preparation method of the modified hollow glass microspheres is as follows: S201: By weight, mix 2-5 parts of 2-5% lanthanum nitrate solution, 1-3 parts of nanocellulose, 4-7 parts of silicon dioxide and 2-3 parts of silicon carbide powder, and stir at 300-500 r / min for 30-60 min at 40-60℃ to obtain an inert liquid. S202: Add 10-15 parts of hollow glass microspheres to 3-5 times their weight of the active liquid, sonicate for 20-40 minutes, stir at 200-300 r / min, and treat at 40-60℃ to obtain a modified microsphere suspension. S203: Filter the modified microsphere suspension and dry it at 80-100℃ for 4-6 hours to obtain the modified hollow glass microsphere agent.

6. The method for preparing the fire-resistant cable insulation layer according to claim 5, characterized in that, The hollow glass microspheres have a particle size of 10-100 μm, a wall thickness of 1-2 μm, and a true density of 0.2-0.5 g / cm³.

7. The method for preparing the fire-resistant cable insulation layer according to claim 5, characterized in that, The nanocellulose has a length of 100-300 nm and a diameter of 5-15 nm; the silica has a particle size of 20-50 nm; the silicon carbide has a particle size of 30-80 nm; and the lanthanum nitrate solution has a mass fraction of 2-5%.

8. The method for preparing the fire-resistant cable insulation layer according to claim 1, characterized in that, The flame retardant is a compound of at least two of aluminum hydroxide, magnesium hydroxide, ammonium polyphosphate, and melamine cyanurate; the crosslinking agent is at least one of dicumyl peroxide and triallyl isocyanurate; and the antioxidant is at least one of antioxidant 1010 and antioxidant 168.

9. The method for preparing the fire-resistant cable insulation layer according to claim 7, characterized in that, In step S304, the length-to-diameter ratio of the twin-screw extruder is 36:1-48:1; in step S305, the thickness of the insulation layer is 0.8-2.5 mm.

10. The method for preparing the fire-resistant cable insulation layer according to claim 7, characterized in that, In step S307, the accelerating voltage for electron beam irradiation crosslinking is 1.0-2.5 MeV, and the beam current is 10-30 mA.

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