Flame-retardant crosslinked polyethylene cable sheath material and preparation method thereof

Abstract

The invention relates to the technical field of cables, in particular to a flame-retardant crosslinked polyethylene cable sheath material and a preparation method thereof. The sheath material is composed of low-density polyethylene, a modified magnesium hydroxide flame retardant, an ethylene-vinyl acetate copolymer, a vulcanizing agent and an auxiliary agent, wherein the modified magnesium hydroxide flame retardant is prepared through a three-step modification process: firstly, sorbitol and epoxidized soybean oil are synthesized into polyhydric alcohol, and then the polyhydric alcohol reacts with branched polyethyleneimine to form a modified adhesive; finally, magnesium hydroxide modification is realized through in-situ synthesis. A three-dimensional network structure of branched polyethyleneimine is creatively adopted to enhance the interfacial compatibility, so that the flame retardant is uniformly dispersed in a matrix, and the flame retardance is improved by forming a stable carbonized layer. The product has excellent flame retardance, mechanical property and processability, is halogen-free and environment-friendly, and meets the technical requirements of high-end cable sheath materials.

Description

Technical Field

[0001] The present invention relates to the technical field of cables, and particularly to a flame-retardant cross-linked polyethylene cable sheath material and a preparation method thereof. Background Art

[0002] With the rapid development of the wire and cable industry and the continuous improvement of safety standards, flame-retardant cross-linked polyethylene cable sheath materials have become an important research direction in the wire and cable industry. Polyethylene materials are widely used in the wire and cable field due to their excellent insulation performance, weather resistance, processing performance and economy, but their flammability has always been an important factor restricting their application. In order to improve the flame-retardant performance of polyethylene materials, the method of adding flame retardants is usually adopted in the industry.

[0003] Traditional flame retardants mainly include two categories: halogen-based flame retardants and inorganic flame retardants. Although halogen-based flame retardants have high flame-retardant efficiency, they will release toxic and harmful gases during combustion, causing adverse effects on the environment and human health, and have been gradually restricted from use. Inorganic flame retardants such as magnesium hydroxide and aluminum hydroxide have the advantages of non-toxicity, smokelessness and environmental protection, and are gradually becoming the mainstream choice of flame-retardant materials. However, inorganic flame retardants usually require a relatively high addition amount (generally 30%-60%) to achieve an ideal flame-retardant effect, which leads to a significant decline in the mechanical properties of the composite material, especially a sharp decrease in the elongation at break, making the material brittle and hard, and not meeting the flexibility requirements of the cable sheath.

[0004] As a commonly used inorganic flame retardant, the flame-retardant mechanism of magnesium hydroxide is mainly to decompose endothermically and release water molecules at high temperatures, dilute the concentration of combustible gases in the combustion area, and at the same time, the generated magnesium oxide can form a protective layer to isolate the transmission of oxygen and heat. However, there is serious interfacial incompatibility between magnesium hydroxide and the polyethylene matrix because the surface of magnesium hydroxide is strongly hydrophilic while polyethylene is a non-polar hydrophobic material, and the interfacial interaction between the two is weak, resulting in the difficulty of magnesium hydroxide to be uniformly dispersed in the polyethylene matrix and easy to form agglomerates, which not only reduces the flame-retardant efficiency but also forms stress concentration points in the material, further deteriorating the mechanical properties.

[0005] At present, the wire and cable industry has higher and higher requirements for flame-retardant materials, which not only need to meet strict flame-retardant standards, but also need to maintain good mechanical properties and processing performance, while considering environmental protection and cost factors. Traditional technologies have obvious deficiencies in balancing these requirements, and there is an urgent need to develop new flame-retardant systems to achieve the synergistic improvement of flame-retardant performance and mechanical properties. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a flame-retardant cross-linked polyethylene cable sheath material and a preparation method thereof to solve the problem that it is difficult for traditional cable sheath materials to simultaneously improve the flame-retardant performance and mechanical properties in a coordinated manner.

[0007] For the above purposes, the present invention provides a flame-retardant cross-linked polyethylene cable sheath material, which is prepared from the following raw materials by weight: 80-120 parts of low-density polyethylene, 30-70 parts of modified magnesium hydroxide flame retardant, 10-20 parts of ethylene-vinyl acetate copolymer, 1.5-4 parts of vulcanizing agent, 0.4-1.2 parts of lubricant, and 0.4-1.2 parts of antioxidant.

[0008] Preferably, the melt index of the low-density polyethylene is 1-3 g / 10 min.

[0009] Furthermore, the preparation steps of the modified magnesium hydroxide flame retardant are as follows: (1) Add sorbitol to dichloromethane, heat up to 63-68 °C, then add epoxy soybean oil and fluoboric acid, stir for 5-7 h, then add sodium carbonate to terminate the reaction, extract, and rotary evaporate to obtain epoxy soybean oil polyol; (2) Add branched polyethyleneimine and epoxy soybean oil polyol to N,N-dimethylformamide, heat up to 58-62 °C, stir and react for 5-7 h, and rotary evaporate to obtain a modified adhesive; (3) Add magnesium sulfate heptahydrate and the modified adhesive to deionized water, heat up to 80-90 °C, dropwise add an aqueous sodium hydroxide solution with a concentration of 2-4 mol / L, stir and react for 5-7 h, perform vacuum filtration with a filter membrane, wash to obtain a filter cake, dry it under vacuum, grind it through a 150-250 mesh sieve to obtain the modified magnesium hydroxide flame retardant.

[0010] Preferably, the weight ratio of sorbitol, dichloromethane, epoxy soybean oil, fluoboric acid, and sodium carbonate in step (1) is 5-6:8-12:9.8:0.1-0.3:0.2-0.8.

[0011] Preferably, the weight ratio of branched polyethyleneimine, epoxy soybean oil polyol, and N,N-dimethylformamide in step (2) is 10:4-7:100-200.

[0012] Preferably, the weight ratio of magnesium sulfate heptahydrate, the modified adhesive, deionized water, and the aqueous sodium hydroxide solution in step (3) is 50-150:1-5:250-800:200-500.

[0013] Preferably, the filter membrane in step (3) is polytetrafluoroethylene with a pore size of 0.4-0.5 μm.

[0014] Preferably, the VA content in the ethylene-vinyl acetate copolymer is 16%-20%.

[0015] Preferably, the vulcanizing agent is dicumyl peroxide.

[0016] Preferably, the lubricant is zinc stearate.

[0017] Preferably, the antioxidant is antioxidant 1010.

[0018] Furthermore, the present invention also provides a preparation method of a flame-retardant crosslinked polyethylene cable sheath material, comprising the following steps: S1: Mix low-density polyethylene, modified magnesium hydroxide flame retardant, ethylene-vinyl acetate copolymer, vulcanizing agent, lubricant and antioxidant for 5 - 10 min to obtain a premix; S2: Feed the premix into a twin-screw extruder for melt extrusion, introduce the extruded melt into a curing tank for vulcanization crosslinking, and granulate the crosslinked melt through a water ring granulator to obtain the flame-retardant crosslinked polyethylene cable sheath material.

[0019] Preferably, in step S2, the temperature of the first zone of the twin-screw extruder is 125 - 135 °C, the temperature of the second zone is 145 - 155 °C, the temperature of the third zone is 160 - 170 °C, the die head temperature is 165 - 175 °C, and the screw speed is 30 - 40 rpm.

[0020] Preferably, in step S2, the vulcanization crosslinking is carried out at a steam pressure of 10 - 15 MPa for 20 - 30 min.

[0021] Advantages of the present invention: First of all, the present invention uses a modified binder prepared by pre-reacting branched polyethyleneimine with epoxy soybean oil polyol to form a macromolecular structure with multiple functional groups, which can form a uniform and stable interfacial layer on the surface of magnesium hydroxide. The synergistic effect between the amino group and the hydroxyl group in the molecular structure of this modified binder enhances the interfacial compatibility between the flame retardant and the polymer matrix, and promotes the uniform dispersion of the flame retardant in the polyethylene matrix.

[0022] Secondly, the three-dimensional structure of branched polyethyleneimine provides more terminal amino functional groups and reaction sites, reacting with epoxy soybean oil polyol to form a complex network structure. This structure can form a more stable char layer during the thermal decomposition process, effectively blocking the transfer of heat and oxygen, and improving the flame retardant performance of the material. At the same time, the branched structure also provides a better steric hindrance effect, restricting the movement of polymer segments when heated, and having a stronger free radical capture ability, effectively interrupting the free radical chain reaction during combustion.

[0023] The present invention prepares a modified magnesium hydroxide flame retardant by an in-situ synthesis method, enabling the modified binder to come into full contact with the magnesium hydroxide crystals being formed, achieving a more thorough and uniform surface modification. This method promotes the chemical bonding between the molecular chains of the modified binder and the surface of the magnesium hydroxide crystals, forming a stable core-shell structure, which not only improves the dispersibility and interfacial compatibility of the flame retardant, but also facilitates the control of the growth process of the magnesium hydroxide crystals, obtaining a more uniform particle size distribution and a higher specific surface area, and enhancing the flame retardancy efficiency.

[0024] In terms of mechanical properties, the network structure formed by the modified binder on the surface of magnesium hydroxide plays a physical reinforcement role. Through the stress transfer and energy absorption mechanisms, the stress concentration phenomenon is reduced, and the overall mechanical strength and toughness of the material are improved. This closely bonded interfacial structure enables the material to better disperse and transfer loads when subjected to mechanical forces, reduces the risk of fracture, and significantly improves the tensile strength while maintaining good ductility.

[0025] In addition, the present invention selects epoxy soybean oil as a raw material, which belongs to renewable plant resources, reduces the dependence on petroleum-based chemicals, and conforms to the development concept of green environmental protection. The modified magnesium hydroxide flame retardant can slowly and uniformly release crystal water and decomposition products at high temperatures, forming a more effective flame retardant barrier, and no toxic or harmful gases are generated during the combustion process, meeting the environmental protection requirements of halogen-free flame retardancy.

[0026] Generally speaking, through the design of the modified binder and the innovation of the in-situ synthesis process, the present invention realizes the synergistic improvement of the flame retardancy and mechanical properties, provides a new technical route for flame retardant cross-linked polyethylene cable sheath materials, and meets the requirements of the wire and cable industry for high-performance and environmentally friendly flame retardant materials. Detailed implementation methods

[0027] To make the purpose, technical solutions and advantages of the present invention clearer, the following further elaborates on the present invention in combination with specific embodiments.

[0028] Epoxy soybean oil was purchased from Shanghai Yuanye Bio-Technology Co., Ltd., product number S50881; branched polyethyleneimine was purchased from Shanghai Yuanye Bio-Technology Co., Ltd., product number S28081, and the weight-average molecular weight was 5000.

[0029] Example 1: (1) Add 5 g of sorbitol to 8 g of dichloromethane, heat up to 63 °C, then add 9.8 g of epoxy soybean oil and 0.1 g of fluoroboric acid, stir for 5 h, and then add 0.2 g of sodium carbonate to terminate the reaction. After extraction and rotary evaporation, epoxy soybean oil polyol with an epoxy value of 0.28% is obtained; (2) Add 10 g of branched polyethyleneimine and 4 g of epoxy soybean oil polyol to 100 g of N,N-dimethylformamide, heat up to 58 °C, stir and react for 5 h, and then perform rotary evaporation to obtain a modified adhesive; (3) Add 50 g of magnesium sulfate heptahydrate and 1 g of the modified adhesive to 250 g of deionized water, heat up to 80 °C, dropwise add 200 g of a sodium hydroxide aqueous solution with a concentration of 2 mol / L, stir and react for 5 h, perform vacuum filtration using a 0.45 μm polytetrafluoroethylene filter membrane, wash alternately with deionized water and absolute ethanol three times in sequence to obtain a filter cake, dry it under vacuum, grind it through a 150-mesh sieve to obtain a modified magnesium hydroxide flame retardant; (4) Mix 80 g of low-density polyethylene (melt index 2.0 g / 10 min), 30 g of the modified magnesium hydroxide flame retardant, 10 g of ethylene-vinyl acetate copolymer (VA content 18%), 1.5 g of diisopropylbenzene peroxide, 0.4 g of zinc stearate, and 0.4 g of antioxidant 1010 at a rotation speed of 600 rpm for 8 min to obtain a premix; (5) Feed the premix into a twin-screw extruder for melt extrusion. Set the temperature of zone 1 to 125 °C, zone 2 to 145 °C, zone 3 to 160 °C, the die head temperature to 165 °C, and the screw speed to 30 rpm. Introduce the extruded melt into a curing tank, maintain it at a steam pressure of 10 MPa for 20 min, and pelletize the cross-linked melt through a water ring pelletizer. The cooling water temperature is 10 °C to obtain a flame-retardant cross-linked polyethylene cable sheath material.

[0030] Example 2: (1) Add 5.5 g of sorbitol to 10 g of dichloromethane, heat up to 65 °C, then add 9.8 g of epoxy soybean oil and 0.2 g of fluoroboric acid, stir for 6 h, and then add 0.5 g of sodium carbonate to terminate the reaction, extract, and perform rotary evaporation to obtain an epoxy soybean oil polyol with an epoxy value of 0.32%; (2) Add 10 g of branched polyethyleneimine and 5 g of epoxy soybean oil polyol to 150 g of N,N-dimethylformamide, heat up to 60 °C, stir and react for 6 h, and then perform rotary evaporation to obtain a modified adhesive; (3) Add 100 g of magnesium sulfate heptahydrate and 3 g of the modified adhesive to 500 g of deionized water, heat up to 85 °C, dropwise add 350 g of a sodium hydroxide aqueous solution with a concentration of 3.0 mol / L, stir and react for 6 h, perform vacuum filtration using a 0.45 μm polytetrafluoroethylene filter membrane, wash alternately with deionized water and absolute ethanol three times in sequence to obtain a filter cake, dry it under vacuum, grind it through a 200-mesh sieve to obtain a modified magnesium hydroxide flame retardant; (4) Mix 100 g of low-density polyethylene (melt index 2.0 g / 10 min), 50 g of modified magnesium hydroxide flame retardant, 15 g of ethylene-vinyl acetate copolymer (VA content 18%), 2.5 g of dicumyl peroxide, 0.8 g of zinc stearate, and 0.8 g of antioxidant 1010 at a rotation speed of 600 rpm for 8 min to obtain a premix; (5) Feed the premix into a twin-screw extruder for melt extrusion. Set the temperature of zone 1 at 130 °C, zone 2 at 150 °C, zone 3 at 165 °C, and the die head temperature at 170 °C. Set the screw rotation speed at 35 rpm. Feed the extruded melt into a vulcanization tank and keep it under a steam pressure of 12 MPa for 25 min. Granulate the crosslinked melt through a water ring pelletizer with the cooling water temperature at 15 °C to obtain the flame-retardant crosslinked polyethylene cable sheath material.

[0031] Example 3: (1) Add 6 g of sorbitol to 12 g of dichloromethane, heat up to 68 °C, then add 9.8 g of epoxidized soybean oil and 0.3 g of fluoroboric acid, stir for 7 h, then add 0.8 g of sodium carbonate to terminate the reaction, extract, and rotary evaporate to obtain epoxidized soybean oil polyol with an epoxy value of 0.36%; (2) Add 10 g of branched polyethyleneimine and 7 g of epoxidized soybean oil polyol to 200 g of N,N-dimethylformamide, heat up to 62 °C, stir and react for 7 h, then rotary evaporate to obtain a modified adhesive; (3) Add 150 g of magnesium sulfate heptahydrate and 5 g of the modified adhesive to 800 g of deionized water, heat up to 90 °C, dropwise add 500 g of a 4 mol / L sodium hydroxide aqueous solution, stir and react for 7 h, vacuum filter using a 0.45 μm polytetrafluoroethylene filter membrane, wash alternately with deionized water and absolute ethanol three times each to obtain a filter cake, dry it under vacuum, grind it through a 250-mesh sieve to obtain the modified magnesium hydroxide flame retardant; (4) Mix 120 g of low-density polyethylene (melt index 2.0 g / 10 min), 70 g of modified magnesium hydroxide flame retardant, 20 g of ethylene-vinyl acetate copolymer (VA content 18%), 4 g of dicumyl peroxide, 1.2 g of zinc stearate, and 1.2 g of antioxidant 1010 at a rotation speed of 600 rpm for 8 min to obtain a premix; (5) Feed the premix into a twin-screw extruder for melt extrusion. Set the temperature of zone 1 at 135 °C, zone 2 at 155 °C, zone 3 at 170 °C, and the die head temperature at 175 °C. Set the screw rotation speed at 40 rpm. Feed the extruded melt into a vulcanization tank and keep it under a steam pressure of 15 MPa for 30 min. Granulate the crosslinked melt through a water ring pelletizer with the cooling water temperature at 20 °C to obtain the flame-retardant crosslinked polyethylene cable sheath material.

[0032] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that the modified adhesive in step (3) is replaced with branched polyethyleneimine; Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that 3 g of the modified adhesive in step (3) is replaced with 2 g of branched polyethyleneimine and 1 g of epoxy soybean oil polyol.

[0033] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the branched polyethyleneimine in step (2) is replaced with linear polyethyleneimine (weight average molecular weight of 5000).

[0034] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that the modified magnesium hydroxide flame retardant in step (4) is replaced with 22 g of magnesium hydroxide passing through a 200-mesh sieve and 3 g of the modified adhesive.

[0035] Performance test: Oxygen index test: According to GB / T 2406.2-2009, standard specimens (length × width × thickness = 120 mm × 10 mm × 4 mm) were prepared using an injection molding machine and tested after conditioning in an environment of 23 ± 2 °C for 24 h; an initial oxygen concentration of 30% was selected, and the oxygen concentration change gradient was adjusted to ±0.5%; the gas flow rate was maintained at 40 ± 10 mm / s, and the oxygen concentration was adjusted until the critical condition of the specimen burning time reaching 180 s or the burning length reaching 50 mm was achieved; at least 15 valid combustion data were recorded, and the oxygen index value was calculated according to the standard formula. The results are shown in Table 1.

[0036] Vertical burning test: According to GB / T 2408-2008, the standard specimen (125 mm × 13 mm × 3 mm) was dried in an oven at 50 °C for 4 h and then placed in a desiccator to cool; flame calibration: adjust the height of the Bunsen burner flame to 20 ± 2 mm, and the tip of the blue flame cone touches the lower edge of the specimen; apply the flame to the specimen for 10 s and then remove it, and record the afterflame time t1; immediately apply the flame again for 10 s after the afterflame extinguishes, and record the afterflame time t2; classification determination: determine V-0 level if t1 + t2 ≤ 30 s, V-1 level if ≤ 60 s, and V-2 level if ≤ 250 s.

[0037] Mechanical property test: According to GB / T 1040.1-2018, dumbbell-shaped specimens (Type I) were injection molded and conditioned in an environment of 23 ± 2 °C and 50 ± 5% RH for 48 h before testing; tensile test: set the tensile rate at 50 mm / min, and record the tensile strength and elongation at break; 5 parallel samples were tested for each group of specimens, and the arithmetic mean was taken after excluding the maximum and minimum values; the results are shown in Table 1.

[0038] Table 1 Performance test results

[0039] Data analysis: From the performance test results of Examples 1 - 3 in Table 1, the flame-retardant crosslinked polyethylene cable sheath material prepared by the present invention shows good performance in both flame-retardant properties and mechanical properties. The improvement in the oxygen index and vertical burning rating may be closely related to the use of modified magnesium hydroxide flame retardant and a specific binder system. This combination effectively enhances the dispersibility of the flame retardant in the polyethylene matrix and improves the material's ability to inhibit flame propagation. At the same time, the tensile strength and elongation at break show stable performance. It is speculated that the synergistic effect of branched polyethyleneimine and epoxy soybean oil polyol improves the interfacial bonding ability between the flame retardant and the polymer, thus taking into account both flame retardancy and mechanical properties.

[0040] By comparing the data of Example 2 and Comparative Example 1 in Table 1, it can be seen that when branched polyethyleneimine is directly used as the binder, the prepared flame-retardant material is inferior to the modified binder in terms of flame retardancy and mechanical properties. The above data trend shows that the modified binder plays a key role in improving the interfacial compatibility between the flame retardant and the polyethylene matrix, enhancing the dispersion stability of the flame retardant, and forming a more uniform and dense network structure in the composite material, thereby improving the flame inhibition effect. At the same time, the tensile strength and toughness of the material are also significantly improved, presumably related to the role of the modified binder in enhancing the interfacial adhesion force and reducing stress concentration. Under the effective combination of the flame retardant and the matrix, the material can better disperse and transfer the load when subjected to mechanical force, reducing the risk of fracture. In addition, this composite modification process helps to give full play to the respective advantages of branched polyethyleneimine and epoxy soybean oil polyol, forming a multi-point chemical crosslinking and physical association network. These synergistic effects jointly promote the optimization of the properties of the flame-retardant material and improve its comprehensive performance in practical applications. In summary, the introduction of the modified binder provides an effective path for enhancing the flame retardant and strengthening effects.

[0041] By comparing the data of Example 2 and Comparative Example 2 in Table 1, the modified binder prepared by pre-reaction has better flame retardancy and mechanical properties than the treatment method of directly mixing branched polyethyleneimine and epoxy soybean oil polyol. This difference may be due to the formation of a more complete chemical bonding network between branched polyethyleneimine and epoxy soybean oil polyol in the modified binder prepared by pre-reaction, resulting in a synergistic effect. The pre-reaction process may enable the hydroxyl groups of the polyol to react more fully with the amino groups of branched polyethyleneimine, forming macromolecules with a specific structure, which form a more uniform and stable interfacial layer on the surface of magnesium hydroxide. In contrast, the physical mixing method in Comparative Example 2 may lead to uneven distribution of the two components on the surface of magnesium hydroxide and weak interfacial bonding force.

[0042] From the data of Example 2 and Comparative Example 3 in Table 1, it can be seen that the modified adhesive prepared with branched polyethyleneimine has significantly superior flame retardant properties and mechanical properties compared to the modified adhesive prepared with linear polyethyleneimine. This performance difference may be due to the molecular structural characteristics of branched polyethyleneimine. The branched structure may provide more terminal amino functional groups and reaction sites, enabling the formation of a more complex and three-dimensional network structure when reacting with epoxy soybean oil polyol. This structure may have stronger interfacial interactions, enhancing the compatibility between the filler and the matrix. In addition, the branched structure may provide better steric hindrance effects, restricting the movement of polymer segments upon heating and forming a more stable char layer. At the same time, the branched structure may have stronger free radical capture ability, effectively interrupting the free radical chain reaction during combustion. In terms of mechanical properties, the branched structure may form a more uniform stress distribution network, reducing stress concentration phenomena, improving the overall mechanical strength and toughness of the material, and enabling the material to still exhibit excellent mechanical properties while maintaining high flame retardancy.

[0043] From the data of Example 2 and Comparative Example 4 in Table 1, it can be seen that the method of in-situ generating a modified magnesium hydroxide flame retardant using a modified adhesive has significant advantages compared to directly mixing magnesium hydroxide with the modified adhesive. This difference may be attributed to the fact that during the in-situ synthesis process, the modified adhesive can come into full contact with the forming magnesium hydroxide crystals, achieving a more thorough and uniform surface modification. The in-situ generation process may promote the chemical bonding between the molecular chains of the modified adhesive and the surface of the magnesium hydroxide crystals, forming a stable core-shell structure, effectively improving the dispersibility and interfacial compatibility of the flame retardant. This tightly bound interfacial structure may reduce stress concentration points, improving the mechanical strength and ductility of the material. In addition, in-situ synthesis may be conducive to controlling the growth process of magnesium hydroxide crystals, obtaining a more uniform particle size distribution and a higher specific surface area, enhancing the flame retardant efficiency. The in-situ synthesized modified magnesium hydroxide may also have better thermal stability, slowly and uniformly releasing crystal water and decomposition products at high temperatures to form a more effective flame retardant barrier. At the same time, the network structure formed by the modified adhesive on the surface of magnesium hydroxide may play a physical reinforcement role, improving the overall mechanical properties of the material through stress transfer and energy absorption mechanisms, especially significantly enhancing the tensile strength while maintaining good ductility.

[0044] Those of ordinary skill in the art should understand that: the discussion of any above embodiment is only exemplary and is not intended to imply that the scope of the present invention is limited to these examples; under the concept of the present invention, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of the present invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A flame-retardant cross-linked polyethylene cable sheath material, characterized in that, It is prepared from the following raw materials by weight parts: 80 - 120 parts of low - density polyethylene, 30 - 70 parts of modified magnesium hydroxide flame retardant, 10 - 20 parts of ethylene - vinyl acetate copolymer, 1.5 - 4 parts of vulcanizing agent, 0.4 - 1.2 parts of lubricant and 0.4 - 1.2 parts of antioxidant; The preparation steps of the modified magnesium hydroxide flame retardant are as follows: (1) Add sorbitol into dichloromethane, heat up to 63 - 68 °C, then add epoxidized soybean oil and fluoboric acid, stir for 5 - 7 h, then add sodium carbonate to stop the reaction, extract, and rotary evaporate to obtain epoxidized soybean oil polyol; (2) Add branched polyethyleneimine and epoxidized soybean oil polyol into N,N - dimethylformamide, heat up to 58 - 62 °C, stir and react for 5 - 7 h, then rotary evaporate to obtain modified adhesive; (3) Add magnesium sulfate heptahydrate and modified adhesive into deionized water, heat up to 80 - 90 °C, dropwise add sodium hydroxide aqueous solution with a concentration of 2 - 4 mol / L, stir and react for 5 - 7 h, perform vacuum filtration with a filter membrane, wash to obtain a filter cake, dry it in vacuum, grind it through a 150 - 250 - mesh sieve to obtain the modified magnesium hydroxide flame retardant; In the step (1), the weight ratio of sorbitol, dichloromethane, epoxidized soybean oil, fluoboric acid and sodium carbonate is 5 - 6:8 - 12:9.8:0.1 - 0.3:0.2 - 0.8; in the step (2), the weight ratio of branched polyethyleneimine, epoxidized soybean oil polyol and N,N - dimethylformamide is 10:4 - 7:100 - 200; in the step (3), the weight ratio of magnesium sulfate heptahydrate, modified adhesive, deionized water and sodium hydroxide aqueous solution is 50 - 150:1 - 5:250 - 800:200 - 500.

2. The flame-retardant crosslinked polyethylene cable sheath material according to claim 1, wherein The melt index of the low - density polyethylene is 1 - 3 g / 10 min.

3. The flame-retardant crosslinked polyethylene cable sheath material according to claim 1, characterized in that, The VA content in the ethylene - vinyl acetate copolymer is 16% - 20%.

4. The flame-retardant crosslinked polyethylene cable sheath material according to claim 1, wherein The vulcanizing agent is dicumyl peroxide.

5. The flame-retardant crosslinked polyethylene cable sheath material according to claim 1, wherein The lubricant is zinc stearate.

6. The flame-retardant crosslinked polyethylene cable sheath material according to claim 1, wherein The antioxidant is antioxidant 1010.

7. A method for preparing a flame-retardant cross-linked polyethylene cable sheath material according to any one of claims 1-6, characterized in that, It includes the following steps: S1: Mix low - density polyethylene, modified magnesium hydroxide flame retardant, ethylene - vinyl acetate copolymer, vulcanizing agent, lubricant and antioxidant for 5 - 10 min to obtain a premix; S2: Feed the premix into a twin - screw extruder for melt extrusion, introduce the extruded melt into a curing tank for vulcanization cross - linking, and pelletize the cross - linked melt through a water ring pelletizer to obtain a flame - retardant cross - linked polyethylene cable sheath material.

8. The preparation method of the flame-retardant crosslinked polyethylene cable sheath material according to claim 7, wherein In the step S2, the temperature of the first zone of the twin - screw extruder is 125 - 135 °C, the temperature of the second zone is 145 - 155 °C, the temperature of the third zone is 160 - 170 °C, the die head temperature is 165 - 175 °C, and the screw speed is 30 - 40 rpm.

9. The preparation method of the flame-retardant crosslinked polyethylene cable sheath material according to claim 7, characterized in that In the step S2, the vulcanization cross - linking is carried out at a steam pressure of 10 - 15 MPa for 20 - 30 min.

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