A flame-retardant cross-linked polyethylene cable sheath material and its preparation method

Abstract

This invention relates to the field of cable technology, specifically to a flame-retardant cross-linked polyethylene cable sheath material and its preparation method. The sheath material is composed of low-density polyethylene, modified magnesium hydroxide flame retardant, ethylene-vinyl acetate copolymer, vulcanizing agent, and additives. The modified magnesium hydroxide flame retardant is prepared through a three-step modification process: first, sorbitol is synthesized into a polyol with epoxidized soybean oil; then, it reacts with branched polyethyleneimine to form a modified binder; finally, magnesium hydroxide modification is achieved through in-situ synthesis. The innovative use of the three-dimensional network structure of branched polyethyleneimine enhances interfacial compatibility, allowing the flame retardant to be uniformly dispersed in the matrix, and improving flame retardant performance by forming a stable char layer. This product combines excellent flame retardancy, mechanical properties, and processing performance, and is halogen-free and environmentally friendly, meeting the technical requirements of high-end cable sheath materials.

Description

Technical Field

[0001] This invention relates to the field of cable technology, and in particular to a flame-retardant cross-linked polyethylene cable sheath material and its preparation method. Background Technology

[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 properties, weather resistance, processing performance, and economy; however, their flammability has always been a significant factor limiting their application. To improve the flame-retardant properties of polyethylene materials, the industry typically employs the method of adding flame retardants.

[0003] Traditional flame retardants mainly fall into two categories: halogenated flame retardants and inorganic flame retardants. While halogenated flame retardants offer high flame retardant efficiency, they release toxic and harmful gases during combustion, negatively impacting the environment and human health, and their use has been gradually restricted. Inorganic flame retardants, such as magnesium hydroxide and aluminum hydroxide, offer advantages such as being non-toxic, smokeless, and environmentally friendly, and are gradually becoming the mainstream choice for flame-retardant materials. However, inorganic flame retardants typically require high addition levels (generally 30%-60%) to achieve the desired flame-retardant effect. This leads to a significant decrease in the mechanical properties of composite materials, especially a sharp reduction in elongation at break, making the material brittle and hard, failing to meet the flexibility requirements of cable sheaths.

[0004] Magnesium hydroxide, a commonly used inorganic flame retardant, primarily functions by decomposing at high temperatures, absorbing heat, and releasing water molecules. This dilutes the concentration of flammable gases in the combustion zone, while the resulting magnesium oxide forms a protective layer, isolating oxygen and heat transfer. However, magnesium hydroxide exhibits severe interfacial incompatibility with the polyethylene matrix. This is because magnesium hydroxide is highly hydrophilic, while polyethylene is a non-polar hydrophobic material. The weak interfacial interaction between the two makes it difficult for magnesium hydroxide to disperse uniformly within the polyethylene matrix, leading to agglomeration. This not only reduces flame retardant efficiency but also creates stress concentration points in the material, further deteriorating its mechanical properties.

[0005] Currently, the wire and cable industry has increasingly stringent requirements for flame-retardant materials. These materials not only need to meet stringent flame-retardant standards but also maintain good mechanical and processing properties, while also considering environmental and cost factors. Traditional technologies are significantly inadequate in balancing these requirements, necessitating the development of novel flame-retardant systems to achieve a synergistic improvement in both flame-retardant and mechanical properties. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a flame-retardant cross-linked polyethylene cable sheath material and its preparation method, so as to solve the problem that traditional cable sheath materials are difficult to simultaneously improve flame retardant performance and mechanical properties.

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

[0010] (1) Add sorbitol to dichloromethane, heat to 63-68℃, then add epoxidized soybean oil and fluoroboric acid, stir for 5-7 hours, then add sodium carbonate to stop the reaction, extract, and rotary evaporate to obtain epoxidized soybean oil polyol.

[0011] (2) Branched polyethyleneimine and epoxidized soybean oil polyol were added to N,N-dimethylformamide, heated to 58-62℃, stirred for 5-7 hours, and then rotary evaporated to obtain the modified adhesive.

[0012] (3) Add magnesium sulfate heptahydrate and modified binder to deionized water, heat to 80-90℃, add sodium hydroxide aqueous solution with a concentration of 2-4 mol / L, stir and react for 5-7 hours, filter by vacuum with filter membrane, wash, obtain filter cake, vacuum dry, grind through 150-250 mesh sieve to obtain modified magnesium hydroxide flame retardant.

[0013] Preferably, in step (1), the weight ratio of sorbitol, dichloromethane, epoxidized soybean oil, fluoroboric acid and sodium carbonate is 5-6:8-12:9.8:0.1-0.3:0.2-0.8.

[0014] Preferably, in step (2), the weight ratio of branched polyethyleneimine, epoxidized soybean oil polyol and N,N-dimethylformamide is 10:4-7:100-200.

[0015] Preferably, in step (3), the weight ratio of magnesium sulfate heptahydrate, modified binder, deionized water and sodium hydroxide aqueous solution is 50-150:1-5:250-800:200-500.

[0016] Preferably, the filter membrane in step (3) is polytetrafluoroethylene with a pore size of 0.4-0.5 μm.

[0017] Preferably, the VA content in the ethylene-vinyl acetate copolymer is 16%-20%.

[0018] Preferably, the vulcanizing agent is dicumyl peroxide.

[0019] Preferably, the lubricant is zinc stearate.

[0020] Preferably, the antioxidant is antioxidant 1010.

[0021] Furthermore, the present invention also provides a method for preparing a flame-retardant cross-linked polyethylene cable sheath material, comprising the following steps:

[0022] S1: Mix low-density polyethylene, modified magnesium hydroxide flame retardant, ethylene-vinyl acetate copolymer, vulcanizing agent, lubricant and antioxidant for 5-10 minutes to obtain a premix;

[0023] S2: The premixed material is fed into a twin-screw extruder for melt extrusion. The extruded melt is then introduced into a vulcanizing tank for vulcanization and crosslinking. The crosslinked melt is then granulated by a water ring pelletizer to obtain flame-retardant crosslinked polyethylene cable sheath material.

[0024] Preferably, in step S2, the temperature of the twin-screw extruder is 125-135℃ in zone one, 145-155℃ in zone two, 160-170℃ in zone three, 165-175℃ in the die head, and 30-40 rpm in screw speed.

[0025] Preferably, in step S2, the vulcanization crosslinking is carried out at a steam pressure of 10-15 MPa for 20-30 minutes.

[0026] The beneficial effects of this invention are:

[0027] First, this invention utilizes a modified adhesive prepared by pre-reaction of branched polyethyleneimine and epoxidized soybean oil polyol, forming a macromolecular structure with multiple functional groups. This structure can form a uniform and stable interfacial layer on the surface of magnesium hydroxide. The synergistic effect between the amino and hydroxyl groups in this modified adhesive molecular structure enhances the interfacial compatibility between the flame retardant and the polymer matrix, promoting the uniform dispersion of the flame retardant in the polyethylene matrix.

[0028] Secondly, the three-dimensional structure of branched polyethyleneimine provides more terminal amino functional groups and reaction sites, forming a complex network structure with epoxidized soybean oil polyols. This structure can form a more stable char layer during thermal decomposition, effectively blocking the transfer of heat and oxygen and improving the flame retardant properties of the material. Simultaneously, the branched structure also provides better steric hindrance, restricting the movement of polymer chain segments under heat and exhibiting stronger free radical scavenging ability, effectively interrupting free radical chain reactions during combustion.

[0029] This invention employs an in-situ synthesis method to prepare modified magnesium hydroxide flame retardants, enabling the modified binder to fully contact the forming magnesium hydroxide crystals, achieving more thorough and uniform surface modification. This method promotes the chemical bonding between the modified binder molecular chains and the surface of the magnesium hydroxide crystals, forming a stable core-shell structure. This not only improves the dispersibility and interfacial compatibility of the flame retardant but also facilitates control over the growth process of the magnesium hydroxide crystals, resulting in a more uniform particle size distribution and a higher specific surface area, thus enhancing flame retardant efficiency.

[0030] In terms of mechanical properties, the network structure formed by the modified binder on the surface of magnesium hydroxide plays a physical reinforcing role. Through stress transfer and energy absorption mechanisms, it reduces stress concentration and improves the overall mechanical strength and toughness of the material. This tightly bonded interfacial structure allows the material to better disperse and transfer loads when subjected to mechanical forces, reducing the risk of fracture and significantly improving tensile strength while maintaining good ductility.

[0031] Furthermore, this invention uses epoxidized soybean oil as a raw material, which is a renewable plant resource, reducing dependence on petroleum-based chemicals and aligning with the concept of green and environmentally friendly development. The modified magnesium hydroxide flame retardant can slowly and uniformly release water of crystallization and decomposition products at high temperatures, forming a more effective flame retardant barrier, and does not produce toxic or harmful gases during combustion, meeting the environmental requirements of halogen-free flame retardancy.

[0032] Overall, this invention achieves a synergistic improvement in flame retardant and mechanical properties through the design of modified adhesives and innovation in in-situ synthesis processes, providing a new technical route for flame-retardant cross-linked polyethylene cable sheath materials and meeting the needs of the wire and cable industry for high-performance, environmentally friendly flame-retardant materials. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0034] Epoxidized soybean oil was purchased from Shanghai Yuanye Biotechnology Co., Ltd., item number S50881; branched polyethyleneimine was purchased from Shanghai Yuanye Biotechnology Co., Ltd., item number S28081, with a weight-average molecular weight of 5000.

[0035] Example 1:

[0036] (1) Add 5g of sorbitol to 8g of dichloromethane, heat to 63℃, then add 9.8g of epoxidized soybean oil and 0.1g of fluoroboric acid, stir for 5h, then add 0.2g of sodium carbonate to stop the reaction, extract, and rotary evaporate to obtain epoxidized soybean oil polyol with an epoxy value of 0.28%;

[0037] (2) Add 10g of branched polyethyleneimine and 4g of epoxidized soybean oil polyol to 100g of N,N-dimethylformamide, heat to 58℃, stir and react for 5h, and then evaporate by rotary evaporation to obtain the modified adhesive.

[0038] (3) Add 50g of magnesium sulfate heptahydrate and 1g of modified binder to 250g of deionized water, heat to 80℃, add 200g of sodium hydroxide aqueous solution with a concentration of 2mol / L, stir and react for 5h, use a 0.45μm polytetrafluoroethylene filter membrane for vacuum filtration, wash with deionized water and anhydrous ethanol three times alternately to obtain filter cake, vacuum dry, grind and pass through a 150-mesh sieve to obtain modified magnesium hydroxide flame retardant;

[0039] (4) Mix 80g of low-density polyethylene (melt index 2.0g / 10min), 30g of modified magnesium hydroxide flame retardant, 10g of ethylene-vinyl acetate copolymer (VA content 18%), 1.5g of dicumyl peroxide, 0.4g of zinc stearate and 0.4g of antioxidant 1010 at 600rpm for 8min to obtain a premix.

[0040] (5) The premixed material is fed into a twin-screw extruder for melt extrusion. The temperature of zone 1 is set to 125°C, zone 2 to 145°C, zone 3 to 160°C, the die temperature to 165°C, and the screw speed to 30 rpm. The extruded melt is introduced into a vulcanizing tank and kept under a steam pressure of 10 MPa for 20 min. The cross-linked melt is then granulated by a water ring pelletizer with a cooling water temperature of 10°C to obtain flame-retardant cross-linked polyethylene cable sheath material.

[0041] Example 2:

[0042] (1) Add 5.5g of sorbitol to 10g of dichloromethane, heat to 65℃, then add 9.8g of epoxidized soybean oil and 0.2g of fluoroboric acid, stir for 6h, then add 0.5g of sodium carbonate to stop the reaction, extract, and rotary evaporate to obtain epoxidized soybean oil polyol with an epoxy value of 0.32%;

[0043] (2) Add 10g of branched polyethyleneimine and 5g of epoxidized soybean oil polyol to 150g of N,N-dimethylformamide, heat to 60℃, stir and react for 6h, and then evaporate by rotary evaporation to obtain the modified adhesive.

[0044] (3) Add 100g magnesium sulfate heptahydrate and 3g modified binder to 500g deionized water, heat to 85℃, add 350g sodium hydroxide aqueous solution with a concentration of 3.0mol / L, stir and react for 6h, use a 0.45μm polytetrafluoroethylene filter membrane for vacuum filtration, wash with deionized water and anhydrous ethanol three times alternately to obtain filter cake, vacuum dry, grind through a 200-mesh sieve to obtain modified magnesium hydroxide flame retardant;

[0045] (4) Mix 100g of low-density polyethylene (melt index 2.0g / 10min), 50g of modified magnesium hydroxide flame retardant, 15g of ethylene-vinyl acetate copolymer (VA content 18%), 2.5g of dicumyl peroxide, 0.8g of zinc stearate and 0.8g of antioxidant 1010 at 600rpm for 8min to obtain a premix.

[0046] (5) The premixed material is fed into a twin-screw extruder for melt extrusion. The temperature of zone 1 is set to 130°C, zone 2 to 150°C, zone 3 to 165°C, the die temperature to 170°C, and the screw speed to 35 rpm. The extruded melt is introduced into a vulcanizing tank and kept under a steam pressure of 12 MPa for 25 min. The cross-linked melt is then granulated by a water ring pelletizer with a cooling water temperature of 15°C to obtain flame-retardant cross-linked polyethylene cable sheath material.

[0047] Example 3:

[0048] (1) Add 6g of sorbitol to 12g of dichloromethane, heat to 68℃, then add 9.8g of epoxidized soybean oil and 0.3g of fluoroboric acid, stir for 7h, then add 0.8g of sodium carbonate to stop the reaction, extract, and rotary evaporate to obtain epoxidized soybean oil polyol with an epoxy value of 0.36%;

[0049] (2) Add 10g of branched polyethyleneimine and 7g of epoxidized soybean oil polyol to 200g of N,N-dimethylformamide, heat to 62℃, stir and react for 7h, and then evaporate by rotary evaporation to obtain the modified adhesive.

[0050] (3) Add 150g of magnesium sulfate heptahydrate and 5g of modified binder to 800g of deionized water, heat to 90℃, add 500g of sodium hydroxide aqueous solution with a concentration of 4mol / L, stir and react for 7h, use a 0.45μm polytetrafluoroethylene filter membrane for vacuum filtration, wash with deionized water and anhydrous ethanol three times alternately to obtain filter cake, vacuum dry, grind through a 250 mesh sieve to obtain modified magnesium hydroxide flame retardant;

[0051] (4) Mix 120g of low-density polyethylene (melt index 2.0g / 10min), 70g of modified magnesium hydroxide flame retardant, 20g of ethylene-vinyl acetate copolymer (VA content 18%), 4g of dicumyl peroxide, 1.2g of zinc stearate and 1.2g of antioxidant 1010 at 600rpm for 8min to obtain a premix.

[0052] (5) The premixed material is fed into a twin-screw extruder for melt extrusion. The temperature of zone 1 is set to 135℃, zone 2 to 155℃, zone 3 to 170℃, the die temperature to 175℃, and the screw speed to 40 rpm. The extruded melt is introduced into a vulcanizing tank and kept under a steam pressure of 15 MPa for 30 min. The cross-linked melt is then granulated by a water ring pelletizer with a cooling water temperature of 20℃ to obtain flame-retardant cross-linked polyethylene cable sheath material.

[0053] Comparative Example 1:

[0054] The difference between Comparative Example 1 and Example 2 is that the modified adhesive in step (3) is replaced with branched polyethyleneimine;

[0055] Comparative Example 2:

[0056] The difference between Comparative Example 2 and Example 2 is that the 3g modified adhesive in step (3) is replaced with 2g branched polyethyleneimine and 1g epoxidized soybean oil polyol.

[0057] Comparative Example 3:

[0058] 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).

[0059] Comparative Example 4:

[0060] The difference between Comparative Example 4 and Example 2 is that the modified magnesium hydroxide flame retardant in step (4) is replaced with 22g of magnesium hydroxide that has passed through a 200-mesh sieve and 3g of modified adhesive.

[0061] Performance testing:

[0062] Oxygen Index Test: According to GB / T 2406.2-2009, standard specimens (length × width × thickness = 120mm × 10mm × 4mm) were prepared using an injection molding machine. The specimens were conditioned at 23±2℃ for 24 hours before testing. An initial oxygen concentration of 30% was selected, and the oxygen concentration gradient was adjusted to ±0.5%. The gas flow rate was maintained at 40±10mm / s, and the oxygen concentration was adjusted until the critical conditions of 180 seconds of combustion time or 50mm of combustion length were reached. 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.

[0063] Vertical burning test: According to GB / T 2408-2008, the standard sample (125mm×13mm×3mm) was dried in an oven at 50℃ for 4 hours and then placed in a desiccator to cool; Flame calibration: Adjust the height of the Bunsen burner flame to 20±2mm, with the tip of the blue flame cone contacting the lower edge of the sample; Apply the flame to the sample for 10 seconds and then remove it, recording the afterflame time t1; After the afterflame extinguishes, immediately apply the flame again for 10 seconds and record the afterflame time t2; Grading judgment: t1+t2≤30 seconds is judged as V-0 grade, ≤60 seconds as V-1 grade, and ≤250 seconds as V-2 grade.

[0064] Mechanical property testing: According to GB / T 1040.1-2018, dumbbell-shaped specimens (Type I) were injection molded and conditioned for 48 hours at 23±2℃ and 50±5%RH before testing; tensile test: the tensile rate was set to 50mm / min, and the tensile strength and elongation at break were recorded; 5 parallel samples were tested for each group of specimens, and the arithmetic mean was taken after removing the maximum and minimum values; the results are shown in Table 1.

[0065] Table 1 Performance Test Results

[0066]

[0067] Data Analysis:

[0068] As shown in Table 1, Examples 1-3, the flame-retardant cross-linked polyethylene cable sheath material prepared in this invention exhibits excellent performance in both flame retardancy and mechanical properties. The improved oxygen index and vertical burning rating are likely closely related to the use of modified magnesium hydroxide flame retardant and a specific adhesive system. This combination effectively enhances the dispersibility of the flame retardant in the polyethylene matrix, improving the material's ability to suppress flame propagation. Simultaneously, the tensile strength and elongation at break remain stable, suggesting that the synergistic effect of branched polyethyleneimine and epoxidized soybean oil polyol improves the bonding ability between the flame retardant and the polymer interface, thus achieving a balance between flame retardancy and mechanical properties.

[0069] By comparing the data from Example 2 and Comparative Example 1 in Table 1, it can be seen that when branched polyethyleneimine is used directly as a binder, the resulting flame-retardant material is inferior to that using a modified binder in both flame retardancy and mechanical properties. The data trends indicate that the modified binder plays a crucial role in improving the interfacial compatibility between the flame retardant and the polyethylene matrix, enhancing the dispersion stability of the flame retardant, and enabling it to form a more uniform and dense network structure in the composite material, thereby improving the flame suppression effect. Simultaneously, the tensile strength and toughness of the material are also significantly improved, presumably related to the modified binder's effect of enhancing interfacial adhesion and reducing stress concentration. With the effective combination of the flame retardant and the matrix, the material can better disperse and transfer loads under mechanical forces, reducing the risk of fracture. Furthermore, this composite modification process helps to leverage the respective strengths of branched polyethyleneimine and epoxidized soybean oil polyol, forming a multi-point chemical crosslinking and physical association network. These synergistic effects jointly promote the optimization of various properties of the flame-retardant material, improving its overall performance in practical applications. In summary, the introduction of modified binders provides an effective path for improving flame retardancy and reinforcement effects.

[0070] By comparing the data from Example 2 and Comparative Example 2 in Table 1, the modified adhesive prepared by pre-reaction exhibits better flame retardant and mechanical properties compared to the treatment method of directly mixing branched polyethyleneimine and epoxidized soybean oil polyol. This difference may be due to the more complete chemical bonding network formed between branched polyethyleneimine and epoxidized soybean oil polyol in the modified adhesive prepared by pre-reaction, resulting in a synergistic effect. The pre-reaction process may allow the hydroxyl groups of the polyol to react more fully with the amino groups of the branched polyethyleneimine, forming macromolecules with specific structures, resulting in a more uniform and stable interfacial layer on the magnesium hydroxide surface. In contrast, the physical mixing method in Comparative Example 2 may lead to uneven distribution of the two components on the magnesium hydroxide surface and weaker interfacial bonding.

[0071] As can be seen from the data in Example 2 and Comparative Example 3 in Table 1, the modified adhesive prepared using branched polyethyleneimine exhibits significantly superior flame retardant and mechanical properties compared to the modified adhesive prepared using linear polyethyleneimine. This performance difference is likely due to the molecular structural characteristics of branched polyethyleneimine. The branched structure may provide more terminal amino functional groups and reaction sites, resulting in a more complex and three-dimensional network structure when reacting with epoxidized soybean oil polyols. This structure may have stronger interfacial interactions, enhancing the compatibility between the filler and the matrix. Furthermore, the branched structure may provide better steric hindrance, restricting the movement of polymer chain segments under heat and forming a more stable char layer. Simultaneously, the branched structure may have a stronger free radical scavenging ability, potentially effectively interrupting free radical chain reactions during combustion. In terms of mechanical properties, the branched structure may form a more uniform stress distribution network, reducing stress concentration and improving the overall mechanical strength and toughness of the material, allowing it to maintain high flame retardancy while still exhibiting excellent mechanical properties.

[0072] As can be seen from the data in Example 2 and Comparative Example 4 in Table 1, the method of in-situ generation of modified magnesium hydroxide flame retardant using a modified binder has significant advantages over directly mixing magnesium hydroxide and the modified binder. This difference may be attributed to the fact that the modified binder can fully contact the forming magnesium hydroxide crystals during the in-situ synthesis process, achieving more thorough and uniform surface modification. The in-situ generation process may promote the chemical bonding between the modified binder molecular chains and the surface of magnesium hydroxide crystals, forming a stable core-shell structure, which effectively improves the dispersibility and interfacial compatibility of the flame retardant. This tightly bonded interfacial structure may reduce stress concentration points and improve the mechanical strength and ductility of the material. In addition, in-situ synthesis may facilitate the control of the growth process of magnesium hydroxide crystals, obtaining a more uniform particle size distribution and a higher specific surface area, thus enhancing the flame retardant efficiency. The modified magnesium hydroxide synthesized in-situ may also have better thermal stability, and can slowly and uniformly release water of crystallization and decomposition products at high temperatures, forming a more effective flame retardant barrier. Meanwhile, the network structure formed on the surface of magnesium hydroxide by the modified adhesive may play a physical reinforcing role, improving the overall mechanical properties of the material through stress transfer and energy absorption mechanisms, especially significantly improving tensile strength while maintaining good ductility.

[0073] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the 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, The product is prepared by weight from the following raw materials: 80-120 parts low-density polyethylene, 30-70 parts modified magnesium hydroxide flame retardant, 10-20 parts ethylene-vinyl acetate copolymer, 1.5-4 parts vulcanizing agent, 0.4-1.2 parts lubricant and 0.4-1.2 parts antioxidant; The preparation steps of the modified magnesium hydroxide flame retardant are as follows: (1) Add sorbitol to dichloromethane, heat to 63-68℃, then add epoxidized soybean oil and fluoroboric acid, stir for 5-7 hours, then add sodium carbonate to stop the reaction, extract, and rotary evaporate to obtain epoxidized soybean oil polyol; the epoxy value of the epoxidized soybean oil polyol is one of 0.28%, 0.32% or 0.36%; (2) Branched polyethyleneimine and epoxidized soybean oil polyol were added to N,N-dimethylformamide, heated to 58-62℃, stirred for 5-7 hours, and then rotary evaporated to obtain the modified adhesive. (3) Add magnesium sulfate heptahydrate and modified binder to deionized water, heat to 80-90℃, add sodium hydroxide aqueous solution with a concentration of 2-4mol / L, stir and react for 5-7h, filter by vacuum with filter membrane, wash, obtain filter cake, vacuum dry, grind through 150-250 mesh sieve to obtain modified magnesium hydroxide flame retardant. In step (1), the weight ratio of sorbitol, dichloromethane, epoxidized soybean oil, fluoroboric acid, and sodium carbonate is 5-6:8-12:9.8:0.1-0.3:0.2-0.8; in step (2), the weight ratio of branched polyethyleneimine, epoxidized soybean oil polyol, and N,N-dimethylformamide is 10:4-7:100-200; in step (3), the weight ratio of magnesium sulfate heptahydrate, modified binder, deionized water, and sodium hydroxide aqueous solution is 50-150:1-5:250-800:200-500.

2. The flame-retardant cross-linked polyethylene cable sheath material according to claim 1, characterized in that, The melt flow index of the low-density polyethylene is 1-3 g / 10 min.

3. The flame-retardant cross-linked 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 cross-linked polyethylene cable sheath material according to claim 1, characterized in that, The vulcanizing agent is dicumyl peroxide.

5. The flame-retardant cross-linked polyethylene cable sheath material according to claim 1, characterized in that, The lubricant is zinc stearate.

6. The flame-retardant cross-linked polyethylene cable sheath material according to claim 1, characterized in that, 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, 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 minutes to obtain a premix; S2: The premixed material is fed into a twin-screw extruder for melt extrusion. The extruded melt is then introduced into a vulcanizing tank for vulcanization and crosslinking. The crosslinked melt is then granulated by a water ring pelletizer to obtain flame-retardant crosslinked polyethylene cable sheath material.

8. The method for preparing the flame-retardant cross-linked polyethylene cable sheath material according to claim 7, characterized in that, In step S2, the temperature of the twin-screw extruder is 125-135℃ in zone one, 145-155℃ in zone two, 160-170℃ in zone three, 165-175℃ in the die head, and the screw speed is 30-40 rpm.

9. The method for preparing the flame-retardant cross-linked polyethylene cable sheath material according to claim 7, characterized in that, In step S2, the vulcanization crosslinking is carried out under a steam pressure of 10-15 MPa for 20-30 minutes.

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