Low-smoke and halogen-free flame-retardant cable for subway and its preparation method

Abstract

The present invention discloses a low-smoke and halogen-free flame-retardant cable for subways and its preparation method, belonging to the technical field of flame-retardant cables; ethylene-vinyl acetate copolymer, high-density polyethylene, low-density polyethylene, reinforcing masterbatch, hydrophobically modified double-coated red phosphorus powder, antioxidant 1010, antioxidant DLTP, ultraviolet absorber, high-phenyl silicone rubber and carbon black are melt-extruded and pelletized by a twin-screw extruder at high temperature to obtain a flame-retardant sheath material; by using a flame-retardant sheath material containing hydrophobically modified double-coated red phosphorus powder as the outer sheath, the present invention modifies it to reduce water erosion, reduce the impact on the mechanical properties of the flame-retardant sheath material, increase flame-retardant stability, help the cable adapt to the high-humidity environment of subway tunnels, and extend the service life of the cable.

Description

Technical Field

[0001] The invention belongs to the technical field of flame-retardant cables, and particularly relates to a low-smoke and halogen-free flame-retardant cable for subways and a preparation method thereof. Background Art

[0002] A cable is usually a rope-like cable formed by stranding several or several groups of wires (at least two wires in each group). The wires in each group are insulated from each other and are often twisted around a center, and the whole is covered with a highly insulating covering layer. A cable has the characteristics of being energized inside and insulated outside. For traditional PVC wires, when a fire occurs, the PVC mixture is easily decomposed upon heating, releasing a large amount of highly toxic and corrosive gases and generating a large amount of highly toxic thick smoke.

[0003] The main characteristics of a low-smoke and halogen-free flame-retardant cable are that all materials do not contain halogens and the amount of smoke released during combustion is very small. The toxicity released by halogen-containing cables in a fire is terrifying. During a fire, the strongly diffused thick smoke makes it difficult for victims to distinguish directions, prolonging the time they stay in the fire. Thus, it is crucial to use halogen-free and low-smoke wires and cables to ensure safety, and halogen-free and low-smoke flame-retardant wires have been widely used in airports, subways, large buildings, and government projects.

[0004] The Chinese invention patent with the publication number CN102751012B discloses a low-smoke and halogen-free flame-retardant cable and a preparation method thereof. The low-smoke and halogen-free flame-retardant cable includes a conducting wire core and an insulating layer in its structure. The insulating layer is a three-layer structure, and its outer layer material is a polyolefin containing a low-smoke and halogen-free flame retardant such as magnesium hydroxide. The insulating layer enables it to have low-smoke and halogen-free flame retardancy while having excellent comprehensive mechanical properties such as tensile strength and elongation at break.

[0005] However, in a humid environment, the erosion of moisture on materials is particularly important. Magnesium hydroxide itself is prone to moisture absorption, which may cause the outer layer material to absorb water, reducing the mechanical properties of the sheath material and thus affecting the long-term durability of the cable. Summary of the Invention

[0006] The purpose of the present invention is to provide a low-smoke and halogen-free flame-retardant cable for subways and a preparation method thereof. The present invention uses a flame-retardant sheath material containing hydrophobically modified double-coated red phosphorus powder as the outer sheath. The hydrophobically modified double-coated red phosphorus powder can reduce moisture erosion, reduce the impact on the mechanical properties of the flame-retardant sheath material, increase flame-retardant stability, help the cable adapt to the high-humidity environment of subway tunnels, and extend the service life of the cable.

[0007] The purpose of the present invention can be achieved through the following technical solutions:

[0008] Preparation method of low-smoke halogen-free flame-retardant cable for subway, the preparation method comprising: uniformly covering a rubber insulation layer, a tinned copper wire braided mesh shielding layer and a flame-retardant sheath material on the surface of a conductor core in sequence from inside to outside, characterized in that the flame-retardant sheath material is prepared by the following steps:

[0009] Step 1: Using tetraethyl orthosilicate as a silicon source, generating silicon dioxide on the surface of magnesium hydroxide-coated red phosphorus powder through hydrolysis and polycondensation reactions to obtain double-coated red phosphorus powder, and modifying it with perfluorodecyltrimethoxysilane to obtain hydrophobically modified double-coated red phosphorus powder.

[0010] Step 2: Melting and extruding ethylene-vinyl acetate copolymer, high-density polyethylene, low-density polyethylene, reinforcing masterbatch, hydrophobically modified double-coated red phosphorus powder, antioxidant 1010, antioxidant DLTP, ultraviolet absorber, high-phenyl silicone rubber and carbon black by a twin-screw machine at a temperature of 140-160 °C, and pelletizing to obtain the flame-retardant sheath material.

[0011] Further, the mass ratio of ethylene-vinyl acetate copolymer, high-density polyethylene, low-density polyethylene, reinforcing masterbatch, hydrophobically modified double-coated red phosphorus powder, antioxidant 1010, antioxidant DLTP, ultraviolet absorber, high-phenyl silicone rubber and carbon black is 5-6:2-3:1-1.5:0.8-1:2-3:0.12-0.16:0.15-0.18:0.08-0.12:0.06-0.08:0.07-0.09.

[0012] Further, the magnesium hydroxide-coated red phosphorus powder is prepared by the following steps:

[0013] Adding red phosphorus powder, sodium hexametaphosphate and distilled water into a reaction kettle, stirring at 55-60 °C and 250-350 rpm for 20-30 min, adding 10 wt% magnesium sulfate solution, then adjusting the pH value to 9 with sodium hydroxide solution and continuing to stir for 2-4 h, centrifuging and filtering, drying the filter cake, naturally cooling, grinding and refining, and passing through a 600-mesh sieve to obtain the magnesium hydroxide-coated red phosphorus powder.

[0014] Further, the dosage ratio of red phosphorus powder, sodium hexametaphosphate, distilled water and magnesium sulfate solution is 8-9 kg:8-9 kg:50-60 L:25-30 L.

[0015] Further, the double-coated red phosphorus powder is specifically prepared by the following steps:

[0016] Add magnesium hydroxide-coated red phosphorus powder, 70 wt% isopropanol solution, and 25 wt% ammonia water into a reaction kettle, ultrasonically disperse for 10 - 20 min, then add tetraethyl orthosilicate, and react for 10 - 12 h under the condition of 50 - 60 °C. Centrifuge and filter, wash the filter cake, dry, grind and refine it, and pass through a 500-mesh sieve to obtain double-coated red phosphorus powder.

[0017] Furthermore, the dosage ratio of magnesium hydroxide-coated red phosphorus powder, isopropanol solution, ammonia water, and tetraethyl orthosilicate is 4 - 6 kg : 500 L : 25 - 45 kg : 30 - 50 kg.

[0018] Furthermore, the hydrophobic modified double-coated red phosphorus powder is specifically prepared through the following steps:

[0019] Add the double-coated red phosphorus powder and absolute ethanol into a reaction kettle, stir at 200 - 500 rpm for 15 - 25 min, dropwise add perfluorodecyltrimethoxysilane, stir for 12 - 14 h, centrifuge, precipitate and filter, wash the filter cake, dry, grind and refine it, and pass through a 500-mesh sieve to obtain hydrophobic modified double-coated red phosphorus powder.

[0020] Furthermore, the dosage ratio of double-coated red phosphorus powder, absolute ethanol, and perfluorodecyltrimethoxysilane is 4 - 5 kg : 50 L : 600 - 700 g.

[0021] Furthermore, the enhanced masterbatch is specifically prepared through the following steps:

[0022] Add ethylene-vinyl acetate copolymer, modified carbon nanotubes, and styrene-maleic anhydride copolymer into a high-speed mixer, stir at a speed of 900 - 1000 rpm for 10 - 20 min, then add it into a twin-screw extruder for plasticization, mixing, extrusion, and pelletizing to obtain the enhanced masterbatch.

[0023] Furthermore, the mass ratio of ethylene-vinyl acetate copolymer, modified carbon nanotubes, and styrene-maleic anhydride copolymer is 40 - 45 : 5 - 7.5 : 4 - 5.

[0024] Furthermore, the modified carbon nanotubes are specifically prepared through the following steps:

[0025] Add carboxylated carbon nanotubes, polyoxyethylene sorbitan monopalmitate, and distilled water into a reaction kettle, stir at 200 - 500 rpm for 8 - 10 min, then add 2,4-diisocyanate and dibutyltin dilaurate at 75 - 85 °C, keep warm and react for 2.5 - 3.5 h, naturally cool to room temperature, centrifuge and filter, wash the filter cake, dry, grind and refine it to obtain modified carbon nanotubes.

[0026] Further, the dosage ratio of carboxylated carbon nanotubes, polyoxyethylene sorbitan monopalmitate, distilled water, 2,4-diisocyanate, and dibutyltin dilaurate is 8 - 10 kg : 700 - 900 g : 100 - 120 L : 2 - 4 kg : 20 - 40 g.

[0027] Advantages of the present invention:

[0028] 1. The low-smoke and halogen-free flame-retardant cable for subways in the present invention uses a flame-retardant sheath material containing hydrophobically modified double-layer coated red phosphorus powder as the outer sheath. The hydrophobically modified double-layer coated red phosphorus powder can reduce moisture erosion, reduce the impact on the mechanical properties of the flame-retardant sheath material, increase flame-retardant stability, help the cable adapt to the high-humidity environment of subway tunnels, and extend the service life of the cable.

[0029] 2. By establishing a three-dimensional flame-retardant system for red phosphorus, a layer of magnesium hydroxide protective film is formed on the surface of red phosphorus. Magnesium hydroxide decomposes into magnesium oxide and water vapor, playing a role in isolating oxygen and suppressing smoke. Red phosphorus decomposes at high temperatures to generate phosphoric acid substances, which can promote the carbonization of polymer materials and form a heat-insulating layer. After the magnesium hydroxide-coated red phosphorus powder is secondarily coated with silica, not only the density of the carbon layer after combustion is increased, but also the structural strength of the double-layer coated red phosphorus powder can be improved, facilitating the maintenance of its structural integrity during the cable processing.

[0030] The hardness of silica (Mohs hardness 7) is greater than that of magnesium hydroxide (Mohs hardness 2.5 - 3). The formation of a soft-hard combined double-layer coating structure can play a buffering role, better disperse stress, and inhibit crack propagation, thereby improving the mechanical properties of the cable sheath.

[0031] 3. The flame-retardant sheath material of the present invention also contains modified carbon nanotubes. A three-dimensional network is formed by the modified carbon nanotubes and filling materials such as hydrophobically modified double-layer coated red phosphorus powder, and the mechanical properties of the sheath material are improved through effects such as interface slip and crack deflection. Among them, the modified carbon nanotubes are obtained by in-situ polymerization of polyurea resin on the surface of carboxylated carbon nanotubes, improving the compatibility of the modified carbon nanotubes in the matrix material, and forming hydrogen bonds between the amino groups in the polyurea resin and the acetic acid groups of ethylene-vinyl acetate copolymer, enhancing the interface strength, thereby further improving the mechanical properties and tensile strength of the cable. Specific embodiments

[0032] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0033] Example 1: Preparation method of low-smoke and halogen-free flame-retardant cable for subway, comprising the following steps:

[0034] S1: Add 8 kg of red phosphorus powder, 8 kg of sodium hexametaphosphate and 50 L of distilled water into a reaction kettle, stir for 20 min under the conditions of 55 °C and 250 rpm, add 25 L of magnesium sulfate solution with a mass fraction of 10%, then adjust the pH value to 9 with sodium hydroxide solution with a mass fraction of 10% and continue stirring for 2 h, carry out centrifugal filtration, dry the filter cake at 80 °C until constant weight, naturally cool to room temperature, grind and refine, and pass through a 600-mesh sieve to obtain magnesium hydroxide-coated red phosphorus powder.

[0035] Sodium hexametaphosphate acts as a ligand, binds to the surface of red phosphorus through phosphate groups to form a stable dispersion system. Magnesium sulfate and sodium hydroxide react to form magnesium hydroxide precipitation. Magnesium hydroxide crystal nuclei preferentially nucleate heterogeneously at the surface defect sites of red phosphorus and grow directionally through hydrogen bonds to form a continuous coating layer.

[0036] S2: Add 4 kg of magnesium hydroxide-coated red phosphorus powder, 500 L of isopropanol solution with a mass fraction of 70% and 25 kg of ammonia water with a mass fraction of 25% into a reaction kettle, carry out ultrasonic dispersion for 10 min, then add 30 kg of tetraethyl orthosilicate, react at 50 °C for 10 h, carry out centrifugal filtration, wash the filter cake 3 times with ethanol and deionized water respectively, dry at 60 °C until constant weight, grind and refine, and pass through a 500-mesh sieve to obtain double-coated red phosphorus powder.

[0037] Tetraethyl orthosilicate acts as a silicon source and undergoes hydrolysis and polycondensation reactions under ammonia water to generate silicon dioxide, forming a silicon dioxide coating layer on the surface of magnesium hydroxide-coated red phosphorus.

[0038] S3: Add 4 kg of double-coated red phosphorus powder and 50 L of absolute ethanol into a reaction kettle, stir for 15 min under the condition of 200 rpm, dropwise add 600 g of perfluorodecyltrimethoxysilane, stir for 12 h, carry out centrifugal precipitation filtration, wash 2 times with ethanol and deionized water, dry at 75 °C for 18 h, grind and refine, and pass through a 500-mesh sieve to obtain hydrophobic modified double-coated red phosphorus powder.

[0039] The structure of perfluorodecyltrimethoxysilane contains three methoxysilyl groups, which hydrolyze in ethanol to generate silanol groups, and then undergo a condensation reaction with the hydroxyl groups on the surface of the silicon dioxide layer to form Si-O-Si bonds, thereby fixing the fluoroalkyl chain on the surface and making the material waterproof.

[0040] S4: Add 8 kg of carboxylated carbon nanotubes, 700 g of polyoxyethylene sorbitan monopalmitate, and 100 L of distilled water into a reaction kettle, stir for 8 min under the condition of 200 rpm, then add 2 kg of 2,4 - diisocyanate and 20 g of dibutyltin dilaurate under the condition of 75 °C, keep the temperature for reaction for 2.5 h, cool naturally to room temperature, centrifuge and filter, wash the filter cake 3 times with an ethanol aqueous solution with a mass fraction of 30%, vacuum dry to constant weight, grind and refine to obtain modified carbon nanotubes.

[0041] There are hydroxyl groups on the surface of carboxylated carbon nanotubes that react rapidly with the groups para - to the benzene ring in 2,4 - diisocyanate to form urethane. 2,4 - diisocyanate continues to react with water to form carbamic acid, which turns into an amino group under heating conditions and continues to react with 2,4 - diisocyanate and water, and finally in - situ polymerizes polyurea resin on the surface of carboxylated carbon nanotubes.

[0042] S5: Add 40 kg of ethylene - vinyl acetate copolymer, 5 kg of modified carbon nanotubes, and 4 kg of styrene - maleic anhydride copolymer into a high - speed mixer, stir for 10 min at a rotation speed of 900 rpm, then add them into a twin - screw extruder, and carry out plasticization, mixing, extrusion, and pelletizing at 180 °C to obtain reinforced masterbatch.

[0043] S6: Melt - extrude 5 kg of ethylene - vinyl acetate copolymer, 2 kg of high - density polyethylene, 1 kg of low - density polyethylene, 0.8 kg of reinforced masterbatch, 2 kg of hydrophobically modified double - layer coated red phosphorus powder, 120 g of antioxidant 1010, 150 g of antioxidant DLTP, 80 g of ultraviolet absorber, 60 g of high - phenyl silicone rubber, and 70 g of carbon black at a temperature of 140 °C using a double - screw machine, and pelletize to obtain a flame - retardant sheath material.

[0044] S7: Obtain the inner core of the conductor by layering and stranding multiple strands of annealed soft copper wire, and sequentially and uniformly coat the rubber insulation layer, tin - plated copper wire braided mesh shielding layer, and flame - retardant sheath material onto the surface of the inner core of the conductor from the inside to the outside to obtain a low - smoke, halogen - free, flame - retardant cable for subway.

[0045] Example 2: A method for preparing a low - smoke, halogen - free, flame - retardant cable for subway, comprising the following steps:

[0046] S1: Add 8.5 kg of red phosphorus powder, 8.5 kg of sodium hexametaphosphate, and 55 L of distilled water into a reaction kettle, stir for 25 min under the conditions of 57.5 °C and 300 rpm, add 27.5 L of a magnesium sulfate solution with a mass fraction of 10%, then adjust the pH value to 9 with a sodium hydroxide solution with a mass fraction of 10% and continue stirring for 3 h, centrifuge and filter, dry the filter cake to constant weight under the condition of 85 °C, cool naturally to room temperature, grind and refine, and pass through a 600 - mesh sieve to obtain magnesium hydroxide - coated red phosphorus powder.

[0047] S2: Add 5 kg of magnesium hydroxide-coated red phosphorus powder, 500 L of isopropanol solution with a mass fraction of 70%, and 35 kg of ammonia water with a mass fraction of 25% into the reaction kettle, ultrasonically disperse for 15 min, then add 40 kg of tetraethyl orthosilicate, react at 55 °C for 11 h, centrifuge and filter, wash the filter cake 4 times with ethanol and deionized water respectively, dry to constant weight at 65 °C, grind and refine, and pass through a 500-mesh sieve to obtain double-coated red phosphorus powder.

[0048] S3: Add 4.5 kg of double-coated red phosphorus powder and 50 L of absolute ethanol into the reaction kettle, stir at 350 rpm for 20 min, dropwise add 650 g of perfluorodecyltrimethoxysilane, stir for 13 h, centrifuge, precipitate and filter, wash 3 times with ethanol and deionized water, dry at 80 °C for 19 h, grind and refine, and pass through a 500-mesh sieve to obtain hydrophobic modified double-coated red phosphorus powder.

[0049] S4: Add 9 kg of carboxylated carbon nanotubes, 800 g of polyoxyethylene sorbitan monopalmitate, and 110 L of distilled water into the reaction kettle, stir at 350 rpm for 9 min, then add 3 kg of 2,4-diisocyanate and 30 g of dibutyltin dilaurate at 80 °C, keep the temperature for reaction for 3 h, naturally cool to room temperature, centrifuge and filter, wash the filter cake 4 times with an ethanol aqueous solution with a mass fraction of 30%, vacuum dry to constant weight, grind and refine to obtain modified carbon nanotubes.

[0050] S5: Add 42.5 kg of ethylene-vinyl acetate copolymer, 6.25 kg of modified carbon nanotubes, and 4.5 kg of styrene-maleic anhydride copolymer into a high-speed mixer, stir at 950 rpm for 15 min, then add into a twin-screw extruder, carry out plasticization, mixing, extrusion and pelletizing at 200 °C to obtain a reinforced masterbatch.

[0051] S6: Use a double-screw machine to melt and extrude 5.5 kg of ethylene-vinyl acetate copolymer, 2.5 kg of high-density polyethylene, 1.25 kg of low-density polyethylene, 0.9 kg of reinforced masterbatch, 2.5 kg of hydrophobic modified double-coated red phosphorus powder, 140 g of antioxidant 1010, 165 g of antioxidant DLTP, 100 g of ultraviolet absorber, 70 g of high-phenyl silicone rubber, and 80 g of carbon black at 150 °C, and pelletize to obtain a flame-retardant sheath material.

[0052] S7: Obtain the inner core of the conductor by performing a hierarchical stranding operation on multiple strands of annealed soft copper wire, and sequentially and evenly coat the rubber insulation layer, the tinned copper wire braided mesh shielding layer, and the flame-retardant sheath material onto the surface of the inner core of the conductor to obtain a low-smoke, halogen-free, flame-retardant cable for subway use.

[0053] Example 3: Preparation method of low-smoke and halogen-free flame-retardant cable for subway, comprising the following steps:

[0054] S1: Add 9 kg of red phosphorus powder, 9 kg of sodium hexametaphosphate and 60 L of distilled water into a reaction kettle, stir for 30 min under the conditions of 60 °C and 350 rpm, add 30 L of magnesium sulfate solution with a mass fraction of 10%, then adjust the pH value to 9 with sodium hydroxide solution with a mass fraction of 10% and continue stirring for 4 h, carry out centrifugal filtration, dry the filter cake to constant weight under the condition of 90 °C, naturally cool to room temperature, grind and refine, and pass through a 600-mesh sieve to obtain magnesium hydroxide-coated red phosphorus powder.

[0055] S2: Add 6 kg of magnesium hydroxide-coated red phosphorus powder, 500 L of isopropyl alcohol solution with a mass fraction of 70% and 45 kg of ammonia water with a mass fraction of 25% into a reaction kettle, carry out ultrasonic dispersion for 20 min, then add 50 kg of tetraethyl orthosilicate, react for 12 h under the condition of 60 °C, carry out centrifugal filtration, wash the filter cake 5 times with ethanol and deionized water respectively, dry to constant weight at 70 °C, grind and refine, and pass through a 500-mesh sieve to obtain double-layer coated red phosphorus powder.

[0056] S3: Add 5 kg of double-layer coated red phosphorus powder and 50 L of absolute ethanol into a reaction kettle, stir for 25 min under the condition of 500 rpm, dropwise add 700 g of perfluorodecyltrimethoxysilane, stir for 14 h, carry out centrifugal precipitation filtration, wash 4 times with ethanol and deionized water, dry for 20 h under the condition of 85 °C, grind and refine, and pass through a 500-mesh sieve to obtain hydrophobic modified double-layer coated red phosphorus powder.

[0057] S4: Add 10 kg of carboxylated carbon nanotubes, 900 g of polyoxyethylene sorbitan monopalmitate and 120 L of distilled water into a reaction kettle, stir for 10 min under the condition of 500 rpm, then add 4 kg of 2,4-diisocyanate and 40 g of dibutyltin dilaurate at 85 °C, keep the temperature for reaction for 3.5 h, naturally cool to room temperature, carry out centrifugal filtration, wash the filter cake 5 times with an ethanol aqueous solution with a mass fraction of 30%, and dry to constant weight under vacuum, grind and refine to obtain modified carbon nanotubes.

[0058] S5: Add 45 kg of ethylene-vinyl acetate copolymer, 7.5 kg of modified carbon nanotubes and 5 kg of styrene-maleic anhydride copolymer into a high-speed mixer, stir for 20 min at a rotation speed of 1000 rpm, then add them into a twin-screw extruder, carry out plasticization, mixing, extrusion and pelletizing at 220 °C to obtain enhanced masterbatch.

[0059] S6: Melt and extrude 6 kg of ethylene-vinyl acetate copolymer, 3 kg of high-density polyethylene, 1.5 kg of low-density polyethylene, 1 kg of reinforcing masterbatch, 3 kg of hydrophobically modified double-layer coated red phosphorus powder, 160 g of antioxidant 1010, 180 g of antioxidant DLTP, 120 g of ultraviolet absorber, 80 g of high-phenyl silicone rubber, and 90 g of carbon black at a temperature of 160 °C using a twin-screw extruder, and pelletize to obtain a flame-retardant sheath material.

[0060] S7: Obtain a conductor inner core by performing a hierarchical stranding operation on multiple strands of annealed soft copper wire, and sequentially and uniformly coat a rubber insulation layer, a tinned copper wire braided mesh shielding layer, and a flame-retardant sheath material onto the surface of the conductor inner core from the inside to the outside to obtain a low-smoke, halogen-free, flame-retardant cable for subway use.

[0061] In Examples 1 - 3, the ultraviolet absorber used is UV-328, and the remaining raw materials are all commercially available products.

[0062] Comparative Example 1: The difference from Example 1 is that step S1 is deleted, and in step S2, magnesium hydroxide-coated red phosphorus powder is replaced with red phosphorus powder, and the remaining steps remain unchanged to prepare a low-smoke, halogen-free, flame-retardant cable for subway use.

[0063] Comparative Example 2: The difference from Example 1 is that step S3 is deleted, and in step S6, hydrophobically modified double-layer coated red phosphorus powder is replaced with double-layer coated red phosphorus powder, and the remaining steps remain unchanged to prepare a low-smoke, halogen-free, flame-retardant cable for subway use.

[0064] Comparative Example 3: The difference from Example 1 is that step S4 is deleted, and in step S5, carbon nanotubes are replaced with modified carbon nanotubes, and the remaining steps remain unchanged to prepare a low-smoke, halogen-free, flame-retardant cable for subway use.

[0065] Referring to the corresponding test standards, prepare specimens from the flame-retardant sheath materials in Examples 1 - 3 and Comparative Examples 1 - 3 respectively, soak different specimens in clean water for 15 days, and then perform performance tests on the specimens; in addition, prepare specimens in Example 1 without soaking in clean water as a blank control for performance testing.

[0066] Referring to GB / T1040.1 - 2008 "Determination of Tensile Properties of Plastics - Part 1: General Principles", detect the tensile strength of the flame-retardant sheath material; referring to GB / T1843 - 2008 "Determination of Izod Impact Strength of Plastics", detect the notched impact strength of the flame-retardant sheath material; referring to GB / T2406 - 2009 "Determination of the Burning Behavior of Plastics by the Oxygen Index Method", detect the limiting oxygen index of the flame-retardant sheath material; perform an electrical test according to Method 9.1 in GB / T2951.13 - 2008. If the specimen is not punctured, it is judged as qualified, otherwise it is judged as unqualified; the results are shown in Table 1: Table 1: Performance Test Results Table

[0067] Project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Blank control Tensile strength / (MPa) 27.8 28.3 27.9 25.7 24.5 22.6 27.9 <![CDATA[Izod impact strength / (kJ / m 2 )]]> 23.3 24.5 23.6 22.8 22.4 22.0 23.5 Limiting oxygen index / (%) 35.6 36.2 35.8 32.9 34.5 34.9 35.8 Waterproofness Qualified Qualified Qualified Qualified Unqualified Qualified Qualified

[0068] As can be seen from Table 1, the samples of low-smoke and halogen-free flame-retardant cables for subways in Examples 1-3 have better waterproof and flame-retardant properties.

[0069] In Comparative Example 1, the limiting oxygen index of the cable decreased. This may be because the red phosphorus was not coated with magnesium hydroxide. The uncoated red phosphorus powder is prone to agglomeration and is easily oxidized in a humid environment, generating phosphorus oxides, which will release harmful gases and at the same time reduce the mechanical properties and flame-retardant effect of the material. The magnesium hydroxide coating layer will decompose into magnesium oxide and water vapor during combustion, playing a role in isolating oxygen and suppressing smoke, not only improving the flame-retardant effect but also enhancing the structural strength of the double-coated red phosphorus powder, facilitating the maintenance of its structural integrity during the cable processing.

[0070] In Comparative Example 2, the waterproof property of the cable decreased significantly. The double-coated red phosphorus powder was not hydrophobically modified. The unmodified double-coated red phosphorus powder may cause the material to expand in volume due to moisture absorption, thereby affecting the waterproof property of the cable sheath. Long-term moisture absorption may also promote the slow oxidation of red phosphorus, reducing its flame-retardant efficiency. The hydrophobically modified double-coated red phosphorus powder can prevent water damage, improve flame retardancy, endow the sheath material with waterproof property, and extend the service life of the cable in a high-humidity environment.

[0071] In Comparative Example 3, the tensile strength and notched impact strength of the cable decreased. The carbon nanotubes were replaced with modified carbon nanotubes. The carbon nanotubes themselves have poor dispersibility, resulting in agglomeration. The agglomerated carbon nanotubes are harder, increasing friction, reducing the mechanical properties of the material and may also exacerbate the aging and embrittlement of the sheath; the modified carbon nanotubes improve the compatibility of the modified carbon nanotubes in the matrix material by in-situ polymerization of polyurea resin on the surface of carboxylated carbon nanotubes, and form hydrogen bonds through the amino groups in the polyurea resin and the acetate groups of ethylene-vinyl acetate copolymer, enhancing the interfacial strength, thereby further improving the mechanical properties and tensile strength of the cable.

[0072] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention.

Claims

1. Preparation method of low-smoke and halogen-free flame-retardant cable for subway, the preparation method comprising: The rubber insulation layer, tinned copper wire braided mesh shielding layer and flame-retardant sheath material are evenly coated on the surface of the inner core of the conductor in sequence from the inside to the outside, and it is characterized in that the flame-retardant sheath material is prepared by the following steps: Step 1: Using tetraethyl orthosilicate as a silicon source, through hydrolysis and polycondensation reactions, silicon dioxide is formed on the surface of magnesium hydroxide-coated red phosphorus powder to obtain double-layer coated red phosphorus powder, which is modified with perfluorodecyltrimethoxysilane to obtain hydrophobic modified double-layer coated red phosphorus powder; Step 2: Ethylene-vinyl acetate copolymer, high-density polyethylene, low-density polyethylene, reinforcing masterbatch, hydrophobic modified double-layer coated red phosphorus powder, antioxidant 1010, antioxidant DLTP, ultraviolet absorber, high-phenyl silicone rubber and carbon black are melt-extruded at a temperature of 140-160 °C using a twin-screw extruder, pelletized, to obtain the flame-retardant sheath material.

2. The preparation method of the low-smoke and halogen-free flame-retardant cable for subway according to claim 1, characterized in that, The mass ratio of the ethylene-vinyl acetate copolymer, high-density polyethylene, low-density polyethylene, reinforcing masterbatch, hydrophobic modified double-layer coated red phosphorus powder, antioxidant 1010, antioxidant DLTP, ultraviolet absorber, high-phenyl silicone rubber, carbon black is 5-6:2-3:1-1.5:0.8-1:2-3:0.12-0.16:0.15-0.18:0.08-0.12:0.06-0.08:0.07-0.

09.

3. The preparation method of the low-smoke and halogen-free flame-retardant cable for subway according to claim 1, characterized in that, The magnesium hydroxide-coated red phosphorus powder is prepared by the following steps: Red phosphorus powder, sodium hexametaphosphate and distilled water are added to a reaction kettle, stirred at 55-60 °C and 250-350 rpm for 20-30 min, 10 wt% magnesium sulfate solution is added, then the pH value is adjusted to 9 with sodium hydroxide solution and stirring is continued for 2-4 h, centrifuged and filtered, the filter cake is dried, naturally cooled, ground and refined, and passed through a 600-mesh sieve to obtain magnesium hydroxide-coated red phosphorus powder; The dosage ratio of the red phosphorus powder, sodium hexametaphosphate, distilled water, magnesium sulfate solution is 8-9 kg:8-9 kg:50-60 L:25-30 L.

4. The preparation method of the low-smoke and halogen-free flame-retardant cable for subway according to claim 1, wherein, The double-layer coated red phosphorus powder is specifically prepared by the following steps: Magnesium hydroxide-coated red phosphorus powder, 70 wt% isopropanol solution and 25 wt% ammonia water are added to a reaction kettle, ultrasonically dispersed for 10-20 min, then tetraethyl orthosilicate is added, and the reaction is carried out at 50-60 °C for 10-12 h, centrifuged and filtered, the filter cake is washed, dried, ground and refined, and passed through a 500-mesh sieve to obtain double-layer coated red phosphorus powder.

5. The preparation method of the low-smoke and halogen-free flame-retardant cable for subway according to claim 4, characterized in that, The dosage ratio of the magnesium hydroxide-coated red phosphorus powder, isopropanol solution, ammonia water, tetraethyl orthosilicate is 4-6 kg:500 L:25-45 kg:30-50 kg.

6. The preparation method of the low-smoke halogen-free flame-retardant cable for subway according to claim 1, characterized in that, The hydrophobic modified double-layer coated red phosphorus powder is specifically prepared by the following steps: The double-layer coated red phosphorus powder and absolute ethanol are added to a reaction kettle, stirred at 200-500 rpm for 15-25 min, perfluorodecyltrimethoxysilane is added dropwise, stirred for 12-14 h, centrifuged, precipitated and filtered, the filter cake is washed, dried, ground and refined, and passed through a 500-mesh sieve to obtain hydrophobic modified double-layer coated red phosphorus powder; The dosage ratio of the double-layer coated red phosphorus powder, absolute ethanol, and perfluorodecyltrimethoxysilane is 4 - 5 kg : 50 L : 600 - 700 g.

7. The preparation method of the low-smoke and halogen-free flame-retardant cable for subway according to claim 1, characterized in that, The reinforcing masterbatch is specifically prepared through the following steps: Add ethylene-vinyl acetate copolymer, modified carbon nanotubes, and styrene-maleic anhydride copolymer into a high-speed mixer, stir at a speed of 900 - 1000 rpm for 10 - 20 min, then add them into a twin-screw extruder for plasticization, mixing, extrusion, and pelletizing to obtain the reinforcing masterbatch.

8. The preparation method of the low-smoke and halogen-free flame-retardant cable for subway according to claim 7, characterized in that, The mass ratio of the ethylene-vinyl acetate copolymer, modified carbon nanotubes, and styrene-maleic anhydride copolymer is 40 - 45 : 5 - 7.5 : 4 - 5.

9. The preparation method of the low-smoke and halogen-free flame-retardant cable for subway according to claim 8, characterized in that, The modified carbon nanotubes are specifically prepared through the following steps: Add carboxylated carbon nanotubes, polyoxyethylene sorbitan monopalmitate, and distilled water into a reaction kettle, stir at 200 - 500 rpm for 8 - 10 min, add 2,4-diisocyanate and dibutyltin dilaurate at 75 - 85 °C, keep the temperature for reaction for 2.5 - 3.5 h, cool naturally, centrifuge and filter, wash the filter cake, dry, and grind and refine to obtain the modified carbon nanotubes; The dosage ratio of the carboxylated carbon nanotubes, polyoxyethylene sorbitan monopalmitate, distilled water, 2,4-diisocyanate, and dibutyltin dilaurate is 8 - 10 kg : 700 - 900 g : 100 - 120 L : 2 - 4 kg : 20 - 40 g.

10. Low-smoke and halogen-free flame-retardant cable for subway, characterized in that, Prepared by the preparation method of the low-smoke and halogen-free flame-retardant cable for subway according to any one of claims 1 - 9.