A halogen-free flame-retardant polyolefin sheathing material for flexible fire-resistant cables and its preparation method

Abstract

This invention relates to the field of cable material technology, specifically disclosing a halogen-free flame-retardant polyolefin sheath material for flexible fireproof cables and its preparation method. The halogen-free flame-retardant polyolefin sheath material provided by this invention uses low-density polyethylene as the main base material. The molecular structure of low-density polyethylene has many long branches, which gives the material excellent flexibility and elasticity. At the same time, by adding composite copolymers and flame-retardant fillers, the mechanical properties, high and low temperature resistance, and flame-retardant properties of the sheath material are improved.

Description

Technical Field

[0001] This invention relates to the field of cable material technology, specifically to a halogen-free flame-retardant polyolefin sheath material for flexible fireproof cables and its preparation method. Background Technology

[0002] As a crucial carrier for electrical energy transmission and distribution, the sheath structure of power cables is essential for ensuring the overall safe and reliable operation of the cable. The power cable sheath, which wraps around the conductor and insulation layer, primarily serves to provide mechanical protection, resist environmental erosion (such as chemical corrosion, moisture penetration, and ultraviolet aging), and extend the cable's service life. It is an indispensable component in ensuring the stable delivery of electricity to end-user equipment. Therefore, selecting appropriate sheath materials directly affects the long-term performance and safety of the cable in complex environments.

[0003] Currently, polyethylene (PE) is frequently used as a material for preparing power cable sheaths due to its good chemical stability, excellent electrical insulation properties, low density, ease of installation and maintenance, and relatively economical cost. However, this polyethylene material still has significant defects in key performance aspects. On the one hand, polyethylene's high-temperature resistance is insufficient. When the temperature rises due to current load or external heat sources during cable operation, the sheath is prone to softening, deformation, or even melting, causing the internal conductors to be exposed, significantly increasing the risk of short circuits and electrical fires. On the other hand, polyethylene's inherent flame-retardant properties are poor, making it difficult to effectively prevent the spread of flames along the cable in the event of a fire, failing to buy valuable time for personnel evacuation and fire rescue, and posing a serious threat to life and property safety.

[0004] For example, Chinese patent document CN202310413407.6 discloses a halogen-free, low-smoke flame-retardant polyolefin sheath material and its preparation process, including the following steps: Step 1: Preparation of halogen-free flame retardant; Step 2: Preparation of flame retardant synergist: Montmorillonite is dispersed in N,N-dimethylformamide and modified with octadecyl isocyanate to obtain a flame retardant synergist; Step 3: Ethylene-octene copolymer elastomer, high-density polyethylene, ethylene-octene copolymer grafted maleic anhydride copolymer, halogen-free flame retardant, flame retardant synergist, lubricant, and organic peroxide are stirred, mixed, compounded, extruded and granulated to obtain the polyolefin sheath material. Although the mechanical properties and flame retardant properties of the prepared sheath material are improved to a certain extent, further improvement is still needed. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a halogen-free flame-retardant polyolefin sheathing material for flexible fire-resistant cables and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A halogen-free flame-retardant polyolefin sheathing material for flexible fireproof cables comprises the following components in parts by weight: 20-40 parts of low-density polyethylene, 10-15 parts of composite copolymer, 5-10 parts of PE grafted maleic anhydride, 10-15 parts of flame-retardant filler, 2-4 parts of lubricant, 1-3 parts of antioxidant, 1-2 parts of ultraviolet absorber, 0.2-0.4 parts of heat stabilizer, and 0.1-0.2 parts of dodecyl peroxide.

[0008] In the technical solution disclosed in this invention, the preparation method of the composite copolymer is as follows: 1,4-butenediol is added to an organic solvent and stirred to dissolve. Then, phosphonic acid-B-styrene ester and benzoyl peroxide are added, and the mixture is heated and stirred to react. After the reaction is completed, the composite copolymer is obtained by vacuum distillation and drying.

[0009] Specifically, the mass ratio of 1,4-butenediol, phosphonic acid-B-styrene ester, and benzoyl peroxide is 3-6:5-10:0.05-0.1, for example, 3:5:0.05, 3:5:0.08, 3:5:0.1, 5:5:0.05, 5:8:0.05, 5:10:0.1, 6:5:0.05, 6:8:0.08, 6:10:0.1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0010] Specifically, the temperature for heating and stirring the reaction is 60-80℃, for example, 60℃, 65℃, 70℃, 75℃, or 80℃ can be selected. The heating and stirring reaction time is 3-5 hours, for example, 3 hours, 3.5 hours, 4 hours, 4.5 hours, or 5 hours can be selected, but it is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0011] In the technical solution disclosed in this invention, 1,4-butenediol and phosphonic acid-B-styrene ester are copolymerized to form a composite copolymer through an addition reaction between double bonds. The composite copolymer is introduced into the sheath material. The phosphonic acid-B-styrene ester molecule contains a benzene ring structure and phosphonic acid groups. The benzene ring structure can improve the thermal stability of the molecular chain, making the material less prone to thermal decomposition at high temperatures, thereby improving the heat resistance of the material. The phosphonic acid groups have good flame retardancy, thereby improving the flame retardancy of the material. The alkyl flexible long-chain structure in 1,4-butenediol improves the low-temperature resistance of the material. At the same time, the hydroxyl groups in 1,4-butenediol can react with maleic anhydride grafted onto PE, improving the mechanical properties of the material.

[0012] In the technical solution disclosed in this invention, the preparation method of the flame-retardant filler is as follows:

[0013] S1. Add alkali lignin to deionized water, stir to dissolve, then add bentonite, ultrasonically disperse evenly, filter, dry and grind to obtain composite bentonite material;

[0014] S2. The composite bentonite material is dispersed in carbon tetrachloride, and then 5-amino-2-mercaptobenzimidazole and epichlorohydrin are added to it. The mixture is heated and stirred to react. After the reaction is completed, it is filtered, washed and dried to obtain the flame-retardant filler.

[0015] Specifically, in step S1, the mass ratio of alkali lignin to bentonite is 4-8:10-15. For example, 4:10, 4:12, 4:15, 6:10, 6:12, 6:15, 8:10, 8:12, and 8:15 can be selected, but it is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0016] Specifically, in step S2, the mass ratio of the composite bentonite material, 5-amino-2-mercaptobenzimidazole, and epichlorohydrin is 5-10:3-5:4-6.

[0017] Specifically, in step S2, the temperature for heating and stirring the reaction is 50-60℃, and the reaction time is 2-4 hours.

[0018] In this invention, alkali lignin is first dissolved in deionized water. The alkali lignin molecules contain a large number of phenolic hydroxyl groups, which, through hydrogen bonding, are loaded onto the surface and interlayer of bentonite to obtain a composite bentonite material. Subsequently, under the action of the crosslinking agent epichlorohydrin, 5-amino-2-mercaptobenzimidazole is grafted onto the composite bentonite material to obtain a flame-retardant filler. This invention, by loading alkali lignin onto the surface and interlayer of bentonite, allows the alkali lignin to form a dense carbon layer at high temperatures, which can isolate oxygen and prevent heat transfer, thereby improving the flame-retardant properties of bentonite. The subsequent grafting of 5-amino-2-mercaptobenzimidazole onto the composite bentonite material improves the dispersion performance of the flame-retardant filler in the raw materials. The benzimidazole groups in 5-amino-2-mercaptobenzimidazole further improve the heat resistance of the material. The thiol groups in 5-amino-2-mercaptobenzimidazole, under the action of the initiator dodecyl peroxide, react with a small number of double bonds contained in low-density polyethylene, further improving the mechanical properties of the material.

[0019] In the technical solution disclosed in this invention, the lubricant is selected from any one of calcium stearate, polyethylene wax, stearic acid, and glyceryl monostearate.

[0020] In the technical solution disclosed in this invention, the antioxidant is selected from antioxidant 1010 or antioxidant 1076.

[0021] In the technical solution disclosed in this invention, the ultraviolet absorber is selected from one or more of titanium dioxide, UV328, UV327, and UV326.

[0022] In the technical solution disclosed in this invention, the heat stabilizer is selected from calcium-zinc heat stabilizer CZ-113 or calcium-zinc heat stabilizer CZ-116.

[0023] The present invention also provides a method for preparing the above-mentioned halogen-free flame-retardant polyolefin sheath material for flexible fireproof cables, comprising the following steps: adding low-density polyethylene, composite copolymer, PE grafted maleic anhydride, flame-retardant filler, lubricant, antioxidant, ultraviolet absorber, heat stabilizer and dodecyl peroxide into a high-speed mixer according to the formula amount, mixing for 30-60 min, and then adding it into a twin-screw extruder, setting the temperature to 160-180℃ and the screw speed to 150-300 r / min, and performing melt extrusion granulation to obtain the sheath material composite material.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The halogen-free flame-retardant polyolefin sheath material provided by the present invention uses low-density polyethylene as the main base material. The molecular structure of low-density polyethylene has many long branches, which gives the material excellent flexibility and elasticity. At the same time, by adding composite copolymers and flame-retardant fillers, the mechanical properties, high and low temperature resistance and flame-retardant properties of the sheath material are improved.

[0026] (2) In this invention, 1,4-butenediol and phosphonic acid-B-styrene ester are copolymerized to form a composite copolymer through an addition reaction between double bonds. The composite copolymer is introduced into the sheath material. The phosphonic acid-B-styrene ester molecule contains a benzene ring structure and a phosphonic acid group. The benzene ring structure can improve the thermal stability of the molecular chain, making the material less prone to thermal decomposition at high temperatures, thereby improving the heat resistance of the material. The phosphonic acid group has good flame retardancy, thereby improving the flame retardancy of the material. The alkyl flexible long chain structure in 1,4-butenediol improves the low temperature resistance of the material. At the same time, the hydroxyl groups in 1,4-butenediol can react with maleic anhydride grafted onto PE, improving the mechanical properties of the material.

[0027] (3) In this invention, alkali lignin is loaded onto the surface and interlayer of bentonite. The alkali lignin forms a dense carbon layer at high temperature, which can isolate oxygen and prevent heat transfer, thereby improving the flame retardant properties of bentonite. Subsequently, 5-amino-2-mercaptobenzimidazole is grafted onto the composite bentonite material, which improves the dispersion performance of the flame retardant filler in the raw material. The benzimidazole group in 5-amino-2-mercaptobenzimidazole further improves the heat resistance of the material. 5-amino-2-mercaptobenzimidazole contains mercapto groups. Under the action of the initiator dodecyl peroxide, the mercapto groups react with a small amount of double bonds contained in low-density polyethylene, which further improves the mechanical properties of the material. Detailed Implementation

[0028] The present invention will be further described in detail below through specific preferred embodiments, but the present invention is not limited to the following embodiments.

[0029] It should be noted that, unless otherwise specified, all chemical reagents involved in this invention were purchased through commercial channels.

[0030] The low-density polyethylene used in this embodiment of the invention is grade VL8005; the grade of PE grafted with maleic anhydride is DuPont 40E529; the CAS of phosphonic acid-B-styrene ester is 1707-08-0; and the bentonite is calcium-based bentonite with a mesh size of 400 mesh.

[0031] Example 1

[0032] A method for preparing a halogen-free flame-retardant polyolefin sheath material for flexible fire-resistant cables includes the following steps:

[0033] 30 parts of low-density polyethylene, 12 parts of composite copolymer, 8 parts of PE grafted maleic anhydride, 12 parts of flame retardant filler, 3 parts of polyethylene wax, 2 parts of antioxidant 1010, 1 part of ultraviolet absorber UV327, 0.3 parts of calcium-zinc heat stabilizer CZ-113, and 0.1 parts of dodecyl peroxide were added to a high-speed mixer and mixed for 30 minutes to obtain a mixture. Then, the mixture was added to a twin-screw extruder, the temperature was set to 170℃, the screw speed was set to 200 r / min, and the mixture was melt extruded and granulated to obtain the sheathing material composite.

[0034] The preparation method of the composite copolymer is as follows: 3g of 1,4-butenediol is added to 100mL of organic solvent DMF and stirred to dissolve. Then, 5g of phosphonic acid-B-styrene ester and 0.05g of benzoyl peroxide are added. The mixture is heated and stirred at 60℃ for 5h. After the reaction is completed, the composite copolymer is obtained by vacuum distillation and drying.

[0035] The method for preparing the flame-retardant filler is as follows:

[0036] S1. Add 4g of alkali lignin to 100mL of deionized water, stir to dissolve, then add 10g of bentonite, ultrasonically disperse evenly, filter, dry, grind through a 400-mesh sieve to obtain composite bentonite material.

[0037] S2. Disperse 5g of composite bentonite material in 100mL of carbon tetrachloride, then add 3g of 5-amino-2-mercaptobenzimidazole and 4g of epichlorohydrin, heat and stir at 50℃ for 3h. After the reaction is complete, filter, wash and dry to obtain flame-retardant filler.

[0038] Example 2

[0039] A method for preparing a halogen-free flame-retardant polyolefin sheath material for flexible fire-resistant cables includes the following steps:

[0040] 20 parts of low-density polyethylene, 10 parts of composite copolymer, 5 parts of PE grafted maleic anhydride, 10 parts of flame retardant filler, 2 parts of calcium stearate, 2 parts of antioxidant 1010, 1 part of ultraviolet absorber UV327, 0.2 parts of calcium-zinc heat stabilizer CZ-113, and 0.1 parts of dodecyl peroxide were added to a high-speed mixer and mixed for 30 minutes to obtain a mixture. Then, the mixture was added to a twin-screw extruder, the temperature was set to 170℃, the screw speed was set to 200 r / min, and the mixture was melt extruded and granulated to obtain the sheathing material composite.

[0041] The composite copolymer is prepared as follows: 6g of 1,4-butenediol is added to 100mL of organic solvent DMF and stirred to dissolve. Then, 10g of phosphonic acid-B-styrene ester and 0.1g of benzoyl peroxide are added. The mixture is heated and stirred at 60℃ for 5h. After the reaction is completed, the composite copolymer is obtained by vacuum distillation and drying.

[0042] The method for preparing the flame-retardant filler is as follows:

[0043] S1. Add 6g of alkali lignin to 100mL of deionized water, stir to dissolve, then add 12g of bentonite, ultrasonically disperse evenly, filter, dry, grind through a 400-mesh sieve to obtain composite bentonite material.

[0044] S2. Disperse 10g of composite bentonite material in 100mL of carbon tetrachloride, then add 5g of 5-amino-2-mercaptobenzimidazole and 6g of epichlorohydrin, heat and stir at 50℃ for 3h. After the reaction is complete, filter, wash and dry to obtain flame-retardant filler.

[0045] Example 3

[0046] A method for preparing a halogen-free flame-retardant polyolefin sheath material for flexible fire-resistant cables includes the following steps:

[0047] 40 parts of low-density polyethylene, 15 parts of composite copolymer, 10 parts of PE grafted maleic anhydride, 15 parts of flame retardant filler, 4 parts of calcium stearate, 3 parts of antioxidant 1010, 2 parts of ultraviolet absorber UV327, 0.4 parts of calcium-zinc heat stabilizer CZ-113, and 0.2 parts of dodecyl peroxide were added to a high-speed mixer and mixed for 30 minutes to obtain a mixture. Then, the mixture was added to a twin-screw extruder, the temperature was set to 170℃, the screw speed was set to 200 r / min, and the mixture was melt extruded and granulated to obtain the sheathing material composite.

[0048] The preparation method of the composite copolymer is as follows: 5g of 1,4-butenediol is added to 100mL of organic solvent DMF and stirred to dissolve. Then, 8g of phosphonic acid-B-styrene ester and 0.1g of benzoyl peroxide are added. The mixture is heated and stirred at 80℃ for 3h. After the reaction is completed, the composite copolymer is obtained by vacuum distillation and drying.

[0049] The method for preparing the flame-retardant filler is as follows:

[0050] S1. Add 8g of alkali lignin to 100mL of deionized water, stir to dissolve, then add 15g of bentonite, ultrasonically disperse evenly, filter, dry, grind through a 400-mesh sieve to obtain composite bentonite material.

[0051] S2. Disperse 8g of composite bentonite material in 100mL of carbon tetrachloride, then add 4g of 5-amino-2-mercaptobenzimidazole and 5g of epichlorohydrin, heat and stir at 50℃ for 3h. After the reaction is complete, filter, wash and dry to obtain flame-retardant filler.

[0052] Example 4

[0053] A method for preparing a halogen-free flame-retardant polyolefin sheath material for flexible fire-resistant cables includes the following steps:

[0054] 25 parts of low-density polyethylene, 10 parts of composite copolymer, 6 parts of PE grafted maleic anhydride, 10 parts of flame retardant filler, 2 parts of calcium stearate, 2 parts of antioxidant 1076, 1 part of ultraviolet absorber UV327, 0.2 parts of calcium-zinc heat stabilizer CZ-113, and 0.1 parts of dodecyl peroxide were added to a high-speed mixer and mixed for 30 minutes to obtain a mixture. Then, the mixture was added to a twin-screw extruder, the temperature was set to 170℃, the screw speed was set to 200 r / min, and the mixture was melt extruded and granulated to obtain the sheathing material composite.

[0055] The preparation method of the composite copolymer is as follows: 4g of 1,4-butenediol is added to 100mL of organic solvent DMF and stirred to dissolve. Then, 6g of phosphonic acid-B-styrene ester and 0.08g of benzoyl peroxide are added. The mixture is heated and stirred at 80℃ for 3h. After the reaction is completed, the composite copolymer is obtained by vacuum distillation and drying.

[0056] The method for preparing the flame-retardant filler is as follows:

[0057] S1. Add 5g of alkali lignin to 100mL of deionized water, stir to dissolve, then add 10g of bentonite, ultrasonically disperse evenly, filter, dry, grind through a 400-mesh sieve to obtain composite bentonite material.

[0058] S2. Disperse 6g of composite bentonite material in 100mL of carbon tetrachloride, then add 4g of 5-amino-2-mercaptobenzimidazole and 4g of epichlorohydrin, heat and stir at 50℃ for 3h. After the reaction is complete, filter, wash and dry to obtain flame-retardant filler.

[0059] Comparative Example 1

[0060] A method for preparing a halogen-free flame-retardant polyolefin sheath material for flexible fire-resistant cables includes the following steps:

[0061] 30 parts of low-density polyethylene, 8 parts of PE grafted maleic anhydride, 12 parts of flame retardant filler, 3 parts of polyethylene wax, 2 parts of antioxidant 1010, 1 part of ultraviolet absorber UV327, 0.3 parts of calcium-zinc heat stabilizer CZ-113 and 0.1 parts of dodecyl peroxide were added to a high-speed mixer and mixed for 30 minutes to obtain a mixture. Then the mixture was added to a twin-screw extruder, the temperature was set to 170℃ and the screw speed was set to 200 r / min, and melt extrusion granulation was performed to obtain the sheath material composite material.

[0062] The preparation method of the flame-retardant filler is as follows:

[0063] S1. Add 4g of alkali lignin to 100mL of deionized water, stir to dissolve, then add 10g of bentonite, ultrasonically disperse evenly, filter, dry, grind through a 400-mesh sieve to obtain composite bentonite material.

[0064] S2. Disperse 5g of composite bentonite material in 100mL of carbon tetrachloride, then add 3g of 5-amino-2-mercaptobenzimidazole and 4g of epichlorohydrin, heat and stir at 50℃ for 3h. After the reaction is complete, filter, wash and dry to obtain flame-retardant filler.

[0065] Compared to Comparative Example 1 and Example 1, no composite copolymer was added.

[0066] Comparative Example 2

[0067] A method for preparing a halogen-free flame-retardant polyolefin sheath material for flexible fire-resistant cables includes the following steps:

[0068] 30 parts of low-density polyethylene, 12 parts of composite copolymer, 8 parts of PE grafted maleic anhydride, 12 parts of flame retardant filler, 3 parts of polyethylene wax, 2 parts of antioxidant 1010, 1 part of ultraviolet absorber UV327, 0.3 parts of calcium-zinc heat stabilizer CZ-113, and 0.1 parts of dodecyl peroxide were added to a high-speed mixer and mixed for 30 minutes to obtain a mixture. Then, the mixture was added to a twin-screw extruder, the temperature was set to 170℃, the screw speed was set to 200 r / min, and the mixture was melt extruded and granulated to obtain the sheathing material composite.

[0069] The preparation method of the composite copolymer is as follows: 3g of 1,4-butenediol is added to 100mL of organic solvent DMF and stirred to dissolve. Then, 5g of phosphonic acid-B-styrene ester and 0.05g of benzoyl peroxide are added. The mixture is heated and stirred at 60℃ for 5h. After the reaction is completed, the composite copolymer is obtained by vacuum distillation and drying.

[0070] The method for preparing the flame-retardant filler is as follows:

[0071] 5g of bentonite was dispersed in 100mL of carbon tetrachloride, and then 3g of 5-amino-2-mercaptobenzimidazole and 4g of epichlorohydrin were added. The mixture was heated and stirred at 50℃ for 3h. After the reaction was completed, the mixture was filtered, washed and dried to obtain the flame-retardant filler.

[0072] Compared with Example 1, Comparative Example 2 did not involve loading bentonite with alkali lignin.

[0073] Comparative Example 3

[0074] A method for preparing a halogen-free flame-retardant polyolefin sheath material for flexible fire-resistant cables includes the following steps:

[0075] 30 parts of low-density polyethylene, 12 parts of composite copolymer, 8 parts of PE grafted maleic anhydride, 12 parts of flame retardant filler, 3 parts of polyethylene wax, 2 parts of antioxidant 1010, 1 part of ultraviolet absorber UV327, 0.3 parts of calcium-zinc heat stabilizer CZ-113, and 0.1 parts of dodecyl peroxide were added to a high-speed mixer and mixed for 30 minutes to obtain a mixture. Then, the mixture was added to a twin-screw extruder, the temperature was set to 170℃, the screw speed was set to 200 r / min, and the mixture was melt extruded and granulated to obtain the sheathing material composite.

[0076] The preparation method of the composite copolymer is as follows: 3g of 1,4-butenediol is added to 100mL of organic solvent DMF and stirred to dissolve. Then, 5g of phosphonic acid-B-styrene ester and 0.05g of benzoyl peroxide are added. The mixture is heated and stirred at 60℃ for 5h. After the reaction is completed, the composite copolymer is obtained by vacuum distillation and drying.

[0077] The method for preparing the flame-retardant filler is as follows:

[0078] S1. Add 4g of alkali lignin to 100mL of deionized water, stir to dissolve, then add 10g of bentonite, ultrasonically disperse evenly, filter, dry, grind through a 400-mesh sieve to obtain composite bentonite material.

[0079] S2. Disperse 5g of composite bentonite material in 100mL of carbon tetrachloride, then add 3g of 4-aminobenzimidazole and 4g of epichlorohydrin, heat and stir at 50℃ for 3h. After the reaction is complete, filter, wash and dry to obtain flame retardant filler.

[0080] Compared with Example 1, 5-amino-2-mercaptobenzimidazole was replaced with 4-aminobenzimidazole in Comparative Example 3.

[0081] The sheath materials prepared in Examples 1-2 and Comparative Examples 1-3 were made into samples that met the testing standards, and then their performance was tested, as follows:

[0082] Tensile strength test: The test was conducted according to GB / T 1040.1-2018 standard, with a tensile speed of 100 mm / min;

[0083] Oxygen index test: Tested according to GB / T 2406.2-2009 standard;

[0084] Heat resistance aging test: Heat aging was carried out in a heat aging oven according to standard GBT 7141-2008. The heat aging conditions were 135℃ for 168h, and then the tensile strength retention rate was tested.

[0085] Unnotched impact strength test: Tested according to GB / T 1843-2008 standard, at a temperature of -40℃;

[0086] The test results are shown in Table 1.

[0087] Table 1 Performance test results for different groups

[0088]

[0089] As can be seen from Table 1, the present invention improves the mechanical properties, high and low temperature resistance, and flame retardant properties of the sheath material through the combined effect of the composite copolymer and the flame retardant filler.

[0090] Finally, it should be noted that the above embodiments do not limit the present invention in any way. Those skilled in the art can make modifications and improvements based on the present invention. Therefore, any modifications or improvements made without departing from the spirit of the present invention are within the scope of protection claimed by the present invention.

Claims

1. A halogen-free flame-retardant polyolefin sheathing material for flexible fire-resistant cables, characterized in that, It comprises the following components in parts by weight: 20-40 parts low-density polyethylene, 10-15 parts composite copolymer, 5-10 parts PE grafted maleic anhydride, 10-15 parts flame retardant filler, 2-4 parts lubricant, 1-3 parts antioxidant, 1-2 parts ultraviolet absorber, 0.2-0.4 parts heat stabilizer, and 0.1-0.2 parts dodecyl peroxide; The preparation method of the composite copolymer is as follows: 1,4-butenediol is added to an organic solvent and stirred to dissolve. Then, phosphonic acid-β-styrene ester and benzoyl peroxide are added, and the mixture is heated and stirred to react. After the reaction is completed, the composite copolymer is obtained by vacuum distillation and drying. The method for preparing the flame-retardant filler is as follows: S1. Add alkali lignin to deionized water, stir to dissolve, then add bentonite, ultrasonically disperse evenly, filter, dry and grind to obtain composite bentonite material; S2. The composite bentonite material is dispersed in carbon tetrachloride, and then 5-amino-2-mercaptobenzimidazole and epichlorohydrin are added to it. The mixture is heated and stirred to react. After the reaction is completed, it is filtered, washed and dried to obtain the flame-retardant filler.

2. The halogen-free flame-retardant polyolefin sheathing material for flexible fire-resistant cables according to claim 1, characterized in that, The mass ratio of 1,4-butenediol, phosphonic acid-β-styrene ester, and benzoyl peroxide is 3-6:5-10:0.05-0.

1.

3. The halogen-free flame-retardant polyolefin sheathing material for flexible fire-resistant cables according to claim 1, characterized in that, The temperature for heating and stirring the reaction is 60-80℃, and the reaction time is 3-5 hours.

4. The halogen-free flame-retardant polyolefin sheathing material for flexible fire-resistant cables according to claim 1, characterized in that, In step S1, the mass ratio of alkali lignin to bentonite is 4-8:10-15.

5. The halogen-free flame-retardant polyolefin sheathing material for flexible fire-resistant cables according to claim 1, characterized in that, In step S2, the mass ratio of the composite bentonite material, 5-amino-2-mercaptobenzimidazole, and epichlorohydrin is 5-10:3-5:4-6.

6. The halogen-free flame-retardant polyolefin sheathing material for flexible fire-resistant cables according to claim 1, characterized in that, The lubricant is selected from any one of calcium stearate, polyethylene wax, stearic acid, and glyceryl monostearate; the antioxidant is selected from antioxidant 1010 or antioxidant 1076.

7. The halogen-free flame-retardant polyolefin sheathing material for flexible fire-resistant cables according to claim 1, characterized in that, The ultraviolet absorber is selected from one or more of titanium dioxide, UV328, UV327, and UV326; the heat stabilizer is selected from calcium-zinc heat stabilizer CZ-113 or calcium-zinc heat stabilizer CZ-116.

8. The method for preparing the halogen-free flame-retardant polyolefin sheath material for flexible fire-resistant cables as described in any one of claims 1-7, characterized in that, The process includes the following steps: Low-density polyethylene, composite copolymer, PE grafted maleic anhydride, flame retardant filler, lubricant, antioxidant, ultraviolet absorber, heat stabilizer and dodecyl peroxide are added to a high-speed mixer according to the formula amount and mixed for 30-60 minutes. Then, it is added to a twin-screw extruder with the temperature set at 160-180℃ and the screw speed set at 150-300 r / min. The mixture is melt-extruded and granulated to obtain the sheathing material composite.

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