A crosslinked polyethylene cable jacketing material and a method for its preparation

Abstract

This invention relates to the field of cable technology, specifically to a cross-linked polyethylene cable sheath material and its preparation method. The sheath material comprises linear low-density polyethylene resin, polyolefin elastomer, ethylene-vinyl acetate copolymer, dual-reactive-site grafted polyethylene masterbatch, encapsulated magnesium hydroxide flame retardant, reactive plasticizer / flame retardant monomer, and antioxidant, etc. By encapsulating the flame retardant with an interface anchoring agent and combining it with the dual-reactive-site grafted masterbatch, the compatibility between the inorganic filler and the resin matrix is ​​significantly improved. The sheath material prepared by this method possesses excellent flame retardancy, low smoke density, oil swelling resistance, mechanical properties, and stable electrical insulation, effectively solving the problem of performance degradation of traditional materials under harsh working conditions.

Description

Technical Field

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

[0002] As a key component protecting the internal conductors of cables, cable sheaths must simultaneously meet multiple performance requirements, including flame retardancy, oil resistance, chemical resistance, and stable electrical insulation, in harsh working conditions such as mines, ports, and industrial robots. Flame-retardant cross-linked polyethylene (XLPE) is widely used as a cable sheath material due to its good processability and electrical insulation. However, when used for extended periods in oily or grease-rich environments, it is prone to swelling and weight gain, flame retardant precipitation, and fluctuations in dielectric properties, which limit its reliability and lifespan in harsh environments. Current technologies often improve oil resistance by increasing cross-linking density or adding oil-resistant additives to suppress swelling. However, these methods often lead to decreased material flexibility, a narrowed processing window, and even sacrifice of flame retardant efficiency. For example, while high-filler flame retardant levels can improve the flame retardancy rating, they can easily induce agglomeration, forming stress concentration points and reducing mechanical strength. Conversely, physically blended plasticizers, while improving processing rheology, tend to migrate and precipitate in oily media, thus accelerating sheath aging.

[0003] Furthermore, traditional cross-linked polyethylene sheaths inherently present a contradiction in balancing flame retardancy and oil resistance. Oil-resistant formulations typically rely on high molecular weight polymers or inert fillers, but these components are difficult to disperse uniformly during cross-linking, easily causing interfacial defects. This not only affects the dehydration and charring effect of the flame retardant but also leads to significant deterioration of electrical properties in humid or oil-immersed environments. Especially in systems with high-filled inorganic flame retardants such as magnesium hydroxide, the high polarity of the particle surface results in poor compatibility with the polyolefin matrix, making them prone to migration during processing or use. This causes cracks in the sheath under mechanical stress or thermal aging, thereby reducing flame retardant durability. At the same time, to maintain processing fluidity, small molecule additives are often added. These additives are easily extracted in oil media, causing volume expansion and fluctuations in electrical properties, creating a vicious cycle.

[0004] For cable sheaths used in mining and port applications, the combined stresses of frequent bending, oil contact, and high temperature and humidity are unavoidable. Existing technologies struggle to simultaneously achieve high flame retardancy, low smoke density, stable dielectric strength, and flexibility. For instance, while some modified polyethylene sheaths improve flame retardancy by introducing halogenated flame retardants, this often results in the release of toxic gases and increased smoke density. Halogen-free flame retardant systems, on the other hand, often suffer from weak interfacial bonding, leading to flame retardant sedimentation after oil immersion and causing localized sheath failure. Furthermore, during processing, the physical mixing of flame retardants and the matrix often fails to form chemical bonds, resulting in phase separation of components during high shear or high-temperature cross-linking, affecting the uniformity and durability of the final sheath. This dilemma of balancing multiple properties creates a bottleneck for existing cross-linked polyethylene sheaths in high-end applications. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose a cross-linked polyethylene cable sheath material and its preparation method, so as to solve the problems that existing cross-linked polyethylene cable sheaths are prone to swelling and weight gain, flame retardant migration and dielectric property degradation in oily environments, while it is difficult to achieve high flame retardant efficiency, processing stability and flexibility at the same time.

[0006] To achieve the above objectives, the present invention provides a cross-linked polyethylene cable sheath material, comprising, by weight parts, the following raw materials: 1200 parts of linear low-density polyethylene resin, 500 parts of polyolefin elastomer, 300 parts of ethylene-vinyl acetate copolymer, 120-200 parts of double-reactive-site grafted polyethylene masterbatch, 2230-2650 parts of encapsulated magnesium hydroxide flame retardant, 150-250 parts of reactive plasticizer / flame retardant monomer, 5 parts of antioxidant 1010, 5 parts of antioxidant 168, and 28-36 parts of dicumyl peroxide.

[0007] The encapsulated magnesium hydroxide flame retardant is a flame retardant in which the surface of magnesium hydroxide flame retardant particles is coated with a dendritic polyester interface anchoring agent with phosphate ester side-linking branches; the mass ratio of the dendritic polyester interface anchoring agent with phosphate ester side-linking branches to the magnesium hydroxide flame retardant is 30-50:2200-2600.

[0008] Preferably, the specific surface area of ​​the magnesium hydroxide flame retardant is 8-10 m². 2 / g.

[0009] Furthermore, the phosphate ester side-linked dendritic polyester interface anchoring agent is obtained by reacting dendritic polyester polyol with reactive plasticizer / flame retardant monomer, and the mass ratio of the dendritic polyester polyol to the reactive plasticizer / flame retardant monomer is 50:30-50.

[0010] Preferably, the dendritic polyester polyol is of the type Boltorn H20.

[0011] Furthermore, the dual-reaction-site grafted polyethylene masterbatch is obtained by grafting linear low-density polyethylene resin with maleic anhydride and glycidyl methacrylate under the action of an initiator. Based on 2000 parts of linear low-density polyethylene resin, the maleic anhydride is 20-28 parts, the glycidyl methacrylate is 12-20 parts, and the initiator is dicumyl peroxide 2-4 parts.

[0012] Furthermore, the reactive plasticizer / flame retardant monomer is obtained by reacting phosphorus oxychloride with 2-ethyl-1-hexanol, allyl alcohol and glycidol, and based on 153 parts of phosphorus oxychloride, 130 parts of 2-ethyl-1-hexanol, 58 parts of allyl alcohol, 74 parts of glycidol and 305 parts of triethylamine as an acid binder.

[0013] Preferably, the linear low-density polyethylene resin is of type DOWLEX 2045G, the polyolefin elastomer is of type ENGAGE 8150, and the ethylene-vinyl acetate copolymer is of type ELVAX 260.

[0014] Furthermore, the present invention also provides a method for preparing cross-linked polyethylene cable sheath material, comprising the following steps: (1) Preparation of double-reaction-site grafted polyethylene masterbatch; (2) Preparation of reactive plasticizers / flame retardants; (3) React dendritic polyester polyol with reactive plasticizer / flame retardant monomer to obtain dendritic polyester interface anchoring agent with phosphate side-linked branches. (4) Mix magnesium hydroxide flame retardant with phosphate ester side-linked dendritic polyester interface anchoring agent, so that the phosphate ester side-linked dendritic polyester interface anchoring agent coats the surface of magnesium hydroxide flame retardant particles to obtain encapsulated magnesium hydroxide flame retardant. (5) In an internal mixer, linear low-density polyethylene resin, polyolefin elastomer, ethylene-vinyl acetate copolymer, double reaction site grafted polyethylene masterbatch, encapsulated magnesium hydroxide flame retardant, reactive plasticizer / flame retardant monomer and antioxidant are mixed and discharged and granulated to obtain interface pre-reaction masterbatch. (6) The interface pre-reaction masterbatch is fed into a twin-screw extruder for melt extrusion, dicumyl peroxide is added to the side feed, and the extrusion is water-cooled and pelletized to obtain cross-linked polyethylene cable sheath material.

[0015] Preferably, the preparation in step (1) is carried out using a twin-screw extruder, with the temperatures of each temperature zone of the extruder being 145℃-155℃, 155℃-165℃, 165℃-175℃, 170℃-180℃, and 170℃-180℃ respectively, and the screw speed being 180rpm-220rpm.

[0016] Preferably, in step (5), the temperature of the internal mixer chamber is 150°C and the rotor speed is 50 rpm. First, the linear low-density polyethylene resin, polyolefin elastomer, ethylene-vinyl acetate copolymer and double-reactive-site grafted polyethylene masterbatch are plasticized and mixed for 2 min. Then, the encapsulated magnesium hydroxide flame retardant is added in 3 batches and mixed for 90 s after each addition. Then, the reactive plasticizer / flame retardant monomer is added and mixed for 5 min-7 min. Finally, antioxidant 1010 and antioxidant 168 are added and mixed for 1 min before being discharged and granulated.

[0017] Preferably, in step (6), the barrel temperature of the twin-screw extruder is 80℃-85℃, 85℃-95℃, 95℃-105℃, 100℃-105℃, and 100℃-105℃, and the screw speed is 140rpm-160rpm.

[0018] The beneficial effects of this invention are: This invention significantly improves the thermal stability and flame-retardant durability of cable sheaths through the synergistic effect of a dual-reaction-site grafted polyethylene masterbatch and reactive plasticizers / flame retardants. The dual-reaction-site grafted polyethylene masterbatch forms active anchoring points in the matrix, allowing the encapsulated magnesium hydroxide flame retardant to uniformly dehydrate and absorb heat during thermal decomposition, and promoting char formation with the phosphate ester structure to construct a continuous insulating layer. This slows down the material's thermal degradation rate and increases high-temperature residual content. Simultaneously, the reactive plasticizers / flame retardants are chemically cured in the cross-linked network, preventing small molecule migration and resulting in low-smoke, drip-free sheathing during combustion with long-term stable flame-retardant performance.

[0019] The pre-coating design of a dendritic polyester interfacial anchoring agent based on phosphate ester side-linking branches effectively improves the interfacial compatibility between the inorganic flame retardant and the resin matrix. The multi-arm structure of the dendritic polyester chemically bonds magnesium hydroxide particles, reducing surface energy and preventing agglomeration and sedimentation during processing. This allows for uniform stress distribution during tension, thereby enhancing the mechanical strength and elongation at break of the sheath. This interfacial optimization not only reduces interfacial defects but also enhances the flexibility and durability of the sheath under dynamic bending conditions.

[0020] Through the network integration of reactive components, the sheath exhibits excellent anti-swelling properties in media such as oil and grease. The active end groups in the reactive plasticizer / flame retardant monomer participate in the peroxide-induced cross-linking reaction, forming a three-dimensional network structure that inhibits oil molecule penetration and the excretion of migratable components, ensuring the sheath maintains dimensional stability and consistent electrical properties even after long-term oil immersion. Furthermore, the epoxy and anhydride sites in the dual-reactive-site grafted polyethylene masterbatch strengthen the interfacial chemical bonding, further blocking media diffusion pathways.

[0021] This invention achieves pre-reaction and uniform dispersion of components through segmented feeding and temperature control, enabling the sheath to maintain both high dielectric strength and low smoke characteristics. The synergistic effect of the encapsulating flame retardant and the interface anchoring agent reduces electric field concentration points, while the network curing of reactive monomers reduces polar migrations, thereby maintaining stable insulation performance. Ultimately, the sheath achieves a balance between flame retardancy, oil resistance, mechanical properties, and electrical insulation, meeting the long-term requirements under complex operating conditions. Detailed Implementation

[0022] 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.

[0023] Example 1: Step 1: Weigh 2000g of linear low-density polyethylene resin (model DOWLEX 2045G, density 0.920g / cm³). 3At 190℃ / 2.16kg, the melt flow rate was 1g / 10min. The melt was fed into the main feed port of a twin-screw extruder. At the same time, 20g of maleic anhydride and 12g of glycidyl methacrylate were weighed and premixed and added by the side feed. Then, 2g of dicumyl peroxide was weighed and added by the independent side feed. The temperatures of each zone of the extruder were set to 145℃, 155℃, 165℃, 170℃ and 170℃ respectively, and the screw speed was 180rpm. After the extruded strip was water-cooled and pelletized, the grafted material was obtained. The obtained grafted material was then subjected to a second vacuum devolatilization extrusion (barrel temperature 165℃, 175℃, 175℃, 175℃, vacuum degree -0.08MPa) to remove unreacted monomers and low molecular weights, and obtain a double-reaction site grafted polyethylene masterbatch. Step 2: In a four-necked flask under dry nitrogen protection, weigh 800g of dichloromethane as a solvent and cool the system to 0℃; weigh 153g of phosphorus oxychloride and add it, stirring to form a homogeneous solution; then weigh 305g of triethylamine as an acid-binding agent, adding it in three portions at 0℃; then weigh 130g... 2-Ethyl-1-hexanol was added dropwise at 0°C over a time of 60 min. After the addition was complete, the mixture was stirred at 10°C for 60 min to form an intermediate containing a single alcohol substitution. Subsequently, 58 g of allyl alcohol was added dropwise at 0°C. After the addition was complete, the temperature was raised to 20°C and stirred for 2 h. The temperature was then lowered to 0°C. 74 g of glycidol was dissolved in 200 g of acetonitrile and added dropwise at 0°C. After the addition was complete, the reaction was continued at 25°C for 4 h. After the reaction was complete, the triethylamine salt was removed by filtration. The filtrate was concentrated under reduced pressure to obtain the crude product. 600 g of ethyl acetate was then weighed to dissolve the crude product and washed three times with 1000 g of deionized water until the aqueous phase was nearly neutral. 100 g of anhydrous sodium sulfate was then weighed, dried, filtered, and the solvent was removed under reduced pressure to obtain the reactive plasticizer / flame retardant monomer. Step 3: Weigh 50g of dendritic polyester polyol (model Boltorn H20, hydroxyl value 510mgKOH / g, number average molecular weight 2152, glass transition temperature 31.2℃) and add it to a four-necked flask and heat it to 110℃ under nitrogen protection; then weigh 0.8g of 1-methylimidazole and add it and stir for 10min; then weigh 30g of reactive plasticizer / flame retardant monomer and add it dropwise at 110℃. After the addition is complete, maintain the reaction at 110℃ for 3h. Finally, remove trace amounts of volatiles from the system at 100℃ and vacuum degree -0.08MPa for 20min to obtain a phosphate ester side-linked dendritic polyester interface anchoring agent; Step 4: Weigh 2200g of magnesium hydroxide flame retardant (model MAGNIFIN H-10A, specific surface area 9.2m²). 2 / g) was placed in a hot air circulating oven and dried at 120℃ for 4 hours, then removed and cooled to 80℃; then it was put into a high-speed mixer and stirred at 800 rpm. 30g of phosphate ester side-linked dendritic polyester interface anchoring agent was weighed, preheated to 80℃ and slowly added along the pot wall. The speed was increased to 1100 rpm and maintained for 8 minutes. The material was discharged to obtain encapsulated magnesium hydroxide flame retardant. Step 5: In the internal mixer, set the chamber temperature to 150℃ and the rotor speed to 50 rpm. Weigh out 1200g of linear low-density polyethylene resin (model DOWLEX 2045G, density 0.920g / cm³). 3 Melt flow rate of 1 g / 10 min at 190℃ / 2.16 kg, 500 g polyolefin elastomer (model ENGAGE 8150, density 0.868 g / cm³) 3 Melt flow rate of 0.5 g / 10 min at 190℃ / 2.16 kg, 300 g ethylene-vinyl acetate copolymer (model ELVAX 260, vinyl acetate content 28%, density 0.955 g / cm³) 3 120g of double-reaction-site grafted polyethylene masterbatch (melt flow rate 6g / 10min at 190℃ / 2.16kg) and 120g of double-reaction-site grafted polyethylene masterbatch were added and plasticized and mixed for 2min; then 2230g of encapsulated magnesium hydroxide flame retardant was added in 3 batches, and mixed for 90s after each addition to avoid agglomeration and control torque stability; after the inorganic filler was evenly dispersed, 150g of reactive plasticizer / flame retardant monomer was weighed and added and mixed for another 5min; finally, 5g of antioxidant 1010 and 5g of antioxidant 168 were weighed and added and mixed for 1min before discharge and granulation to obtain the interface pre-reaction masterbatch; Step 6: Feed the interface pre-reaction masterbatch obtained in Step 5 into a twin-screw extruder, set the barrel temperature to 80℃, 85℃, 95℃, 100℃ and 100℃ in sequence, and the screw speed to 140 rpm; then weigh 28g of dicumyl peroxide and add it from the side feed, extrude and water-cool pelletize to obtain flame-retardant cross-linked polyethylene cable sheath material. Step 7: Add the flame-retardant cross-linked polyethylene cable sheath material obtained in Step 6 to the hopper of the cable sheath extrusion production line. Set the temperatures of each zone of the extruder to 108℃, 118℃, 123℃, and 123℃ respectively, and the die head temperature to 123℃. Extrude the sheath material to cover the conductor / cable core according to the target sheath thickness. Then, put the extruded sheath into the continuous vulcanization tube and keep it at 210℃ for 7 minutes to complete the cross-linking. After exiting the tube, water cool and shape it and rewind it to obtain a cable with a cross-linked polyethylene cable sheath.

[0024] Example 2: Step 1: Weigh 2000g of linear low-density polyethylene resin (model DOWLEX 2045G, density 0.920g / cm³). 3At 190℃ / 2.16kg, the melt flow rate was 1g / 10min. The melt was fed into the main feed port of a twin-screw extruder. At the same time, 24g of maleic anhydride and 16g of glycidyl methacrylate were weighed and premixed and added by the side feed. Then, 3g of dicumyl peroxide was weighed and added by the independent side feed. The temperatures of each zone of the extruder were set to 150℃, 160℃, 170℃, 175℃ and 175℃ respectively, and the screw speed was 200rpm. After the extruded strip was water-cooled and pelletized, the grafted material was obtained. The obtained grafted material was then subjected to a second vacuum devolatilization extrusion (barrel temperature 170℃, 180℃, 180℃, 180℃, vacuum degree -0.08MPa) to remove unreacted monomers and low molecular weights, and obtain a double-reaction site grafted polyethylene masterbatch. Step 2: In a four-necked flask under dry nitrogen protection, weigh 800g of dichloromethane as a solvent and cool the system to 0℃; weigh 153g of phosphorus oxychloride and add it, stirring to form a homogeneous solution; then weigh 305g of triethylamine as an acid-binding agent, adding it in three portions at 0℃; then weigh 130g... 2-Ethyl-1-hexanol was added dropwise at 0°C over a time of 60 min. After the addition was complete, the mixture was stirred at 10°C for 60 min to form an intermediate containing a single alcohol substitution. Subsequently, 58 g of allyl alcohol was added dropwise at 0°C. After the addition was complete, the temperature was raised to 20°C and stirred for 2 h. The temperature was then lowered to 0°C. 74 g of glycidol was dissolved in 200 g of acetonitrile and added dropwise at 0°C. After the addition was complete, the reaction was continued at 25°C for 4 h. After the reaction was complete, the triethylamine salt was removed by filtration. The filtrate was concentrated under reduced pressure to obtain the crude product. 600 g of ethyl acetate was then weighed to dissolve the crude product and washed three times with 1000 g of deionized water until the aqueous phase was nearly neutral. 100 g of anhydrous sodium sulfate was then weighed, dried, filtered, and the solvent was removed under reduced pressure to obtain the reactive plasticizer / flame retardant monomer. Step 3: Weigh 50g of dendritic polyester polyol (model Boltorn H20, hydroxyl value 510mgKOH / g, number average molecular weight 2152, glass transition temperature 31.2℃) and add it to a four-necked flask and heat it to 110℃ under nitrogen protection; then weigh 1g of 1-methylimidazole and add it and stir for 10min; then weigh 40g of reactive plasticizer / flame retardant monomer and add it dropwise at 110℃. After the addition is complete, maintain the reaction at 110℃ for 4h. Finally, remove trace amounts of volatiles from the system at 100℃ and vacuum degree -0.08MPa for 30min to obtain a phosphate ester side-linked dendritic polyester interface anchoring agent; Step 4: Weigh 2400g of magnesium hydroxide flame retardant (model MAGNIFIN H-10A, specific surface area 9.2m²). 2 / g) was placed in a hot air circulating oven and dried at 120℃ for 4 hours, then removed and cooled to 80℃; then it was put into a high-speed mixer and stirred at 800 rpm. 40g of phosphate ester side-linked dendritic polyester interface anchoring agent was weighed, preheated to 80℃ and slowly added along the pot wall. The speed was increased to 1200 rpm and maintained for 10 minutes. The material was discharged to obtain encapsulated magnesium hydroxide flame retardant. Step 5: In the internal mixer, set the chamber temperature to 150℃ and the rotor speed to 50 rpm. Weigh out 1200g of linear low-density polyethylene resin (model DOWLEX 2045G, density 0.920g / cm³). 3 Melt flow rate of 1 g / 10 min at 190℃ / 2.16 kg, 500 g polyolefin elastomer (model ENGAGE 8150, density 0.868 g / cm³) 3 Melt flow rate of 0.5 g / 10 min at 190℃ / 2.16 kg, 300 g ethylene-vinyl acetate copolymer (model ELVAX 260, vinyl acetate content 28%, density 0.955 g / cm³) 3 160g of double-reaction-site grafted polyethylene masterbatch (melt flow rate 6g / 10min at 190℃ / 2.16kg) and 2440g of encapsulated magnesium hydroxide flame retardant were added in three batches, mixing for 90s after each addition to prevent agglomeration and control torque stability. After the inorganic filler was evenly dispersed, 200g of reactive plasticizer / flame retardant monomer was weighed and added, and mixing continued for 6min. Finally, 5g of antioxidant 1010 and 5g of antioxidant 168 were weighed and added, and mixed for 1min before discharge and granulation to obtain the interface pre-reaction masterbatch. Step 6: Feed the interface pre-reaction masterbatch obtained in Step 5 into a twin-screw extruder. Set the barrel temperature to 80℃, 90℃, 100℃, 105℃ and 105℃ in sequence, and the screw speed to 150 rpm. Then weigh 32g of dicumyl peroxide and add it from the side feed. Extrude and water-cool pelletize to obtain flame-retardant cross-linked polyethylene cable sheath material. Step 7: Add the flame-retardant cross-linked polyethylene cable sheath material obtained in Step 6 into the hopper of the cable sheath extrusion production line. Set the temperatures of each zone of the extruder to 110℃, 120℃, 125℃, and 125℃ respectively, and the die head temperature to 125℃. Extrude the sheath material to cover the conductor / cable core according to the target sheath thickness. Then, put the extruded sheath into the continuous vulcanization tube and keep it at 210℃ for 8 minutes to complete the cross-linking. After exiting the tube, water cool and shape it and rewind it to obtain a cable with a cross-linked polyethylene cable sheath.

[0025] Example 3: Step 1: Weigh 2000g of linear low-density polyethylene resin (model DOWLEX 2045G, density 0.920g / cm³). 3A melt flow rate of 1g / 10min at 190℃ / 2.16kg was fed into the main feed port of a twin-screw extruder. At the same time, 28g of maleic anhydride and 20g of glycidyl methacrylate were weighed and premixed and added by side feed. Then, 4g of dicumyl peroxide was weighed and added by independent side feed. The temperatures of each zone of the extruder were set sequentially to 155℃, 165℃, 175℃, 180℃, and 180℃, and the screw speed was 220rpm. After the extruded strip was water-cooled and pelletized, the grafted material was obtained. Subsequently, the obtained grafted material was subjected to a second vacuum devolatilization extrusion (barrel temperature 175℃, 185℃, 185℃, 185℃, vacuum degree -0.08MPa) to remove unreacted monomers and low molecular weights, and obtain a double-reaction site grafted polyethylene masterbatch. Step 2: In a four-necked flask under dry nitrogen protection, weigh 800g of dichloromethane as a solvent and cool the system to 0℃; weigh 153g of phosphorus oxychloride and add it, stirring to form a homogeneous solution; then weigh 305g of triethylamine as an acid-binding agent, adding it in three portions at 0℃; then weigh 130g... 2-Ethyl-1-hexanol was added dropwise at 0°C over a time of 60 min. After the addition was complete, the mixture was stirred at 10°C for 60 min to form an intermediate containing a single alcohol substitution. Subsequently, 58 g of allyl alcohol was added dropwise at 0°C. After the addition was complete, the temperature was raised to 20°C and stirred for 2 h. The temperature was then lowered to 0°C. 74 g of glycidol was dissolved in 200 g of acetonitrile and added dropwise at 0°C. After the addition was complete, the reaction was continued at 25°C for 4 h. After the reaction was complete, the triethylamine salt was removed by filtration. The filtrate was concentrated under reduced pressure to obtain the crude product. 600 g of ethyl acetate was then weighed to dissolve the crude product and washed three times with 1000 g of deionized water until the aqueous phase was nearly neutral. 100 g of anhydrous sodium sulfate was then weighed, dried, filtered, and the solvent was removed under reduced pressure to obtain the reactive plasticizer / flame retardant monomer. Step 3: Weigh 50g of dendritic polyester polyol (model Boltorn H20, hydroxyl value 510mgKOH / g, number average molecular weight 2152, glass transition temperature 31.2℃) and add it to a four-necked flask and heat it to 110℃ under nitrogen protection; then weigh 1.2g of 1-methylimidazole and add it and stir for 10min; then weigh 50g of reactive plasticizer / flame retardant monomer and add it dropwise at 110℃. After the addition is complete, maintain the reaction at 110℃ for 5h. Finally, remove trace volatiles from the system at 100℃ and vacuum degree -0.08MPa for 40min to obtain a phosphate ester side-linked dendritic polyester interface anchoring agent; Step 4: Weigh 2600g of magnesium hydroxide flame retardant (model MAGNIFIN H-10A, specific surface area 9.2m²). 2 / g) was placed in a hot air circulating oven and dried at 120℃ for 4 hours, then removed and cooled to 80℃; then it was put into a high-speed mixer and stirred at 800 rpm. 50g of phosphate ester side-linked dendritic polyester interface anchoring agent was weighed, preheated to 80℃ and slowly added along the pot wall. The speed was increased to 1300 rpm and maintained for 12 minutes. The material was discharged to obtain the encapsulated magnesium hydroxide flame retardant. Step 5: In the internal mixer, set the chamber temperature to 150℃ and the rotor speed to 50 rpm. Weigh out 1200g of linear low-density polyethylene resin (model DOWLEX 2045G, density 0.920g / cm³). 3 Melt flow rate of 1 g / 10 min at 190℃ / 2.16 kg, 500 g polyolefin elastomer (model ENGAGE 8150, density 0.868 g / cm³) 3 Melt flow rate of 0.5 g / 10 min at 190℃ / 2.16 kg, 300 g ethylene-vinyl acetate copolymer (model ELVAX 260, vinyl acetate content 28%, density 0.955 g / cm³) 3 200g of double-reaction-site grafted polyethylene masterbatch (melt flow rate 6g / 10min at 190℃ / 2.16kg) and plasticized and mixed for 2min were added; then 2650g of encapsulated magnesium hydroxide flame retardant was added in 3 batches, and mixed for 90s after each addition to avoid agglomeration and control torque stability; after the inorganic filler was evenly dispersed, 250g of reactive plasticizer / flame retardant monomer was weighed and added and mixed for 7min; finally, 5g of antioxidant 1010 and 5g of antioxidant 168 were weighed and added and mixed for 1min before discharge and granulation to obtain the interface pre-reaction masterbatch; Step 6: Feed the interface pre-reaction masterbatch obtained in Step 5 into a twin-screw extruder, set the barrel temperature to 85℃, 95℃, 105℃, 105℃, and 105℃ in sequence, and the screw speed to 160 rpm; then weigh 36g of dicumyl peroxide and add it from the side feed, extrude and water-cool pelletize to obtain flame-retardant cross-linked polyethylene cable sheath material. Step 7: Add the flame-retardant cross-linked polyethylene cable sheath material obtained in Step 6 into the hopper of the cable sheath extrusion production line. Set the temperatures of each zone of the extruder to 112℃, 122℃, 127℃, and 127℃ respectively, and set the die head temperature to 127℃. Extrude the sheath material to cover the conductor / cable core according to the target sheath thickness. Then, put the extruded sheath into the continuous vulcanization tube and keep it at 210℃ for 9 minutes to complete the cross-linking. After exiting the tube, water cool and shape it and rewind it to obtain a cable with a cross-linked polyethylene cable sheath.

[0026] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that: in step 1, when preparing the dual-reaction-site grafted polyethylene masterbatch, glycidyl methacrylate was not added, but the 16g glycidyl methacrylate in step 1 was replaced by 16g linear low-density polyethylene resin (model DOWLEX 2045G); the other conditions were the same as in Example 2.

[0027] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that maleic anhydride is not added when preparing the dual-reaction-site grafted polyethylene masterbatch in step 1. Instead, 24g of maleic anhydride in step 1 is replaced by 24g of linear low-density polyethylene resin (model DOWLEX 2045G). The other conditions are the same as in Example 2.

[0028] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that 74g of glycidol was not added during the synthesis of the reactive plasticizer / flame retardant monomer in step 2; the other conditions were the same as in Example 2.

[0029] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that the high-speed pre-coating treatment in step 4 is not performed; in step 5, 2400g of uncoated magnesium hydroxide flame retardant and 40g of interface anchoring agent are added to the internal mixer and mixed with the remaining components, so that the interface anchoring agent is introduced by direct mixing.

[0030] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that: in step 5, reactive plasticizer / flame retardant monomer is not added, but 200g of reactive plasticizer / flame retardant monomer in step 5 is replaced by 200g of linear low-density polyethylene resin (model DOWLEX2045G); the other conditions are the same as in Example 2.

[0031] Comparative Example 6: The difference between Comparative Example 6 and Example 2 is that, in step 5, no dual-reaction-site grafted polyethylene masterbatch is added. Instead, 160g of dual-reaction-site grafted polyethylene masterbatch in step 5 is replaced by 160g of linear low-density polyethylene resin (model DOWLEX 2045G). The other conditions are the same as in Example 2.

[0032] Performance testing Sample preparation: The flame-retardant cross-linked polyethylene cable sheath material obtained in step 6 according to the respective formulations and processes of the examples and comparative examples was used to prepare test sheets by flat sheet pressing: first, it was preheated and pressed for 3 minutes at 125℃ and 10MPa to form a uniform molten sheet, then hot-pressed for 8 minutes at 210℃ and 10MPa, and then water-cooled to 40℃ at 10MPa to demold; cross-linked sheets with a thickness of 1.0mm (for dielectric and resistance testing) and a thickness of 2.0mm (for mechanical, oil resistance and combustion testing) were prepared respectively, and dumbbell-shaped samples and burning strip samples were cut from the 2.0mm thick sheet; all samples were conditioned in an environment of (23±2)℃ and (50±5)% relative humidity for 48h before various tests were performed.

[0033] Thermogravimetric analysis: 10.0 mg of each cross-linked sheet was cut and placed in an alumina crucible. The temperature was increased from 30 °C to 800 °C at a rate of 20 °C / min under nitrogen flow rate of 50 mL / min. After holding at 800 °C for 10 min, the temperature was switched to air flow rate of 50 mL / min and held for another 10 min. The 5% weight loss temperature, the temperature corresponding to the maximum weight loss rate, and the residual mass fraction at 700 °C were recorded. Tensile properties at room temperature: Tensile properties were tested according to GB / T 2951.11-2008. Dumbbell-shaped specimens (gauge length 20 mm, width 4 mm, thickness 2 mm) were used. The test was conducted at (23±2)℃ with a tensile speed of 250 mm / min. Five parallel specimens were tested for each sample and the average value was taken to obtain the tensile strength and elongation at break. Mineral oil swelling resistance: The mineral oil immersion test was carried out according to GB / T 2951.21-2008. Each sample was cut from a 2.0 mm sheet into 25 mm × 25 mm specimens and the initial mass and thickness were accurately measured. The specimens were immersed in mineral oil at 100 °C for 168 h. After removal, the surface oil film was quickly absorbed with non-woven fabric and placed at (23 ± 2) °C for 30 min. The mass and thickness were remeasured and the mass change rate and volume change rate were calculated. Oxygen index: The oxygen index was tested according to GB / T 2406.2-2009. A strip sample with a size of 130mm×6.5mm×3.0mm was prepared. The sample was ignited at room temperature (23±2)℃ and the minimum oxygen concentration required to maintain combustion was determined by adjusting the oxygen volume fraction in the oxygen-nitrogen mixture in increments of 0.2%. Each sample was tested 3 times and the average value was taken. Vertical burning of plastics: Vertical burning test was conducted according to GB / T 2408-2021. Samples with dimensions of 125mm×13mm×3.0mm were prepared. Under the specified flame height of 20mm, each sample was ignited for 10s, with a 10s interval, and then ignited for another 10s. The afterflame time, afterburn time, and drip ignition were recorded for the first and second tests to evaluate the vertical burning rating. Five parallel samples were tested for each sample. Smoke density: Smoke density was determined according to GB / T 17651.1-2021 and GB / T 17651.2-2021. Three cable samples with a length of 1.0m were cut from the finished cable sheath section and conditioned for 16 hours at (23±2)℃ and (50±5)% relative humidity. The samples were then arranged in a 3m... 3 The smoke density test chamber was filled and ignited. The light transmittance curve of the entire test process was recorded and the minimum light transmittance was taken as the characterization index. Power frequency dielectric strength: The power frequency dielectric strength was determined according to GB / T 1408.1-2016. Each sample was a 1.0 mm cross-linked sheet, and a 50 mm diameter disc was punched out. A 25 mm diameter flat electrode was used. A 50 Hz power frequency voltage was applied at a voltage increase rate of 500 V per second under (23±2)℃ conditions until breakdown. The breakdown voltage was recorded and the dielectric strength was calculated. Five parallel samples were tested for each sample and the average value was taken. The test results are shown in Table 1.

[0034]

[0035] Data Analysis: As can be seen from the data in Examples 1-3 of Table 1, the flame-retardant cross-linked polyethylene cable sheath material prepared by this invention exhibits a high initial decomposition temperature and maintains a high high-temperature residual amount in thermogravimetric analysis. This indicates that the encapsulated magnesium hydroxide flame retardant forms a stable inorganic residue while undergoing heat absorption and dehydration, and the phosphate ester structure in the reactive plasticizer / flame retardant monomer is beneficial for promoting char formation and the formation of the insulation layer, thus achieving a balance between thermal stability, flame retardancy, and low smoke in the material. Meanwhile, the tensile strength and elongation at break of each example remain at a high level, indicating that the dual-reaction-site grafted polyethylene masterbatch and the phosphate ester-side-branched dendritic polyester interface anchoring agent can jointly improve the interfacial bonding of linear low-density polyethylene resin, ethylene-vinyl acetate copolymer, polyolefin elastomer, and inorganic flame retardant, resulting in more uniform stress transmission and reducing defects caused by filler agglomeration. Furthermore, the changes in mass and volume after immersion in mineral oil were both at a low level, suggesting that the cross-linking network and interfacial chemical bonding inhibited the precipitation of migratable small molecules and the penetration of oil molecules. On this basis, the material can still maintain a high power frequency dielectric strength, demonstrating a comprehensive balance of flame retardancy, dielectric resistance and electrical insulation properties.

[0036] As can be seen from the data in Table 1 for Example 2 and Comparative Examples 1 and 2, when the dual-reaction-site grafted polyethylene masterbatch lacks either glycidyl methacrylate or maleic anhydride reaction sites, the mechanical properties, resistance to mineral oil swelling, and overall flame retardant and low-smoke performance of the material all deteriorate to varying degrees. The main reason is that the epoxy reaction sites provided by glycidyl methacrylate and the anhydride reaction sites provided by maleic anhydride correspond to the synergistic anchoring of the dendritic polyester interface anchoring agent grafted to the phosphate ester side and the surface active groups of the encapsulated magnesium hydroxide flame retardant, respectively. The absence of either reaction site reduces the interfacial chemical bond density, leading to uneven dispersion of the inorganic flame retardant and an increase in interfacial micro-defects, thus making it difficult to simultaneously optimize pyrolysis to char and stress transfer.

[0037] As can be seen from the data in Example 2 and Comparative Example 3 in Table 1, when glycidol is not added during the synthesis of reactive plasticizers / flame retardants, the mineral oil swelling resistance and long-term stability of the materials decrease more significantly, and some indicators show characteristics that are not entirely consistent with the conventional swelling pattern. The possible reason is that the active end group introduced by glycidol makes the reactive plasticizers / flame retardants more prone to chemical curing and network binding during the crosslinking process initiated by dicumyl peroxide; without this end group, the phosphate ester structure tends to exist in a physical blending form, dissolving or migrating in the mineral oil medium, resulting in more significant swelling in the volume direction and the formation of micropores, while the mass change is affected by the mutual cancellation of dissolution and oil absorption.

[0038] As can be seen from the data in Example 2 and Comparative Example 4 in Table 1, when the introduction method of the interface anchoring agent for magnesium hydroxide is changed from pre-coating to direct mixing, the tensile properties, vertical flammability rating, smoke density, light transmittance, and power frequency dielectric strength of the material are all adversely affected. The main reason is that the uncoated magnesium hydroxide flame retardant has a strong surface polarity and is prone to agglomeration. Direct mixing will form hard agglomerates and interfacial voids in the resin matrix, which weakens stress transmission and becomes an initiation defect for electric field concentration and thermal decomposition. In contrast, the pre-coating of the phosphate ester side-linked dendritic polyester interface anchoring agent can reduce the particle surface energy and improve wetting and dispersion, so that the inorganic dehydration endothermic effect and the phosphate ester char-promoting effect occur simultaneously at the microscale, thus exhibiting an unexpected synergistic effect.

[0039] As can be seen from the data in Example 2 and Comparative Example 5 in Table 1, although the material can still maintain a certain level of mechanical and electrical insulation without the addition of reactive plasticizer / flame retardant monomer, the overall performance of flame retardancy and resistance to mineral oil swelling is significantly reduced. This may be because the reactive plasticizer / flame retardant monomer provides both a phosphate ester flame-retardant structure and reactive end groups, which can be network-cured during the crosslinking process initiated by dicumyl peroxide, thereby reducing migratable components and promoting char formation during combustion, synergistically constructing a heat insulation layer with the encapsulated magnesium hydroxide flame retardant. Without this monomer, the system relies more on the single mechanism of inorganic flame retardant, making it difficult to achieve simultaneous improvement in flame retardancy, resistance to media, and electrical properties.

[0040] As can be seen from the data in Table 1 for Example 2 and Comparative Example 6, when only linear low-density polyethylene resin is used instead of adding the dual-reaction-site grafted polyethylene masterbatch in step 5, the mechanical properties, flame retardancy, low smoke, and electrical insulation of the material all decrease simultaneously, and the stability after the action of mineral oil is even worse. The possible reason is that the dual-reaction-site grafted polyethylene masterbatch can form interfacial chemical bonds through the reaction sites provided by maleic anhydride and glycidyl methacrylate, thereby effectively coupling the encapsulated magnesium hydroxide flame retardant with the resin phase. Without this masterbatch, the interface mainly relies on physical interlocking, and filler agglomeration and interfacial voids are more likely to form and amplify into macroscopic defects, making it difficult to simultaneously meet the requirements for flame retardancy and insulation.

[0041] 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 cross-linked polyethylene cable sheath material, characterized in that, By weight, it includes the following raw materials: 1200 parts linear low-density polyethylene resin, 500 parts polyolefin elastomer, 300 parts ethylene-vinyl acetate copolymer, 120-200 parts dual-reactive-site grafted polyethylene masterbatch, 2230-2650 parts encapsulated magnesium hydroxide flame retardant, 150-250 parts reactive plasticizer / flame retardant monomer, 5 parts antioxidant 1010, 5 parts antioxidant 168, and 28-36 parts dicumyl peroxide; The encapsulated magnesium hydroxide flame retardant is a flame retardant in which the surface of magnesium hydroxide flame retardant particles is coated with a dendritic polyester interface anchoring agent with phosphate ester side links, and the mass ratio of the dendritic polyester interface anchoring agent with phosphate ester side links to the magnesium hydroxide flame retardant is 30-50:2200-2600. The phosphate ester side-linked dendritic polyester interface anchoring agent is obtained by reacting dendritic polyester polyol with reactive plasticizer / flame retardant monomer, and the mass ratio of the dendritic polyester polyol to the reactive plasticizer / flame retardant monomer is 50:30-50. The dual-reaction-site grafted polyethylene masterbatch is obtained by grafting linear low-density polyethylene resin with maleic anhydride and glycidyl methacrylate under the action of an initiator. Based on 2000 parts of linear low-density polyethylene resin, the maleic anhydride is 20-28 parts, the glycidyl methacrylate is 12-20 parts, and the initiator is dicumyl peroxide 2-4 parts. The reactive plasticizer / flame retardant monomer is obtained by reacting phosphorus oxychloride with 2-ethyl-1-hexanol, allyl alcohol and glycidol, and based on 153 parts of phosphorus oxychloride, 130 parts of 2-ethyl-1-hexanol, 58 parts of allyl alcohol, 74 parts of glycidol and 305 parts of triethylamine as an acid binder.

2. The cross-linked polyethylene cable sheath material according to claim 1, characterized in that, The specific surface area of ​​the magnesium hydroxide flame retardant is 8-10 m². 2 / g.

3. The cross-linked polyethylene cable sheath material according to claim 1, characterized in that, The dendritic polyester polyol is designated as Boltorn H20.

4. The cross-linked polyethylene cable sheath material according to claim 1, characterized in that, The linear low-density polyethylene resin is designated as DOWLEX 2045G, the polyolefin elastomer as ENGAGE 8150, and the ethylene-vinyl acetate copolymer as ELVAX 260.

5. A method for preparing cross-linked polyethylene cable sheath material according to any one of claims 1-4, characterized in that, Includes the following steps: (1) Preparation of double-reaction-site grafted polyethylene masterbatch; (2) Preparation of reactive plasticizers / flame retardants; (3) React dendritic polyester polyol with reactive plasticizer / flame retardant monomer to obtain dendritic polyester interface anchoring agent with phosphate side-linked branches. (4) Mix magnesium hydroxide flame retardant with phosphate ester side-linked dendritic polyester interface anchoring agent, so that the phosphate ester side-linked dendritic polyester interface anchoring agent coats the surface of magnesium hydroxide flame retardant particles to obtain encapsulated magnesium hydroxide flame retardant. (5) In an internal mixer, linear low-density polyethylene resin, polyolefin elastomer, ethylene-vinyl acetate copolymer, double reaction site grafted polyethylene masterbatch, encapsulated magnesium hydroxide flame retardant, reactive plasticizer / flame retardant monomer and antioxidant are mixed and discharged and granulated to obtain interface pre-reaction masterbatch. (6) The interface pre-reaction masterbatch is fed into a twin-screw extruder for melt extrusion, dicumyl peroxide is added to the side feed, and the extrusion is water-cooled and pelletized to obtain cross-linked polyethylene cable sheath material.

6. The method for preparing cross-linked polyethylene cable sheath material according to claim 5, characterized in that, The preparation in step (1) is carried out using a twin-screw extruder. The temperatures of each temperature zone of the extruder are 145℃-155℃, 155℃-165℃, 165℃-175℃, 170℃-180℃, and 170℃-180℃, respectively, and the screw speed is 180rpm-220rpm.

7. The method for preparing cross-linked polyethylene cable sheath material according to claim 5, characterized in that, In step (5), the temperature of the internal mixer chamber is 150℃ and the rotor speed is 50rpm. First, the linear low-density polyethylene resin, polyolefin elastomer, ethylene-vinyl acetate copolymer and double-reactive-site grafted polyethylene masterbatch are plasticized and mixed for 2min. Then, the encapsulated magnesium hydroxide flame retardant is added in 3 parts and mixed for 90s after each addition. Then, the reactive plasticizer / flame retardant monomer is added and mixed for 5min-7min. Finally, antioxidant 1010 and antioxidant 168 are added and mixed for 1min before being discharged and granulated.

8. The method for preparing cross-linked polyethylene cable sheath material according to claim 5, characterized in that, In step (6), the barrel temperature of the twin-screw extruder is 80℃-85℃, 85℃-95℃, 95℃-105℃, 100℃-105℃, and 100℃-105℃, and the screw speed is 140rpm-160rpm.

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