Preparation method of high-strength and high-flame-retardant XLPE (cross linked polyethylene) insulated power cable

Abstract

The invention discloses a preparation method of a high-strength and high-flame-retardant XLPE insulated power cable, relates to the technical field of power cables, and solves the technical problems that an existing XLPE insulated power cable is insufficient in toughness, and flame retardance and insulativity are mutually restricted. According to the preparation process, the conductor, the inner semiconductor layer, the wire core insulating layer, the outer semiconductor layer, the armor layer and the sheath layer are sequentially arranged from inside to outside, and the inner semiconductor layer, the heat-conducting flame-retardant buffer layer, the wire core insulating layer, the outer semiconductor layer, the armor layer, the flame-retardant buffer layer and the sheath layer are sequentially extruded on the conductor from inside to outside. On the basis of keeping the original excellent electrical performance of the XLPE cable, the tensile strength, the impact resistance and the high flame retardance of the XLPE cable are remarkably improved fundamentally, so that the high-performance comprehensive power cable which can meet the requirements of harsh laying and operating environments and can also guarantee safe power supply under the condition of fire disasters is manufactured.

Description

Technical Field

[0001] This invention relates to the field of power cable technology, and in particular to a method for preparing a high-strength, high-flame-retardant XLPE insulated power cable. Background Technology

[0002] Cross-linked polyethylene (XLPE) has become the preferred insulation material for low-voltage power cables due to its excellent electrical properties, heat resistance, and processing performance. With modern urban construction and industrial development, power cables are widely used in complex environments with extremely high requirements for the safety and stability of power supply, such as subways, tunnels, integrated utility tunnels, large commercial complexes, and industrial plants. These applications place extremely high demands on the mechanical properties of cables, especially tensile strength, impact resistance, flame retardancy, and reliability. For example, during cable laying, cables often need to withstand significant traction and lateral pressure; during operation, they may face unexpected mechanical stress impacts such as external extrusion and collisions.

[0003] Currently, most mainstream fire-resistant and flame-retardant XLPE insulated cables on the market focus their performance improvements on optimizing the flame-retardant system and fire-resistant structure, such as using ceramicized silicone rubber and mica tape wrapping to ensure the integrity of the line under fire conditions. However, while these technical solutions significantly improve the fire resistance and flame-retardant rating of the cables, they often fail to simultaneously and effectively address the problem of insufficient mechanical strength, especially impact resistance.

[0004] To improve the mechanical properties of XLPE cables, those skilled in the art typically employ the following methods: Material modification: Inorganic materials (such as nano-silica, magnesium hydroxide, etc.), micron-sized reinforcing fibers (such as chopped glass fibers), micron-sized reinforcing fibers (such as nano-montmorillonite, carbon nanotubes, etc.), or elastomers (such as POE, EPDM) are added to the XLPE matrix for blending modification. While adding micron-sized reinforcing fibers can significantly improve the rigidity and tensile strength of the material, it often leads to a sharp decrease in toughness, reduced cable flexibility, and brittle fracture under impact or pressure. Excessive addition can also impair its electrical insulation performance. Adding nano-sized reinforcing fibers, inorganic materials, or elastomers can achieve toughening and reinforcement to some extent, but nanoparticles are prone to agglomeration and uneven dispersion due to their high surface energy, making dispersion in the matrix difficult and easily forming stress concentration points. This results in unstable effects and poor process reproducibility.

[0005] Structural design: Thick insulation layers or additional metal armor layers (such as steel tape armor) are employed. Thick insulation layers increase the cable's outer diameter, raising material and installation costs; while metal armor provides excellent mechanical protection, it suffers from increased weight, poor flexibility, large bending radius, and may affect the cable's flame retardancy (metal conducts heat) and current carrying capacity, limiting its application in space-constrained situations or where non-magnetic, lightweight designs are required.

[0006] Process optimization: The crystal morphology and crosslinking density of the insulation layer are optimized by adjusting crosslinking process parameters (such as temperature, pressure, and cooling rate). This method has limited improvement on the performance of XLPE base material and a narrow process window, making it difficult to achieve a significant synergistic enhancement of strength and toughness.

[0007] To improve flame retardancy, those skilled in the art commonly employ the method of adding large amounts of inorganic hydroxides (such as aluminum hydroxide and magnesium hydroxide). However, to achieve a high flame retardancy rating (such as UL-94 V-0), a high level of flame retardant is often required, which severely deteriorates the mechanical properties of the material (making the product brittle) and processing flowability, and leads to a significant increase in cable weight, while also reducing insulation resistivity.

[0008] Of particular note is the challenge of interfacial compatibility when attempting to simultaneously add microfibers and nanofillers to synergistically enhance overall performance. A single coupling agent system struggles to simultaneously achieve chemical bonding with inorganic fibers and physical entanglement and compatibility with organic fibers, leading to phase separation of components within the matrix and ineffective stress transfer. This not only fails to produce a synergistic effect but may even result in performance inferior to a single filler system. Furthermore, traditional simple melt blending extrusion processes cannot address the issues of uniform dispersion and interfacial control of multi-scale, multi-component fillers, easily introducing defects and causing significant fluctuations in the final product's performance, making it difficult to meet the stringent consistency and reliability requirements of high-end applications.

[0009] It is evident that existing XLPE insulated power cables suffer from insufficient toughness and a trade-off between flame retardancy and insulation. Therefore, there is an urgent need in this field for an innovative method to fundamentally and significantly improve the tensile strength, impact resistance, and high flame retardancy of high-performance integrated power cables, ensuring safe power supply in the event of a fire. Summary of the Invention

[0010] To address the above shortcomings, this invention provides a method for preparing a high-strength, high-flame-retardant XLPE insulated power cable. This method significantly improves the tensile strength, impact resistance, and high flame retardancy of the XLPE cable while maintaining its original excellent electrical properties. This results in a high-performance, comprehensive power cable that meets both stringent installation and operating environment requirements and ensures safe power supply in case of fire. The specific technical solution is as follows: A method for manufacturing a high-strength, high-flame-retardant XLPE insulated power cable, the manufacturing process from the inside out being: conductor, inner semiconductor layer, core insulation layer, outer semiconductor layer, armor layer, and sheath layer, wherein the preparation of the core insulation layer includes the following steps: (1) Preparation of insulating composite materials: ① Fiber modification: Organic nanofibers and inorganic nanofibers are immersed in cerium chloride aqueous solution and ultrasonically treated. Then they are taken out and placed in L-glutamic acid aqueous solution for reaction. After filtration, washing and drying, modified composite filler is obtained. ② Gradient melt blending: The modified composite filler obtained in step ① is fed into a twin-screw extruder along with XLPE matrix, crosslinking agent, and additives. Gradient melt blending is carried out under vacuum, and the composite material is obtained by extrusion granulation. (2) The insulating composite material obtained in step (1) is extruded onto the inner semiconductor layer and cross-linked through a high-temperature and high-pressure cross-linking vulcanizing tube to form the core insulation layer.

[0011] A thermally conductive buffer layer is filled between the inner semiconductor layer and the wire core insulation layer; A flame-retardant buffer layer is filled between the armor layer and the sheath layer; The high-strength, high-flame-retardant XLPE insulated power cable is manufactured by extruding the inner semiconductor layer, thermally conductive and flame-retardant buffer layer, core insulation layer, outer semiconductor layer, armor layer, flame-retardant buffer layer, and sheath layer onto the conductor in sequence from the inside out.

[0012] Preferably, the thermally conductive buffer layer is one of thermally conductive silicone rubber and boron nitride (BN) and aluminum hydroxide, with a mass ratio of (2-2.5):1, and the thickness of the thermally conductive buffer layer is 0.3-0.5 mm. The preparation method of the flame-retardant buffer layer is as follows: polyethylene grafted maleic anhydride (PE-g-MA) and nanocellulose crystals with a mass ratio of (2-4):1 are mixed evenly and fed into the feed port of a twin-screw extruder preheated to 150-180°C and 300-500 r / min. After extrusion, the mixture is cooled and pelletized to form a PE-g-MA coated CNC composite masterbatch. The composite masterbatch is then fed into a twin-screw extruder preheated to 160-190°C and directly and evenly extruded onto the outer surface of the formed armor layer through the crosshead of the extruder. After cooling, a dense and uniform flame-retardant buffer layer is formed.

[0013] In the molten state, the high-speed rotation of the twin-screw extruder generates strong shear force, which on the one hand breaks up the agglomerates of cellulose nanocrystals (CNC), and on the other hand forces the molten polyethylene-grafted maleic anhydride (PE-g-MA) molecular chains to undergo esterification with the hydroxyl groups on the surface of the cellulose nanocrystals (CNC) through the maleic anhydride groups at their ends, forming a strong covalent bond and achieving "chemical coating". That is, under the high heat and high shear force environment, the molten PE-g-MA molecular chains are forced to be squeezed and penetrate into the CNC aggregates, thus successfully coating the surface of the cellulose nanocrystals (CNC) with polyethylene-grafted maleic anhydride (PE-g-MA).

[0014] Preferably, the insulating composite material is composed of the following raw materials by weight: 100-105 parts of XLPE substrate, 5-10 parts of organic nanofibers, 3-8 parts of inorganic nanofibers, 5-8 parts of cerium chloride, 25-35 parts of L-glutamic acid, 1.5-2.5 parts of crosslinking agent, and 0.5-1.5 parts of additives.

[0015] Preferably, in step ①, the ultrasonic treatment is performed at 40–60°C for 30–60 min; the reaction is performed at 50–70°C for 60–120 min; and the drying is performed under vacuum at 80–100°C.

[0016] Preferably, in step ①, the concentration of the cerium chloride (CeCl3) aqueous solution is 0.05–0.2 mol / L; the concentration of the L-glutamic acid (L-Glutamic Acid) aqueous solution is 0.1–0.4 mol / L, and the pH value is 5.5–7.0.

[0017] Preferably, in step ②, the gradient melt blending specifically involves: the twin-screw extruder employing a five-stage temperature control system from the feed inlet to the die head, consisting of "low temperature - medium temperature - high temperature - medium temperature - low temperature," with temperatures sequentially ranging from 180℃ to 185℃, 195℃ to 200℃, 215℃ to 220℃, 195℃ to 200℃, and 180℃ to 185℃, while the die head temperature is 200℃ to 205℃; the screw speeds corresponding to the five temperature stages are 500 to 550 r / min, 450 to 500 r / min, 400 to 450 r / min, 450 to 500 r / min, and 500 to 550 r / min.

[0018] Preferably, in step ②, the screw length-to-diameter ratio of the twin-screw extruder is (40-48):1, and the vacuum degree is -0.04MPa to -0.06MPa.

[0019] Preferably, the organic nanofibers are aramid nanofibers or ultra-high molecular weight polyethylene nanofibers, with a fiber diameter of 50-200 nm and a length of 5-20 μm; the inorganic nanofibers are silica nanofibers or alumina nanofibers, with a fiber diameter of 50-500 nm and a length of 1-10 μm.

[0020] Preferably, the crosslinking agent is dicumyl peroxide (DCP).

[0021] Preferably, the additive is selected from at least one of antioxidants and copper inhibitors; the antioxidant is a compound system prepared by feeding antioxidant 1010 and antioxidant 168 into a high-speed mixer at a mass ratio of 1:(1-2) and stirring for 3-5 minutes; the copper inhibitor is N,N'-bis(β-naphthyl)-p-phenylenediamine (DNP).

[0022] Preferably, a high-strength, high-flame-retardant XLPE insulated power cable prepared by the method described above has the following properties: tensile strength ≥ 26 MPa; breaking strength ≥ 45 kN / m; compressive strength ≥ 500 N / cm². 2 The tensile strength of the insulation layer is ≥16MPa; the impact strength is ≥90kJ / m. 2 Elongation under heat stretching load (200℃, 0.2MPa, 15min) ≤80%; Elongation after cooling ≤5%; Flame retardant rating: A; Volume resistivity at 20℃ ≥1×10⁻⁶ 15 Ω·m, insulation resistance change rate after bending ≤10%.

[0023] Preferably, the thickness of the flame-retardant buffer layer is 0.1 to 0.2 mm, which can effectively block heat transfer and improve the flame-retardant performance of the cable.

[0024] Preferably, the conductor is high-purity electrolytic copper with a purity of 99.95% or higher and a conductivity of not less than 58 MS / m. The conductor adopts a stranded structure, which is composed of multiple copper wires with a diameter of 0.1 to 0.3 mm twisted together to improve the flexibility of the wire core.

[0025] Preferably, the thickness of the wire core insulation layer is 0.8–1.2 mm, and its volume resistivity is not less than 1 × 10¹. 4 Ω・cm.

[0026] Preferably, the sheath layer thickness is 1.5 to 2 mm, and it is tightly wrapped around the outside of the mica tape using an extrusion process to ensure that there are no gaps between the inner sheath and the mica tape, thereby improving the cable's moisture resistance and mechanical protection performance.

[0027] Preferably, the head pressure during the extrusion is controlled at 15-20 MPa, and the ratio of extrusion speed to cable core traction speed is 1.05-1.10.

[0028] Due to their extremely high specific surface area and surface energy, nanofibers are highly prone to agglomeration (forming "rice grains"). In simple blending, these agglomerates become stress defect points in the XLPE matrix, leading to a significant decrease in mechanical properties (especially toughness) and electrical properties (breakdown field strength). This invention employs a synergistic approach of cerium chloride pretreatment and L-glutamic acid grafting to modify the fibers. This process first involves cerium chloride pretreatment to... 3+ Under ultrasonic assistance, ions preferentially adsorb onto the surfaces of organic and inorganic nanofibers through electrostatic adsorption and coordination. This not only effectively breaks the initial aggregation state of the fibers but also provides uniformly distributed active sites for the second-step reaction. The crucial next step is the grafting of L-glutamic acid to construct a "bridging" molecular layer: L-glutamic acid is an amphiphilic molecule containing both hydrophilic carboxyl groups (-COOH) and amino groups (-NH2), and hydrophobic methylene segments (-CH2-CH2-). The two carboxyl groups (-COOH) in the L-glutamic acid molecule react with the Ce groups adsorbed on the fiber surface. 3+ The ions undergo a strong chelation reaction to form a stable five- or six-membered cyclic complex, via Ce. 3+ The "bridge" firmly anchors the L-glutamic acid molecule to the fiber surface, allowing the hydrophilic end (-COOH) of the L-glutamic acid molecule to pass through Ce. 3+ The nanofibers are firmly anchored to the fiber surface, while their hydrophobic methylene chains (-CH2-CH2-) naturally face outwards. Therefore, after modification using the above method, the fiber surface is essentially coated with a molecular layer of outward-facing hydrophobic methylene chains (-CH2-CH2-). During subsequent melt blending, these outward-facing hydrophobic methylene chains (-CH2-CH2-) are highly similar in structure to the molecular chain of the XLPE matrix. Based on the principle of "like dissolves like," this significantly improves the interfacial compatibility between the fiber and the hydrophobic polymer matrix. This makes the nanofibers easier to wet and disperse with XLPE resin during melt blending, effectively inhibiting agglomeration and achieving uniform dispersion at the nanoscale, thus solving the problems of dispersion and interfacial compatibility of nanofillers in the matrix. Ultimately, this uniform dispersion and strong interfacial bonding work together to significantly improve the mechanical properties of the cable material (simultaneously achieving toughening and reinforcement) and synergistically enhance flame retardant properties.

[0029] When cable materials are subjected to external impact or microcracks begin to form, uniformly dispersed nanofibers can bridge the cracks, preventing further crack propagation. This is because the fibers connect to the matrix via L-glutamic acid / Ce... 3+The interfacial layer forms a strong interfacial bond (far exceeding physical adsorption). When the external force is large enough, the fibers do not simply slip out of the matrix, but require a large amount of energy to be "pulled out" or broken. This "pulling out" process greatly absorbs the impact energy, thus significantly improving the material's impact resistance and fracture toughness. It can be seen that the strong interfacial bond ensures that the stress applied to the matrix can be effectively transferred through the interfacial layer to the nanofibers with extremely strong interfacial bonds (especially aramid or UHMWPE organic fibers). The network structure of the nanofibers collectively bears and disperses the stress, thereby significantly improving the tensile strength, compressive strength, and extrusion resistance of the composite material.

[0030] The uniform dispersion of modified fibers provides more nucleation and attachment points for the large amount of added flame retardants (such as aluminum hydroxide), which helps disperse the flame retardant particles, prevents them from agglomerating, and forms a denser and more stable flame-retardant char layer. During combustion, organic fibers (such as aramid) carbonize, while inorganic fibers (such as alumina) remain unchanged. Together, they act as a physical reinforcing "skeleton" in the char layer formed during combustion. This makes the char layer stronger and denser, less easily broken by the flame flow, and more effectively isolates oxygen and heat, thereby improving flame retardant efficiency. This makes it possible to reduce the total amount of flame retardant added when achieving the same flame retardant rating (such as Class A bundled combustion), indirectly mitigating the negative impact on mechanical properties. Therefore, modification itself does not directly provide flame retardancy, but it creates conditions for highly efficient flame retardancy and has a synergistic effect on flame retardant performance.

[0031] Traditional unmodified nanofillers are prone to agglomeration, which creates microscopic air gaps or defects between the agglomerates and the matrix. These areas concentrate electric fields, easily inducing partial discharge, leading to a decrease in insulation resistance and breakdown field strength. This invention modifies the layer (L-glutamic acid / Ce). 3+ The nanofibers bond well with both the fibers and the matrix, eliminating microscopic defects caused by poor compatibility. The uniformly dispersed fibers themselves do not cause severe distortion of the electric field distribution. Therefore, despite the addition of nanofibers, the modified composite material still maintains the extremely high bulk insulation properties of XLPE (volume resistivity ≥1×10⁻⁶). 15 Ω·m).

[0032] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention uses cerium chloride pretreatment + L-glutamic acid grafting to modify fibers, constructing a robust "bridge" between nanofibers and the XLPE matrix. This achieves uniform dispersion and strong interfacial bonding of the nanofiller in the matrix, thereby improving the mechanical properties (achieving toughening and reinforcement), flame retardancy of the cable material, and synergistically enhancing both flame retardancy and electrical insulation properties. Ultimately, this results in a good balance between cable strength, toughness, flame retardancy, and insulation.

[0033] 2. The present invention uses gradient melt blending to prepare the modified fiber, which has better flowability and is more compatible with the melt viscosity of the matrix. This allows the "gradient heating-cooling" process carried out in a twin-screw extruder to complete the entire process of dispersion, wetting and degassing more efficiently, ensuring the stability of the final product performance.

[0034] 3. This invention can fundamentally and significantly improve the tensile strength, impact resistance and high flame retardancy of XLPE cables while maintaining their original excellent electrical properties, thereby manufacturing a high-performance integrated power cable that can meet the requirements of harsh laying and operating environments and ensure safe power supply in the event of a fire. Detailed Implementation

[0035] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. Unless otherwise defined, all technical terms used below have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of the present invention. Unless otherwise specifically stated, all raw materials, reagents, instruments, and equipment used in the present invention are commercially available or can be prepared by existing methods.

[0036] Example 1 A method for manufacturing a high-strength, high-flame-retardant XLPE insulated power cable, the manufacturing process from the inside out being: conductor, inner semiconductor layer, core insulation layer, outer semiconductor layer, armor layer, and sheath layer. The preparation of the core insulation layer includes the following steps: (1) Preparation of insulating composite materials: ① Fiber modification: Five parts of aramid nanofibers with a diameter of 50 nm and a length of 5 μm, and three parts of silica nanofibers with a diameter of 50 nm and a length of 1 μm were immersed in five parts of cerium chloride aqueous solution with a concentration of 0.05 mol / L and ultrasonically treated at 40 °C for 30 min. Then, they were taken out and placed in 25 parts of L-glutamic acid aqueous solution with a concentration of 0.1 mol / L and a pH of 5.5 and reacted at 50 °C for 60 min. After filtration, washing, and vacuum drying at 80 °C, the modified composite filler was obtained. ② Gradient melt blending: The modified composite filler obtained in step ①, 100 parts of XLPE matrix, 1.5 parts of dicumyl peroxide (DCP), and 0.5 parts of antioxidant additive are fed into a twin-screw extruder with a screw length-to-diameter ratio of 40:1. Gradient melt blending is carried out under a vacuum of -0.04 MPa. Specifically, the twin-screw extruder adopts a five-stage temperature control from the feed port to the die head: "low temperature-medium temperature-high temperature-medium temperature-low temperature". The temperatures are 180℃, 195℃, 215℃, 195℃, and 180℃ respectively, and the die head temperature is 200℃. The screw speeds corresponding to the five temperature stages are 500 r / min, 450 r / min, 400 r / min, 450 r / min, and 500 r / min. After extrusion granulation, an insulating composite material is obtained. The antioxidant is a compound system prepared by mixing antioxidant 1010 and antioxidant 168 in a high-speed mixer at a mass ratio of 1:1 for 3 minutes. (2) The insulating composite material obtained in step (1) is extruded onto the inner semiconductor layer and cross-linked through a high-temperature and high-pressure cross-linking vulcanizing tube to form the wire core insulation layer; A 0.3mm thick thermally conductive buffer layer is filled between the inner semiconductor layer and the wire core insulation layer. The thermally conductive buffer layer is a mixture of thermally conductive silicone rubber and boron nitride (BN) with a mass ratio of 2:1. A flame-retardant buffer layer is filled between the armor layer and the sheath layer. The flame-retardant buffer layer is prepared by mixing polyethylene grafted maleic anhydride (PE-g-MA) and nanocellulose crystals at a mass ratio of 2:1, feeding them into the feed port of a twin-screw extruder preheated to 150°C and 300r / min, cooling and pelletizing them to form a PE-g-MA coated CNC composite masterbatch, and then feeding the composite masterbatch into a twin-screw extruder preheated to 160°C. The masterbatch is then directly and evenly extruded onto the outer surface of the formed armor layer through the crosshead of the extruder. After cooling, a dense and uniform flame-retardant buffer layer is formed. The inner semiconductor layer, thermally conductive and flame-retardant buffer layer, core insulation layer, outer semiconductor layer, armor layer, flame-retardant buffer layer, and sheath layer are extruded onto the conductor from the inside out to obtain a high-strength and high-flame-retardant XLPE insulated power cable.

[0037] A high-strength, high-flame-retardant XLPE insulated power cable prepared by the above method has the following properties: tensile strength ≥ 26 MPa; breaking strength ≥ 45 kN / m; compressive strength ≥ 500 N / cm. 2 The tensile strength of the insulation layer is ≥16MPa; the impact strength is ≥90kJ / m. 2Elongation under heat stretching load (200℃, 0.2MPa, 15min) ≤80%; Elongation after cooling ≤5%; Flame retardant rating: A; Volume resistivity at 20℃ ≥1×10⁻⁶ 15 Ω·m.

[0038] Example 2 A method for manufacturing a high-strength, high-flame-retardant XLPE insulated power cable, the manufacturing process from the inside out being: conductor, inner semiconductor layer, core insulation layer, outer semiconductor layer, armor layer, and sheath layer. The preparation of the core insulation layer includes the following steps: (1) Preparation of insulating composite materials: ① Fiber modification: Ten parts of ultra-high molecular weight polyethylene nanofibers with a diameter of 200 nm and a length of 20 μm, and eight parts of alumina nanofibers with a diameter of 500 nm and a length of 10 μm were immersed in eight parts of cerium chloride aqueous solution with a concentration of 0.2 mol / L and ultrasonically treated at 60 °C for 60 min. Then, they were taken out and placed in 35 parts of L-glutamic acid aqueous solution with a concentration of 0.4 mol / L and a pH of 7.0 and reacted at 70 °C for 120 min. After filtration, washing and vacuum drying at 100 °C, the modified composite filler was obtained. ② Gradient melt blending: The modified composite filler obtained in step ①, along with 105 parts of XLPE matrix, 2.5 parts of dicumyl peroxide (DCP), and 1.5 parts of N,N'-bis(β-naphthyl)-p-phenylenediamine (DNP), are fed into a twin-screw extruder with a screw length-to-diameter ratio of 48:1. Gradient melt blending is performed under a vacuum of -0.06 MPa. Specifically, the twin-screw extruder uses a five-stage temperature control from the feed port to the die head: low temperature - medium temperature - high temperature - medium temperature - low temperature. The temperatures are 185℃, 200℃, 220℃, 200℃, and 185℃, respectively, with the die head temperature at 205℃. The screw speeds corresponding to the five temperature stages are 550 r / min, 500 r / min, 450 r / min, 500 r / min, and 550 r / min. After extrusion granulation, an insulating composite material is obtained. (2) The insulating composite material obtained in step (1) is extruded onto the inner semiconductor layer and cross-linked through a high-temperature and high-pressure cross-linking vulcanizing tube to form the wire core insulation layer; A 0.5mm thick thermally conductive buffer layer is filled between the inner semiconductor layer and the wire core insulation layer. The thermally conductive buffer layer is composed of thermally conductive silicone rubber and aluminum hydroxide in a mass ratio of 2.5:1. A flame-retardant buffer layer is filled between the armor layer and the sheath layer. The flame-retardant buffer layer is prepared by mixing polyethylene grafted maleic anhydride (PE-g-MA) and nanocellulose crystals at a mass ratio of 4:1, feeding the mixture into the feed port of a twin-screw extruder preheated to 180°C and rotating at 500 r / min, cooling and pelletizing the mixture to form a PE-g-MA coated CNC composite masterbatch, and then feeding the composite masterbatch into a twin-screw extruder preheated to 190°C. The masterbatch is then directly and evenly extruded onto the outer surface of the formed armor layer through the crosshead of the extruder. After cooling, a dense and uniform flame-retardant buffer layer is formed. The inner semiconductor layer, thermally conductive and flame-retardant buffer layer, core insulation layer, outer semiconductor layer, armor layer, flame-retardant buffer layer, and sheath layer are extruded onto the conductor from the inside out to obtain a high-strength and high-flame-retardant XLPE insulated power cable.

[0039] A high-strength, high-flame-retardant XLPE insulated power cable prepared by the above method has the following properties: tensile strength ≥ 26 MPa; breaking strength ≥ 45 kN / m; compressive strength ≥ 500 N / cm. 2 The tensile strength of the insulation layer is ≥16MPa; the impact strength is ≥90kJ / m. 2 Elongation under heat stretching load (200℃, 0.2MPa, 15min) ≤80%; Elongation after cooling ≤5%; Flame retardant rating: A; Volume resistivity at 20℃ ≥1×10⁻⁶ 15 Ω·m.

[0040] Example 3 A method for manufacturing a high-strength, high-flame-retardant XLPE insulated power cable, the manufacturing process from the inside out being: conductor, inner semiconductor layer, core insulation layer, outer semiconductor layer, armor layer, and sheath layer. The preparation of the core insulation layer includes the following steps: (1) Preparation of insulating composite materials: ① Fiber modification: Six parts of aramid nanofibers with a diameter of 70 nm and a length of 10 μm, and four parts of silica nanofibers with a diameter of 100 nm and a length of 2 μm were immersed in six parts of 0.1 mol / L cerium chloride aqueous solution and ultrasonically treated at 45 °C for 40 min. Then, they were taken out and placed in 28 parts of L-glutamic acid aqueous solution with a concentration of 0.2 mol / L and a pH of 6.0 and reacted at 55 °C for 110 min. After filtration, washing, and vacuum drying at 85 °C, the modified composite filler was obtained. ② Gradient melt blending: The modified composite filler obtained in step ①, along with 101 parts of XLPE matrix, 1.8 parts of dicumyl peroxide (DCP), and 0.8 parts of additives, are fed into a twin-screw extruder with a screw length-to-diameter ratio of 42:1. Gradient melt blending is carried out under a vacuum of -0.04 MPa. Specifically, the twin-screw extruder uses a five-stage temperature control from the feed port to the die head: "low temperature-medium temperature-high temperature-medium temperature-low temperature". The temperatures are 182℃, 196℃, 217℃, 196℃, and 182℃, respectively, with the die head temperature at 202℃. The screw speeds corresponding to the five temperature stages are 510 r / min, 460 r / min, 410 r / min, 460 r / min, and 510 r / min. After extrusion granulation, an insulating composite material is obtained. The additives are antioxidants and copper inhibitors in a mass ratio of 1:1; the antioxidant is a compound system prepared by mixing antioxidant 1010 and antioxidant 168 in a mass ratio of 1:1.2 in a high-speed mixer for 4 minutes; the copper inhibitor is N,N'-bis(β-naphthyl)-p-phenylenediamine (DNP). (2) The insulating composite material obtained in step (1) is extruded onto the inner semiconductor layer and cross-linked through a high-temperature and high-pressure cross-linking vulcanizing tube to form a core insulation layer.

[0041] A 0.5mm thick thermally conductive buffer layer is filled between the inner semiconductor layer and the core insulation layer. The thermally conductive buffer layer is composed of thermally conductive silicone rubber and boron nitride (BN) in a mass ratio of 2.2:1. A flame-retardant buffer layer is filled between the armor layer and the sheath layer. The preparation method of the flame-retardant buffer layer is as follows: polyethylene grafted maleic anhydride (PE-g-MA) and nanocellulose crystals with a mass ratio of 2.5:1 are mixed evenly and fed into the feed port of a twin-screw extruder preheated to 160℃ and 350r / min. After extrusion, the mixture is cooled and pelletized to form a PE-g-MA coated CNC composite masterbatch. The composite masterbatch is then fed into a twin-screw extruder preheated to 170℃ and directly and evenly extruded onto the outer surface of the formed armor layer through the cross head of the extruder. After cooling, a dense and uniform flame-retardant buffer layer is formed. The inner semiconductor layer, thermally conductive and flame-retardant buffer layer, core insulation layer, outer semiconductor layer, armor layer, flame-retardant buffer layer, and sheath layer are extruded onto the conductor from the inside out to obtain a high-strength and high-flame-retardant XLPE insulated power cable.

[0042] A high-strength, high-flame-retardant XLPE insulated power cable prepared by the method described above has the following properties: tensile strength ≥ 26 MPa; breaking strength ≥ 45 kN / m; compressive strength ≥ 500 N / cm. 2 The tensile strength of the insulation layer is ≥16MPa; the impact strength is ≥90kJ / m.2 Elongation under heat stretching load (200℃, 0.2MPa, 15min) ≤80%; Elongation after cooling ≤5%; Flame retardant rating: A; Volume resistivity at 20℃ ≥1×10⁻⁶ 15 Ω·m.

[0043] Example 4 A method for manufacturing a high-strength, high-flame-retardant XLPE insulated power cable, the manufacturing process from the inside out being: conductor, inner semiconductor layer, core insulation layer, outer semiconductor layer, armor layer, and sheath layer. The preparation of the core insulation layer includes the following steps: (1) Preparation of insulating composite materials: ① Fiber modification: Nine parts of ultra-high molecular weight polyethylene nanofibers with a diameter of 180 nm and a length of 13 μm, and seven parts of alumina nanofibers with a diameter of 300 nm and a length of 5 μm were immersed in seven parts of 0.15 mol / L cerium chloride aqueous solution and ultrasonically treated at 55 °C for 50 min. Then, they were taken out and placed in 33 parts of L-glutamic acid aqueous solution with a concentration of 0.3 mol / L and a pH of 6.5 and reacted at 65 °C for 80 min. After filtration, washing, and vacuum drying at 95 °C, the modified composite filler was obtained. ② Gradient melt blending: The modified composite filler obtained in step ①, along with 104 parts of XLPE matrix, 2.2 parts of dicumyl peroxide (DCP), and 1.3 parts of additives, are fed into a twin-screw extruder with a screw length-to-diameter ratio of 46:1. Gradient melt blending is carried out under a vacuum of -0.05 MPa. Specifically, the twin-screw extruder uses a five-stage temperature control from the feed port to the die head: "low temperature-medium temperature-high temperature-medium temperature-low temperature". The temperatures are 184℃, 199℃, 219℃, 199℃, and 184℃ respectively, with a die head temperature of 203℃. The screw speeds corresponding to the five temperature stages are 540 r / min, 490 r / min, 440 r / min, 490 r / min, and 540 r / min. After extrusion granulation, an insulating composite material is obtained. The additives are antioxidants and copper inhibitors in a mass ratio of 1:1; the antioxidant is a compound system prepared by mixing antioxidant 1010 and antioxidant 168 in a mass ratio of 1:(1~2) in a high-speed mixer for 3~5 minutes; the copper inhibitor is N,N'-bis(β-naphthyl)-p-phenylenediamine (DNP). (2) The insulating composite material obtained in step (1) is extruded onto the inner semiconductor layer and cross-linked through a high-temperature and high-pressure cross-linking vulcanizing tube to form the core insulation layer.

[0044] A thermally conductive buffer layer with a thickness of 0.45 mm is filled between the inner semiconductor layer and the core insulation layer. The thermally conductive buffer layer is composed of thermally conductive silicone rubber and boron nitride (BN) with a mass ratio of 2.4:1. A flame-retardant buffer layer is filled between the armor layer and the sheath layer. The flame-retardant buffer layer is prepared by mixing polyethylene grafted maleic anhydride (PE-g-MA) and nanocellulose crystals at a mass ratio of 3.5:1. The mixture is fed into the feed port of a twin-screw extruder preheated to 170°C and 450 r / min. After extrusion, the mixture is cooled and pelletized to form a PE-g-MA coated CNC composite masterbatch. The composite masterbatch is then fed into a twin-screw extruder preheated to 180°C and directly and evenly extruded onto the outer surface of the formed armor layer through the crosshead of the extruder. After cooling, a dense and uniform flame-retardant buffer layer is formed. The inner semiconductor layer, thermally conductive and flame-retardant buffer layer, core insulation layer, outer semiconductor layer, armor layer, flame-retardant buffer layer, and sheath layer are extruded onto the conductor from the inside out to obtain a high-strength and high-flame-retardant XLPE insulated power cable.

[0045] A high-strength, high-flame-retardant XLPE insulated power cable prepared by the above method has the following properties: tensile strength ≥ 26 MPa; breaking strength ≥ 45 kN / m; compressive strength ≥ 500 N / cm. 2 The tensile strength of the insulation layer is ≥16MPa; the impact strength is ≥90kJ / m. 2 Elongation under heat stretching load (200℃, 0.2MPa, 15min) ≤80%; Elongation after cooling ≤5%; Flame retardant rating: A; Volume resistivity at 20℃ ≥1×10⁻⁶ 15 Ω·m.

[0046] Example 5 A method for manufacturing a high-strength, high-flame-retardant XLPE insulated power cable, the manufacturing process from the inside out being: conductor, inner semiconductor layer, core insulation layer, outer semiconductor layer, armor layer, and sheath layer. The preparation of the core insulation layer includes the following steps: (1) Preparation of insulating composite materials: ① Fiber modification: Eight parts of high molecular weight polyethylene nanofibers with a diameter of 150 nm and a length of 15 μm, and five parts of silica nanofibers with a diameter of 250 nm and a length of 9 μm were immersed in six parts of 0.12 mol / L cerium chloride aqueous solution and ultrasonically treated at 50 °C for 45 min. Then, they were taken out and placed in 30 parts of L-glutamic acid aqueous solution with a concentration of 0.3 mol / L and a pH of 6.3 and reacted at 60 °C for 90 min. After filtration, washing, and vacuum drying at 90 °C, the modified composite filler was obtained. ② Gradient melt blending: The modified composite filler obtained in step ①, along with 103 parts of XLPE matrix, 2 parts of dicumyl peroxide, and 1 part of additives, are fed into a twin-screw extruder with a screw length-to-diameter ratio of 44:1. Gradient melt blending is carried out under a vacuum of -0.05 MPa. Specifically, the twin-screw extruder uses a five-stage temperature control from the feed port to the die head: "low temperature-medium temperature-high temperature-medium temperature-low temperature". The temperatures are 183℃, 198℃, 218℃, 198℃, and 183℃ respectively, with a die head temperature of 203℃. The screw speeds corresponding to the five temperature stages are 530 r / min, 480 r / min, 430 r / min, 480 r / min, and 530 r / min. After extrusion granulation, an insulating composite material is obtained. The additives are antioxidants and copper inhibitors in a mass ratio of 1:1; the antioxidant is a compound system prepared by mixing antioxidant 1010 and antioxidant 168 in a high-speed mixer at a mass ratio of 1:1.5 for 4 minutes; the copper inhibitor is N,N'-di(β-naphthyl)-p-phenylenediamine. (2) The insulating composite material obtained in step (1) is extruded onto the inner semiconductor layer and cross-linked through a high-temperature and high-pressure cross-linking vulcanizing tube to form the core insulation layer.

[0047] A 0.4mm thick thermally conductive buffer layer is filled between the inner semiconductor layer and the wire core insulation layer. The thermally conductive buffer layer is composed of thermally conductive silicone rubber and aluminum hydroxide in a mass ratio of 2.3:1. A flame-retardant buffer layer is filled between the armor layer and the sheath layer. The flame-retardant buffer layer is prepared by mixing polyethylene grafted maleic anhydride (PE-g-MA) and nanocellulose crystals at a mass ratio of 3:1. The mixture is fed into the feed port of a twin-screw extruder preheated to 150-180℃ and 300-500r / min. After extrusion, the mixture is cooled and pelletized to form a PE-g-MA coated CNC composite masterbatch. The composite masterbatch is then fed into a twin-screw extruder preheated to 175℃ and directly and evenly extruded onto the outer surface of the formed armor layer through the crosshead of the extruder. After cooling, a dense and uniform flame-retardant buffer layer is formed. The inner semiconductor layer, thermally conductive and flame-retardant buffer layer, core insulation layer, outer semiconductor layer, armor layer, flame-retardant buffer layer, and sheath layer are extruded onto the conductor from the inside out to obtain a high-strength and high-flame-retardant XLPE insulated power cable.

[0048] A high-strength, high-flame-retardant XLPE insulated power cable prepared by the above method has the following properties: tensile strength ≥ 26 MPa; breaking strength ≥ 45 kN / m; compressive strength ≥ 500 N / cm. 2 The tensile strength of the insulation layer is ≥16MPa; the impact strength is ≥90kJ / m. 2Elongation under heat stretching load (200℃, 0.2MPa, 15min) ≤80%; Elongation after cooling ≤5%; Flame retardant rating: A; Volume resistivity at 20℃ ≥1×10⁻⁶ 15 Ω·m.

[0049] Comparative Example 1 The difference from Example 1 is that no fiber modification treatment is performed. Instead, organic nanofibers and inorganic nanofibers are directly fed into a twin-screw extruder with XLPE substrate, crosslinking agent, and additives for gradient melt blending, while other conditions remain unchanged.

[0050] Comparative Example 2 The difference from Example 1 is that in step ②, instead of gradient melt blending, the reaction is carried out at a screw speed of 500-550 r / min and a temperature of 180℃-220℃, while other conditions remain unchanged.

[0051] Comparative Example 3 The difference from Example 1 is that no thermally conductive buffer layer is filled between the inner semiconductor layer and the core insulation layer, while other conditions remain the same.

[0052] Comparative Example 4 The difference from Example 1 is that no flame-retardant buffer layer is filled between the armor layer and the sheath layer, while other conditions remain the same.

[0053] Comparative Example 5 The difference from Example 1 is that only cerium chloride aqueous solution was used to modify the muscle fibers, while other conditions remained unchanged.

[0054] Comparative Example 6 The difference from Example 1 is that only L-glutamic acid aqueous solution was used to modify the muscle fibers, while other conditions remained unchanged.

[0055] Comparative Example 7 The difference from Example 1 is that in step ①, fiber modification: organic nanofibers and inorganic nanofibers are immersed in a mixed aqueous solution of cerium chloride and L-glutamic acid, ultrasonically treated, filtered, washed and dried to obtain modified composite filler, with other conditions remaining unchanged.

[0056] The XLPE insulated power cables prepared in Examples 1-5 and Comparative Examples 1-4 were subjected to performance tests, including flame retardancy rating, impact strength (kJ / m²), tensile strength (MPa), breaking strength (kN / m), and compressive strength (N / cm²). 2The insulation layer's tensile strength (MPa) and elongation after cooling were tested. Among these, impact resistance and extrusion resistance (according to GB / T2951.31 standard), flame retardancy (according to GB / T19666-2019 standard), tensile strength, fracture strength and elongation after cooling (according to GB / T 17650 standard), and tensile strength of the insulation layer (according to GB / T 12706.1-2008 standard) were also tested. The test results are shown in Table 1. Table 1. Test results of cable protection pipe material performance for each group As shown in Table 1, the XLPE insulated power cables prepared in Examples 1-5 of this invention have a flame retardant rating of Class A, a tensile strength ≥26MPa, a breaking strength ≥45kN / m, and a compressive strength ≥500N / cm. 2 The insulation layer has the following properties: tensile strength ≥ 16 MPa; impact strength ≥ 90 kJ / m²; elongation under thermal extension load (200℃, 0.2 MPa, 15 min) ≤ 80%; elongation after cooling ≤ 5%; and volume resistivity at 20℃ ≥ 1 × 10⁻⁶. 15 It has excellent mechanical properties (Ω·m) and is not easily deformed when heated.

[0057] In summary, this invention employs cerium chloride pretreatment combined with L-glutamic acid grafting to modify the fibers, constructing a robust "bridge" between the nanofibers and the XLPE matrix. This achieves uniform dispersion and strong interfacial bonding of the nanofillers within the matrix, thereby enhancing the mechanical properties (achieving toughening and reinforcement), flame retardancy, and synergistic effects on flame retardancy and electrical insulation. Ultimately, it achieves a good balance between cable strength, toughness, flame retardancy, and insulation. Furthermore, the gradient melt blending process allows for more efficient completion of the dispersion, impregnation, and degassing processes during the "gradient heating-cooling" process in a twin-screw extruder, ensuring the stability of the final product's performance. This invention, while maintaining the original excellent electrical properties of XLPE cables, fundamentally and significantly improves their tensile strength, impact resistance, and high flame retardancy, thus manufacturing a high-performance integrated power cable that meets the requirements of demanding laying and operating environments while ensuring safe power supply in the event of a fire.

[0058] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A method for preparing a high-strength, high-flame-retardant XLPE insulated power cable, characterized in that: The fabrication process, from the inside out, consists of: conductor, inner semiconductor layer, core insulation layer, outer semiconductor layer, armor layer, and sheath layer. The fabrication of the core insulation layer includes the following steps: (1) Preparation of insulating composite materials: ① Fiber modification: Organic nanofibers and inorganic nanofibers are immersed in cerium chloride aqueous solution and ultrasonically treated. Then they are taken out and placed in L-glutamic acid aqueous solution for reaction. After filtration, washing and drying, modified composite filler is obtained. ② Gradient melt blending: The modified composite filler obtained in step ① is fed into a twin-screw extruder along with XLPE matrix, crosslinking agent, and additives. Gradient melt blending is carried out under vacuum, and the composite material is obtained by extrusion granulation. (2) The insulating composite material obtained in step (1) is extruded onto the inner semiconductor layer and cross-linked through a high-temperature and high-pressure cross-linking vulcanizing tube to form the wire core insulation layer; A thermally conductive buffer layer is filled between the inner semiconductor layer and the insulating core layer; A flame-retardant buffer layer is filled between the armor layer and the sheath layer; The high-strength, high-flame-retardant XLPE insulated power cable is manufactured by extruding the inner semiconductor layer, thermally conductive and flame-retardant buffer layer, core insulation layer, outer semiconductor layer, armor layer, flame-retardant buffer layer, and sheath layer onto the conductor in sequence from the inside out.

2. The preparation method according to claim 1, characterized in that, The insulating composite material is composed of the following raw materials by weight: 100-105 parts XLPE substrate, 5-10 parts organic nanofibers, 3-8 parts inorganic nanofibers, 5-8 parts cerium chloride, 25-35 parts L-glutamic acid, 1.5-2.5 parts crosslinking agent, and 0.5-1.5 parts additives.

3. The preparation method according to claim 1, characterized in that, The thermally conductive buffer layer is one of thermally conductive silicone rubber and boron nitride or aluminum hydroxide, with a mass ratio of (2-2.5):1; the flame-retardant buffer layer is prepared by grafting maleic anhydride onto the surface of nanocellulose crystals with polyethylene.

4. The preparation method according to claim 1, characterized in that, In step ①, the ultrasonic treatment is performed at 40–60°C for 30–60 min; the reaction is performed at 50–70°C for 60–120 min; the drying is vacuum drying at 80–100°C; in step ①, the concentration of the cerium chloride aqueous solution is 0.05–0.2 mol / L; the concentration of the L-glutamic acid aqueous solution is 0.1–0.4 mol / L, and the pH value is 5.5–7.

0.

5. The preparation method according to claim 1, characterized in that, In step ②, the gradient melt blending specifically involves the twin-screw extruder employing a five-stage temperature control system from the feed inlet to the die head: "low temperature - medium temperature - high temperature - medium temperature - low temperature". The temperatures are as follows: 180℃~185℃, 195℃~200℃, 215℃~220℃, 195℃~200℃, and 180℃~185℃, with the die head temperature being 200~205℃. The screw speeds corresponding to the five temperature stages are: 500~550r / min, 450~500r / min, 400~450r / min, 450~500r / min, and 500~550r / min.

6. The preparation method according to claim 1, characterized in that, In step ②, the screw length-to-diameter ratio of the twin-screw extruder is (40~48):1, and the vacuum degree is -0.04MPa~-0.06MPa.

7. The preparation method according to claim 2, characterized in that, The organic nanofibers are aramid nanofibers or ultra-high molecular weight polyethylene nanofibers, with a fiber diameter of 50–200 nm and a length of 5–20 μm; the inorganic nanofibers are silica nanofibers or alumina nanofibers, with a fiber diameter of 50–500 nm and a length of 1–10 μm.

8. The preparation method according to claim 2, characterized in that, The crosslinking agent is dicumyl peroxide.

9. The preparation method according to claim 2, characterized in that, The additive is selected from at least one of antioxidants and copper inhibitors; the antioxidant is a compound system prepared by feeding antioxidant 1010 and antioxidant 168 into a high-speed mixer at a mass ratio of 1:(1-2) and stirring for 3-5 minutes; the copper inhibitor is N,N'-di(β-naphthyl)-p-phenylenediamine.

10. A high-strength, high-flame-retardant XLPE insulated power cable manufactured by the method according to any one of claims 1 to 9, characterized in that: The performance of the high-strength, high-flame-retardant XLPE insulated power cable is as follows: Tensile strength ≥ 26 MPa; Fracture strength ≥ 45 kN / m; Compressive strength ≥500 N / cm 2 ; The tensile strength of the insulation layer is ≥16MPa; Impact strength ≥90kJ / m²; Elongation under thermal stretching load (200℃, 0.2MPa, 15min) ≤80%; Elongation after cooling ≤5%; Flame retardant rating: A; Volume resistivity ≥ 1 × 10 at 20℃ 15 Ω·m.

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