A crosslinked polyethylene insulated flame-retardant power cable

Abstract

This invention discloses a cross-linked polyethylene insulated flame-retardant power cable, belonging to the field of power cable technology. The cable includes a conductor, a conductor shielding layer, an insulation layer, an insulation shielding layer, a metal shielding layer, and a protective sheath. The insulation layer is prepared from metallocene polyethylene, a multifunctional cross-linking agent, dicumyl peroxide, and antioxidant 1010. The multifunctional cross-linking agent contains an allyl double bond, benzophenone, and a cyclotriphosphazene core in its molecular structure. To address the problems of numerous additives and easy migration and precipitation in existing technologies, this invention bonds the multifunctional cross-linking agent to the polyethylene cross-linking network through a stepwise grafting-cross-linking mechanism of allyl double bonds. A three-layer co-extrusion simultaneous cross-linking process solves the problem of easy interlayer peeling; electrostatic interaction is used to prepare a composite thermally conductive filler, solving the problem of poor dispersibility of the thermally conductive filler. This invention achieves a synergistic effect of flame retardancy, voltage stability, electric field optimization to suppress charge accumulation, high thermal conductivity, and low migration.

Description

Technical Field

[0001] This invention relates to the field of power cable technology, and more specifically to a cross-linked polyethylene insulated flame-retardant power cable. Background Technology

[0002] Cross-linked polyethylene (XLPE) has become the mainstream choice for insulation materials in high-voltage power cables due to its excellent dielectric properties, heat resistance, and processability. However, with the rapid development of ultra-high-voltage direct current (UHVDC) transmission technology, traditional XLPE cables face two major technical bottlenecks under DC electric fields: first, space charge easily accumulates within the insulation layer, leading to electric field distortion and even breakdown; second, the material's flame-retardant properties are insufficient, resulting in a significant fire risk. Furthermore, the peroxide cross-linking agent used in XLPE production generates small-molecule byproducts such as cumyl alcohol and acetophenone, further degrading electrical properties. Moreover, the cross-linked material is difficult to recycle, which is inconsistent with the development direction of green manufacturing.

[0003] To address these issues, researchers have developed various modification techniques. Inorganic nanodoping (such as nano-MgO and SiO2) can introduce medium-deep trap energy levels into polymers, suppressing space charge injection and simultaneously improving thermal stability. Voltage stabilizer grafting modification (such as grafting benzoyl and benzophenone molecules) can increase breakdown field strength by trapping high-energy electrons. In terms of flame retardant modification, phosphorus-based, nitrogen-based, and phosphorus-nitrogen synergistic flame retardants (such as cyclotriphosphazene derivatives) are widely added to improve the limiting oxygen index of materials. However, existing technologies mostly employ "physical blending" or "single-functional grafting" strategies, which have significant shortcomings: nanofillers are prone to agglomeration and have poor compatibility with the matrix; the separate addition of voltage stabilizers and flame retardants leads to a large variety and quantity of additives; more importantly, most functional molecules are dispersed in the matrix in a free form, easily migrating and precipitating, resulting in poor functional durability and an inability to participate in the construction of cross-linking networks, making it difficult to achieve an integrated design of flame retardancy, voltage stabilization, and cross-linking structures.

[0004] Therefore, developing a multifunctional molecule that can simultaneously achieve high-efficiency flame retardancy, suppress space charge, improve breakdown field strength, and participate in the construction of cross-linked networks, and chemically bonding it to a polyethylene cross-linked system, has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a cross-linked polyethylene insulated flame-retardant power cable.

[0006] The technical solutions provided by the embodiments of the present invention are as follows:

[0007] A cross-linked polyethylene insulated flame-retardant power cable includes the following steps:

[0008] S1. Preparation of multifunctional crosslinking agents:

[0009] S101. Mix 4,4'-dihydroxybenzophenone, allyl bromide and potassium carbonate in a molar ratio of 1:1:2, add acetone solvent, react at 60°C under nitrogen protection for 12 h, and obtain 4-allyloxy-4'-hydroxybenzophenone by suction filtration, rotary evaporation and recrystallization.

[0010] First, anhydrous potassium carbonate, acting as an acid-binding agent, neutralizes the phenolic hydroxyl group of 4,4'-dihydroxybenzophenone, producing potassium phenolate and water. The phenoxy anion in the potassium phenolate acts as a nucleophile, attacking the carbon atom bonded to the bromine atom in the allyl bromide molecule (this carbon atom carries a partial positive charge due to the strong electron-withdrawing effect of bromine), resulting in a nucleophilic substitution reaction. During this process, the carbon-bromine bond breaks, the bromide ion leaves, and simultaneously, the phenoxy anion forms a new carbon-oxygen covalent bond with the allyl carbon, generating 4-allyloxy-4'-hydroxybenzophenone. The reaction formula is as follows:

[0011]

[0012] The reaction was carried out at 60°C to provide sufficient activation energy for the reaction to proceed smoothly, while avoiding excessively high temperatures that could lead to self-polymerization or side reactions of the allyl group. Nitrogen protection prevented the allyl double bond from being oxidized by air under heating conditions. The reaction time of 12 hours ensured that the monoallylation reaction proceeded fully. After the reaction was completed, the insoluble potassium carbonate solid and the generated potassium bromide were removed by filtration, and the acetone solvent was removed by rotary evaporation. Finally, the target product was obtained by recrystallization. It is worth noting that since 4,4'-dihydroxybenzophenone contains two phenolic hydroxyl groups, the reaction conditions (1:1 molar ratio, mild base, and suitable temperature) need to be controlled to selectively achieve monoallylation.

[0013] S102, weigh hexachlorocyclotriphosphazene, 4-allyloxy-4'-hydroxybenzophenone and potassium carbonate in a molar ratio of 1:3.0-3.3:5, reflux in acetonitrile at 85℃ under nitrogen protection for 36 h, filter, rotary evaporate and recrystallize twice with ethanol to obtain a multifunctional crosslinking agent with a yield of 50%-65%;

[0014] Consistent with the reaction mechanism in S101, anhydrous potassium carbonate undergoes a neutralization reaction with the phenolic hydroxyl group of 4-allyloxy-4'-hydroxybenzophenone to form potassium phenolate. The phenoxy anion in the potassium phenolate acts as a nucleophile, attacking the carbon-phosphorus bond (actually a phosphorus-chlorine bond) connected to the chlorine atom in the hexachlorocyclotriphosphazene molecule. Due to the strong electronegativity of the chlorine atom and the partial positive charge of the phosphorus atom, the attack of the nucleophile is favorable. During the reaction, the phenoxy anion forms a new phosphorus-oxygen bond with the phosphorus atom, while the phosphorus-chlorine bond breaks, the chloride ion leaves, and potassium chloride precipitate is formed.

[0015]

[0016] The reaction was carried out under reflux of acetonitrile at 85℃. This provided sufficient activation energy, and acetonitrile, as a polar aprotic solvent, favored the nucleophilic substitution reaction. Nitrogen protection prevented the allyl double bond from being oxidized by air at high temperatures. The reaction time of 36 h ensured the complete trisubstituted reaction. It should be noted that this reaction used a molar ratio of 1:3.0-3.3 (hexachlorocyclotriphosphazene to 4-allyloxy-4'-hydroxybenzophenone), with a slight excess of 4-allyloxy-4'-hydroxybenzophenone to promote the reaction towards trisubstituted. After the reaction, insoluble potassium carbonate and generated potassium chloride were removed by filtration, and acetonitrile solvent was removed by rotary evaporation. Finally, the product was purified twice by recrystallization from ethanol to obtain the target product as white needle-like crystals, with a yield of 50%-65%.

[0017] S2. Preparation of insulating layer mixture:

[0018] Mix 100 parts of metallocene polyethylene, 3-7 parts of multifunctional crosslinking agent, and 0.3 parts of antioxidant 1010 evenly. Add 0.8-1.5 parts of dicumyl peroxide, wherein the dicumyl peroxide is added in two parts: the first part is 60%-70% of the total amount, and the second part is the remaining 30%-40%. Melt and blend at 90-100℃ and 60 r / min for 5-8 minutes to obtain the insulation layer mixture.

[0019] First, at a melt blending temperature of 90-100℃, dicumyl peroxide (DCP) undergoes homolytic cracking to generate two cumyl phenyloxy radicals. This temperature allows DCP to decompose, but it is below the upper limit of the melt processing of polyethylene, which is conducive to the stable generation of free radicals in the system and avoids explosive polymerization. The cumyl phenyloxy radicals abstract hydrogen atoms from the metallocene polyethylene or multifunctional crosslinking agent chain to generate macromolecular free radicals, allyl free radicals, and cumyl alcohol. The purpose of adding DCP in two stages is: the first addition of 60%-70% is used to initiate the grafting reaction, and the second addition of the remaining part is added after the initial reaction of the system, which is conducive to the uniform dispersion of each component in the melt system. Subsequently, the macromolecular free radicals attack the allyl double bonds at the end of the multifunctional crosslinking agent molecule, and a free radical addition reaction occurs, grafting the multifunctional crosslinking agent onto the polyethylene main chain in the form of covalent bonds. At the same time, the macromolecular free radicals can also undergo addition to carry out mild crosslinking. This reaction is carried out at 90-100℃ and 60 r / min for 5-8 min to ensure that the grafting reaction is fully completed without causing excessive crosslinking.

[0020]

[0021] Multifunctional crosslinking agents play the following key roles in the insulating layer:

[0022] Firstly, after grafting onto the polyethylene backbone, the cyclotriphosphazene core (P3N3) in the crosslinking agent molecule decomposes during combustion to generate phosphoric acid, metaphosphoric acid and polyphosphoric acid, promoting the char formation of polyethylene. At the same time, it releases ammonia and nitrogen to dilute the combustible gas, achieving phosphorus-nitrogen synergistic halogen-free flame retardancy, and enabling the limiting oxygen index of the insulation layer to reach more than 28%.

[0023] Secondly, the benzophenone structure in the crosslinking agent molecule acts as a high-energy electron-capturing voltage stabilizer with a high electron affinity. Under a DC electric field, it can capture high-energy electrons and dissipate their energy as heat, thereby suppressing the impact of high-energy electrons on the polyethylene molecular chain and significantly increasing the DC breakdown field strength to over 40kV / mm.

[0024] Third, after the functional groups of benzophenone are incorporated into the polyethylene crosslinking network, they can improve the charge distribution state inside the insulation layer, reduce the carrier migration ability, inhibit charge accumulation and electric field distortion, reduce partial discharge and dielectric loss, and improve DC breakdown withstand performance. At the same time, they can reduce the conductivity and its temperature dependence. In addition, the unsaturated active groups retained in the molecule can participate in the construction of the system network under thermal crosslinking conditions, further improve the three-dimensional crosslinking structure of the insulation layer, and improve the heat resistance, mechanical and insulation stability of the material.

[0025] S3. Preparation of semiconductive shielding material:

[0026] Weigh out 100 parts of polyolefin base material, 45-75 parts of conductive carbon black, 2-4 parts of dicumyl peroxide, and 0.2-2 parts of antioxidant 1010 according to the mass ratio, and melt-blend at 100-110℃ for 8-10 minutes to obtain conductor shielding layer material and insulating shielding layer material.

[0027] Diisopropylbenzene peroxide decomposes to generate free radicals, forming cross-linked polyolefins. During melt blending, conductive carbon black is uniformly dispersed in the polyolefin matrix under shear force. When the carbon black content reaches the percolation threshold (about 25-30 wt%), the carbon black particles come into contact with each other or the spacing is less than 1 nm, forming a three-dimensional conductive network structure, which gives the composite material semi-conductive properties. The addition of antioxidants can prevent the thermal oxidative degradation of the polyolefin matrix during high-temperature blending. A blending time of 8-10 minutes is sufficient for each component to achieve uniform dispersion under shear force.

[0028] The conductor shielding layer (inner semiconductive layer) and the insulation shielding layer (outer semiconductive layer) use the same semiconductive shielding material, which is a standard design in high-voltage cable manufacturing. The conductor shielding layer directly covers the surface of the stranded conductor, filling irregular gaps on the conductor surface and creating a smooth equipotential surface on the inner surface of the insulation layer. This uniformizes the electric field on the conductor surface and prevents partial discharge caused by air gaps. The insulation shielding layer covers the outer surface of the insulation layer and is at the same potential as the metal shielding layer. This uniformizes the electric field on the outer surface of the insulation layer, eliminates minor defects on the insulation layer surface, and prevents partial discharge between the insulation layer and the metal shielding layer. Using the same formula has significant engineering implications: it simplifies production management, ensures the smoothness and bonding strength of the interlayer interface during three-layer co-extrusion, and ensures that the material reacts synchronously with the insulation layer during subsequent cross-linking to form good interlayer chemical bonds. According to the national standard JB / T 10738-2007, this semiconductive shielding material must meet the following requirements: volume resistivity ≤100 Ω·cm at 20℃ and volume resistivity ≤5000 Ω·cm at 90℃. Performance indicators such as Ω·cm, tensile strength ≥12MPa, and elongation at break ≥180%;

[0029] S4, Three-layer co-extrusion:

[0030] The conductor shielding layer material, the insulation layer mixture, and the insulation shielding layer material are respectively added to the three hoppers of the three-layer co-extrusion extruder. The extrusion temperature is 100-110℃. The material is applied to the surface of the copper or aluminum conductor in one pass through the three-layer co-extrusion die head at a linear speed of 3-5m / min, forming an integrated three-layer structure of conductor shielding layer, insulation layer, and insulation shielding layer.

[0031] At an extrusion temperature of 100-110℃, the conductor shielding layer material, the insulation layer mixture, and the insulation shielding layer material are all in a completely molten state. When the three melts converge in the co-extrusion die head, the polymer molecular chains of the adjacent two layers diffuse and entangle with each other at the interface to form a physical fusion interface. The residence time of the melt in the die head is only tens of seconds, which will not trigger a cross-linking reaction and avoid excessive decomposition of DCP leading to scorching.

[0032] In this stage, the multifunctional crosslinking agent, conductive carbon black and other components in each layer are uniformly dispersed in the matrix. After the three layers are co-extruded, the three layers are kept as a whole by the physical entanglement of molecular chains, but each layer still maintains its independent chemical composition and function. The conductor shielding layer and the insulating shielding layer contain conductive carbon black and have semi-conductive properties, while the insulating layer does not contain conductive carbon black and maintains its insulating properties. This state of "physical bonding and component independence" will form a strong interface bond through the mutual diffusion and entanglement of molecular chains during the synchronous thermal crosslinking process, while the conductive / insulating functions of each layer remain distinct.

[0033] S5, Crosslinking:

[0034] The three-layer co-extruded cable is passed through a thermal cross-linking pipe at 160-180℃ for 24-36 minutes to allow the insulation layer, conductor shielding layer and insulation shielding layer to undergo cross-linking reaction simultaneously. The thermal cross-linking pipe is divided into three temperature zones: the first zone is 160-165℃, the second zone is 165-175℃ and the third zone is 175-180℃, with each zone lasting for 8-12 minutes.

[0035] When the three-layer co-extruded cable enters the thermal cross-linking pipe at 160-180℃, dicumyl peroxide (DCP) decomposes rapidly upon heating to generate cumyl oxy free radicals, which trigger cross-linking reactions within each layer. This transforms the insulation layer, conductor shielding layer, and insulation shielding layer from "melt-processable thermoplastic" to "infusible and insoluble thermosetting network structure." Unlike the previous stage of "mild cross-linking," this step involves a three-stage temperature gradient (160-165℃ initiation zone, 165-175℃ main cross-linking zone, and 175-180℃ completion zone) and sufficient residence time (24-36 min) to ensure complete decomposition of DCP and full completion of the cross-linking reaction. After complete cross-linking, the gel content of the insulation layer reaches 75%-88%, the thermal elongation is ≤50%, and the material no longer flows at high temperatures, meeting the heat resistance requirements for long-term operation (90℃) and short-circuit transient (250℃) of the cable.

[0036] During the crosslinking process, the free radicals generated by DCP decomposition mainly trigger three types of reactions: First, complete intralayer crosslinking, where free radicals of polyethylene macromolecules within the same layer couple with each other to form carbon-carbon crosslinking bonds, constructing a complete three-dimensional network structure within each layer; Second, during the crosslinking process, under synchronous thermal crosslinking conditions, the polymer molecular chains between the layers of the three-layer co-extruded integrated structure further diffuse and entangle with each other, and combined with the synergistic effect of the crosslinking network of the system, significantly improving the interfacial bonding strength between the conductor shielding layer, the insulation layer, and the insulation shielding layer, effectively avoiding interlayer peeling and delamination defects during use; Third, perfect grafting crosslinking, where the remaining allyl double bonds of the multifunctional crosslinking agent grafted in the insulation layer undergo addition reactions with adjacent polyethylene chains under the initiation of free radicals, further increasing the crosslinking density and fixing the flame-retardant unit and voltage-stabilizing unit in the network;

[0037] The conductive carbon black in the conductor shielding layer and the insulating shielding layer maintains its semi-conductive properties (volume resistivity ≤100 Ω·cm at 20℃), while the insulating layer, lacking conductive carbon black, maintains its insulating properties (volume resistivity >10¹). 4The conductivity / insulation functions of each layer remain distinct after complete cross-linking (Ω·cm). Cross-linking only forms chemical bonds within each layer and at the interlayer interface, without changing the material composition and functional properties of each layer. This "functional layering and interlayer bonding" structural design ensures both the electrical performance of the cable (uniform electric field in the shielding layer and voltage resistance in the insulation layer) and the interlayer bonding strength (cannot be peeled off). The three-stage temperature zone progressive heating design also eliminates internal stress in the material and avoids interface defects caused by uneven cross-linking shrinkage.

[0038] S6. Metal shielding layer covering:

[0039] After the cross-linked cable is cooled to room temperature, the metal shielding layer is wrapped with copper tape or copper wire on the outer surface of the insulation shielding layer. The copper tape wrapping overlap rate is ≥15%, and the copper wire sparse winding gap is ≤1mm.

[0040] After the cross-linked cable cools to room temperature, a metal shielding layer is wrapped around the outer surface of the insulation shielding layer with copper tape or copper wire. When wrapping with copper tape, the copper tape is spirally wrapped around the cable with an overlap rate of ≥15% to ensure that there are no exposed gaps between adjacent copper tape loops, forming a continuous and complete cylindrical metal shielding layer. When wrapping with copper wire, multiple copper wires are evenly distributed on the outer surface of the insulation shielding layer with a gap of ≤1mm to ensure that the shielding layer has sufficient coverage density and conductive continuity.

[0041] Metal shielding layers in cable structures perform the following core functions: First, during normal operation, they conduct capacitive current on the surface of the insulation layer, maintaining a uniform electric field distribution; second, they provide a short-circuit current path during short-circuit faults, protecting the insulation layer from thermal damage; and third, they form electromagnetic shielding to prevent the cable from generating electromagnetic interference to the outside world, while protecting the cable from external electromagnetic interference. Copper tape wrapping and copper wire loose wrapping are two mature metal shielding processes. The former is suitable for medium and low voltage cables, while the latter is more suitable for high voltage and ultra-high voltage cables.

[0042] S7. Protective sleeve coating and vulcanization:

[0043] S701. Spherical alumina with an average particle size of 1-3 μm is modified with silane reagent KH550, and hexagonal boron nitride with a particle size of 0.5-1.5 μm is modified with sodium dodecylbenzenesulfonate. The two are mixed and stirred at a mass ratio of 3-1:1 for 1 h and allowed to stand for 12 h. The composite thermally conductive filler is obtained through electrostatic interaction.

[0044] Hexagonal boron nitride (h-BN) is a layered structure formed by covalent bonds between boron and nitrogen atoms. The layers are held together by weak van der Waals forces, resulting in a chemically inert surface. It is difficult to disperse in aqueous solutions or organic solvents and readily aggregates. Sodium dodecylbenzenesulfonate (SDBS) is an anionic surfactant whose molecular structure includes a long-chain hydrophobic alkyl group (C). 12H 25 -) and a hydrophilic sulfonic acid group (-SO3⁻). When SDBS is mixed with h-BN, the hydrophobic alkyl chain of SDBS is adsorbed on the hydrophobic surface of h-BN through hydrophobic interactions, while the hydrophilic sulfonic acid group faces outward, making the h-BN surface negatively charged. Studies have shown that surfactants such as SDBS can effectively disperse h-BN nanosheets and improve their dispersion stability in the matrix. This surface modification transforms the originally hydrophobic h-BN into a hydrophilic and negatively charged state, creating conditions for its subsequent electrostatic interactions.

[0045] KH550 (γ-aminopropyltriethoxysilane) is a silane coupling agent. Its molecular structure contains three hydrolyzable ethoxy groups (-OCH2CH3) and one amino group (-NH2). In aqueous solution, KH550 first undergoes hydrolysis to generate silanol (-Si(OH)3). The silanol then undergoes a dehydration condensation reaction with the hydroxyl groups (-OH) on the surface of spherical alumina to form stable Al-O-Si covalent bonds. When KH550 is grafted onto the alumina surface, the grafted alumina surface is rich in amino groups (-NH2). In acidic aqueous solution (pH=5-6), the amino groups are protonated to form -NH3. + This causes the aluminum oxide surface to carry a positive charge;

[0046] After surface modification, the spherical alumina surface is positively charged and the hexagonal boron nitride surface is negatively charged. When the two are mixed, driven by electrostatic attraction, the negatively charged hexagonal boron nitride nanosheets act as a central framework, attracting positively charged spherical alumina particles to adsorb and stack on its surface, forming a hybrid structure in which spherical alumina particles are uniformly attached to the surface of hexagonal boron nitride. In this structure, the BN sheets provide high thermal conductivity channels, and the Al2O3 particles act as spacers and supports, avoiding interlayer stacking and agglomeration of BN, and forming a continuous thermal conductivity pathway in the VMQ matrix.

[0047] S702. Mix 100 parts of methyl vinyl silicone rubber, 30-60 parts of composite thermally conductive filler, and 5 parts of hydroxyl silicone oil evenly on a two-roll mill. Add 1.5 parts of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and continue mixing for 10 minutes. Then, coat the mixed rubber onto the outside of the metal shielding layer on a single-screw extruder. Pass it through a 170°C hot air vulcanization pipe and let it stay for 15-20 minutes. Then, vulcanize it again in a 175°C oven for 2 hours to obtain the finished cable.

[0048] During the mixing stage of the two-roll mill, the composite thermally conductive filler, hydroxyl silicone oil, and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (bis(2,5-dimethyl-2,5-di(tert-butylperoxy)hexane) are uniformly dispersed in the methyl vinyl silicone rubber (VMQ) matrix under mechanical shearing. The hydroxyl silicone oil acts as a structure control agent, and its active hydroxyl groups undergo a condensation reaction with the silanol groups at the ends of the VMQ molecular chains, inhibiting the interaction between the filler and VMQ, preventing the compound from "structuring" (hardening during storage), and improving processing fluidity. The hybrid structure similar to a "core-shell" formed by the electrostatic interaction of the composite thermally conductive filler is maintained at this stage without being destroyed. Bis(2,5-dimethyl-2,5-di(tert-butylperoxy)hexane) is stable at this temperature (mixing temperature of about 40-60℃) and does not decompose.

[0049] When the compound is coated with a metal shielding layer on a single-screw extruder and enters a 170°C hot air vulcanization pipe, the bis(2,5-dimethyl) rubber undergoes homolytic cracking upon heating to generate tert-butyloxy radicals, which abstract hydrogen atoms from the methyl groups on the side chains of the VMQ molecular chain to generate silicon-based radicals. The silicon-based radicals couple with each other to form carbon-carbon crosslinks, connecting the linear VMQ molecular chains into a three-dimensional network structure, thus transforming silicone rubber from a plastic material into an elastomer.

[0050] The hot air vulcanization pipe is held for 15-20 minutes to allow the cross-linking reaction to be initially completed, and the material acquires basic mechanical properties. Subsequently, a second vulcanization is carried out in a 175℃ oven for 2 hours. The purpose of this second vulcanization is to: 1) completely decompose the residual bis(2,5-diphenyltrimethylammonium chloride) and eliminate small molecule by-products; 2) promote further rearrangement and cross-linking of silicone rubber molecular chains and improve cross-linking density; and 3) volatilize low molecular weight substances and eliminate product odor. After the second vulcanization, the tensile strength, elongation at break, and heat aging resistance of the VMQ protective sleeve all reach their optimal state, and the thermal conductivity is ≥0.5W / (m·K), which meets the usage requirements of power cable protective sleeves.

[0051] Preferably, the conductor diameter is 2.5-30mm, the conductor shielding layer and the insulating shielding layer are both 0.5-1.2mm thick, the insulating layer is 3.0-9.0mm thick, the metal shielding layer is 0.1-0.3mm thick, and the protective sleeve is 2.0-4.0mm thick.

[0052] Preferably, the metallocene polyethylene is a copolymer of ethylene with 1-butene, 1-hexene or 1-octene, and its molecular weight distribution index Mw / Mn is 1.5-3.0.

[0053] Compared with the prior art, the beneficial effects of the present invention are:

[0054] 1. This invention employs a trisubstituted cyclotriphosphazene-benzophenone multifunctional crosslinking agent molecule design, combined with an allyl double bond stepwise grafting-crosslinking mechanism, to solve the problems of existing technologies where separate addition of flame retardants and voltage stabilizers leads to a large number of additives, large dosages, and easy migration and precipitation. It achieves integrated chemical bonding of flame retardancy, voltage stabilization, medium-deep traps, and a crosslinking network. This invention designs and synthesizes a multifunctional crosslinking agent with cyclotriphosphazene as the core, benzophenone as the side group, and allyl as the end group. This molecule undergoes a two-step reaction via allyl double bonds: first, grafting onto the polyethylene backbone at 90–100℃; second, participating in crosslinking network construction at 160–180℃. The cyclotriphosphazene core provides phosphorus-nitrogen synergistic halogen-free flame retardancy (limiting oxygen index ≥28%), while the benzophenone structure acts as a voltage stabilizer, increasing the DC breakdown field strength (≥40kV / mm). Its carbonyl structure may introduce specific energy level trap states into the polyethylene band gap, thereby helping to suppress space charge accumulation. The entire functional molecule is covalently fixed in the cross-linked network and does not migrate or precipitate. The three functional units are integrated into one molecule, which simplifies the formulation system.

[0055] 2. This invention employs a three-layer co-extrusion synchronous cross-linking process and a cross-layer co-cross-linking mechanism to solve the problems of insufficient interlayer bonding strength and easy delamination between the shielding layer and insulation layer in traditional cables, achieving synergistic optimization of functional layering and interface bonding. This invention encapsulates the conductor shielding layer, insulation layer, and insulating shielding layer in a single three-layer co-extrusion process. The free radicals generated by the decomposition of DCP in the thermal cross-linking pipeline not only initiate cross-linking within each layer but also trigger cross-layer free radical coupling reactions at the interfaces, forming cross-layer chemical bonds. After cross-linking, the shielding layer retains its semi-conductive properties (volume resistivity ≤ 100 Ω·cm), and the insulation layer retains its insulating properties (volume resistivity > 10). 14 (Ω·cm), each layer has independent functions, the interface molecular chains are tightly entangled, and the interlayer bonding strength is high. At the same time, the multifunctional crosslinking agent grafted in the insulating layer fixes the flame retardant and voltage stabilizing units to the network nodes.

[0056] 3. This invention employs electrostatic interaction to construct a hybrid thermally conductive network, solving the problems of poor dispersibility and easy agglomeration of traditional thermally conductive fillers, and achieving high thermal conductivity and insulation performance with low filler content. This invention combines KH550-modified positively charged spherical alumina with SDBS-modified negatively charged hexagonal boron nitride to form a composite thermally conductive filler through electrostatic interaction, avoiding interlayer stacking and agglomeration of BN, significantly improving dispersion and compatibility in silicone rubber, facilitating the construction of continuous and stable thermally conductive pathways, achieving high thermal conductivity with low filler content, a protective sheath thermal conductivity ≥0.5 W / (m·K), and maintaining a volume resistivity of 10. 14 Ω·cm or higher. Attached Figure Description

[0057] Figure 1 This is a flowchart illustrating the manufacturing process of the power cable of the present invention.

[0058] Figure 2 The 1H NMR spectrum of the multifunctional crosslinking agent of this invention;

[0059] Figure 3 This is the carbon NMR spectrum of the multifunctional crosslinking agent of this invention. Detailed Implementation

[0060] The technical solutions of this invention are described below. It should also be noted that, to make the embodiments more detailed, the following embodiments are the best and preferred embodiments; those skilled in the art can also use other alternative methods to implement some well-known technologies.

[0061] Example 1: Preparation of cross-linked polyethylene insulated flame-retardant power cable:

[0062] S1. Preparation of multifunctional crosslinking agents:

[0063] S101. Mix 4,4'-dihydroxybenzophenone, allyl bromide and potassium carbonate in a molar ratio of 1:1:2, add acetone solvent, react at 60°C under nitrogen protection for 12 h, and obtain 4-allyloxy-4'-hydroxybenzophenone by suction filtration, rotary evaporation and recrystallization.

[0064] S102, hexachlorocyclotriphosphazene, 4-allyloxy-4'-hydroxybenzophenone and potassium carbonate were weighed in a molar ratio of 1:3.2:5 and refluxed in acetonitrile at 85°C under nitrogen protection for 36 h. After filtration, rotary evaporation and recrystallization with ethanol twice, a multifunctional crosslinking agent was obtained with a yield of 55%.

[0065] S2. Preparation of insulating layer mixture:

[0066] Mix 100 parts of metallocene polyethylene, 3 parts of multifunctional crosslinking agent, and 0.3 parts of antioxidant 1010 evenly, and add 1 part of dicumyl peroxide. The dicumyl peroxide is added in two parts: 65% of the total amount is added in the first part and the remaining 35% is added in the second part. The mixture is melt-blended at 100°C and 60 r / min for 7 min to obtain the insulation layer mixture.

[0067] S3. Preparation of semiconductive shielding material:

[0068] Weigh out 100 parts of polyolefin base material, 50 parts of conductive carbon black, 3 parts of dicumyl peroxide, and 1 part of antioxidant 1010 according to the mass ratio, and melt-blend them at 105℃ for 10 min to obtain conductor shielding layer material and insulating shielding layer material.

[0069] S4, Three-layer co-extrusion:

[0070] The conductor shielding layer material, the insulation layer mixture, and the insulation shielding layer material are respectively added to the three hoppers of the three-layer co-extrusion extruder. The extrusion temperature is 105℃. The material is coated onto the surface of the copper or aluminum conductor in one go through the three-layer co-extrusion die head at a linear speed of 4m / min, forming an integrated three-layer structure of conductor shielding layer, insulation layer, and insulation shielding layer.

[0071] S5, Crosslinking:

[0072] The three-layer co-extruded cable is passed through a 160-180℃ thermal cross-linking pipe for 30 minutes to allow the insulation layer, conductor shielding layer and insulation shielding layer to undergo cross-linking reaction simultaneously. The thermal cross-linking pipe is divided into three temperature zones: the first zone is 160℃, the second zone is 170℃ and the third zone is 180℃, with each zone lasting for 10 minutes.

[0073] S6. Metal shielding layer covering:

[0074] After the cross-linked cable is cooled to room temperature, the metal shielding layer is wrapped with copper tape or copper wire on the outer surface of the insulation shielding layer. The copper tape wrapping overlap rate is ≥15%, and the copper wire sparse winding gap is ≤1mm.

[0075] S7. Protective sleeve coating and vulcanization:

[0076] S701. Spherical alumina with an average particle size of 2μm is modified with silane reagent KH550, and hexagonal boron nitride with a flake size of 1μm is modified with sodium dodecylbenzenesulfonate. The two are mixed and stirred at a mass ratio of 2:1 for 1 hour and left to stand for 12 hours. The composite thermally conductive filler is obtained through electrostatic interaction.

[0077] S702. Mix 100 parts of methyl vinyl silicone rubber, 45 parts of composite thermally conductive filler, and 5 parts of hydroxyl silicone oil evenly on a two-roll mill. Add 1.5 parts of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and continue mixing for 10 minutes. Then, coat the mixed rubber onto the outside of the metal shielding layer on a single-screw extruder. Pass it through a 170°C hot air vulcanization pipe and let it stay for 20 minutes. Then, vulcanize it again in a 175°C oven for 2 hours to obtain the finished cable.

[0078] Example 2: Preparation of cross-linked polyethylene insulated flame-retardant power cable:

[0079] S1. Preparation of multifunctional crosslinking agents:

[0080] S101. Mix 4,4'-dihydroxybenzophenone, allyl bromide and potassium carbonate in a molar ratio of 1:1:2, add acetone solvent, react at 60°C under nitrogen protection for 12 h, and obtain 4-allyloxy-4'-hydroxybenzophenone by suction filtration, rotary evaporation and recrystallization.

[0081] S102, hexachlorocyclotriphosphazene, 4-allyloxy-4'-hydroxybenzophenone and potassium carbonate were weighed in a molar ratio of 1:3.2:5 and refluxed in acetonitrile at 85°C under nitrogen protection for 36 h. After filtration, rotary evaporation and recrystallization with ethanol twice, a multifunctional crosslinking agent was obtained with a yield of 55%.

[0082] S2. Preparation of insulating layer mixture:

[0083] Mix 100 parts of metallocene polyethylene, 5 parts of multifunctional crosslinking agent, and 0.3 parts of antioxidant 1010 evenly, and add 1 part of dicumyl peroxide. The dicumyl peroxide is added in two parts: the first part is 65% of the total amount, and the second part is the remaining 35%. Melt and blend at 100℃ and 60 r / min for 7 min to obtain the insulation layer mixture.

[0084] S3. Preparation of semiconductive shielding material:

[0085] Weigh out 100 parts of polyolefin base material, 50 parts of conductive carbon black, 3 parts of dicumyl peroxide, and 1 part of antioxidant 1010 according to the mass ratio, and melt-blend them at 105℃ for 10 min to obtain conductor shielding layer material and insulating shielding layer material.

[0086] S4, Three-layer co-extrusion:

[0087] The conductor shielding layer material, the insulation layer mixture, and the insulation shielding layer material are respectively added to the three hoppers of the three-layer co-extrusion extruder. The extrusion temperature is 105℃. The material is coated onto the surface of the copper or aluminum conductor in one go through the three-layer co-extrusion die head at a linear speed of 4m / min, forming an integrated three-layer structure of conductor shielding layer, insulation layer, and insulation shielding layer.

[0088] S5, Crosslinking:

[0089] The three-layer co-extruded cable is passed through a 160-180℃ thermal cross-linking pipe for 30 minutes to allow the insulation layer, conductor shielding layer and insulation shielding layer to undergo cross-linking reaction simultaneously. The thermal cross-linking pipe is divided into three temperature zones: the first zone is 160℃, the second zone is 170℃ and the third zone is 180℃, with each zone lasting for 10 minutes.

[0090] S6. Metal shielding layer covering:

[0091] After the cross-linked cable is cooled to room temperature, the metal shielding layer is wrapped with copper tape or copper wire on the outer surface of the insulation shielding layer. The copper tape wrapping overlap rate is ≥15%, and the copper wire sparse winding gap is ≤1mm.

[0092] S7. Protective sleeve coating and vulcanization:

[0093] S701. Spherical alumina with an average particle size of 2μm is modified with silane reagent KH550, and hexagonal boron nitride with a flake size of 1μm is modified with sodium dodecylbenzenesulfonate. The two are mixed and stirred at a mass ratio of 2:1 for 1 hour and left to stand for 12 hours. The composite thermally conductive filler is obtained through electrostatic interaction.

[0094] S702. Mix 100 parts of methyl vinyl silicone rubber, 30 parts of composite thermally conductive filler, and 5 parts of hydroxyl silicone oil evenly on a two-roll mill. Add 1.5 parts of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and continue mixing for 10 minutes. Then, coat the mixed rubber onto the outside of the metal shielding layer on a single-screw extruder. Pass it through a 170°C hot air vulcanization pipe and let it stay for 20 minutes. Then, vulcanize it again in a 175°C oven for 2 hours to obtain the finished cable.

[0095] Example 3: Preparation of cross-linked polyethylene insulated flame-retardant power cable:

[0096] S1. Preparation of multifunctional crosslinking agents:

[0097] S101. Mix 4,4'-dihydroxybenzophenone, allyl bromide and potassium carbonate in a molar ratio of 1:1:2, add acetone solvent, react at 60°C under nitrogen protection for 12 h, and obtain 4-allyloxy-4'-hydroxybenzophenone by suction filtration, rotary evaporation and recrystallization.

[0098] S102, hexachlorocyclotriphosphazene, 4-allyloxy-4'-hydroxybenzophenone and potassium carbonate were weighed in a molar ratio of 1:3.2:5 and refluxed in acetonitrile at 85°C under nitrogen protection for 36 h. After filtration, rotary evaporation and recrystallization with ethanol twice, a multifunctional crosslinking agent was obtained with a yield of 55%.

[0099] S2. Preparation of insulating layer mixture:

[0100] Mix 100 parts of metallocene polyethylene, 5 parts of multifunctional crosslinking agent, and 0.3 parts of antioxidant 1010 evenly, and add 1 part of dicumyl peroxide. The dicumyl peroxide is added in two parts: the first part is 65% of the total amount, and the second part is the remaining 35%. Melt and blend at 100℃ and 60 r / min for 7 min to obtain the insulation layer mixture.

[0101] S3. Preparation of semiconductive shielding material:

[0102] Weigh out 100 parts of polyolefin base material, 50 parts of conductive carbon black, 3 parts of dicumyl peroxide, and 1 part of antioxidant 1010 according to the mass ratio, and melt-blend them at 105℃ for 10 min to obtain conductor shielding layer material and insulating shielding layer material.

[0103] S4, Three-layer co-extrusion:

[0104] The conductor shielding layer material, the insulation layer mixture, and the insulation shielding layer material are respectively added to the three hoppers of the three-layer co-extrusion extruder. The extrusion temperature is 105℃. The material is coated onto the surface of the copper or aluminum conductor in one go through the three-layer co-extrusion die head at a linear speed of 4m / min, forming an integrated three-layer structure of conductor shielding layer, insulation layer, and insulation shielding layer.

[0105] S5, Crosslinking:

[0106] The three-layer co-extruded cable is passed through a 160-180℃ thermal cross-linking pipe for 30 minutes to allow the insulation layer, conductor shielding layer and insulation shielding layer to undergo cross-linking reaction simultaneously. The thermal cross-linking pipe is divided into three temperature zones: the first zone is 160℃, the second zone is 170℃ and the third zone is 180℃, with each zone lasting for 10 minutes.

[0107] S6. Metal shielding layer covering:

[0108] After the cross-linked cable is cooled to room temperature, the metal shielding layer is wrapped with copper tape or copper wire on the outer surface of the insulation shielding layer. The copper tape wrapping overlap rate is ≥15%, and the copper wire sparse winding gap is ≤1mm.

[0109] S7. Protective sleeve coating and vulcanization:

[0110] S701. Spherical alumina with an average particle size of 2μm is modified with silane reagent KH550, and hexagonal boron nitride with a flake size of 1μm is modified with sodium dodecylbenzenesulfonate. The two are mixed and stirred at a mass ratio of 2:1 for 1 hour and left to stand for 12 hours. The composite thermally conductive filler is obtained through electrostatic interaction.

[0111] S702. Mix 100 parts of methyl vinyl silicone rubber, 45 parts of composite thermally conductive filler, and 5 parts of hydroxyl silicone oil evenly on a two-roll mill. Add 1.5 parts of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and continue mixing for 10 minutes. Then, coat the mixed rubber onto the outside of the metal shielding layer on a single-screw extruder. Pass it through a 170°C hot air vulcanization pipe and let it stay for 20 minutes. Then, vulcanize it again in a 175°C oven for 2 hours to obtain the finished cable.

[0112] Comparative Example 1:

[0113] Compared with Example 3, no multifunctional crosslinking agent was added to the insulating layer in Comparative Example 1, while other conditions remained unchanged.

[0114] Comparative Example 2:

[0115] Compared with Example 3, Comparative Example 2 added 5 parts of commercially available cyclotriphosphazene flame retardant and 5 parts of benzophenone voltage stabilizer to the insulation layer, while keeping other conditions unchanged.

[0116] Comparative Example 3:

[0117] Compared with Example 3, in Comparative Example 3, spherical alumina and hexagonal boron nitride were directly added to the protective sleeve, while other conditions remained unchanged.

[0118] Comparative Example 4:

[0119] Compared with Example 3, in Comparative Example 4, no dicumyl peroxide was added to the insulating layer, and other conditions remained unchanged.

[0120] Performance testing:

[0121] 1. Test the DC resistance of the conductor using the four-terminal method according to GB / T 3048.4; test the insulation resistance according to GB / T 3048.5; perform the power frequency voltage test according to GB / T 3048.8; perform partial discharge detection according to GB / T 3048.12; and test the dielectric loss tangent according to GB / T 3048.11.

[0122] 2. The tensile strength and elongation at break of the insulation layer and protective sheath shall be tested using dumbbell-shaped specimens in accordance with GB / T 2951.11; and the bending test shall be conducted in accordance with GB / T 12706.

[0123] 3. Perform a thermal elongation test according to GB / T 2951.21; perform a thermal aging test according to GB / T 2951.12 to test the elongation at break retention rate; perform a thermal shrinkage test according to GB / T 2951.13 to measure the shrinkage rate; and perform a high-temperature pressure test according to GB / T 2951.31 to measure the indentation depth.

[0124] 4. Perform bundled burning test (Class A) according to GB / T 18380.33-36 and measure the char height; perform single vertical burning test according to GB / T18380.12, with flame application time of 60s and measure self-extinguishing time; perform smoke density test according to GB / T 17651, with light transmittance ≥60%.

[0125] 5. Test the peel strength between the shielding layer and the insulating layer according to GB / T 2791 (adhesive T peel strength test method).

[0126] 6. Refer to GB / T 31818 or use the solvent immersion method to test the migration and precipitation performance of functional molecules and measure the mass loss rate of the insulation layer.

[0127] 7. Thermal conductivity test of the protective sleeve:

[0128] Test item: Thermal conductivity measurement

[0129] Test method: Refer to ASTM D5470 or ISO 22007-2 (hot plate method)

[0130] Test steps:

[0131] (1) Cut three circular samples with a diameter of 25.4 mm and a thickness of 2 mm from the protective sleeve;

[0132] (2) Place the sample between the hot plate and the cold plate of the thermal conductivity tester;

[0133] (3) Apply a contact pressure of 50 kPa, set the hot plate temperature to 60°C and the cold plate temperature to 30°C;

[0134] (5) Record the heat flux density Q (W / m³) after stabilization. 2 ), measure the sample thickness d (m), calculate the thermal resistance R=ΔT / Q, and the thermal conductivity λ=d / R, in W / (m·K).

[0135] Judgment criterion: λ≥0.5 W / (m·K).

[0136] The performance test data of the above embodiments and comparative examples are shown in Tables 1, 2, 3, and 4 below.

[0137] Table 1 Electrical performance test data

[0138] Group DC resistance of conductor Insulation resistance Power frequency voltage test Partial discharge quantity Dielectric loss tangent unit Ω / km MΩ·km 35kV / 5min pC tanδ Example 1 1.83 525 pass 7.2 0.0008 Example 2 1.82 545 pass 6.5 0.0007 Example 3 1.8 575 pass 5.8 0.0006 Comparative Example 1 1.85 485 pass 10.5 0.0012 Comparative Example 2 1.83 505 pass 9.2 0.001 Comparative Example 3 1.81 555 pass 6.2 0.0007 Comparative Example 4 1.84 180 breakdown 17.6 0.0035

[0139] Data Analysis:

[0140] Figure 1 This is a flowchart illustrating the production process of the power cable of the present invention. The flowchart fully presents the entire manufacturing process of the cable. First, a multifunctional crosslinking agent of trisubstituted cyclotriphosphazene-benzophenone is synthesized, and insulation layer mixture and semiconductive shielding material are prepared simultaneously. Then, the conductor shielding layer, insulation layer and insulation shielding layer are coated onto the conductor in one step through a three-layer co-extrusion process, and intralayer crosslinking and interfacial bonding are achieved through three-stage thermal crosslinking. After cooling, a metal shielding layer is coated. Finally, a silicone rubber protective sheath is prepared using electrostatic interaction composite thermally conductive filler. After extrusion and vulcanization, the finished cable is obtained. The process is simple and efficient, and each step works together to achieve flame retardant, high insulation and high thermal conductivity.

[0141] The nuclear magnetic resonance (NMR) spectrum of the multifunctional crosslinking agent of the present invention (e.g., Figure 2 As shown in the figure, the characteristic hydrogen signals of the allyl double bond are at δ 5.8–6.0 ppm and δ 5.1–5.3 ppm, confirming the existence of the double bond structure that can participate in grafting and crosslinking; the hydrogen signal of the benzene ring of benzophenone is at δ 6.8–7.2 ppm, proving that the voltage stabilizing unit was successfully introduced; the hydrogen signal of the methylene group connected by the phosphorus-oxygen bond is at δ 4.4–4.6 ppm; there are no obvious impurity peaks, indicating that the product has high purity.

[0142] Figure 3In the carbon spectrum, except for the solvent peak at δ77ppm, the other characteristic peaks correspond to the carbon skeleton of cyclotriphosphazene, the carbonyl carbon of benzophenone, the aromatic carbon of the benzene ring, the carbon of the allyl double bond, and the saturated carbon of the alkoxy group, respectively. The chemical shift peaks of the carbon spectrum correspond one-to-one with the carbon atoms of the designed cyclotriphosphazene-benzophenone-allyl molecular skeleton. Combined with the above proton spectrum analysis results, these findings jointly confirm the successful synthesis of the target molecular structure of the multifunctional crosslinking agent of this invention.

[0143] Based on the electrical performance test data in Table 1, the following conclusions can be drawn:

[0144] First, regarding the DC resistance of the conductor, the values ​​of all embodiments and comparative examples are within the range of 1.80-1.85 Ω / km, with very little difference. This indicates that the adjustment of the insulation layer formula and cross-linking process in this invention has almost no impact on the conductivity of the conductor itself, and the current carrying capacity of the cable is not affected by the modification measures.

[0145] Secondly, regarding insulation resistance, the values ​​of Examples 1-3 (525-575 MΩ·km) were significantly higher than those of Comparative Example 1 (485 MΩ·km) and Comparative Example 2 (505 MΩ·km), with Example 3 showing the highest value. This indicates that the multifunctional crosslinking agent BMP designed in this invention can effectively improve the resistance characteristics of the insulation layer, and the insulation resistance increases with the increase of BMP addition (from 3 parts in Example 1 to 5 parts in Example 3). Comparative Example 4, which did not undergo crosslinking due to the absence of dicumyl peroxide, had the lowest insulation resistance (330 MΩ·km), demonstrating that crosslinking is a necessary condition for ensuring good insulation performance.

[0146] Third, in the power frequency voltage test, all samples except Comparative Example 4 passed the 35kV / 5min test, while Comparative Example 4 broke down, further confirming the crucial role of the cross-linked structure in withstanding high voltage. Uncross-linked insulation layers are prone to breakdown failure under high voltage.

[0147] Fourth, the partial discharge data showed a clear pattern: Example 3 (5.8 pC) < Comparative Example 3 (6.2 pC) < Example 2 (6.5 pC) < Example 1 (7.2 pC) < Comparative Example 2 (9.2 pC) < Comparative Example 1 (10.5 pC) < Comparative Example 4 (17.6 pC). Example 3 had the lowest partial discharge, indicating that the medium-deep trap energy level introduced after BMP chemical grafting effectively captured the charge generated by partial discharge and suppressed its development. Comparative Example 1, without BMP, had a higher partial discharge; Comparative Example 2, using a physically blended flame retardant and voltage stabilizer, had an intermediate effect, but not as good as the chemically bonded examples; Comparative Example 4, without cross-linking, had the most severe partial discharge.

[0148] Finally, the order of dielectric loss tangent (tanδ) was consistent with the partial discharge magnitude: Example 3 had the lowest (0.0006), followed by Examples 2 (0.0007) and 1 (0.0008) which were slightly higher, Comparative Example 1 (0.0012) and Comparative Example 2 (0.0010) which were relatively high, and Comparative Example 4 had the highest (0.0035). This indicates that the carbonyl and benzoyl structures in the chemically grafted BMP molecules introduce medium-deep trap levels in the polyethylene band gap, effectively reducing carrier mobility and dielectric loss. While physical blending showed some effect, it was not as significant as chemical bonding.

[0149] In summary, this invention, by chemically grafting the multifunctional crosslinking agent BMP into the crosslinked polyethylene network, can significantly improve the electrical performance of the cable insulation layer, including increasing insulation resistance, reducing partial discharge, reducing dielectric loss, and ensuring qualified power frequency withstand voltage. Example 3 (with 5 parts BMP added) exhibits the best overall electrical performance. In contrast, the uncrosslinked cable (Comparative Example 4) shows severely deteriorated electrical properties, demonstrating the necessity of the crosslinking process.

[0150] Table 2 Mechanical Performance Test Data

[0151] Group Tensile strength of insulation layer Elongation at break of insulation layer tensile strength of protective sleeve Elongation at break of protective sleeve Bending test (6 times diameter) unit MPa % MPa % - Example 1 14.2 425 11.5 525 No cracks Example 2 15.5 475 10.8 485 No cracks Example 3 16.5 510 12.2 550 No cracks Comparative Example 1 13.5 385 11.8 535 No cracks Comparative Example 2 14 405 11.2 515 No cracks Comparative Example 3 16.2 505 10.5 425 No cracks Comparative Example 4 7.2 120 11.5 525 cracking

[0152] Data Analysis:

[0153] According to the data in Table 2, in terms of the mechanical properties of the insulation layer, the tensile strength and elongation at break of Examples 1-3 are significantly better than those of Comparative Examples 1 and 2. Furthermore, both indicators show a continuous upward trend with the increase of the amount of the multifunctional crosslinking agent BMP. This indicates that BMP molecules are chemically grafted onto the polyethylene backbone through allyl double bonds and participate in the construction of the crosslinking network under the initiation of dicumyl peroxide, forming a denser three-dimensional network structure, thereby significantly enhancing the tensile strength and flexibility of the insulation layer. In contrast, Comparative Example 1 did not add BMP and relied solely on dicumyl peroxide for crosslinking, resulting in a lower crosslinking density and lower mechanical properties. Comparative Example 2 used a physically blended cyclotriphosphazene flame retardant and benzophenone voltage stabilizer. These functional molecules are dispersed in the matrix in small molecule form, which not only cannot participate in the crosslinking network reinforcement but may also disrupt the regular arrangement of polyethylene chain segments. Therefore, its mechanical properties are only slightly better than those of Comparative Example 1 but significantly lower than those of the other examples.

[0154] Comparative Example 3 uses a direct physical blend of alumina and boron nitride, which lacks a hybrid structure formed by electrostatic interactions. However, its insulation layer formulation is the same as that of Example 3, therefore, the mechanical properties of the insulation layer are basically equivalent to those of Example 3, indicating that the modification measures of the protective sleeve do not affect the mechanical properties of the insulation layer itself. Comparative Example 4, because it did not add dicumyl peroxide, did not undergo cross-linking in its insulation layer. Its tensile strength and elongation at break were much lower than all cross-linked samples, and it cracked during the bending test. This fully demonstrates that cross-linking is a necessary condition for imparting sufficient mechanical strength and flexibility to the cable insulation layer.

[0155] Regarding the mechanical properties of the protective sleeves, the tensile strength and elongation at break of all samples met the national standard requirements. The protective sleeve of Example 3 exhibited the highest tensile strength and elongation at break, attributed to the moderate amount of composite thermally conductive filler added, which formed a uniform "core-shell" hybrid structure. The filler and silicone rubber matrix showed good interfacial bonding, providing reinforcement. The protective sleeve of Comparative Example 3 used a direct physical blend filler, which was unevenly dispersed and prone to agglomeration, resulting in a significantly lower elongation at break and a decrease in tensile strength compared to Example 3.

[0156] Combining the data in Tables 1 and 2, it can be found that Example 3 exhibits the best performance in both electrical properties (highest insulation resistance, lowest partial discharge, and lowest dielectric loss) and mechanical properties (highest tensile strength and highest elongation at break). This indicates that the BMP chemical grafting strategy not only improves electrical properties by introducing deep trap energy levels in benzophenone, but also enhances mechanical properties by participating in the construction of the crosslinking network, achieving a synergistic improvement in both electrical and mechanical properties. Comparative Example 4 shows the worst performance in both electrical and mechanical properties, further verifying that the crosslinking structure is fundamental to the performance of cable insulation materials. Comparative Example 2 has better electrical properties than Comparative Example 1 but worse than Example 3, while its mechanical properties are similar to Comparative Example 1. This indicates that the physical blending scheme can only partially improve electrical properties and cannot simultaneously enhance mechanical properties, further highlighting the advantages of the integrated chemical bonding design of this invention.

[0157] Table 3 Thermal performance test data

[0158] Group Thermal elongation under load Thermal elongation permanent deformation rate Elongation at break due to heat aging thermal shrinkage rate High temperature pressure indentation depth unit % % % % % Example 1 45 12 86 2.5 32 Example 2 38 10 88 2.1 28 Example 3 35 8 91 1.8 25 Comparative Example 1 42 11 83 2.8 35 Comparative Example 2 40 10 85 2.5 30 Comparative Example 3 36 9 90 2 26 Comparative Example 4 Melting - 48 10.5 78

[0159] Data Analysis:

[0160] Based on the thermal performance test data in Table 3, the following conclusions can be drawn:

[0161] Regarding thermal elongation properties, the load elongation and permanent deformation of Examples 1-3 decreased significantly with increasing addition of the multifunctional crosslinking agent BMP. This indicates that BMP molecules are chemically grafted onto the polyethylene backbone via allyl double bonds and participate in the construction of the crosslinking network, gradually increasing the crosslinking density of the insulation layer. Higher crosslinking density makes chain slippage more difficult at high temperatures, resulting in lower load elongation and lower permanent deformation after unloading. Comparative Example 1, without BMP, relied solely on dicumyl peroxide crosslinking, resulting in a lower crosslinking density, significantly higher load elongation, and slightly higher permanent deformation than the examples. Comparative Example 2 used physically blended functional molecules, which do not participate in the crosslinking network and contribute little to the crosslinking density; therefore, its thermal elongation properties were similar to Comparative Example 1 but still inferior to the examples. Comparative Example 3 had the same insulation layer formulation as Example 3, with comparable thermal elongation data, further demonstrating that the modification of the protective sleeve does not affect the crosslinking structure of the insulation layer. Comparative Example 4, due to the absence of dicumyl peroxide, did not undergo cross-linking of the insulation layer and melted directly during the thermal elongation test, making it impossible to measure data. This fully demonstrates that cross-linking is the fundamental guarantee for giving the cable heat resistance and deformation resistance.

[0162] Regarding thermal aging performance, the elongation at break retention rates of Examples 1-3 increased with increasing BMP addition, indicating that increased crosslinking density helps suppress molecular chain breakage during thermo-oxidative aging. The crosslinked network restricts molecular chain movement, reducing oxygen diffusion and attack, thereby slowing down the aging process. The retention rates of Comparative Examples 1 and 2 were lower than those of the examples, and the retention rate of the uncrosslinked Comparative Example 4 was less than half, indicating the most severe aging.

[0163] Regarding heat shrinkage, the data for Examples 1-3 decreased with increasing BMP addition, while Comparative Example 4 showed the highest heat shrinkage, nearly six times that of Example 3. This is because the cross-linked network effectively constrains the recovery movement of polyethylene molecular chains at high temperatures, while the uncross-linked material undergoes free molecular chain rearrangement after melting, resulting in significant shrinkage upon cooling. This indicator is directly related to the dimensional stability of the cable under fluctuating operating temperatures.

[0164] Regarding the indentation depth under high-temperature pressure, the indentation depth of Examples 1-3 decreased with increasing crosslinking density, indicating that the crosslinking network endowed the material with higher resistance to compressive deformation. The uncrosslinked Comparative Example 4 exhibited the largest indentation depth, and the material underwent severe softening and deformation under high-temperature pressure.

[0165] Table 4 Flame retardant properties and interlayer peel strength, thermal conductivity and migration / exudation test data

[0166] Group Bundle combustion carbonization height Single vertical combustion self-extinguishing time Smoke density and light transmittance interlayer peel strength thermal conductivity of protective sleeve Insulation layer mass loss rate unit m s % N / mm (W / (m·K)) % Example 1 2.1 18 72 12.5 0.56 0.6 Example 2 1.8 15 78 13.2 0.53 0.5 Example 3 1.6 12 82 14.8 0.63 0.4 Comparative Example 1 3.5 35 55 11.8 0.61 0.3 Comparative Example 2 1.9 16 75 12 0.59 2.8 Comparative Example 3 1.7 13 80 14.2 0.32 0.4 Comparative Example 4 1.8 14 80 2.5 0.61 0.3

[0167] Data Analysis:

[0168] Based on the flame retardant performance, interlayer peel strength, thermal conductivity, and migration precipitation test data in Table 4, it can be observed that: in terms of flame retardant performance, the char height of bundled combustion in Examples 1-3 significantly decreased with the increase of the amount of multifunctional crosslinking agent BMP, the self-extinguishing time of single vertical combustion was correspondingly shortened, and the smoke density and light transmittance gradually increased. This indicates that the cyclotriphosphazene nucleus in the BMP molecule decomposes during combustion to generate substances such as phosphoric acid and metaphosphoric acid, promoting the formation of a dense char layer from polyethylene, while releasing non-combustible gases such as ammonia and nitrogen to dilute the oxygen concentration, achieving efficient phosphorus-nitrogen synergistic flame retardancy. Comparative Example 1, without the addition of BMP, had the highest char height, the longest self-extinguishing time, and the lowest smoke density, with flame retardant performance significantly inferior to all examples. Comparative Example 2, using a physically blended cyclotriphosphazene flame retardant and benzophenone voltage stabilizer, although showing some flame retardant effect, still had slightly lower smoke density and self-extinguishing time than Example 3, indicating that the chemical bonding method makes the flame retardant units more evenly distributed and longer-lasting. The flame retardant properties of Comparative Examples 3 and 4 are similar to those of Examples 2-3, indicating that flame retardancy mainly depends on the addition of BMP, and whether or not cross-linking is involved has little effect on the flame retardant effect.

[0169] Regarding interlayer peel strength, the values ​​in Examples 1-3 significantly increased with increasing BMP addition, with Example 3 reaching the highest value. This is because after BMP is grafted onto the polyethylene backbone via allyl double bonds, during subsequent crosslinking, the polyethylene macromolecular free radicals at the interface undergo translayer coupling reactions, forming translayer covalent bonds between the conductor shielding layer and the insulating layer, and between the insulating layers and the insulating shielding layer, firmly sealing the three layers. The peel strength of Comparative Examples 1 and 2 is slightly lower than that of the examples, Comparative Example 3 is comparable to Example 3, while Comparative Example 4, due to lack of crosslinking, has extremely low peel strength. This fully demonstrates that the combined effect of the "three-layer co-extrusion simultaneous crosslinking" process and BMP chemical grafting in this invention significantly enhances the interlayer bonding strength.

[0170] Regarding the thermal conductivity of the protective sleeve, Examples 1-3 all achieved above 0.5 W / (m·K), with Example 3 showing the highest conductivity. Comparative Example 3, using a direct physical blend of spherical alumina and hexagonal boron nitride without modification, exhibited a significantly lower thermal conductivity than all other examples. This is because the present invention modifies the spherical alumina with KH550 to make its surface positively charged and modifies the hexagonal boron nitride with SDBS to make its surface negatively charged. Driven by electrostatic attraction, these two materials self-assemble to form a hybrid structure similar to a "core-shell." Utilizing the high thermal conductivity of BN and the spherical support of Al2O3, interlayer stacking and agglomeration of BN are avoided, forming a continuous and efficient thermal conductivity pathway within the VMQ matrix.

[0171] Regarding migration and precipitation performance, the insulation layer mass loss rates of Examples 1-3 were all very low, while the mass loss rate of Comparative Example 2 was several times higher than that of the Examples. This is because the cyclotriphosphazene flame retardant and benzophenone voltage stabilizer in Comparative Example 2 were only physically blended and dispersed in the polyethylene matrix, and the small molecules were easily migrated and precipitated under high temperature or solvent immersion. In contrast, the BMP in the Examples was chemically grafted and cross-linked into a three-dimensional network structure through allyl double bond, and the functional molecules were firmly locked by covalent bonds and could not migrate. This result directly proves the core advantage of the "chemical bonding integration" design of this invention.

[0172] Based on the overall analysis of Tables 4 and 1-3, it can be concluded that this invention achieves comprehensive improvements in multiple dimensions, including flame retardancy, interlayer bonding strength, thermal conductivity, and anti-migration properties, through chemical grafting of BMP multifunctional crosslinking agent, three-layer co-extrusion simultaneous crosslinking process, and electrostatic interaction to construct a hybrid thermally conductive network. Example 3 shows the best performance in all indicators, further verifying that a BMP addition of 5 parts achieves the best overall performance balance. While the physical blending scheme of Comparative Example 2 can partially improve flame retardancy and electrical properties, it suffers from severe functional molecule migration and precipitation, resulting in insufficient long-term reliability, highlighting the irreplaceable nature of the chemical bonding scheme of this invention.

[0173] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A cross-linked polyethylene insulated flame-retardant power cable, characterized in that, It includes a copper or aluminum conductor, a conductor shielding layer covering the outside of the conductor, an insulating layer covering the outside of the conductor shielding layer, an insulating shielding layer covering the outside of the insulating layer, a metal shielding layer covering the outside of the insulating shielding layer, and a protective sleeve covering the outside of the metal shielding layer. Both the conductor shielding layer and the insulating shielding layer are peroxide cross-linked semi-conductive shielding materials, comprising the following mass fractions: 100 parts polyolefin base material, 45-75 parts conductive carbon black, 2-4 parts dicumyl peroxide, and 0.2-2 parts antioxidant 1010. The metal shielding layer is a copper strip wrapping layer or a loosely wrapped copper wire layer; The insulating layer comprises the following components by weight: 100 parts metallocene polyethylene, 3-7 parts multifunctional crosslinking agent, 0.8-1.5 parts dicumyl peroxide, and 0.3 parts antioxidant 1010; The protective sleeve comprises the following components by weight: 100 parts methyl vinyl silicone rubber, 30-60 parts composite thermally conductive filler, 5 parts hydroxyl silicone oil, and 1.5 parts 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.

2. The cross-linked polyethylene insulated flame-retardant power cable according to claim 1, characterized in that, The conductor has a diameter of 2.5-30 mm, the conductor shielding layer and the insulating shielding layer each have a thickness of 0.5-1.2 mm, the insulating layer has a thickness of 3.0-9.0 mm, the metal shielding layer has a thickness of 0.1-0.3 mm, and the protective sleeve has a thickness of 2.0-4.0 mm.

3. The cross-linked polyethylene insulated flame-retardant power cable according to claim 1, characterized in that, The preparation steps of the multifunctional crosslinking agent are as follows: 4,4'-dihydroxybenzophenone, allyl bromide, and potassium carbonate were mixed in a molar ratio of 1:1:2, and acetone was added as solvent. The mixture was reacted at 60°C under nitrogen protection for 12 h. After filtration, rotary evaporation, and recrystallization, 4-allyloxy-4'-hydroxybenzophenone was obtained. Hexachlorocyclotriphosphazene, 4-allyloxy-4'-hydroxybenzophenone, and potassium carbonate were weighed in a molar ratio of 1:3.0-3.3:5 and refluxed in acetonitrile at 85°C under nitrogen protection for 36 h. After filtration, rotary evaporation, and recrystallization twice with ethanol, a multifunctional crosslinking agent was obtained with a yield of 50%-65%.

4. The cross-linked polyethylene insulated flame-retardant power cable according to claim 1, characterized in that, The preparation steps of the composite thermally conductive filler are as follows: Spherical alumina with an average particle size of 1-3 μm was modified with silane reagent KH550, and hexagonal boron nitride with a particle size of 0.5-1.5 μm was modified with sodium dodecylbenzenesulfonate. The two were mixed and stirred at a mass ratio of 3-1:1 for 1 h and then allowed to stand for 12 h. The composite thermally conductive filler was obtained through electrostatic interaction.

5. A cross-linked polyethylene insulated flame-retardant power cable according to claim 1, characterized in that, The metallocene polyethylene is a copolymer of ethylene with 1-butene, 1-hexene or 1-octene, and its molecular weight distribution index Mw / Mn is 1.5-3.

0.

6. The cross-linked polyethylene insulated flame-retardant power cable according to claim 1, characterized in that, The preparation of cables also includes the following steps: S1. Preparation of semiconductive shielding material: Weigh out polyolefin base material, conductive carbon black, diisopropylbenzene peroxide and antioxidant 1010 according to the mass fractions, and melt blend them at 100-110℃ for 8-10 minutes to obtain conductor shielding layer material and insulating shielding layer material. S2. Preparation of insulating layer mixture: Metallocene polyethylene, multifunctional crosslinking agent, and antioxidant 1010 are mixed evenly, and dicumyl peroxide is added. The dicumyl peroxide is added in two parts: the first part is 60%-70% of the total amount, and the second part is the remaining 30%-40%. The mixture is melt-blended at 90-100℃ and 60r / min for 5-8 minutes to obtain the insulation layer mixture. S3, Three-layer co-extrusion: The conductor shielding layer material, the insulation layer mixture, and the insulation shielding layer material are respectively added to the three hoppers of the three-layer co-extrusion extruder. The extrusion temperature is 100-110℃. The material is applied to the surface of the copper or aluminum conductor in one pass through the three-layer co-extrusion die head at a linear speed of 3-5m / min, forming an integrated three-layer structure of conductor shielding layer, insulation layer, and insulation shielding layer. S4, cross-linking: The three-layer co-extruded cable is passed through a thermal cross-linking pipe at 160-180℃ for 24-36 minutes to allow the insulation layer, conductor shielding layer and insulation shielding layer to undergo cross-linking reaction simultaneously. The thermal cross-linking pipe is divided into three temperature zones: the first zone is 160-165℃, the second zone is 165-175℃ and the third zone is 175-180℃, with each zone lasting for 8-12 minutes. S5. Metal shielding layer covering: After the cross-linked cable is cooled to room temperature, the metal shielding layer is wrapped with copper tape or copper wire on the outer surface of the insulation shielding layer. The copper tape wrapping overlap rate is ≥15%, and the copper wire sparse winding gap is ≤1mm. S6. Covering and vulcanization of the protective sleeve: Methyl vinyl silicone rubber, composite thermally conductive filler, and hydroxyl silicone oil are mixed evenly on a two-roll mill. 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane is added and mixing is continued for 10 minutes. The mixed rubber is then coated onto the outside of the metal shielding layer on a single-screw extruder. It is then placed in a 170°C hot air vulcanization pipe for 15-20 minutes and then vulcanized again in a 175°C oven for 2 hours to obtain the finished cable.

7. A cross-linked polyethylene insulated flame-retardant power cable according to claim 1, characterized in that, The finished cable has an insulation gel content of 75%-88%, a thermal elongation of ≤50%, a DC breakdown field strength of ≥40kV / mm, a limiting oxygen index of ≥28%, and a protective sheath thermal conductivity of ≥0.5W / (m·K).

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