Direct-current composite submarine cable and preparation method thereof

Abstract

The invention relates to the technical field of direct-current submarine cables, and particularly discloses a direct-current composite submarine cable and a preparation method thereof. According to the submarine cable, the biphenyl-acetophenone epoxy oligomer is added into the low-density polyethylene to form the insulating layer of the submarine cable, the modified magnesium hydroxide and the modified aluminum hypophosphite are used as flame retardants of the inner sheath layer, and the modified polyolefin and the fluorinated nano silicon dioxide are added into the outer sheath layer. The prepared direct-current composite submarine cable has good insulativity, flame retardance and hydrolysis and aging resistance.

Description

Technical Field

[0001] This invention relates to the field of DC submarine cable technology, specifically to a DC composite submarine cable and its manufacturing method. Background Technology

[0002] Offshore wind power, as an important clean energy source, has shown enormous development potential and technological prospects against the backdrop of the increasing depletion of traditional fossil fuels and the transformation of the energy structure towards green and low-carbon energy. With the large-scale advancement of offshore wind power projects, especially towards deep-sea and large-scale centralized development, the demand for power transmission between land and sea inter-regional power grids has risen rapidly, and the demand for DC submarine cables has also increased significantly.

[0003] In applications such as offshore wind power grid connection and inter-regional power interconnection, high-voltage direct current (HVDC) submarine cables have become key equipment for efficient offshore wind power transmission and flexible grid interconnection due to their outstanding advantages, including low loss, large transmission capacity, minimal corridor occupation, and suitability for long-distance, high-capacity power transmission. Compared with onshore cables, submarine cables have a more complex structure and higher manufacturing requirements. They not only need to maintain excellent electrical insulation performance and transmission efficiency under high-voltage transmission conditions but also must be able to withstand the complex and harsh marine environment for extended periods. This dictates that submarine cables must possess excellent high-voltage resistance and corrosion resistance to ensure safe, stable, and reliable operation throughout their entire lifecycle. Therefore, developing high-voltage resistant and corrosion-resistant submarine cables to improve the transmission efficiency and reliability of offshore wind power and ensure the long-term safe and stable operation of submarine cables in extreme marine environments is of great significance for promoting the sustainable and high-quality development of the offshore wind power industry. Summary of the Invention

[0004] The purpose of this invention is to provide a DC composite submarine cable and its preparation method. This DC submarine cable has excellent high voltage resistance and corrosion resistance, promoting the sustainable and high-quality development of the offshore wind power industry.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: A method for preparing a DC composite submarine cable, specifically comprising: Step 1: Prepare biphenyl-acetophenone epoxy oligomers using 3,5-dihydroxyacetophenone, epichlorohydrin, and biphenyl dicarboxylic acid; Step 2: Mix low-density polyethylene with biphenyl-acetophenone epoxy oligomer and melt extrude to obtain biphenyl-acetophenone modified cross-linked XLPE insulating granules, and melt them to form an insulating layer; Step 3: Mix high-density polyethylene, NDZ-201 modified aluminum hypophosphite and NDZ-201 modified magnesium hydroxide and melt extrude to obtain flame-retardant HDPE granules, and melt them to form an inner sheath layer. Step 4: Mix the polyolefin and vinyltriethoxysilane and melt extrude to obtain silane-modified polyolefin masterbatch; Step 5: Mix high-density polyethylene, silane-modified polyolefin masterbatch and fluorinated nano-silica and melt extrude to obtain fluorosilicone-modified polyolefin granules. Melt these granules to form an outer sheath layer to obtain a DC composite submarine cable.

[0006] As a limitation of this invention, the preparation method of the biphenyl-acetophenone epoxy oligomer is as follows: 3,5-Dihydroxyacetophenone and epichlorohydrin were mixed, and tetrabutylammonium bromide was added as a catalyst. The mixture was stirred and refluxed at 80-90℃ and 300-400 rpm for 4-6 hours, with the pH maintained at 10-11 during the reaction. After the reaction was completed, the mixture was washed with deionized water until the pH of the aqueous phase was neutral. The organic phase was dried with anhydrous sodium sulfate, filtered, and the filtrate was evaporated by rotary evaporation to obtain acetylresorcinol diglycidyl ether. Biphenyl dicarboxylic acid and the catalyst tetrabutylammonium bromide were mixed and reacted under nitrogen protection at 110-120℃ and 300-400 rpm for 0.5-1 h. After the reaction was completed, the mixture was cooled to 100-110℃ and acetylresorcinol diglycidyl ether was added dropwise. The mixture was then stirred and reacted at 120-130℃ and 300-400 rpm for 2-4 h. After the reaction was completed, the end-capping agent phenyl glycidyl ether was added and the mixture was stirred and reacted for another 0.5-1 h. After the reaction was completed, the mixture was cooled to obtain biphenyl-acetophenone epoxy oligomer.

[0007] Under alkaline conditions, tetrabutylammonium bromide catalyzes the reaction of 3,5-dihydroxyacetophenone with epichlorohydrin. The phenolic hydroxyl group of 3,5-dihydroxyacetophenone forms a phenoxy anion under alkaline conditions, which undergoes a ring-opening reaction with the epoxy ring of epichlorohydrin. Subsequently, under alkaline conditions, the intramolecular ring closes to form a new epoxy group, resulting in a molecule containing a rigid benzene ring, a polar carbonyl group, and two epoxy end groups. This molecule then undergoes an epoxy-carboxylic acid ring-opening copolymerization reaction with biphenyl dicarboxylic acid. The carboxylic acid and epoxy group undergo addition polymerization under the action of a catalyst, and the end is capped with a phenyl glycidyl ether with a single reaction site to form a linear biphenyl-acetophenone epoxy oligomer linked by ester and ether bonds.

[0008] Biphenyl-acetophenone epoxy oligomers contain biphenyl and acetophenone groups. The rigid large π-bond structure of the biphenyl group and the strongly polar carbonyl group of acetophenone act as deep traps, capturing and binding injected free electrons or ions, significantly inhibiting the accumulation of space charge and suppressing the initiation and growth of electrical trees. This enhances the intrinsic insulation strength of the composite material while also improving its current resistance and insulation properties. Furthermore, the rigid structure of biphenyl increases the glass transition temperature and modulus of the material, enhancing its resistance to deformation. The terminal epoxy groups can also react with polyethylene chains or peroxide crosslinking byproducts, providing in-situ compatibilization, improving the interfacial bonding between inorganic nanoparticles and the polyethylene matrix, and reducing defects. The acetophenone structure can assist the crosslinking agent dicumyl peroxide in crosslinking, optimizing the crosslinking network and improving the long-term stability of the composite material.

[0009] As a limitation of the present invention, the mass ratio of 3,5-dihydroxyacetophenone, epichlorohydrin and the catalyst tetrabutylammonium bromide is (5-7):(12-14):(0.5-0.8); the mass ratio of biphenyl dicarboxylic acid, acetylresorcinol diglycidyl ether, the catalyst tetrabutylammonium bromide and the capping agent phenyl glycidyl ether is (5-7):(9-11):(0.1-0.3):(0.5-1.0).

[0010] As a limitation of this invention, the biphenyl-acetophenone modified crosslinked XLPE insulating granules, by weight, comprise 80-100 parts of low-density polyethylene, 8-10 parts of compatibilizer ethylene-acrylic acid copolymer, 3-5 parts of biphenyl-acetophenone epoxy oligomer, 3-5 parts of KH-560 modified nano-alumina, 1-1.5 parts of dicumyl peroxide, and 0.2-0.4 parts of antioxidant 168; the melt extrusion process conditions include a screw speed of 200-300 rpm, a zone 1 temperature of 130-140°C, a zone 2 temperature of 150-160°C, a zone 3 temperature of 160-170°C, and a zone 4 temperature of 155-165°C.

[0011] Preferably, the preparation method of the KH-560 modified nano-alumina is as follows: Nano-alumina was added to an ethanol-water solution, stirred evenly, and ultrasonically dispersed for 10 min. Silane coupling agent KH-560 was added, and the pH was adjusted to 4-5. Under nitrogen protection, the reaction was carried out at 70-80℃ and 300-400 rpm for 5-6 h. After the reaction was completed, the nano-alumina was washed with ethanol and deionized water and vacuum dried at 60-70℃ for 6-8 h to obtain KH-560 modified nano-alumina. The mass ratio of nano-alumina to silane coupling agent KH-560 is (8-12):(1-1.2).

[0012] As a limitation of the present invention, the flame-retardant HDPE granules, by weight, comprise 80-100 parts high-density polyethylene, 4-6 parts NDZ-201 modified aluminum hypophosphite, 18-20 parts NDZ-201 modified magnesium hydroxide, 3-5 parts compatibilizer maleic anhydride grafted polyolefin, 0.4-0.5 parts antioxidant 168, and 1-1.5 parts lubricant calcium stearate; the melt extrusion process conditions include a screw speed of 250-350 rpm, a zone 1 temperature of 160-170℃, a zone 2 temperature of 180-190℃, a zone 3 temperature of 190-200℃, and a zone 4 temperature of 185-195℃.

[0013] Preferably, the preparation method of the NDZ-201 modified aluminum hypophosphite is as follows: Add titanate coupling agent NDZ-201 to anhydrous ethanol, stir evenly, add aluminum hypophosphite, ultrasonically disperse for 5-10 min, react at 70-80℃ and 300-400 rpm for 1-2 h, cool after reaction, filter, wash with ethanol, dry at 70-80℃ for 3-4 h to obtain NDZ-201 modified aluminum hypophosphite. The mass ratio of aluminum hypophosphite to titanate coupling agent NDZ-201 is (90-110):(2-3).

[0014] Preferably, the preparation method of the NDZ-201 modified magnesium hydroxide is as follows: Add titanate coupling agent NDZ-201 to anhydrous ethanol, stir evenly, add magnesium hydroxide, ultrasonically disperse for 5-10 min, react at 70-80℃ and 300-400 rpm for 1-2 h, cool after reaction, filter, wash with ethanol, dry at 70-80℃ for 3-4 h to obtain NDZ-201 modified magnesium hydroxide. The mass ratio of magnesium hydroxide to titanate coupling agent NDZ-201 is (90-110):(3-4).

[0015] The titanate coupling agent NDZ-201 has a pyrophosphoryloxy group at one end of its molecular structure that reacts chemically with the hydroxyl groups on the surface of the flame retardant components (aluminum hypophosphite and magnesium hydroxide) to form a strong titanium-oxygen-aluminum or titanium-oxygen-magnesium covalent bond, which is grafted onto the surface of the flame retardant components. The long-chain alkyl structure at the other end of its molecule is highly similar to the HDPE molecular chain. Through intermolecular forces and chain segment entanglement, it enhances the compatibility between the flame retardant components and the HDPE matrix, and prevents the flame retardant components from agglomerating in the HDPE matrix.

[0016] Magnesium hydroxide, as the primary flame retardant, undergoes endothermic decomposition at approximately 340°C, generating magnesium oxide and water vapor. This process effectively reduces the surface temperature of the material, delaying thermal degradation; simultaneously, the generated water vapor dilutes the concentration of combustible gases and oxygen, and acts as a cooling agent in the gas phase. Aluminum hypophosphite decomposes at approximately 240°C, generating acidic substances such as polymetaphosphoric acid. These substances catalyze the dehydration of the polymer matrix into carbon, forming a dense carbon layer covering the material surface, providing thermal and oxygen insulation. More importantly, the gaseous products generated from the decomposition of aluminum hypophosphite contain phosphorus free radicals, which can capture highly reactive free radicals in the combustion chain reaction, interrupting the combustion reaction. The synergistic effect of these two substances enhances the flame retardant properties of the composite material and the long-term stability of the submarine cable structure.

[0017] As a limitation of the present invention, the silane-modified polyolefin comprises, by weight, 80-100 parts polyolefin, 4-6 parts vinyltriethoxysilane, 0.1-0.2 parts dicumyl peroxide and 0.3-0.5 parts antioxidant 168; the melt extrusion process conditions include a screw speed of 200-300 rpm, a zone 1 temperature of 140-150°C, a zone 2 temperature of 160-170°C, a zone 3 temperature of 170-180°C and a zone 4 temperature of 165-175°C.

[0018] Under the initiator dicumyl peroxide, the vinyl groups of vinyltriethoxysilane open and graft onto the polyolefin molecular chain, generating a modified polymer with triethoxysilyl groups on the side chains, fundamentally improving the flexibility and cohesive strength of the composite material. The long silane chains form a stretchable, flexible network within the polyolefin molecules. When the sheath layer is subjected to external mechanical stress, these flexible segments effectively absorb and disperse the stress, preventing brittle cracking caused by stress concentration. Simultaneously, the silane structural units exhibit good compatibility and entanglement with the polyethylene matrix, enhancing the material's integrity and tear resistance. In subsequent processing, this silane-modified polyolefin also acts as a key interfacial compatibilizer, promoting the bonding between high-density polyethylene and fluorinated nanofillers, improving the uniformity and stability of the composite system.

[0019] As a limitation of the present invention, the fluorosilicone modified polyolefin granules, by weight, comprise 40-50 parts high-density polyethylene, 20-25 parts silane modified polyolefin masterbatch, 28-32 parts fluorinated nano-silica, 3-5 parts maleic anhydride grafted polyolefin compatibilizer, and 0.5-1 parts calcium stearate lubricant; the melt extrusion process conditions include a screw speed of 250-350 rpm, a zone 1 temperature of 150-160°C, a zone 2 temperature of 160-170°C, a zone 3 temperature of 170-180°C, and a zone 4 temperature of 165-175°C.

[0020] Fluorinated nano-silica is incorporated into the outer sheath layer. The nanoscale silica particles are uniformly dispersed within the matrix, significantly enhancing the sheath's hardness, compressive strength, and abrasion resistance, enabling it to withstand long-term wear from seabed sand and rocks. More importantly, the fluorinated layer on the surface imparts extremely low surface energy, giving the sheath a durable and stable hydrophobic and oleophobic property. This effectively prevents the attachment and proliferation of marine organisms, avoiding damage to the sheath structure from biological corrosion and reducing additional mechanical loads caused by increased water flow resistance due to deposits. Furthermore, the nano-silica particles reflect and scatter ultraviolet light and block the penetration of water vapor and corrosive ions, synergistically improving the outer sheath's weather resistance, resistance to UV aging, and resistance to seawater corrosion.

[0021] As a limitation of this invention, the preparation method of the fluorinated nano-silica is as follows: Nano-silica was added to an ethanol aqueous solution, stirred evenly, and ultrasonically dispersed for 5-10 min. Hexafluorohexyltriethoxysilane was added, and the pH was adjusted to 4-5. Under nitrogen protection, the reaction was carried out at 70-80℃ and 300-400 rpm for 4-6 h. After the reaction was completed, the mixture was washed with ethanol and deionized water, and vacuum dried at 60-70℃ for 6-8 h to obtain fluorinated nano-silica.

[0022] Preferably, the mass ratio of the nano-silica to hexafluorohexyltriethoxysilane is (8-12):(0.8-1.2).

[0023] A DC composite submarine cable, wherein the cable structure comprises, from the inside out, a conductor, a conductor shielding layer, an insulation layer, an insulation shielding layer, a metal sheath layer, an inner sheath layer, an armor layer, and an outer sheath layer; the insulation layer, inner sheath layer, and outer sheath layer of the DC composite submarine cable are prepared by any of the preparation methods described above.

[0024] As a limitation of the present invention, the material of the conductor shielding layer is a semi-conductive cross-linked polyolefin; the material of the insulating shielding layer is a semi-conductive cross-linked polyolefin; the metal sheath layer includes an aluminum-plastic composite strip and a metal sheath outside the aluminum-plastic composite strip, and the material of the metal sheath is at least one of copper, aluminum, lead and steel; the material of the armor layer is at least one of galvanized steel and stainless steel.

[0025] Compared with the prior art, the beneficial effects of the present invention are: This invention incorporates a biphenyl-acetophenone epoxy oligomer into low-density polyethylene to form an insulating layer. This oligomer contains biphenyl and acetophenone groups, which enhances the intrinsic insulation strength of the composite material while also improving its current resistance and insulation properties. Furthermore, it improves the composite material's thermal stability, compatibility, and long-term stability.

[0026] This invention uses magnesium hydroxide and aluminum hypophosphite modified with titanate coupling agent NDZ-201 as flame retardants for the inner sheath layer. By catalyzing the dehydration of the polymer matrix to form carbon, a dense carbon layer covering the material surface is formed, which has the functions of heat insulation and oxygen isolation. The phosphorus free radicals generated by decomposition capture highly active free radicals in the combustion chain reaction, interrupting the combustion reaction, and endothermic decomposition, generating substances such as magnesium oxide and water vapor to reduce the temperature of the material surface, delay thermal degradation, enhance the flame retardant performance of the composite material and the long-term stability of the submarine cable structure.

[0027] This invention incorporates triethoxysilane-modified polyolefin and fluorinated nano-silica into the outer sheath layer. The silane-modified polyolefin contains flexible silane chains, which can effectively absorb and disperse stress, preventing brittle cracking caused by stress concentration and enhancing the material's integrity and tear resistance. It also enhances the compatibility between high-density polyethylene and fluorinated nano-silica. The fluorinated nano-silica imparts extremely low surface energy to the material, giving the sheath surface durable and stable hydrophobic and oleophobic properties. This effectively prevents the attachment and proliferation of marine organisms, avoiding damage to the sheath structure from biological corrosion and reducing additional mechanical loads caused by increased water flow resistance due to attached organisms. Furthermore, the nano-silica particles can reflect and scatter ultraviolet rays and block the penetration of water vapor and corrosive ions, synergistically improving the outer sheath's weather resistance, UV aging resistance, and seawater corrosion resistance. Detailed Implementation

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Nano-alumina (particle size: 50nm, specific surface area: 100m²) 2 / g), aluminum hypophosphite (particle size: 2μm, moisture: ≤0.5%, phosphorus content: 28wt%), magnesium hydroxide (particle size: 2μm, specific surface area: 10m²). 2 / g, purity: ≥98%), nano-silica (particle size: 40nm, specific surface area 150m²) 2 / g), low-density polyethylene (density: 0.92g / cm³) 3 Melt index (190℃, 2.16kg): 1.0g / 10min), ethylene-acrylic acid copolymer (acrylic acid content: 15wt%), high-density polyethylene (density: 0.96g / cm³). 3 Melt flow index (190℃, 5kg): 0.4g / 10min; Polyolefin (density: 0.86g / cm³) 3Melt index (190℃, 2.16kg): 2.5g / 10min).

[0030] The preparation method of KH-560 modified nano-alumina is as follows: 10g of nano-alumina was added to 100mL of ethanol-water solution (vethanol:vwater=4:1), stirred evenly, and ultrasonically dispersed for 10min. 1g of silane coupling agent KH-560 was added, and the pH was adjusted to 4. The reaction was carried out at 70℃ and 300rpm for 5h under nitrogen protection. After the reaction was completed, the nano-alumina was washed with ethanol and deionized water and dried under vacuum at 60℃ for 8h to obtain KH-560 modified nano-alumina.

[0031] The preparation method of NDZ-201 modified aluminum hypophosphite is as follows: 2g of titanate coupling agent NDZ-201 was added to 80mL of anhydrous ethanol and stirred until homogeneous. Then, 100g of aluminum hypophosphite was added and ultrasonically dispersed for 10min. The mixture was then reacted at 70℃ and 300rpm for 2h. After the reaction was completed, the mixture was cooled, filtered, washed with ethanol, and dried at 80℃ for 4h to obtain NDZ-201 modified aluminum hypophosphite.

[0032] The preparation method of NDZ-201 modified magnesium hydroxide is as follows: Add 3g of titanate coupling agent NDZ-201 to 80mL of anhydrous ethanol, stir well, add 100g of magnesium hydroxide, sonicate for 10min, react at 70℃ and 300rpm for 2h, cool after reaction, filter, wash with ethanol, dry at 80℃ for 4h to obtain NDZ-201 modified magnesium hydroxide.

[0033] The preparation method of fluorinated nano-silica is as follows: Add 10g of nano-silica to 100mL of ethanol aqueous solution (v 乙醇 :v 水 Mix the ingredients in a 4:1 ratio, stir until homogeneous, ultrasonically disperse for 10 min, add 1 g of hexafluorohexyltriethoxysilane, adjust the pH to 4, and react at 70 °C and 300 rpm for 5 h under nitrogen protection. After the reaction is complete, wash with ethanol and deionized water, and vacuum dry at 60 °C for 8 h to obtain fluorinated nano-silica.

[0034] Example 1: A method for preparing a DC composite submarine cable, specifically as follows: Step 1: Mix 5g of 3,5-dihydroxyacetophenone and 12g of epichlorohydrin, add 0.25g of tetrabutylammonium bromide catalyst, and stir and reflux at 90℃ and 300rpm for 5h. Maintain the pH at 10 during the reaction. After the reaction is complete, add deionized water to wash and separate the liquid until the pH of the aqueous phase is neutral. Dry the organic phase with anhydrous sodium sulfate, filter, and evaporate the filtrate by rotary evaporation to obtain acetylresorcinol diglycidyl ether. Step 2: Mix 5g of biphenyl dicarboxylic acid and 0.1g of tetrabutylammonium bromide catalyst, and stir at 120℃ and 300rpm for 0.5h under nitrogen protection. After the reaction is complete, cool the mixture to 110℃ and add 9g of acetylresorcinol diglycidyl ether dropwise. Continue stirring at 130℃ and 300rpm for 3h. After the reaction is complete, add 0.5g of phenyl glycidyl ether end-capping agent and continue stirring for 0.5h. After the reaction is complete, cool the mixture to obtain biphenyl-acetophenone epoxy oligomer. Step 3: Mix 100g of low-density polyethylene, 8g of compatibilizer ethylene-acrylic acid copolymer, 3g of biphenyl-acetophenone epoxy oligomer, 3g of KH-560 modified nano-alumina, 1.2g of dicumyl peroxide, and 0.2g of antioxidant 168 evenly, and add them to a twin-screw extruder for extrusion granulation. Set the screw speed to 200 rpm, the zone 1 temperature to 130℃, the zone 2 temperature to 150℃, the zone 3 temperature to 160℃, and the zone 4 temperature to 155℃ to obtain biphenyl-acetophenone modified cross-linked XLPE insulating granules. Melt the biphenyl-acetophenone modified cross-linked XLPE insulating granules and extrude them to form an insulating layer. Step 4: Mix 100g of high-density polyethylene, 4g of NDZ-201 modified aluminum hypophosphite, 18g of NDZ-201 modified magnesium hydroxide, 3g of compatibilizer maleic anhydride grafted polyolefin, 0.4g of antioxidant 168 and 1g of lubricant calcium stearate evenly, add to a twin-screw extruder for extrusion granulation, set the screw speed to 250rpm, the temperature of zone 1 to 160℃, the temperature of zone 2 to 180℃, the temperature of zone 3 to 190℃, and the temperature of zone 4 to 185℃ to obtain flame-retardant HDPE granules. Melt the flame-retardant HDPE granules and extrude them to form an inner sheath layer. Step 5: Mix 100g of polyolefin, 4g of vinyltriethoxysilane, 0.1g of dicumyl peroxide and 0.3g of antioxidant 168 evenly, add it to a twin-screw extruder for extrusion granulation, set the screw speed to 250rpm, the temperature of zone 1 to 140℃, the temperature of zone 2 to 160℃, the temperature of zone 3 to 170℃, and the temperature of zone 4 to 165℃ to obtain silane-modified polyolefin masterbatch; Step 6: Mix 50g of high-density polyethylene, 20g of silane-modified polyolefin masterbatch, 29.5g of fluorinated nano-silica, and 0.5g of calcium stearate lubricant evenly, and add it to a twin-screw extruder for extrusion granulation. Set the screw speed to 250rpm, the zone 1 temperature to 150℃, the zone 2 temperature to 160℃, the zone 3 temperature to 170℃, and the zone 4 temperature to 165℃ to obtain fluorosilicone-modified polyolefin granules. Melt the fluorosilicone-modified polyolefin granules and extrude them to form the outer sheath layer to obtain the DC composite submarine cable.

[0035] Example 2: A method for preparing a DC composite submarine cable, specifically as follows: Step 1: Mix 6g of 3,5-dihydroxyacetophenone and 13g of epichlorohydrin, add 0.25g of tetrabutylammonium bromide catalyst, and reflux at 90℃ and 300rpm for 5h with stirring. Maintain the pH at 10 during the reaction. After the reaction is complete, add deionized water to wash and separate the liquid until the pH of the aqueous phase is neutral. Dry the organic phase with anhydrous sodium sulfate, filter, and evaporate the filtrate by rotary evaporation to obtain acetylresorcinol diglycidyl ether. Step 2: Mix 6g of biphenyl dicarboxylic acid and 0.1g of tetrabutylammonium bromide catalyst, and stir at 120℃ and 300rpm for 0.5h under nitrogen protection. After the reaction is complete, cool the mixture to 110℃ and add 9g of acetylresorcinol diglycidyl ether dropwise. Continue stirring at 130℃ and 300rpm for 3h. After the reaction is complete, add 0.5g of phenyl glycidyl ether end-capping agent and continue stirring for 0.5h. After the reaction is complete, cool the mixture to obtain biphenyl-acetophenone epoxy oligomer. Step 3: Mix 90g of low-density polyethylene, 8g of compatibilizer ethylene-acrylic acid copolymer, 3g of biphenyl-acetophenone epoxy oligomer, 3g of KH-560 modified nano-alumina, 1.2g of dicumyl peroxide, and 0.2g of antioxidant 168 evenly, and add them to a twin-screw extruder for extrusion granulation. Set the screw speed to 200 rpm, the zone 1 temperature to 130℃, the zone 2 temperature to 150℃, the zone 3 temperature to 160℃, and the zone 4 temperature to 155℃ to obtain biphenyl-acetophenone modified crosslinked XLPE insulating granules. Melt the biphenyl-acetophenone modified crosslinked XLPE insulating granules and extrude them to form an insulating layer. Step 4: Mix 90g of high-density polyethylene, 4g of NDZ-201 modified aluminum hypophosphite, 18g of NDZ-201 modified magnesium hydroxide, 3g of compatibilizer maleic anhydride grafted polyolefin, 0.4g of antioxidant 168 and 1g of lubricant calcium stearate evenly, add to a twin-screw extruder for extrusion granulation, set the screw speed to 250rpm, the temperature of zone 1 to 160℃, the temperature of zone 2 to 180℃, the temperature of zone 3 to 190℃, and the temperature of zone 4 to 185℃ to obtain flame-retardant HDPE granules. Melt the flame-retardant HDPE granules and extrude them to form an inner sheath layer. Step 5: Mix 90g of polyolefin, 4g of vinyltriethoxysilane, 0.1g of dicumyl peroxide and 0.3g of antioxidant 168 evenly, add it to a twin-screw extruder for extrusion granulation, set the screw speed to 250rpm, the temperature of zone 1 to 140℃, the temperature of zone 2 to 160℃, the temperature of zone 3 to 170℃, and the temperature of zone 4 to 165℃ to obtain silane-modified polyolefin masterbatch; Step 6: Mix 50g of high-density polyethylene, 20g of silane-modified polyolefin masterbatch, 29.5g of fluorinated nano-silica, and 0.5g of calcium stearate lubricant evenly, and add it to a twin-screw extruder for extrusion granulation. Set the screw speed to 250rpm, the zone 1 temperature to 150℃, the zone 2 temperature to 160℃, the zone 3 temperature to 170℃, and the zone 4 temperature to 165℃ to obtain fluorosilicone-modified polyolefin granules. Melt the fluorosilicone-modified polyolefin granules and extrude them to form the outer sheath layer to obtain the DC composite submarine cable.

[0036] Example 3: A method for preparing a DC composite submarine cable, specifically as follows: Step 1: Mix 7g of 3,5-dihydroxyacetophenone and 14g of epichlorohydrin, add 0.25g of tetrabutylammonium bromide catalyst, and stir and reflux at 90℃ and 300rpm for 5h. Maintain the pH at 10 during the reaction. After the reaction is complete, add deionized water to wash and separate the liquid until the pH of the aqueous phase is neutral. Dry the organic phase with anhydrous sodium sulfate, filter, and evaporate the filtrate by rotary evaporation to obtain acetylresorcinol diglycidyl ether. Step 2: Mix 7g of biphenyl dicarboxylic acid and 0.1g of tetrabutylammonium bromide catalyst, and stir at 120℃ and 300rpm for 0.5h under nitrogen protection. After the reaction is complete, cool the mixture to 110℃ and add 9g of acetylresorcinol diglycidyl ether dropwise. Continue stirring at 130℃ and 300rpm for 3h. After the reaction is complete, add 0.5g of phenyl glycidyl ether end-capping agent and continue stirring for 0.5h. After the reaction is complete, cool the mixture to obtain biphenyl-acetophenone epoxy oligomer. Step 3: Mix 80g of low-density polyethylene, 8g of compatibilizer ethylene-acrylic acid copolymer, 3g of biphenyl-acetophenone epoxy oligomer, 3g of KH-560 modified nano-alumina, 1.2g of dicumyl peroxide, and 0.2g of antioxidant 168 evenly, and add them to a twin-screw extruder for extrusion granulation. Set the screw speed to 200 rpm, the zone 1 temperature to 130℃, the zone 2 temperature to 150℃, the zone 3 temperature to 160℃, and the zone 4 temperature to 155℃ to obtain biphenyl-acetophenone modified crosslinked XLPE insulating granules. Melt the biphenyl-acetophenone modified crosslinked XLPE insulating granules and extrude them to form an insulating layer. Step 4: Mix 80g of high-density polyethylene, 4g of NDZ-201 modified aluminum hypophosphite, 18g of NDZ-201 modified magnesium hydroxide, 3g of compatibilizer maleic anhydride grafted polyolefin, 0.4g of antioxidant 168 and 1g of lubricant calcium stearate evenly, add to a twin-screw extruder for extrusion granulation, set the screw speed to 250rpm, the temperature of zone 1 to 160℃, the temperature of zone 2 to 180℃, the temperature of zone 3 to 190℃, and the temperature of zone 4 to 185℃ to obtain flame-retardant HDPE granules. Melt the flame-retardant HDPE granules and extrude them to form an inner sheath layer. Step 5: Mix 80g of polyolefin, 4g of vinyltriethoxysilane, 0.1g of dicumyl peroxide and 0.3g of antioxidant 168 evenly, add it to a twin-screw extruder for extrusion granulation, set the screw speed to 250rpm, the temperature of zone 1 to 140℃, the temperature of zone 2 to 160℃, the temperature of zone 3 to 170℃, and the temperature of zone 4 to 165℃ to obtain silane-modified polyolefin masterbatch; Step 6: Mix 50g of high-density polyethylene, 20g of silane-modified polyolefin masterbatch, 29.5g of fluorinated nano-silica, and 0.5g of calcium stearate lubricant evenly, and add it to a twin-screw extruder for extrusion granulation. Set the screw speed to 250rpm, the zone 1 temperature to 150℃, the zone 2 temperature to 160℃, the zone 3 temperature to 170℃, and the zone 4 temperature to 165℃ to obtain fluorosilicone-modified polyolefin granules. Melt the fluorosilicone-modified polyolefin granules and extrude them to form the outer sheath layer to obtain the DC composite submarine cable.

[0037] Based on Example 1, the following comparative experiments were conducted, specifically Comparative Example 1, Comparative Example 2, and Comparative Example 3, as described below: Comparative Example 1: This comparative example relates to a method for preparing a DC composite submarine cable. The difference from Example 1 is that dihydroxyacetophenone is added to low-density polyethylene to form an insulating layer, specifically: 100g of low-density polyethylene, 8g of compatibilizer ethylene-acrylic acid copolymer, 3g of 3,5-dihydroxyacetophenone, 3g of KH-560 modified nano-alumina, 1.2g of dicumyl peroxide, and 0.2g of antioxidant 168 were mixed evenly and added to a twin-screw extruder for granulation. The screw speed was set to 200 rpm, the temperature of zone 1 was 130℃, the temperature of zone 2 was 150℃, the temperature of zone 3 was 160℃, and the temperature of zone 4 was 155℃ to obtain biphenyl-acetophenone modified crosslinked XLPE insulating granules. The biphenyl-acetophenone modified crosslinked XLPE insulating granules were melted and extruded to form an insulating layer. The remaining conditions are the same as in Example 1.

[0038] Comparative Example 2: This comparative example relates to a method for preparing a DC composite submarine cable. The difference from Example 1 is that aluminum hypophosphite and magnesium hydroxide are added to high-density polyethylene to form an inner sheath layer, specifically: 100g of high-density polyethylene, 4g of aluminum hypophosphite, 18g of magnesium hydroxide, 3g of maleic anhydride-grafted polyolefin, 0.4g of antioxidant 168 and 1g of lubricant calcium stearate were mixed evenly and added to a twin-screw extruder for granulation. The screw speed was set to 250 rpm, the temperature of zone 1 was 160℃, the temperature of zone 2 was 180℃, the temperature of zone 3 was 190℃ and the temperature of zone 4 was 185℃ to obtain flame-retardant HDPE granules. The flame-retardant HDPE granules were melted and extruded to form an inner sheath layer. The remaining conditions are the same as in Example 1.

[0039] Comparative Example 3: This comparative example relates to a method for preparing a DC composite submarine cable. The difference from Example 1 is that nano-silica is formed into an outer sheath layer in high-density polyethylene, specifically: 50g of high-density polyethylene, 20g of silane-modified polyolefin masterbatch, 29.5g of nano-silica and 0.5g of lubricant calcium stearate were mixed evenly and added to a twin-screw extruder for granulation. The screw speed was set to 250 rpm, the temperature of zone 1 was 150℃, the temperature of zone 2 was 160℃, the temperature of zone 3 was 170℃ and the temperature of zone 4 was 165℃ to obtain fluorosilicone-modified polyolefin granules. The fluorosilicone-modified polyolefin granules were melted and extruded to form an outer sheath layer. The remaining conditions are the same as in Example 1.

[0040] Testing experiment: The insulation layer, inner sheath layer, and outer sheath layer composite materials of DC composite submarine cables were prepared according to the methods in each embodiment and comparative example, and the following tests were conducted.

[0041] Insulation performance test: The insulation performance of the DC composite submarine cable was tested. The test was conducted in accordance with the "Electrical strength test method for insulating materials - Part 1: Power frequency test" (GB / T 1408.1-2016). The composite material sample of the insulation layer was cured at 25℃ and 50%RH relative humidity for 24 hours. Then, it was immersed in insulating oil together with two stainless steel rod electrodes with a diameter of 6mm. The position of the sample was adjusted so that the sample was located between the two electrodes and the axis was aligned. The voltage source was started and the voltage was increased uniformly at a rate of 500V / s from zero. The equipment automatically recorded the breakdown voltage value of the sample and calculated the dielectric strength of the composite material of the insulation layer.

[0042] Flame retardant performance test: The flame retardant performance of the inner sheath of the DC composite submarine cable was tested. The test was conducted in accordance with the "Determination of Burning Performance of Plastics - Horizontal and Vertical Methods" (GB / T 2408-2021). The inner sheath composite material sample was placed at 25℃ and 50%RH relative humidity for 24 hours, and then vertically clamped on the fixture. A 20mm high-powered torch with a blue flame was used to contact the center of the lower edge of the composite material for 10 seconds. The central axis of the flame was at a 45° angle to the material. The flame retardant rating of the inner sheath material was then tested.

[0043] Hydrolysis resistance test: The hydrolysis resistance of the outer sheath layer was tested. First, the tensile strength of the outer sheath layer composite material was tested. Then, the outer sheath layer composite material was placed in 90℃ deionized water for 7 days for hydrolysis. After 7 days, it was taken out, the surface of the composite material was wiped dry, and it was placed in an environment of 25℃ and 50%RH relative humidity for 24 hours. The tensile strength of the outer sheath layer composite material after hydrolysis aging was measured, and the tensile strength retention rate was calculated.

[0044]

[0045] Conclusion: The test data shows that, compared with Example 1, the dielectric strength of the DC composite submarine cable in Comparative Example 1, which directly adds dihydroxyacetophenone as an antistatic agent to its insulation layer, is significantly lower than that of Example 1; the flame retardant component in the inner sheath of the DC composite submarine cable in Comparative Example 2, which is not modified with titanate coupling agent, is significantly lower than that of Example 1; and the tensile strength and tensile strength retention rate after 7 days of hydrolytic aging at 90°C in the outer sheath of the DC composite submarine cable in Comparative Example 3, which is not fluorinated in its nano-silica layer, are significantly lower than those of Example 1. The DC composite submarine cable provided by this invention has good insulation, flame retardancy, and resistance to hydrolytic aging.

[0046] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for preparing a DC composite submarine cable, characterized in that: Specifically: Step 1: Prepare biphenyl-acetophenone epoxy oligomers using 3,5-dihydroxyacetophenone, epichlorohydrin, and biphenyl dicarboxylic acid; Step 2: Mix low-density polyethylene with biphenyl-acetophenone epoxy oligomer and melt extrude to obtain biphenyl-acetophenone modified cross-linked XLPE insulating granules, and melt them to form an insulating layer; Step 3: Mix high-density polyethylene, NDZ-201 modified aluminum hypophosphite and NDZ-201 modified magnesium hydroxide and melt extrude to obtain flame-retardant HDPE granules, and melt them to form an inner sheath layer. Step 4: Mix the polyolefin and vinyltriethoxysilane and melt extrude to obtain silane-modified polyolefin masterbatch; Step 5: Mix high-density polyethylene, silane-modified polyolefin masterbatch and fluorinated nano-silica and melt extrude to obtain fluorosilicone-modified polyolefin granules. Melt these granules to form an outer sheath layer to obtain a DC composite submarine cable.

2. The method for preparing a DC composite submarine cable according to claim 1, characterized in that: The preparation method of biphenyl-acetophenone epoxy oligomer is as follows: 3,5-Dihydroxyacetophenone and epichlorohydrin were mixed, and tetrabutylammonium bromide was added as a catalyst. The mixture was stirred and refluxed at 80-90℃ and 300-400 rpm for 4-6 hours, with the pH maintained at 10-11 during the reaction. After the reaction was completed, the mixture was washed with deionized water until the pH of the aqueous phase was neutral. The organic phase was dried with anhydrous sodium sulfate, filtered, and the filtrate was evaporated by rotary evaporation to obtain acetylresorcinol diglycidyl ether. Biphenyl dicarboxylic acid and the catalyst tetrabutylammonium bromide were mixed and reacted under nitrogen protection at 110-120℃ and 300-400 rpm for 0.5-1 h. After the reaction was completed, the mixture was cooled to 100-110℃ and acetylresorcinol diglycidyl ether was added dropwise. The mixture was then stirred and reacted at 120-130℃ and 300-400 rpm for 2-4 h. After the reaction was completed, the end-capping agent phenyl glycidyl ether was added and the mixture was stirred and reacted for another 0.5-1 h. After the reaction was completed, the mixture was cooled to obtain biphenyl-acetophenone epoxy oligomer.

3. The method for preparing a DC composite submarine cable according to claim 2, characterized in that: The mass ratio of 3,5-dihydroxyacetophenone, epichlorohydrin, and catalyst tetrabutylammonium bromide is (5-7):(12-14):(0.5-0.8); the mass ratio of biphenyl dicarboxylic acid, acetylresorcinol diglycidyl ether, catalyst tetrabutylammonium bromide, and end-capping agent phenyl glycidyl ether is (5-7):(9-11):(0.1-0.3):(0.5-1.0).

4. The method for preparing a DC composite submarine cable according to claim 1, characterized in that: By weight, the biphenyl-acetophenone modified cross-linked XLPE insulating granules include 80-100 parts low-density polyethylene, 8-10 parts compatibilizer ethylene-acrylic acid copolymer, 3-5 parts biphenyl-acetophenone epoxy oligomer, 3-5 parts KH-560 modified nano alumina, 1-1.5 parts dicumyl peroxide, and 0.2-0.4 parts antioxidant 168. The melt extrusion process conditions include a screw speed of 200-300 rpm, a zone 1 temperature of 130-140℃, a zone 2 temperature of 150-160℃, a zone 3 temperature of 160-170℃, and a zone 4 temperature of 155-165℃.

5. The method for preparing a DC composite submarine cable according to claim 1, characterized in that: By weight, the flame-retardant HDPE granules include 80-100 parts high-density polyethylene, 4-6 parts NDZ-201 modified aluminum hypophosphite, 18-20 parts NDZ-201 modified magnesium hydroxide, 3-5 parts compatibilizer maleic anhydride grafted polyolefin, 0.4-0.5 parts antioxidant 168, and 1-1.5 parts lubricant calcium stearate. The melt extrusion process conditions include a screw speed of 250-350 rpm, a zone 1 temperature of 160-170℃, a zone 2 temperature of 180-190℃, a zone 3 temperature of 190-200℃, and a zone 4 temperature of 185-195℃.

6. The method for preparing a DC composite submarine cable according to claim 1, characterized in that: By weight, the silane-modified polyolefin comprises 80-100 parts polyolefin, 4-6 parts vinyltriethoxysilane, 0.1-0.2 parts dicumyl peroxide, and 0.3-0.5 parts antioxidant 168; the melt extrusion process conditions include a screw speed of 200-300 rpm, a zone 1 temperature of 140-150℃, a zone 2 temperature of 160-170℃, a zone 3 temperature of 170-180℃, and a zone 4 temperature of 165-175℃.

7. The method for preparing a DC composite submarine cable according to claim 1, characterized in that: By weight, the fluorosilicone modified polyolefin granules comprise 40-50 parts high-density polyethylene, 20-25 parts silane modified polyolefin masterbatch, 28-32 parts fluorinated nano-silica, 3-5 parts maleic anhydride grafted polyolefin compatibilizer, and 0.5-1 parts calcium stearate lubricant; the melt extrusion process conditions include a screw speed of 250-350 rpm, a zone 1 temperature of 150-160℃, a zone 2 temperature of 160-170℃, a zone 3 temperature of 170-180℃, and a zone 4 temperature of 165-175℃.

8. The method for preparing a DC composite submarine cable according to claim 7, characterized in that: The preparation method of fluorinated nano-silica is as follows: Nano-silica was added to an ethanol aqueous solution, stirred evenly, and ultrasonically dispersed for 5-10 min. Hexafluorohexyltriethoxysilane was added, and the pH was adjusted to 4-5. Under nitrogen protection, the reaction was carried out at 70-80℃ and 300-400 rpm for 4-6 h. After the reaction was completed, the nano-silica was washed with ethanol and deionized water and dried under vacuum at 60-70℃ for 6-8 h to obtain fluorinated nano-silica.

9. A DC composite submarine cable, characterized in that: The submarine cable structure consists of, from the inside out, a conductor, a conductor shielding layer, an insulation layer, an insulation shielding layer, a metal sheath layer, an inner sheath layer, an armor layer, and an outer sheath layer; the insulation layer and the outer sheath layer of the DC composite submarine cable are prepared using the preparation method described in any one of claims 1-8.

10. A DC composite submarine cable according to claim 9, characterized in that: As a limitation of the present invention, the material of the conductor shielding layer is a semi-conductive cross-linked polyolefin; the material of the insulating shielding layer is a semi-conductive cross-linked polyolefin; the metal sheath layer includes an aluminum-plastic composite strip and a metal sheath outside the aluminum-plastic composite strip, and the material of the metal sheath is at least one of copper, aluminum, lead and steel; the material of the armor layer is at least one of galvanized steel and stainless steel.

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