High-heat-resistant polypropylene insulated high-voltage cable and preparation method thereof

Abstract

The invention discloses a high-heat-resistance polypropylene insulated high-voltage cable and a preparation method thereof. The cable insulating layer is formed by compounding co-grafted modified polypropylene, a composite flexibilizer, a synergist, modified boron nitride and a functional additive. The co-grafted modified polypropylene is subjected to synergistic grafting modification through maleic anhydride and styrene, so that a polar group is introduced to improve the interfacial compatibility, and the heat resistance is remarkably improved through a rigid benzene ring structure. The modified boron nitride adopts a three-step modification process, so that uniform dispersion and strong interface bonding of the filler in the matrix are realized. The composite toughening agent is prepared by ternary compounding of an ethylene-vinyl acetate copolymer, a polyolefin elastomer and metallocene polyethylene, so that rigid-tough balance is realized. Compared with traditional polypropylene, the long-term use temperature of the cable insulation layer is greatly increased, the heat conduction performance is remarkably improved, meanwhile, excellent mechanical toughness and electrical insulation performance are kept, and the cable insulation layer meets the power transmission requirement under the high-load and high-temperature working conditions.

Description

Technical Field

[0001] This invention belongs to the field of cable manufacturing technology, specifically a high heat-resistant polypropylene insulated high voltage cable and its manufacturing method. Background Technology

[0002] As a critical component in power transmission, the performance of the insulation material in high-voltage cables directly affects their service life and operational safety. Polypropylene (PP) is considered an ideal choice for next-generation high-voltage cable insulation materials due to its excellent electrical insulation properties, low dielectric constant, low dielectric loss, and good processing performance. However, traditional polypropylene materials still face many technical bottlenecks in high-voltage cable applications.

[0003] In existing technologies, the long-term service temperature of ordinary polypropylene is only 90-105℃, which is insufficient to meet the heat resistance requirements of high-voltage cables under high-load conditions. Current technologies often employ single maleic anhydride grafting modification or the addition of inorganic fillers to improve heat resistance, but the effects are limited. While single maleic anhydride grafting modification can improve polarity, its effect on improving heat resistance is not significant; and simply adding inorganic fillers leads to a decrease in material toughness and a deterioration in processing performance.

[0004] High-voltage cables generate a significant amount of heat during operation, requiring insulation materials with excellent thermal conductivity for effective heat dissipation. Boron nitride has been extensively studied due to its superior thermal conductivity and electrical insulation properties. However, unmodified boron nitride exhibits poor compatibility with the polypropylene matrix, easily agglomerating within it. This not only fails to fully utilize its thermal conductivity but also creates defects at the interface, leading to a significant decline in the material's mechanical and electrical properties. Existing surface modification methods for boron nitride are limited, often employing simple silane coupling agents, resulting in unsatisfactory modification effects and difficulty in achieving uniform dispersion and strong interfacial bonding of boron nitride within the polypropylene matrix.

[0005] To obtain insulating materials with excellent overall performance, it is often necessary to add a variety of functional components to the polypropylene matrix, including toughening agents, thermally conductive fillers, crosslinking agents, and antioxidants. However, these components have poor compatibility with each other and with the polypropylene matrix, which can easily lead to problems such as phase separation and interface defects, making it impossible to achieve synergistic effects on the performance of each component. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a high heat-resistant polypropylene insulated high-voltage cable and its preparation method, which not only has excellent heat resistance and thermal conductivity, but also good mechanical toughness and electrical insulation properties. At the same time, through innovative co-grafting modification technology and boron nitride multi-level modification technology, a multi-component synergistic insulation system is achieved.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A high-heat resistant polypropylene insulated high-voltage cable includes a conductor core, an insulation layer disposed on the outer wall of the conductor core, and a protective sheath disposed on the outer wall of the insulation layer. The insulation layer comprises the following components by weight: 60-70 parts of co-grafted modified polypropylene, 18-25 parts of composite toughening agent, 9-14 parts of synergist, 12-18 parts of modified boron nitride, and 5-8 parts of functional additives.

[0008] Based on the aforementioned technical methods, by selecting co-grafted modified polypropylene as the matrix material and utilizing the synergistic grafting modification of maleic anhydride and styrene, not only are polar groups introduced to improve interfacial compatibility with fillers, but the rigid benzene ring structure of styrene also significantly enhances heat resistance. Furthermore, the thermally conductive network of modified boron nitride and the toughness adjustment of the composite toughening agent achieve a comprehensive balance of heat resistance, thermal conductivity, and toughness. The combined use of peroxides, crosslinking aids, and compatibilizers in the synergistic agent can form a moderately crosslinked network during processing and use, further improving the long-term heat resistance stability and creep resistance of the material.

[0009] Preferably, the co-grafted modified polypropylene is prepared by reactive extrusion, wherein polypropylene, co-grafted monomer and initiator are mixed in a mass ratio of 18-22:4.5-6.0:0.20-0.30 and reactively extruded in a twin-screw extruder at 185-210℃ and a screw speed of 180-220 r / min, with a residence time of 90-120 s.

[0010] More preferably, the co-grafted monomer is a mixture of maleic anhydride and styrene in a mass ratio of 2.8-3.8:1.0-1.6.

[0011] More preferably, the initiator is a mixture of di-tert-butyl peroxide and benzoyl peroxide in a mass ratio of 2.0-3.0:1.

[0012] Based on the aforementioned techniques, maleic anhydride, as a polar monomer, can introduce carboxyl and anhydride groups into the polypropylene backbone, significantly improving the interfacial bonding between polypropylene and polar fillers. Styrene, through grafting its rigid benzene ring structure, effectively increases the rigidity of the polypropylene molecular chain and its glass transition temperature, thereby enhancing its heat resistance. Compared to single grafting, co-grafting of these two monomers achieves the dual effects of improved polarity and enhanced heat resistance. A combined initiation system using di-tert-butyl peroxide and benzoyl peroxide, utilizing their different decomposition temperatures, enables continuous initiation over a wide temperature range, ensuring the high efficiency and uniformity of the co-grafting reaction.

[0013] Preferably, the modified boron nitride is prepared by a three-step modification process, including: Step 1, mixing hexagonal boron nitride micro powder and anhydrous ethanol and then ultrasonically dispersing them, adding tetrabutyl titanate solution, stirring and hydrolyzing at 60-80℃, adjusting the pH value, transferring to a hydrothermal reactor for hydrothermal crystallization, cooling and centrifuging, washing, drying and then high-temperature annealing under a nitrogen atmosphere to obtain titanium dioxide in-situ supported boron nitride composite powder; Step 2: Disperse the composite powder in toluene, add a bifunctional silane coupling agent, heat under reflux in an inert atmosphere, cool, centrifuge, wash, and dry to obtain silanized composite powder. Step 3: The silanized composite powder, toluene, functional monomer mixture and initiator are mixed and then subjected to in-situ polymerization under an inert atmosphere. After cooling, the mixture is centrifuged and extracted with toluene for 48-72 hours using a Soxhlet extractor to remove ungrafted polymer. After drying, modified boron nitride is obtained.

[0014] More preferably, in step one, hexagonal boron nitride micro powder, anhydrous ethanol, tetrabutyl titanate solution, and ammonia are mixed sequentially at a mass ratio of 10-14:100-120:8-12:3-5. The ultrasonic dispersion is performed by ultrasonication at 400-500W power for 20-30 minutes. The hydrolysis stirring speed is 300-500 r / min for 2-3 hours. The hydrothermal crystallization is performed by reacting at 160-190℃ for 10-15 hours at a reaction pressure of 1.0-2.0 MPa. The pH value is adjusted to 8.5-10.5 using ammonia. The washing process involves first washing with deionized water until neutral, then washing twice with anhydrous ethanol. The drying process involves vacuum drying at 100-110℃ to constant weight. The high-temperature annealing treatment is performed by annealing at 550-650℃ under a nitrogen atmosphere for 3-5 hours at a heating rate of 3-5℃ / min.

[0015] More preferably, the hexagonal boron nitride micropowder has an average particle size of 0.5-2.0 μm and a specific surface area of ​​15-35 m² / g; the tetrabutyl titanate solution has a concentration of 15-25 wt%.

[0016] Using the aforementioned techniques, uniformly distributed titanium dioxide nanoparticles are generated in situ on the surface of hexagonal boron nitride through the hydrolysis and hydrothermal crystallization of tetrabutyl titanate. Titanium dioxide possesses excellent polarity and surface activity, acting as a "bridge" to improve the surface properties of boron nitride and providing active sites for subsequent silanization and polymer grafting. Under hydrothermal crystallization conditions, titanium dioxide grows in the anatase crystal form, exhibiting high surface energy and reactivity. High-temperature annealing enhances the interfacial bonding strength between titanium dioxide and boron nitride and removes adsorbed moisture and impurities from the surface, creating favorable conditions for subsequent modification.

[0017] Preferably, in step two, the composite powder obtained in step one, toluene, and the bifunctional silane coupling agent are mixed sequentially at a mass ratio of 16-20:80-100:6-10. The heating and reflux are carried out under nitrogen protection at a temperature of 85-100°C for 4-6 hours. The washing is carried out by washing twice with toluene and then twice with anhydrous ethanol. The drying is carried out by vacuum drying at a temperature of 90-105°C to constant weight. The bifunctional silane coupling agent is γ-methacryloyloxypropyltrimethoxysilane.

[0018] Based on the aforementioned techniques, the trimethoxysilane end of the bifunctional silane coupling agent can undergo a condensation reaction with the hydroxyl groups on the titanium dioxide surface to form a stable Si-O-Ti chemical bond; while the methacryloyloxy group at the other end provides polymerizable double bonds for subsequent polymer grafting. This bifunctional design achieves the transformation of the inorganic filler surface from hydrophilic to oleophilic, and provides sufficient active sites for the subsequent in-situ polymerization reaction.

[0019] Preferably, in step three, the silanized composite powder, toluene, functional monomer mixture, and initiator are mixed sequentially at a mass ratio of 18-24:100-130:12-18:0.30-0.45 and reacted at 70-85°C for 6-9 hours under nitrogen protection. The functional monomer mixture is composed of glycidyl methacrylate, methyl methacrylate, and acrylic acid mixed at a mass ratio of 3.0-4.0:1.5-2.5:0.8-1.5, and the initiator is azobisisobutyronitrile.

[0020] More preferably, the washing is performed by washing twice with toluene and then twice with anhydrous ethanol; the drying is performed by vacuum drying at a temperature of 90-105°C to constant weight.

[0021] Based on the aforementioned technical methods, precise chemical modification of the boron nitride surface is achieved through the compounding of three functional monomers. Glycidyl methacrylate provides epoxy groups, which can undergo ring-opening reactions with the carboxyl and anhydride groups in the co-grafted polypropylene to form chemical bonds; methyl methacrylate provides hydrophobicity and flexibility, improving compatibility with the polypropylene matrix; acrylic acid introduces a small amount of carboxyl groups, providing additional reaction sites and polarity regulation. The functional polymer layer formed by the copolymerization of these three monomers coats the boron nitride surface, significantly improving dispersibility and forming a strong interfacial bond with the matrix. Soxhlet extraction ensures that only the polymer truly grafted onto the boron nitride surface is retained, avoiding the negative impact of free polymers on product performance.

[0022] Preferably, the synergist is a mixture of dicumyl peroxide, triallyl isocyanurate and maleic anhydride-grafted polypropylene in a mass ratio of 1.0-1.3:2.8-3.5:1.5-2.2.

[0023] Based on the aforementioned technical methods, dicumyl peroxide, acting as a crosslinking initiator, gradually decomposes during processing and use to generate free radicals, initiating crosslinking reactions between polypropylene molecular chains and forming a three-dimensional network structure. Triallyl isocyanurate, as a multifunctional crosslinking aid, has three allyl groups that can participate in the crosslinking reaction, significantly improving crosslinking efficiency and density. Maleic anhydride-grafted polypropylene, acting as a compatibilizer, improves the interfacial compatibility between components and promotes uniform dispersion. The combined use of these three components achieves a comprehensive effect of moderate crosslinking, interfacial reinforcement, and synergistic enhancement, significantly improving the material's long-term heat resistance and creep resistance.

[0024] Preferably, the functional additive is a mixture of heat stabilizer, antioxidant and lubricant in a mass ratio of 2.5-3.0:1.5-2.0:1.2-1.6.

[0025] More preferably, the heat stabilizer is a mixture of β-diketone metal stabilizer and calcium stearate in a mass ratio of 1.0-1.8:1.

[0026] More preferably, the antioxidant is a mixture of hindered phenolic antioxidants and phosphite antioxidants in a mass ratio of 1.5-2.5:1.

[0027] More preferably, the lubricant is a mixture of oxidized polyethylene wax and stearamide in a mass ratio of 2.0-3.0:1.

[0028] Based on the aforementioned technical methods, β-diketone metal stabilizers can capture acidic substances generated during polypropylene degradation, preventing autocatalytic degradation, while calcium stearate provides additional thermal stability and lubrication. Together, they significantly improve the long-term thermal stability of the material. Hindered phenolic antioxidants act as primary antioxidants to capture free radicals, while phosphite antioxidants act as secondary antioxidants to decompose hydrogen peroxides; their combination achieves comprehensive antioxidant protection. Oxidized polyethylene wax provides external lubrication to improve mold release performance, while stearamide provides internal lubrication to reduce melt viscosity; together, they optimize processing performance.

[0029] Preferably, the composite toughening agent is a mixture of ethylene-vinyl acetate copolymer, polyolefin elastomer and metallocene polyethylene in a mass ratio of 3.5-4.5:2.0-3.0:1.0-1.5.

[0030] More preferably, the VA content of the ethylene-vinyl acetate copolymer is 18-28 wt%, and the Mooney viscosity of the polyolefin elastomer is 15-35.

[0031] Based on the aforementioned technical methods, the toughening effect is optimized through the compounding of three elastomers. Ethylene-vinyl acetate copolymer, through its polar vinyl acetate segments, can form good interfacial compatibility with modified polypropylene and modified boron nitride, while providing excellent low-temperature toughness. Polyolefin elastomers have a similar molecular structure to polypropylene, exhibiting good compatibility and effectively dispersing stress concentration, thus improving impact strength. Metallocene polyethylene, with its narrow molecular weight distribution and uniform comonomer distribution, can maintain high melting point and rigidity while preserving toughness. The synergistic effect of these three elastomers achieves a comprehensive balance between toughness, heat resistance, and processability.

[0032] This application also discloses a method for preparing a high heat-resistant polypropylene insulated high-voltage cable, comprising the following steps: S1. Preparation of insulating material: Weigh 60-70 parts by weight of co-grafted modified polypropylene, 18-25 parts by weight of composite toughening agent, 9-14 parts by weight of synergist, 12-18 parts by weight of modified boron nitride and 5-8 parts by weight of functional additives, mix them in an internal mixer at 155-165℃ for 30-40 min at a speed of 40-60 r / min, exhaust the air 3-5 times, and extrude and granulate to obtain the insulating material.

[0033] In a further preferred embodiment, during the mixing process, the co-grafted modified polypropylene and functional additives are first added and premixed for 3-5 minutes, and then the composite toughening agent, modified boron nitride and synergist are added in sequence to ensure that each component is uniformly dispersed.

[0034] S2. Extrusion molding: Preheat the conductor core wire to 70-90℃, place the insulating material in a three-layer co-extrusion extruder, melt and extrude at 205-225℃, and coat the outer wall of the conductor core wire. The extrusion speed is 8-15m / min to form an insulating layer.

[0035] In a further optimized configuration, the temperature of each zone of the three-layer co-extrusion extruder increases sequentially from the feed section to the die head, with the temperature gradient controlled at 10-15℃ / zone, ensuring uniform plasticization of the melt and stable extrusion.

[0036] S3. Crosslinking treatment: The conductor core wire covered with the insulation layer is continuously passed through the crosslinking tube and dynamically crosslinked at a temperature of 190-210℃ for 15-25 minutes, with the degree of crosslinking controlled at 65-85%.

[0037] Further preferably, the cross-linking tube adopts a three-stage heating method, with temperatures of 190-195℃, 200-205℃, and 205-210℃ respectively, to achieve gradual heating and cross-linking, and avoid increased brittleness caused by excessive cross-linking.

[0038] S4. Cooling and Shaping: The cross-linked product is cooled to 40-60℃ in a cooling water bath at a rate of 8-15℃ / min.

[0039] Further preferably, the cooling water tank adopts multi-stage cooling, with the first stage water temperature at 60-70℃, the second stage water temperature at 40-50℃, and the third stage water temperature at 25-35℃, to achieve gradient cooling and avoid internal stress and surface defects caused by rapid cooling.

[0040] S5. Sheath Extrusion: The cooled and shaped product is extruded to form a protective sheath, cooled and shaped again, and then wound up to obtain a high-heat-resistant polypropylene insulated high-voltage cable.

[0041] More preferably, the sheath material is high-density polyethylene or polyvinyl chloride, the extrusion temperature is 160-180℃, and the extrusion speed is synchronized with the extrusion speed of the insulation layer.

[0042] Through the above technical solutions, the high heat-resistant polypropylene insulated high-voltage cable of the present invention has excellent heat resistance, thermal conductivity, mechanical toughness and electrical insulation properties. The manufacturing process is scientific and reasonable, the product performance is stable, and it is suitable for power transmission needs under high load and high temperature conditions.

[0043] The beneficial effects of this invention are as follows: 1. This invention achieves the dual goals of improving the polarity and enhancing the heat resistance of the polypropylene matrix through an innovative maleic anhydride-styrene co-grafting modification technology. Compared to traditional single maleic anhydride grafting modification, this invention employs synergistic grafting of maleic anhydride and styrene. This not only significantly improves the interfacial compatibility with inorganic fillers and polar toughening agents through the polar groups introduced by maleic anhydride, but also effectively increases the rigidity of the polypropylene molecular chain and its glass transition temperature through the grafting of the rigid benzene ring structure of styrene, thereby raising the long-term service temperature of the material. A combined initiation system of di-tert-butyl peroxide and benzoyl peroxide is used, leveraging their different decomposition characteristics to achieve continuous and efficient initiation over a wide temperature range, ensuring the uniformity and efficiency of the co-grafting reaction, and laying a solid foundation for constructing a high-performance insulation system.

[0044] 2. This invention employs a three-step modification strategy—in-situ titanium dioxide loading, silanization modification, and functional polymer grafting—to achieve precise surface engineering of boron nitride fillers. The first step involves in-situ generation of uniformly distributed anatase titanium dioxide nanoparticles on the boron nitride surface via a hydrothermal method, improving surface polarity and providing active sites. The second step utilizes a bifunctional silane coupling agent to achieve surface lipophilic modification and introduce polymerizable double bonds. The third step involves in-situ copolymerization of glycidyl methacrylate, methyl methacrylate, and acrylic acid to construct a functional polymer coating layer on the boron nitride surface, achieving dual enhancement of chemical bonding and physical compatibility with the matrix. Compared to traditional simple silanization treatment, this three-step modification strategy improves the dispersion uniformity and interfacial bonding strength of boron nitride in the polypropylene matrix, achieving a comprehensive improvement in thermal conductivity, mechanical properties, and electrical properties.

[0045] 3. This invention achieves an optimal balance of heat resistance, thermal conductivity, toughness, and processing performance through a meticulously designed multi-component synergistic system. The composite toughening agent is a ternary compound of ethylene-vinyl acetate copolymer, polyolefin elastomer, and metallocene polyethylene, taking into account polar compatibility, stress dispersion, and high-temperature strength retention. The synergist is a compound of peroxide, crosslinking aid, and compatibilizer, achieving moderate crosslinking and interfacial reinforcement. The functional additives are a compound of heat stabilizer, antioxidant, and lubricant, ensuring long-term stability and processing performance. The components achieve effective interfacial compatibility and synergistic effects through co-grafting modified polypropylene polar groups and a modified boron nitride functional polymer layer. The final product's comprehensive performance surpasses existing technologies and has broad industrial application prospects. Detailed Implementation

[0046] The present invention will be further described below with reference to specific embodiments, but the invention is not limited to these embodiments. Those skilled in the art should recognize that the present invention covers all possible alternatives, improvements, and equivalents included within the scope of the claims.

[0047] Description of raw materials used in this invention: Polypropylene: Grade T30S, homopolymer polypropylene, melt index 3.0 g / 10 min (230℃, 2.16 kg), molecular weight approximately 280,000; Hexagonal boron nitride micro powder: average particle size 1.0 μm, specific surface area 25 m² / g, purity ≥99.5%; Ethylene-vinyl acetate copolymer: Grade Elvax260, VA content 28wt%, melt index 2.5g / 10min (190℃, 2.16kg). Polyolefin elastomer: Grade ENGAGE8100, density 0.87 g / cm³, Mooney viscosity 25 (ML1+4, 125℃). Dicumyl peroxide: purity ≥98%, active oxygen content 5.8-6.2%; All other raw materials are analytical grade or industrial grade reagents, purchased through formal channels. Example 1

[0048] I. Preparation of Co-grafted Modified Polypropylene In a high-speed mixer, polypropylene, co-grafted monomers, and initiator were weighed and premixed at a mass ratio of 20:5.25:0.25. The co-grafted monomers consisted of 3.3 parts maleic anhydride and 1.3 parts styrene, and the initiator consisted of 2.5 parts di-tert-butyl peroxide and 1 part benzoyl peroxide. Premixing was carried out at 800 rpm for 5 minutes to ensure uniform distribution. The premixed material was fed into a co-rotating twin-screw extruder (screw diameter 35 mm, L / D ratio 40:1). The temperatures of each zone from the feed section to the die were set as follows: 170℃, 185℃, 195℃, 198℃, 200℃, 198℃, 195℃, 190℃. The screw speed was 200 rpm, and the feed rate was controlled to ensure a residence time of 105 seconds. A vacuum of -0.08 MPa was maintained during extrusion to remove volatiles. After extrusion, the polypropylene is granulated by water cooling and dried in a vacuum oven at 80°C for 8 hours to obtain co-grafted modified polypropylene.

[0049] II. Preparation of Modified Boron Nitride Add 12 parts of hexagonal boron nitride micro powder and 110 parts of anhydrous ethanol to a 1000 mL beaker, and ultrasonically disperse for 25 minutes using an ultrasonic disperser (450W power, 20kHz frequency). Add 10 parts of tetrabutyl titanate solution (20wt%), and hydrolyze the mixture in a constant temperature water bath at 70℃ and 400 rpm for 2.5 hours. During the reaction, slowly add 4 parts of ammonia water to adjust the pH to 9.5. Transfer the mixture to a 500 mL stainless steel hydrothermal reactor lined with polytetrafluoroethylene, with a filling degree controlled at 70%, and hydrothermally crystallize at 175℃ for 12 hours at a reaction pressure of approximately 1.5 MPa. After cooling to room temperature, centrifuge at 4000 rpm for 15 minutes, wash with deionized water until neutral (pH=7), then wash twice with anhydrous ethanol, and vacuum dry at 105℃ to constant weight (approximately 10 hours). The dried powder was placed in a tube furnace and heated to 600°C at a rate of 4°C / min under a nitrogen atmosphere (flow rate 200 mL / min). The temperature was held for annealing for 4 hours and then naturally cooled to room temperature to obtain TiO2-supported BN composite powder.

[0050] Eighteen parts of the above composite powder, 90 parts of toluene, and eight parts of γ-methacryloxypropyltrimethoxysilane were added to a 500 mL three-necked flask equipped with a condenser and a thermometer. Nitrogen gas was introduced for protection, and the mixture was refluxed at 92 °C for 5 hours, with a stirring speed of 300 rpm during the reaction. After cooling to room temperature, the mixture was centrifuged at 3500 rpm for 10 minutes, washed twice with toluene and twice with anhydrous ethanol, and then vacuum dried at 100 °C to constant weight to obtain the silanized composite powder.

[0051] A functional monomer mixture was prepared by uniformly mixing 3.5 parts glycidyl methacrylate, 2.0 parts methyl methacrylate, and 1.2 parts acrylic acid. In a 1000 mL three-necked flask equipped with a condenser, thermometer, and nitrogen protection device, 21 parts silanized composite powder, 115 parts toluene, 15 parts functional monomer mixture, and 0.38 parts azobisisobutyronitrile (AIB) were added. Nitrogen protection was introduced, and the mixture was heated to 75°C and reacted for 7.5 hours. After cooling to room temperature, the mixture was centrifuged at 3500 rpm and washed twice with toluene and twice with anhydrous ethanol. The solid product was placed in a Soxhlet extractor and continuously extracted with toluene for 60 hours to remove ungrafted free polymers. After extraction, the product was dried under vacuum at 95°C to constant weight to obtain 15 parts modified boron nitride.

[0052] III. Preparation of Compound Components Ethylene-vinyl acetate copolymer (VA content 23wt%), polyolefin elastomer (Mooney viscosity 25), and metallocene polyethylene were weighed in a mass ratio of 4.0:2.5:1.2 and mixed at 80°C for 10 minutes in a high-speed mixer to obtain 22 parts of composite toughening agent. Dicumyl peroxide, triallyl isocyanurate, and maleic anhydride-grafted polypropylene (grafting rate 0.8-1.0%) were weighed in a mass ratio of 1.15:3.2:1.85 and mixed evenly to obtain 12 parts of synergist.

[0053] A heat stabilizer was prepared by mixing 1.4 parts of an aluminum-based β-diketone metal stabilizer with 1 part of calcium stearate; an antioxidant was prepared by mixing 2.0 parts of a hindered phenolic antioxidant (antioxidant 1010) with 1 part of a phosphite antioxidant (antioxidant 168); and a lubricant was prepared by mixing 2.5 parts of oxidized polyethylene wax with 1 part of stearamide. The above three additives were mixed in a mass ratio of 2.8:1.8:1.4 to obtain 7 parts of functional additives.

[0054] IV. Insulation Material Preparation and Cable Molding Weigh out 65 parts of co-grafted modified polypropylene, 22 parts of composite toughening agent, 12 parts of synergist, 15 parts of modified boron nitride, and 7 parts of functional additives according to the formula. In a Banbury internal mixer (5L capacity, rotor type), first add the co-grafted modified polypropylene and functional additives, and premix for 4 minutes at 160℃ and 50 rpm. Then, sequentially add the composite toughening agent and mix for 5 minutes, followed by the modified boron nitride and then the synergist for 5 minutes. During the mixing process, venting is performed at minutes 10, 18, 25, and 32, for a total mixing time of 35 minutes. The discharge temperature is 165℃. Extrude and granulate using a twin-screw extruder to obtain insulating material granules.

[0055] The conductor core (copper core, cross-sectional area 120mm²) was preheated to 80℃ in a preheating furnace. The insulation material was fed into a three-layer co-extrusion extruder (screw diameter 90mm). The temperatures of each zone were set as follows: feeding zone 205℃, compression zone 210℃, metering zone 215℃, die head 220℃, and die opening 218℃. The extrusion speed was 12m / min, the die temperature was 215℃, and the insulation layer thickness was 4.5mm. The insulated cable was continuously passed through a three-section cross-linking pipe (total length 60m), with temperatures of 193℃, 203℃, and 208℃ for each section, respectively. The cross-linking time was 20 minutes, and the cross-linking degree was measured to be 75%. The cross-linked cable was then passed through three cooling water tanks: the first section had a water temperature of 65℃, the second section 45℃, and the third section 30℃. The total cooling time was approximately 8 minutes, the cooling rate was approximately 12℃ / min, and the final product temperature was 50℃. A high-density polyethylene sheath (2.0 mm thick) was extruded using a single-screw extruder at an extrusion temperature of 170°C. After cooling and setting, the sheath was wound up to obtain a high-heat-resistant polypropylene insulated high-voltage cable. Example 2

[0056] The mixture consisted of 60 parts of co-grafted modified polypropylene, 18 parts of composite toughening agent, 9 parts of synergist, 12 parts of modified boron nitride, and 5 parts of functional additives.

[0057] Preparation parameters for co-grafted modified polypropylene: polypropylene: co-grafted monomer: initiator = 18: 4.5: 0.20, where maleic anhydride: styrene = 2.8: 1.0, di-tert-butyl peroxide: benzoyl peroxide = 2.0: 1, reaction temperature 185℃, screw speed 180 rpm, residence time 90s.

[0058] The preparation parameters for modified boron nitride were adjusted accordingly: In step one, BN:ethanol:butyl titanate solution:ammonia = 10:100:8:3, hydrothermal temperature 160℃, time 10h, annealing temperature 550℃, time 3h; In step two, composite powder:toluene:silane coupling agent = 16:80:6; In step three, silanized powder:toluene:functional monomer mixture:initiator = 18:100:12:0.30, wherein in the functional monomer mixture, glycidyl methacrylate:methyl methacrylate:acrylic acid = 3.0:1.5:0.8.

[0059] Synergist composition: dicumyl peroxide: triallyl isocyanurate: maleic anhydride-grafted PP = 1.0: 2.8: 1.5.

[0060] The composite toughening agent composition is: EVA:POE:mPE = 3.5:2.0:1.0.

[0061] Functional additive composition: heat stabilizer: antioxidant: lubricant = 2.5:1.5:1.2, including heat stabilizer (β-diketone: calcium stearate = 1.0:1), antioxidant (hindered phenol: phosphite = 1.5:1), and lubricant (oxidized PE wax: stearamide = 2.0:1).

[0062] The mixing time was 30 minutes, the extrusion temperature was 205℃, and the degree of crosslinking was 65%. Since the dosage of each component was at the lower limit, the order of ingredients and the mixing time needed to be precisely controlled to ensure uniform dispersion. Example 3

[0063] The preparation process is basically the same as in Example 1, the main difference being that the raw material ratio is at the upper limit of the claims: The mixture consisted of 70 parts of co-grafted modified polypropylene, 25 parts of composite toughening agent, 14 parts of synergist, 18 parts of modified boron nitride, and 8 parts of functional additives.

[0064] Preparation parameters for co-grafted modified polypropylene: polypropylene: co-grafted monomer: initiator = 22:6.0:0.30, where maleic anhydride: styrene = 3.8:1.6, di-tert-butyl peroxide: benzoyl peroxide = 3.0:1, reaction temperature 210℃, screw speed 220rpm, residence time 120s, grafting rate 4.8%.

[0065] The preparation parameters for modified boron nitride were adjusted accordingly: In step one, BN:ethanol:butyl titanate solution:ammonia = 14:120:12:5, hydrothermal temperature 190℃, time 15h, annealing temperature 650℃, time 5h; In step two, composite powder:toluene:silane coupling agent = 20:100:10; In step three, silanized powder:toluene:functional monomer mixture:initiator = 24:130:18:0.45, wherein in the functional monomer mixture, glycidyl methacrylate:methyl methacrylate:acrylic acid = 4.0:2.5:1.5.

[0066] Synergist composition: dicumyl peroxide: triallyl isocyanurate: maleic anhydride-grafted PP = 1.3:3.5:2.2.

[0067] The composite toughening agent composition is: EVA:POE:mPE = 4.5:3.0:1.5.

[0068] Composition of functional additives: heat stabilizer: antioxidant: lubricant = 3.0: 2.0: 1.6, including heat stabilizer (β-diketone: calcium stearate = 1.8: 1), antioxidant (hindered phenol: phosphite = 2.5: 1), and lubricant (oxidized PE wax: stearamide = 3.0: 1).

[0069] The mixing time was 40 minutes, the extrusion temperature was 225℃, and the degree of crosslinking was 85%. Due to the high component content, the viscosity of the system increased, and the mixing time and mixing temperature needed to be appropriately extended to ensure sufficient plasticization. Example 4

[0070] The preparation process was exactly the same as in Example 1, except that the composition of the co-grafted monomers was adjusted to a mixture of maleic anhydride and styrene at a mass ratio of 3.0:1.1 to verify the effect of the co-grafted monomer ratio on performance. The remaining component ratios and process parameters were the same as in Example 1. Example 5

[0071] The preparation process was exactly the same as in Example 1, except that the composition of the synergist was adjusted to dicumyl peroxide: triallyl isocyanurate: maleic anhydride-grafted PP = 1.1:3.0:1.7, to verify the effect of the synergist ratio on the degree of crosslinking and heat resistance. The remaining component ratios and process parameters were the same as in Example 1. Example 6

[0072] The preparation process was exactly the same as in Example 1, except that the composition of the functional monomer mixture in step three of the modified boron nitride preparation was changed to glycidyl methacrylate: methyl methacrylate: acrylic acid = 3.2:1.8:1.0, to verify the effect of the degree of surface functionalization on thermal conductivity and interfacial bonding. All other process parameters were the same as in Example 1.

[0073] Comparative Example 1 The same formulation and process as in Example 1 were used, but the co-grafted modified polypropylene was replaced with ordinary homopolymer polypropylene (using the same raw materials as in Example 1, but without co-grafting modification), in an amount of 65 parts. The remaining components included 22 parts of a composite toughening agent, 12 parts of a synergist, 15 parts of modified boron nitride, and 7 parts of a functional additive; the total formulation and preparation process were exactly the same. This was to verify the necessity of co-grafting modification for heat resistance and interfacial compatibility.

[0074] Comparative Example 2 The preparation process was basically the same as in Example 1, but the modified boron nitride underwent only simple silanization treatment, skipping the in-situ TiO2 loading in step one and the in-situ polymerization grafting in step three. Specifically, 15 parts of unmodified hexagonal boron nitride micropowder were directly mixed with 1.2 parts of γ-methacryloyloxypropyltrimethoxysilane in toluene and refluxed at 85°C for 4 hours. After washing and drying, it was used. The remaining components included 65 parts of co-grafted modified polypropylene, 22 parts of composite toughening agent, 12 parts of synergist, and 7 parts of functional additives, with the dosage and process being exactly the same as in Example 1. This verified the necessity of the multi-stage modification technology of boron nitride.

[0075] Comparative Example 3 The preparation process was basically the same as in Example 1, but without the addition of a synergist. Only 65 parts of co-grafted modified polypropylene, 22 parts of composite toughening agent, 15 parts of modified boron nitride, and 7 parts of functional additives were used. The amount of co-grafted modified polypropylene was increased to 77 parts to maintain a consistent total amount. The crosslinking treatment steps were kept under the same conditions to verify the effect of the synergist on the crosslinking system and long-term heat resistance.

[0076] Comparative Example 4 The preparation process was basically the same as in Example 1, but the reaction temperature of the co-grafted modified polypropylene was increased to 230°C (outside the claim range of 185-210°C), the screw speed was 220 rpm, and the residence time was 120 s. At this temperature, the polypropylene underwent partial degradation, resulting in a decrease in molecular weight. Simultaneously, the grafted monomers underwent excessive homopolymerization, leading to a grafting rate of 5.2% but low grafting efficiency and numerous byproducts. The remaining components and processes were the same as in Example 1, verifying the adverse effects of exceeding the reaction temperature range.

[0077] Comparative Example 5 The preparation process was basically the same as in Example 1, but without the addition of a composite toughening agent. Only 87 parts of co-grafted modified polypropylene, 12 parts of a synergist, 15 parts of modified boron nitride, and 7 parts of a functional additive were used. The amount of co-grafted modified polypropylene was increased accordingly to maintain a consistent total amount. The amounts of the remaining components, the modification methods, and the process parameters were exactly the same as in Example 1, verifying the effect of the composite toughening agent on the rigidity-toughness balance. Performance testing

[0078] The following performance tests were performed on the cable insulation layers prepared in Examples 1-6 and Comparative Examples 1-4: Test methods: 1. Long-term use temperature test: The test is conducted in accordance with IEC 60216-1:2013. The samples are placed in aging chambers at different temperatures (110℃, 115℃, 120℃, 125℃, 130℃) for 1000 hours. The tensile strength retention rate is then tested. The highest temperature at which the tensile strength retention rate is ≥50% is taken as the long-term use temperature.

[0079] 2. Thermal conductivity test: The test was conducted in accordance with GB / T 32064-2015 "Determination of thermal conductivity and thermal diffusivity of plastics - Part 2: Transient plane heat source method". A circular sample with a diameter of 50 mm and a thickness of 3 mm was prepared. The test temperature was 25℃ and the relative humidity was 50%.

[0080] 3. Tensile strength and elongation at break test: The test was conducted in accordance with GB / T 1040.2-2022 "Determination of tensile properties of plastics - Part 2: Test conditions for molded and extruded plastics". A type 1A dumbbell-shaped specimen was prepared, the tensile rate was 50 mm / min, and the test temperature was 23±2℃.

[0081] 4. Dielectric strength test: The test is conducted in accordance with GB / T 1408.1-2016 "Electrical strength test method for insulating materials - Part 1: Test at power frequency". A circular sample with a diameter of 100 mm and a thickness of 1 mm is prepared. A ball-to-ball electrode (diameter of 25 mm) is used, and the voltage rise rate is 2 kV / s. The test is carried out in transformer oil.

[0082] The test results are shown in Table 1:

[0083] Data analysis: Example 1 showed the best performance, with a tensile strength of 32.5 MPa, elongation at break of 485%, long-term service temperature of 125℃, and thermal conductivity of 0.82 W / m·K, verifying the rationality of the median formulation. Examples 2-3 verified the feasibility of the extreme values ​​of the formulation range, with performance lower than Example 1, consistent with the median optimality principle. Examples 4-6 verified the influence of each technical parameter through single-variable adjustment, with performance slightly lower than Example 1.

[0084] The comparative examples fully demonstrate the necessity of each key technology. Comparative Example 1 (without co-grafting modification) has a heat resistance temperature reduced to 95℃ and a thermal conductivity reduced to 0.50 W / m·K, with a performance decrease of 24-39%; Comparative Example 2 (simplified BN modification) has a thermal conductivity of only 0.52 W / m·K; Comparative Example 3 (without synergist) has insufficient crosslinking degree, and its heat resistance temperature has dropped to 108℃; Comparative Example 5 (without toughening agent) has a sharp drop in elongation at break to 305%, and although the strength is slightly increased, it is too brittle and unsuitable.

[0085] The comparison between the examples and the comparative examples shows that the present invention achieves a comprehensive balance of heat resistance, thermal conductivity and toughness through the synergistic effect of co-grafting modification, boron nitride three-step modification and composite toughening agent, and its performance is superior to the prior art in all aspects.

[0086] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A highly heat-resistant polypropylene insulated high-voltage cable, characterized in that, The high-heat-resistance polypropylene insulated high-voltage cable comprises a conductor core, an insulation layer arranged on the outer wall of the conductor core, and a protective sheath arranged on the outer wall of the insulation layer, and the insulation layer comprises the following components by weight fraction: Co-grafting modified polypropylene 60-70 parts Composite toughening agent 18-25 parts Synergist 9-14 parts Modified boron nitride 12-18 parts Functional auxiliary agent 5-8 parts The preparation method of the co-grafting modified polypropylene comprises the following steps: After the polypropylene, the co-grafting monomer and the initiator are pre-mixed in a high-speed mixer, reaction extrusion is performed through a double-screw extruder, and extrusion granulation is performed to obtain the co-grafting modified polypropylene; the temperature of the reaction extrusion is 185-210 DEG C, the screw rotation speed is 180-220 r / min, and the residence time is 90-120 s; the mass ratio of the polypropylene, the co-grafting monomer and the initiator is 18-22:4.5-6.0:0.20-0.30; the co-grafting monomer is mixed by maleic anhydride and styrene in a mass ratio of 2.8-3.8:1.0-1.6; and the initiator is mixed by di-t-butyl peroxide and benzoyl peroxide in a mass ratio of 2.0-3.0:

1.

2. A high heat resistant polypropylene insulated high voltage cable according to claim 1, characterized in that The preparation method of the modified boron nitride comprises the following steps: Step one: hexagonal boron nitride powder and anhydrous ethanol are mixed and then ultrasonic dispersion is performed, a butyl titanate solution is added, hydrolysis is performed under stirring at 60-80 DEG C, the pH value is adjusted, then the mixture is transferred to a hydrothermal reaction kettle for hydrothermal crystallization, centrifugal separation is performed after cooling, washing and drying are performed, and high-temperature annealing treatment is performed in a nitrogen atmosphere to obtain a titanium dioxide in-situ loaded boron nitride composite powder; Step two: the composite powder is dispersed in toluene, a bifunctional silane coupling agent is added, heating reflux is performed under inert atmosphere protection, centrifugal separation is performed after cooling, washing and drying are performed to obtain a silanized composite powder; Step three: the silanized composite powder, toluene, a functional monomer mixed solution and an initiator are mixed, in-situ polymerization reaction is performed under inert atmosphere protection, centrifugal separation is performed after cooling, a Soxhlet extractor is used to extract the ungrafted polymer with toluene for 48-72 h, and drying is performed to obtain the modified boron nitride; the functional monomer mixed solution is mixed by glycidyl methacrylate, methyl methacrylate and acrylic acid in a mass ratio of 3.0-4.0:1.5-2.5:0.8-1.5; the mass ratio of the silanized composite powder, toluene, the functional monomer mixed solution and the initiator is 18-24:100-130:12-18:0.30-0.45; and the initiator is azobisdimethyl isobutyronitrile.

3. A high heat resistant polypropylene insulated high voltage cable according to claim 1, characterized in that: The synergist is mixed by dicumyl peroxide, triallyl isocyanurate and maleic anhydride grafted polypropylene in a mass ratio of 1.0-1.3:2.8-3.5:1.5-2.2; and the functional auxiliary agent is mixed by a heat stabilizer, an antioxidant and a lubricant in a mass ratio of 2.5-3.0:1.5-2.0:1.2-1.

6.

4. A high heat resistant polypropylene insulated high voltage cable according to claim 3, characterized in that: The heat stabilizer is a mixture of β-diketone metal stabilizer and calcium stearate with a mass ratio of 1.0-1.8:1; the antioxidant is a mixture of hindered phenolic antioxidant and phosphite antioxidant with a mass ratio of 1.5-2.5:1; the lubricant is a mixture of oxidized polyethylene wax and stearic acid amide with a mass ratio of 2.0-3.0:

1.

5. A high heat resistant polypropylene insulated high voltage cable according to claim 1, characterized in that: The composite toughening agent is a mixture of ethylene-vinyl acetate copolymer, polyolefin elastomer and metallocene polyethylene with a mass ratio of 3.5-4.5:2.0-3.0:1.0-1.

5.

6. A high heat resistant polypropylene insulated high voltage cable according to claim 2, characterized in that: In step one, the ultrasonic dispersion is performed at a power of 400-500W for 20-30min; the stirring speed for hydrolysis is 300-500r / min for 2-3h; the hydrothermal crystallization is performed at a temperature of 160-190℃ for 10-15h under a pressure of 1.0-2.0MPa; the pH value is adjusted to 8.5-10.5 using ammonia; the washing is performed by first washing with deionized water until neutral, and then washing twice with anhydrous ethanol; the drying is performed at a temperature of 100-110℃ under vacuum until constant weight; the high-temperature annealing treatment is performed at a temperature of 550-650℃ for 3-5h under a nitrogen atmosphere, with a heating rate of 3-5℃ / min.

7. A high heat resistant polypropylene insulated high voltage cable according to claim 2, characterized in that: In step one, the mass ratio of the hexagonal boron nitride micro-powder, anhydrous ethanol, butyl titanate solution and ammonia is 10-14:100-120:8-12:3-5.

8. A high heat resistant polypropylene insulated high voltage cable according to claim 2, characterized in that: In step two, the heating reflux is performed at a temperature of 85-100℃ for 4-6h under nitrogen protection; the washing is performed twice with toluene, and then twice with anhydrous ethanol; the drying is performed at a temperature of 90-105℃ under vacuum until constant weight; the mass ratio of the composite powder, toluene and bifunctional silane coupling agent is 16-20:80-100:6-10; the bifunctional silane coupling agent is γ-methacryloyloxypropyl trimethoxysilane.

9. A high heat resistant polypropylene insulated high voltage cable according to claim 2, characterized in that: In step three, the in-situ polymerization reaction is performed at a temperature of 70-85℃ for 6-9h under nitrogen protection; the washing is performed twice with toluene, and then twice with anhydrous ethanol; the drying is performed at a temperature of 90-105℃ under vacuum until constant weight.

10. A process for the production of a highly heat resistant polypropylene insulated high voltage cable according to any one of claims 1 to 9, characterized in that The preparation method comprises the following steps: 1) Insulating material preparation: the co-grafting modified polypropylene, composite toughening agent, synergist, modified boron nitride and functional additives are weighed according to the weight parts, mixed and kneaded in a Banbury mixer at 155-165℃ for 30-40min, with a rotation speed of 40-60r / min and exhaust 3-5 times, and then extruded and granulated to obtain the insulating material; 2) Extrusion molding: the conductor core wire is preheated to 70-90℃, the insulating material is placed in a three-layer co-extrusion machine, and is melt-extruded at 205-225℃ to coat the outer wall of the conductor core wire, with an extrusion speed of 8-15m / min to form an insulating layer; 3) Crosslinking treatment: the conductor core wire coated with the insulating layer is continuously passed through a crosslinking pipe for dynamic crosslinking at a temperature of 190-210℃ for 15-25min, with a crosslinking degree controlled at 65-85%. 4) cooling and setting: the crosslinked product is cooled to 40-60℃ through a cooling water tank, and the cooling rate is 8-15℃ / min; 5) sheath extrusion: the cooled and set product is extruded with a protective sheath, cooled and set again, and wound to obtain a high heat-resistant polypropylene insulated high-voltage cable.

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