A cross-linked polyethylene sheath material for power cables and its manufacturing process

Abstract

This invention relates to the field of cable technology, specifically to a cross-linked polyethylene sheath material for power cables and its manufacturing process. The sheath material is composed of pretreated particles, modified nano-silica, modified aluminum hydroxide, perfluorooctyltriethoxysilane, carbon black, hindered amine light stabilizer, benzotriazole UV absorber, and microencapsulated dibutyltin dilaurate. Through a sequential process of "pre-extrusion grafting – steam pre-activation – hydrophobic end-sealing – wet cross-linking," the material's resistance to electrical tracking, flashover, and long-term electrical stability in wet salt environments is significantly improved, while maintaining good low-temperature flexibility. Core innovations include synergistic modification of nano-silica with dual silanes, deeply hydrophobic aluminum hydroxide, and delayed release of microencapsulated catalysts, optimizing the interface structure and network density. This invention features a simple process, a wide operating window, and is suitable for the preparation of high-reliability power cable sheaths.

Description

Technical Field

[0001] This invention relates to the field of cable technology, and in particular to a cross-linked polyethylene sheath material for power cables and its manufacturing process. Background Technology

[0002] Power cable sheaths commonly use polyolefin-based materials, constructed through wet crosslinking with peroxides or silanes to create a three-dimensional network, providing both mechanical and environmental protection. In actual operation, the coexistence of humidity, heat, salt spray, and contamination can form a conductive water film on the sheath surface. This, combined with electric field distortion and surface micro-defects, easily induces tracking and surface discharge, thus reducing insulation safety margin. To improve tracking resistance and weather resistance, the industry often introduces fillers such as aluminum hydroxide and nano-silica, supplemented with coupling agents for surface modification. However, aluminum hydroxide has a naturally hydrophilic surface, easily attracting water; if modification is insufficient, it may still form a water penetration interface and ion channels. Nano-silica has a high specific surface area and surface hydroxyl groups, making it prone to aggregation and lacking compatibility with non-polar matrices, leading to weakened interfacial adhesion and defect accumulation.

[0003] Existing modification strategies mostly employ single-functional silanes: polar coupling is beneficial for condensation and anchoring, but may increase hydrophilicity and media adsorption; hydrophobic alkyl or aromatic end-capping can reduce surface energy, but in the absence of active sites, it is difficult to form a continuous, dense interfacial network, limiting long-term durability and electrical stability. Furthermore, the hydrolysis, condensation, and crosslinking of silane systems are constrained by multiple factors such as water activity, pH, temperature, and diffusion pathways, leading to uneven condensation under production cycle conditions, resulting in weak interfaces and localized stress concentrations. Regarding crosslinking catalysis, traditional catalysts release their activity too early, easily inducing self-condensation or localized crosslinking in the initial stage of mixing, causing uneven gel distribution and melt rheological fluctuations, resulting in apparent instability during extrusion. Subsequent wet crosslinking cannot completely repair the resulting structural defects.

[0004] To suppress electrical tracking, shielding and energy dissipation are often enhanced by increasing filler content and crosslinking density. However, this often sacrifices the flexibility and low-temperature impact resistance of the sheath. Micro-cracks during bending or installation can become the starting point for electric field concentration and water film penetration, thus weakening insulation stability. On the other hand, if measures to enhance hydrophobicity only reduce surface energy without considering interface anchoring and network continuity, the hydrophobic layer is prone to instability under thermal-humidity-electrical coupling stress, leading to performance degradation over time. At the process level, without effective control of surface activation before end-sealing, the depth and uniformity of silane end-sealing are insufficient, making it difficult to establish a durable low-energy interface. The narrow processing window also challenges batch-to-batch consistency and large-scale replication.

[0005] In summary, while existing technologies improve resistance to tracking and flashover in wet saline environments, they often struggle to simultaneously ensure low-temperature flexibility and processing stability of the sheath. The lack of synergistic optimization between materials, interfaces, and processes is a key bottleneck restricting the long-term reliable operation and lightweight design of cables. Therefore, a solution that achieves synergistic control over interface chemistry, network structure, and processing history is needed to simultaneously realize stable hydrophobic and anti-fouling capabilities with excellent flexibility, fundamentally alleviating the performance contradictions. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a cross-linked polyethylene sheath material for power cables and its manufacturing process, so as to improve the long-term resistance to electrical tracking in wet and salty environments while maintaining the low-temperature flexibility of the sheath.

[0007] To achieve the above objectives, the present invention provides a cross-linked polyethylene sheath material for power cables, which is prepared by weight from the following raw materials: 10,000 parts of pretreated particles, 225-325 parts of modified nano-silica, 3,000-4,000 parts of modified aluminum hydroxide, 20-40 parts of perfluorooctyltriethoxysilane, 30-50 parts of carbon black, 20-40 parts of hindered amine light stabilizer, 15-25 parts of benzotriazole ultraviolet absorber, and 15-21 parts of microencapsulated dibutyltin dilaurate.

[0008] The pretreated particles are obtained by mixing and pelletizing 6500-7500 parts of low-density polyethylene, 2500-3500 parts of ethylene-octene copolymer, 120-180 parts of vinyltriethoxysilane, and 4.5-7.5 parts of dicumyl peroxide; the modified nano-silica is nano-silica synergistically modified with 3-aminopropyltriethoxysilane and phenyltriethoxysilane; the aluminum hydroxide is aluminum hydroxide surface modified with perfluorooctyltriethoxysilane; and the microencapsulated dibutyltin dilaurate is obtained by in-situ polymerization of urea, paraformaldehyde, and ammonium chloride in an acidic aqueous phase to coat dibutyltin dilaurate.

[0009] Preferably, the average particle size of the nano-silica is 20-100 nm.

[0010] Preferably, the aluminum hydroxide has an average particle size of 1-5 μm.

[0011] Preferably, the average particle size of the carbon black is 50-200 nm.

[0012] Preferably, the weight ratio of 3-aminopropyltriethoxysilane, phenyltriethoxysilane and nano-silica in the raw materials for preparing the modified nano-silica is 24-45:35-55:150.

[0013] Preferably, the weight ratio of perfluorooctyltriethoxysilane to aluminum hydroxide in the raw materials for preparing the modified aluminum hydroxide is 70-140:3500.

[0014] Preferably, the weight ratio of urea, paraformaldehyde, ammonium chloride and dibutyltin dilaurate in the raw materials for preparing the microencapsulated dibutyltin dilaurate is 8:4:2:9-12.

[0015] Furthermore, the present invention provides a manufacturing process for cross-linked polyethylene sheath material for power cables, comprising the following steps:

[0016] 1) Pre-extrusion grafting: Low-density polyethylene, ethylene-octene copolymer, vinyltriethoxysilane and dicumyl peroxide are melt-mixed and pelletized to obtain pre-treated granules;

[0017] 2) Internal mixing and sequential condensation: Pretreated particles and modified nano-silica are added to the internal mixer for a first mixing. After pre-activation by passing saturated steam at 140-150℃, perfluorooctyltriethoxysilane is added dropwise. Then, modified aluminum hydroxide, carbon black, hindered amine light stabilizer and benzotriazole UV absorber are added for a second mixing. Subsequently, dibutyltin dilaurate microcapsule catalyst is added at 133-137℃ for a third mixing. The mixture is then extruded and pelletized to obtain sheath material masterbatch.

[0018] 3) Sheath extrusion and wet crosslinking: The sheath masterbatch is extruded onto the conductor to form a sheath layer, followed by continuous steam crosslinking and subsequent curing to obtain crosslinked polyethylene sheath material for power cables.

[0019] Preferably, in step 1), the cylinder temperature for melting and mixing is 165-175℃, the screw speed is 280-320rpm, and the melting and mixing time is 2-4min.

[0020] Preferably, in step 2), the first mixing temperature is 150-160℃ and the steam pre-activation time is 6-10 min; the second mixing temperature is 148-152℃ and the time is 5-6 min; the third mixing temperature is 133-137℃ and the time is 2-4 min.

[0021] Preferably, in step 3), the screw temperature range for extrusion is 148-178℃; the temperature for continuous steam crosslinking is 65-75℃, the relative humidity is 94-98%, and the residence time is 8-12h; the temperature for post-curing is 20-30℃, the relative humidity is 45-55%, and the time is 7-14d.

[0022] The beneficial effects of this invention are:

[0023] This scheme uses a sequential coupling of "pre-extrusion grafting - steam pre-activation - hydrophobic end sealing - wet crosslinking" to advance silane hydrolysis and condensation in a controlled manner, significantly enhancing the availability of interfacial hydroxyl groups and the depth of end sealing, thereby constructing a dense, continuous and hierarchical silicon-oxygen network.

[0024] Nano-silica is modified with bissilane containing amino and phenyl groups. One end promotes condensation and interfacial anchoring with polar sites, while the other end constructs a stable barrier with a hydrophobic aromatic layer. This achieves bidirectional compatibility between polar condensates and non-polar polyolefin matrices, significantly reduces interfacial porosity and microphase separation, and inhibits electric field concentration and interfacial water permeation.

[0025] Deep hydrophobic end-sealing of aluminum hydroxide transforms the hydrophilic surface into a low surface energy interface. Combined with the network framework of the nanophase, this disrupts the continuity of the water film and the pathways for ion channels, thereby weakening the tendency of electrical trace initiation and propagation in a wet saline environment from the source.

[0026] Microencapsulated organotin catalysts enable delayed release and gradual supply, significantly inhibiting self-condensation and premature cross-linking in the initial stage of mixing, reducing the accumulation of micro-defects caused by uneven gel distribution; at the same time, they broaden the processing window, making the extrusion process more stable and the sheath layer more dense and smooth.

[0027] By precisely controlling the interface and homogenizing the network structure, the sheath material can maintain good dielectric stability while remaining flexible and elastic. The electrical properties in both dry and wet states are more balanced, and the sensitivity of thickness fluctuations to insulation reliability is reduced, which is conducive to thin-walled and lightweight designs. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0029] Example 1:

[0030] (1) 7500g of low-density polyethylene (purchased from Sinopec Yanshan Petrochemical Company, grade 1C7A), 2500g of ethylene-octene copolymer elastomer (purchased from Dow Chemical Company, grade ENGAGE8180), 120g of vinyltriethoxysilane and 4.5g of dicumyl peroxide were put into the hopper of a co-rotating twin-screw extruder with the temperature set at 165℃. The mixture was melt-mixed for 3 minutes at a screw speed of 280rpm. After extrusion, the mixture was water-cooled and pelletized to obtain pretreated pellets.

[0031] (2) 3500g of aluminum hydroxide micro powder (average particle size 2.4μm) was added to a mixture of 5000g of anhydrous ethanol and 300g of deionized water, sonicated for 25min, and glacial acetic acid was added to adjust the pH of the solution to 4.0. Then 70g of perfluorooctyltriethoxysilane was added, and the mixture was refluxed at 76℃ for 75min. After the reaction was completed, the mixture was filtered under reduced pressure, and the free silane was removed by rinsing with anhydrous ethanol three times. Finally, the mixture was dried in a vacuum oven at 80℃ for 8h to obtain surface-modified aluminum hydroxide.

[0032] (3) Add 9g of dibutyltin dilaurate and 12g of liquid paraffin to 180g of deionized water and stir at 1800rpm for 25min to obtain an emulsion. Then dissolve 8g of urea, 4g of paraformaldehyde and 2g of ammonium chloride in 50g of deionized water to obtain an aqueous solution. Slowly add the aqueous solution to the above emulsion under stirring. Adjust the pH value to 2.6 with dilute hydrochloric acid. React in a 52℃ water bath for 60min. After filtration and washing, dry under reduced pressure at 45℃ for 12h to obtain the dibutyltin dilaurate microcapsule catalyst.

[0033] (4) 150g of nano silica (average particle size 50nm) was added to 2000g of anhydrous isopropanol and ultrasonically dispersed for 25min. Then, 25g of 3-aminopropyltriethoxysilane, 35g of phenyltriethoxysilane and 40g of deionized water were added in sequence. The mixture was stirred at 80℃ for 100min. After the reaction was completed, the solvent was removed by vacuum distillation and dried under vacuum at 80℃ for 6h to obtain surface-modified nano silica.

[0034] (5) 10,000 g of pretreated particles were put into a mixer with the internal temperature set at 150°C. After starting the rotor, 225 g of surface-modified nano silica was added and mixed for 4 min. Then the temperature of the mixer was reduced to 140°C and saturated steam was introduced for 6 min. After that, the temperature of the mixer was kept at 140°C and 20 g of perfluorooctyltriethoxysilane was added dropwise and mixed for 3 min. Then 3,000 g of surface-modified aluminum hydroxide, 30 g of carbon black (average particle size 120 nm), 20 g of hindered amine light stabilizer and 15 g of benzotriazole UV absorber were added and mixed at 148°C for 5 min. Then 15 g of dibutyltin dilaurate microcapsule catalyst was added and mixed at 133°C for 3 min. The mixture was extruded into strips and water-cooled and pelletized to obtain sheath material masterbatch.

[0035] (6) The sheathing masterbatch is extruded onto the outside of the copper conductor on a sheathing extrusion production line with a screw temperature of 148°C to 170°C to form a sheathing layer. The extruded cable is immediately passed through a continuous steam crosslinking chamber with a temperature of 65°C and a relative humidity of 94% for 8 hours to complete the crosslinking and curing. Then it is placed in an environment with a temperature of 25°C and a relative humidity of 50% for 7 days for post-curing to obtain crosslinked polyethylene sheathing material for power cables.

[0036] Example 2:

[0037] (1) 7000g of low-density polyethylene (purchased from Sinopec Yanshan Petrochemical Company, grade 1C7A), 3000g of ethylene-octene copolymer elastomer (purchased from Dow Chemical Company, grade ENGAGE8180), 150g of vinyltriethoxysilane and 6g of dicumyl peroxide were put into the hopper of a co-rotating twin-screw extruder with the temperature set at 170℃. The mixture was melt-mixed for 3min at a screw speed of 300rpm. After extrusion, the mixture was water-cooled and pelletized to obtain pretreated pellets.

[0038] (2) 3500g of aluminum hydroxide micro powder (average particle size 2.4μm) was added to a mixture of 5000g of anhydrous ethanol and 300g of deionized water, sonicated for 30min, and glacial acetic acid was added to adjust the pH of the solution to 4.2. Then 105g of perfluorooctyltriethoxysilane was added, and the mixture was refluxed at 78℃ for 90min. After the reaction was completed, the mixture was filtered under reduced pressure, and the free silane was removed by rinsing with anhydrous ethanol three times. Finally, the mixture was dried in a vacuum oven at 80℃ for 8h to obtain surface-modified aluminum hydroxide.

[0039] (3) Add 10g of dibutyltin dilaurate and 12g of liquid paraffin to 180g of deionized water and stir at 2000rpm for 30min to obtain an emulsion. Then dissolve 8g of urea, 4g of paraformaldehyde and 2g of ammonium chloride in 50g of deionized water to obtain an aqueous solution. Slowly add the aqueous solution to the above emulsion under stirring. Adjust the pH value to 2.8 with dilute hydrochloric acid and react in a 55℃ water bath for 75min. After filtration and washing, dry under reduced pressure at 45℃ for 12h to obtain the dibutyltin dilaurate microcapsule catalyst.

[0040] (4) 150g of nano silica (average particle size 50nm) was added to 2000g of anhydrous isopropanol and ultrasonically dispersed for 30min. Then, 35g of 3-aminopropyltriethoxysilane, 45g of phenyltriethoxysilane and 50g of deionized water were added in sequence. The mixture was stirred at 82℃ for 120min. After the reaction was completed, the solvent was removed by vacuum distillation and dried under vacuum at 80℃ for 6h to obtain surface-modified nano silica.

[0041] (5) 10,000 g of pretreated particles were put into a mixer with the internal temperature set at 155°C. After starting the rotor, 275 g of surface-modified nano silica was added and mixed for 4 min. Then the temperature of the mixer was lowered to 145°C and saturated steam was introduced for 8 min. After that, the temperature of the mixer was kept at 145°C and 30 g of perfluorooctyltriethoxysilane was added dropwise and mixed for 3 min. Then 3,500 g of surface-modified aluminum hydroxide, 40 g of carbon black (average particle size 120 nm), 30 g of hindered amine light stabilizer and 20 g of benzotriazole UV absorber were added and mixed at 150°C for 5 min. Then 18 g of dibutyltin dilaurate microcapsule catalyst was added and mixed at 135°C for 3 min. The mixture was extruded into strips and water-cooled and pelletized to obtain sheath material masterbatch.

[0042] (6) The sheathing masterbatch is extruded onto the outside of the copper conductor on a sheathing extrusion production line with a screw temperature of 150°C to 175°C to form a sheathing layer. The extruded cable is immediately passed through a continuous steam crosslinking chamber with a temperature of 70°C and a relative humidity of 96% for 10 hours to complete the crosslinking and curing. Then it is placed in an environment with a temperature of 25°C and a relative humidity of 50% for 10 days for post-curing to obtain crosslinked polyethylene sheathing material for power cables.

[0043] Example 3:

[0044] (1) 6500g of low-density polyethylene (purchased from Sinopec Yanshan Petrochemical Company, grade 1C7A), 3500g of ethylene-octene copolymer elastomer (purchased from Dow Chemical Company, grade ENGAGE8180), 180g of vinyltriethoxysilane and 7.5g of dicumyl peroxide were put into the hopper of a co-rotating twin-screw extruder with the temperature set at 175℃. The mixture was melt-mixed for 3min at a screw speed of 320rpm. After extrusion, the mixture was water-cooled and pelletized to obtain pretreated pellets.

[0045] (2) 3500g of aluminum hydroxide micro powder (average particle size 2.4μm) was added to a mixture of 5000g of anhydrous ethanol and 300g of deionized water, sonicated for 35min, and glacial acetic acid was added to adjust the pH of the solution to 4.4. Then 140g of perfluorooctyltriethoxysilane was added, and the mixture was refluxed at 80℃ for 120min. After the reaction was completed, the mixture was filtered under reduced pressure, and the free silane was removed by rinsing with anhydrous ethanol three times. Finally, the mixture was dried in a vacuum oven at 80℃ for 8h to obtain surface-modified aluminum hydroxide.

[0046] (3) Add 12g of dibutyltin dilaurate and 12g of liquid paraffin to 180g of deionized water and stir at 2400rpm for 40min to obtain an emulsion. Then dissolve 8g of urea, 4g of paraformaldehyde and 2g of ammonium chloride in 50g of deionized water to obtain an aqueous solution. Slowly add the aqueous solution to the above emulsion under stirring. Adjust the pH value to 3.0 with dilute hydrochloric acid and react in a 58℃ water bath for 90min. After filtration and washing, dry under reduced pressure at 45℃ for 12h to obtain the dibutyltin dilaurate microcapsule catalyst.

[0047] (4) 150g of nano silica (average particle size 50nm) was added to 2000g of anhydrous isopropanol and ultrasonically dispersed for 35min. Then, 45g of 3-aminopropyltriethoxysilane, 55g of phenyltriethoxysilane and 60g of deionized water were added in sequence. The mixture was stirred at 85℃ for 150min. After the reaction was completed, the solvent was removed by vacuum distillation and dried under vacuum at 80℃ for 6h to obtain surface-modified nano silica.

[0048] (5) 10,000 g of pretreated particles were put into a mixer with the internal temperature set at 160°C. After starting the rotor, 325 g of surface-modified nano silica was added and mixed for 4 min. Then the temperature of the mixer was lowered to 150°C and saturated steam was introduced for 10 min. After that, the temperature of the mixer was kept at 150°C and 40 g of perfluorooctyltriethoxysilane was added dropwise and mixed for 3 min. 4,000 g of surface-modified aluminum hydroxide, 50 g of carbon black (average particle size 120 nm), 40 g of hindered amine light stabilizer and 25 g of benzotriazole UV absorber were added and mixed at 152°C for 6 min. Then 21 g of dibutyltin dilaurate microcapsule catalyst was added and mixed at 137°C for 3 min. The mixture was extruded into strips and water-cooled and pelletized to obtain sheath material masterbatch.

[0049] (6) The sheathing masterbatch is extruded onto the outside of the copper conductor on a sheathing extrusion production line with a screw temperature of 152°C to 178°C to form a sheathing layer. The extruded cable is immediately passed through a continuous steam crosslinking chamber with a temperature of 75°C and a relative humidity of 98% for 12 hours to complete the crosslinking and curing. Then it is placed in an environment with a temperature of 25°C and a relative humidity of 50% for 14 days for post-curing to obtain crosslinked polyethylene sheathing material for power cables.

[0050] Comparative Example 1:

[0051] The difference between Comparative Example 1 and Example 2 is that the nano-silica was modified only with 3-aminopropyltriethoxysilane, without the addition of phenyltriethoxysilane, while the other conditions were the same as in Example 2.

[0052] Comparative Example 2:

[0053] The difference between Comparative Example 2 and Example 2 is that: only phenyltriethoxysilane was used to modify the nano silica, and 3-aminopropyltriethoxysilane was not added, while the other conditions were the same as in Example 2;

[0054] Comparative Example 3:

[0055] The difference between Comparative Example 3 and Example 2 is that the saturated steam pre-activation step in the mixing stage is omitted, and perfluorooctyltriethoxysilane is added dropwise directly at 145°C under steam-free conditions, while other conditions are the same as in Example 2.

[0056] Comparative Example 4:

[0057] The difference between Comparative Example 4 and Example 2 is that the aluminum hydroxide micro powder is not modified, while other conditions are the same as in Example 2;

[0058] Comparative Example 5:

[0059] The difference between Comparative Example 5 and Example 2 is that unmicroencapsulated dibutyltin dilaurate (with the same amount as in Example 2) was used as the catalyst, while other conditions were the same as in Example 2.

[0060] Performance testing:

[0061] Tracking index (CTI) (GB / T 4207-2022): Standard samples were prepared for each sample and equilibrated at 23±2℃ and 50±5% relative humidity for 24 h. The step voltage method was used for testing. The standard test solution containing NH4Cl was added dropwise at 5 V per level and 3 times per level. The highest voltage at which no tracking occurred was taken as the CTI. The results are shown in Table 1.

[0062] Salt spray resistance (GB / T 10125-2021): The sample was placed in a test chamber and sprayed continuously at 35℃ for 1000 h with 5% NaCl mass fraction. The contact angle reduction rate and tracking index (CTI) reduction rate were measured. The results are shown in Table 1.

[0063] Electrical properties: The samples were tested under two conditions: dry (23℃×24 h) and water immersion (23℃×168 h). The volume resistivity and breakdown field strength were recorded, and the retention rate was calculated. The results are shown in Table 1.

[0064] Table 1 Performance Test Results

[0065]

[0066] Data Analysis:

[0067] In Examples 1-3, when the tracking index was in the range of 575-600 V, the contact angle decrease rate and the tracking index decrease rate remained at a low level of about 7.5%-6.3% and 1.5%-0.8% respectively after the salt spray test. The dry volume resistivity and breakdown field strength retention rate remained basically stable at around 99%, and the volume resistivity and breakdown field strength retention rate after water immersion also remained above 95%. The above data generally indicate that the sequential combination of vinyltriethoxysilane grafting and wet crosslinking in the pre-extrusion stage, supplemented by nano-silica synergistically modified with 3-aminopropyltriethoxysilane and phenyltriethoxysilane, and aluminum hydroxide deeply hydrophobically modified with perfluorooctyltriethoxysilane, may have constructed a hierarchical silicon-oxygen network and a low surface energy interface inside and outside the matrix, suppressing the continuity of the water film and the concentration of local electric fields. The delayed release of dibutyltin dilaurate microcapsules helps to reduce early self-condensation and avoid micro-defects caused by uneven gelation. At the same time, the synergistic shielding and quenching effects of carbon black, hindered amine light stabilizers, and benzotriazole UV absorbers may suppress the degradation of mechanical and electrical properties after hygrothermal and UV coupled aging, thus exhibiting a high and balanced electrical retention rate in both dry and wet states.

[0068] Example 2 showed better performance than Comparative Example 1, which only used 3-aminopropyltriethoxysilane modification, in terms of tracking index, contact angle and tracking index reduction after salt spray, volume resistivity and breakdown field strength retention under water immersion conditions. This indicates that simultaneously introducing amino sites and a phenyl hydrophobic layer onto the surface of nano-silica may, on the one hand, impart bidirectional compatibility between the filler and the polar condensate and non-polar polyolefin matrix, reducing interfacial porosity and microphase separation; on the other hand, amino sites can promote silane condensation, while the phenyl layer stabilizes the hydrophobic barrier, thereby effectively suppressing the continuity of water film and ion channels under salt spray conditions and reducing the probability of tracking path formation. The improved electrical performance retention after water immersion may be related to dense interfacial bonding, local electric field passivation, and controlled defect size, indicating that synergistic modification with dual silanes is more conducive to forming a uniform and moisture-resistant interfacial network.

[0069] Compared to Comparative Example 2, which only uses phenyltriethoxysilane modification, Example 2 shows superior performance in terms of tracking index, the rate of decrease in both salt spray tests, and water immersion retention. While hydrophobic phenyl end-capping alone can reduce surface energy, the lack of amino active sites may result in insufficient silane hydrolysis and condensation near the interface, leading to low continuity and cross-linking of the functional layer on the filler surface, making it difficult to form a stable energy dissipation and charge passivation network. The decrease in resistivity and breakdown field strength retention after water immersion in Comparative Example 2 indicates that the interfacial anchoring force is insufficient to block micro-permeability and electric field concentration during wet medium penetration; while the amino / phenyl synergistic layer in Example 2 accelerates and improves interfacial condensation, combining hydrophobic barrier and structural anchoring, thereby maintaining lower performance degradation under high humidity and electrical stress coupling conditions.

[0070] Comparative Example 3, which omitted the saturated steam pre-activation during the mixing stage, showed weaker tracking index, salt spray degradation rate, and water immersion electrical retention rate compared to Example 2. This indicates that steam pre-activation before adding perfluorooctyltriethoxysilane may increase the degree of surface hydrolysis and hydroxyl abundance, thereby promoting the depth and uniformity of subsequent end-sealing and condensation. Without this step, some incompletely hydrolyzed or condensed intermediates may remain on the interface, resulting in incomplete hydrophobic layer coverage and insufficient crosslinking network layers.

[0071] Comparative Example 4, which did not undergo perfluorooctyltriethoxysilane modification of aluminum hydroxide, exhibited a significantly lower tracking index. The contact angle and tracking index decreased considerably after salt spraying, and the volume resistivity and breakdown field strength retention rate also decreased significantly after water immersion. This indicates that the unmodified aluminum hydroxide surface is hydrophilic, easily forming a continuous water film and introducing ion conduction pathways. Simultaneously, its interfacial compatibility with the polyolefin matrix is ​​insufficient, making it more prone to debonding and voids under electrothermal stress, thereby amplifying the local electric field and reducing tracking resistance. Example 2, through deep hydrophobic end-sealing, may establish a stable low surface energy layer on the powder surface, both blocking the continuity of the water film and improving interfacial bonding with the matrix, thus simultaneously benefiting both wet electrical properties and salt spray resistance.

[0072] Comparative Example 5 directly used unmicroencapsulated dibutyltin dilaurate. Table 1 shows that its tracking index and wet-state electrical performance retention were lower than those of Example 2, and it performed poorly in the two indicators that decreased after salt spray. This indicates that the high activity of the catalyst in the early stage of mixing may induce local premature crosslinking and self-condensation, resulting in uneven gel distribution and accumulation of micro-defects. Some indicators are still high in the dry state, indicating that the overall degree of crosslinking may not be insufficient, but the spatial uniformity and interfacial integrity are damaged, making it easier to form tracking channels when water penetrates and electrical stress couples. The delayed release and gradual supply of the microcapsule shell may widen the processing window, suppress early side reactions, and make network formation and interfacial condensation more synchronous and uniform, thus maintaining higher electrical stability under humid heat and salt spray conditions.

[0073] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A cross-linked polyethylene sheath material for power cables, characterized in that, The product is prepared by weight from the following raw materials: 10,000 parts of pretreated particles, 225-325 parts of modified nano-silica, 3,000-4,000 parts of modified aluminum hydroxide, 20-40 parts of perfluorooctyltriethoxysilane, 30-50 parts of carbon black, 20-40 parts of hindered amine light stabilizer, 15-25 parts of benzotriazole ultraviolet absorber, and 15-21 parts of microencapsulated dibutyltin dilaurate. The pretreated granules are obtained by melt mixing 6500-7500 parts of low-density polyethylene, 2500-3500 parts of ethylene-octene copolymer, 120-180 parts of vinyltriethoxysilane, and 4.5-7.5 parts of dicumyl peroxide, followed by water cooling and pelletizing. The modified nano-silica is nano-silica synergistically modified with 3-aminopropyltriethoxysilane and phenyltriethoxysilane; the modified aluminum hydroxide is aluminum hydroxide surface modified with perfluorooctyltriethoxysilane. The microencapsulated dibutyltin dilaurate was obtained by in-situ polymerization of urea, paraformaldehyde, and ammonium chloride in an acidic aqueous phase to encapsulate dibutyltin dilaurate. The production process of the cross-linked polyethylene sheath material for power cables is characterized by the following steps: 1) Pre-extrusion grafting: Low-density polyethylene, ethylene-octene copolymer, vinyltriethoxysilane and dicumyl peroxide are melt-mixed and pelletized to obtain pre-treated granules; 2) Internal mixing and sequential condensation: Pretreated particles and modified nano-silica are added to the internal mixer for a first mixing. After pre-activation by passing saturated steam at 140-150℃, perfluorooctyltriethoxysilane is added dropwise. Then, modified aluminum hydroxide, carbon black, hindered amine light stabilizer and benzotriazole UV absorber are added for a second mixing. Subsequently, dibutyltin dilaurate microcapsule catalyst is added at 133-137℃ for a third mixing. The mixture is then extruded and pelletized to obtain sheath material masterbatch. 3) Sheath extrusion and wet crosslinking: The sheath masterbatch is extruded onto the conductor to form a sheath layer, followed by continuous steam crosslinking and post-curing to obtain crosslinked polyethylene sheath material for power cables; In step 2), the temperature of the first mixing is 150-160℃ and the time is 4 min, and the steam pre-activation time is 6-10 min; the temperature of the second mixing is 148-152℃ and the time is 5-6 min; and the temperature of the third mixing is 133-137℃ and the time is 2-4 min.

2. The cross-linked polyethylene sheath material for power cables according to claim 1, characterized in that, The average particle size of the nano-silica is 20-100 nm; the average particle size of the aluminum hydroxide is 1-5 μm; and the average particle size of the carbon black is 50-200 nm.

3. The cross-linked polyethylene sheath material for power cables according to claim 1, characterized in that, The weight ratio of 3-aminopropyltriethoxysilane, phenyltriethoxysilane and nano-silica in the raw materials for preparing the modified nano-silica is 24-45:35-55:

150.

4. The cross-linked polyethylene sheath material for power cables according to claim 1, characterized in that, The weight ratio of perfluorooctyltriethoxysilane to aluminum hydroxide in the raw materials for preparing the modified aluminum hydroxide is 70-140:3500.

5. The cross-linked polyethylene sheath material for power cables according to claim 1, characterized in that, The weight ratio of urea, paraformaldehyde, ammonium chloride, and dibutyltin dilaurate is 8:4:2:9-12.

6. The production process of cross-linked polyethylene sheath material for power cables according to claim 1, characterized in that, In step 1), the cylinder temperature for melting and mixing is 165-175℃, the screw speed is 280-320rpm, and the melting and mixing time is 2-4min.

7. The production process of cross-linked polyethylene sheath material for power cables according to claim 1, characterized in that, In step 3), the screw temperature range for extrusion is 148-178℃; the temperature for continuous steam crosslinking is 65-75℃, the relative humidity is 94-98%, and the residence time is 8-12h; the temperature for post-curing is 20-30℃, the relative humidity is 45-55%, and the time is 7-14d.

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