A high-voltage-resistant direct-current cable insulation material and a method for producing the same

Abstract

The application relates to the technical field of insulating materials, in particular to a high-voltage DC cable insulating material and a preparation method thereof. The material comprises a crosslinked cable material and modified nano silicon dioxide, the modified nano silicon dioxide has a directional grafting structure composed of an inner layer of a low-polarity long alkyl brush layer and an outer layer of an epoxy-rich brush layer, and the outer layer of epoxy groups is reacted with long-chain alkyl mercaptan to construct a gradient deep-trap shell layer. The structure can effectively inhibit space charge injection and deeply capture the injected charges in a high energy level, significantly improves the DC breakdown strength and volume resistivity of the material, keeps a low dielectric loss, and solves the problem that traditional modification methods are difficult to improve charge injection and migration in coordination.

Description

Technical Field

[0001] This invention relates to the field of insulation materials technology, and in particular to a high-voltage DC cable insulation material and its preparation method. Background Technology

[0002] High-voltage direct current (HVDC) cables are crucial equipment for long-distance, high-capacity power transmission, and the long-term reliability of their insulation systems directly affects the safe and stable operation of the power grid. Unlike alternating current (AC) electric fields, the electric field experienced by the insulating medium under a DC electric field is constant in direction. The injection, migration, and accumulation of charge carriers (space charge) become the core factors affecting insulation performance. Traditional cross-linked polyethylene (XLPE) materials, as the mainstream cable insulation, are prone to space charge injection at the electrode-medium interface under DC high voltage. This space charge then migrates and accumulates within the bulk phase, leading to severe local electric field distortion and significantly reducing the actual insulation strength of the material. This is a major hidden danger that induces DC breakdown and shortens cable life.

[0003] To suppress space charge, the industry commonly employs a modification strategy of adding inorganic nanofillers (such as silica) to the polymer matrix. The introduction of nanoparticles aims to utilize the interfacial region between them and the matrix to form charge traps, capturing and binding migrating charge carriers. However, unmodified inorganic nanofillers are rich in polar hydroxyl groups, resulting in poor compatibility with the low-polarity polyethylene matrix. They readily agglomerate during melt blending and subsequent processing, forming micron-scale defects. These defects not only become electric field concentration points, but the resulting interfacial traps also have shallow energy levels, resulting in weak charge binding capacity. This easily leads to charge detachment and re-migration, making it impossible to form an effective and uniform charge suppression region along the insulation thickness direction. Furthermore, agglomeration defects also degrade the mechanical properties of the material.

[0004] Furthermore, amination of nano-silica with silane coupling agents (such as 3-aminopropyltriethoxysilane) is a common method to improve dispersibility and introduce chemical functional groups. While the introduction of amino groups enhances the interaction with the matrix, the amino groups themselves are highly polar, increasing the overall polarity of the filler / matrix interface. This increased polarity may promote interfacial polarization under a DC electric field and provide additional migration channels for charge carriers, leading to increased bulk current and dielectric losses, partially offsetting or even masking the potential benefits of introducing traps. In other words, simple polarity modification often results in a contradictory situation where it suppresses charge accumulation but exacerbates charge migration.

[0005] A deeper challenge lies in the need for precise nanoscale control of the interfacial structure in ideal DC cable insulation materials to simultaneously suppress charge injection from electrodes and enhance deep-level trapping of injected charges. Traditional single chemical modifications (such as introducing only polar groups or only hydrophobic treatment) are insufficient to synergistically achieve these two mechanisms. For example, hydrophobic long-chain treatment helps reduce interfacial energy, improve dispersion, and suppress charge injection, but may weaken trap depth; while the construction of deep traps usually relies on strongly polar or polarizable chemical groups, which, as mentioned earlier, may increase interfacial polarity and conductivity. Therefore, how to construct a composite interfacial structure on the surface of nanofillers that combines low injection characteristics and deep trapping capabilities, and can be stably and directionally distributed in specific phase regions of the polymer (such as spherulite / amorphous interfaces), has become a critical technical bottleneck that urgently needs to be overcome to improve the insulation performance of high-voltage DC cables. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a high-voltage DC cable insulation material and its preparation method, so as to solve the problem of field strength distortion caused by space charge injection and migration in traditional cross-linked polyethylene DC insulation materials, and the problem that inorganic nanofillers are difficult to construct stable and continuous deep trap networks to synergistically suppress charge behavior due to shallow interface traps and poor dispersion.

[0007] To achieve the above objectives, the present invention provides a high-voltage DC cable insulation material, comprising: cross-linked cable material and modified nano-silica; The modified nano-silica includes nano-silica and a directional grafting layer coating the surface of the nano-silica. The directional grafting layer includes an inner low-polarity long alkyl brush layer connected to the surface of the nano-silica, and an outer epoxy enrichment brush layer connected to the outside of the inner low-polarity long alkyl brush layer. The outer epoxy enrichment brush layer contains epoxy groups, and at least a portion of the epoxy groups are linked to long-chain alkyl thiols-derived alkyl sulfide groups through a ring-opening reaction, while retaining residual epoxy groups.

[0008] Preferably, the mass ratio of the cross-linked cable material to the modified nano-silica is 100:1.

[0009] Preferably, the average particle size of the nano-silica is 10-50 nm.

[0010] Furthermore, the inner low-polarity long alkyl brush layer is formed by graft polymerization of lauryl methacrylate; the outer epoxy enriched brush layer is formed by graft polymerization of glycidyl methacrylate.

[0011] Preferably, the weight ratio of nano-silica, lauryl methacrylate, glycidyl methacrylate, and long-chain alkyl thiols in the raw materials for preparing the modified nano-silica is 20:120:100:8-14.

[0012] Furthermore, the long-chain alkyl thiol is n-decyl thiol, dodecanethiol, or octadecethiol.

[0013] Furthermore, another portion of the epoxy group is further linked to an aniline derivative containing trifluoromethyl via a ring-opening reaction.

[0014] Preferably, the mass ratio of the long-chain alkyl thiol to the aniline derivative containing trifluoromethyl is 12:1-3.

[0015] Preferably, the trifluoromethyl-containing aniline derivative is 3,5-bis(trifluoromethyl)aniline or p-trifluoromethylaniline.

[0016] Preferably, the ring-opening reaction is carried out in the order of first reacting with the long-chain alkyl thiol and then with the aniline derivative containing trifluoromethyl, and some residual epoxy groups are retained after the ring-opening reaction.

[0017] Preferably, the melt flow rate of the cross-linked cable material at 190℃ / 2.16kg is 2g / 10min.

[0018] Furthermore, the present invention also provides a method for preparing modified nano-silica, comprising the following steps: S1. Aminated nano-silica was obtained by aminated nano-silica using 3-aminopropyltriethoxysilane. S2. React the aminated nano-silica with α-bromoisobutyryl bromide to graft atom transfer radical polymerization initiating groups onto the surface of the aminated nano-silica to obtain surface-initiated nano-silica. S3. Using the surface-initiated nano-silica as an initiator, atom transfer radical polymerization grafting polymerization is carried out on lauryl methacrylate monomer to obtain nano-silica grafted with an inner low-polarity long alkyl brush layer. S4. On the surface of the nano silica obtained in step S3, glycidyl methacrylate monomer is subjected to atom transfer radical polymerization grafting polymerization to obtain nano silica grafted with an outer epoxy enriched brush layer. S5. The epoxy groups of the nano-silica obtained in step S4 are subjected to a ring-opening reaction with long-chain alkyl thiols to obtain modified nano-silica.

[0019] Furthermore, the present invention also provides a method for preparing high-voltage DC cable insulation material, comprising the following steps: (1) The cross-linked cable material and modified nano-silica are mixed at a mass ratio of 1000:10 and melt-kneaded to obtain a compound; (2) Granulate the mixture; (3) The granulation product is cross-linked and degassed to obtain a high voltage DC cable insulation material.

[0020] Preferably, in step (1), the cross-linked cable material is dried at 70°C for 6 hours and then added to a mixer. It is pre-melted and mixed at 125°C and a rotor speed of 60 rpm for 6 minutes, and then the modified nano-silica is added and mixed for another 6 minutes.

[0021] Preferably, in step (3), the crosslinking includes: pre-pressing at 130°C for 5 min in a flat vulcanizing machine to remove air bubbles, then transferring to 180°C and maintaining for 20 min to complete the crosslinking; the degassing includes: placing the crosslinked product under vacuum at 80°C for 48 h.

[0022] The beneficial effects of this invention are: This invention employs a directional grafting structure, comprising an inner low-polarity long alkyl brush layer and an outer epoxy-enriched brush layer, to preferentially position epoxy groups on the outer layer of nanoparticles. This structure promotes the enrichment of modified particles in the spherulitic / amorphous interface region of the polymer during melt processing. The long alkyl chains provide excellent hydrophobic shielding and steric hindrance effects, effectively reducing the interfacial energy between the nanoparticles and the polymer matrix. This not only significantly improves the dispersion and compatibility of nano-silica in the matrix and reduces microscopic defects caused by agglomeration, but more importantly, it forms an energy barrier at the interface, significantly suppressing the injection of charge carriers (especially electrons) from electrodes or conductive impurities, thereby reducing the amount of space charge generated at the source.

[0023] Furthermore, by utilizing the high reactivity of epoxy groups on the outer epoxy-enriched brush layer, long-chain alkyl thiols and aniline derivatives containing strong electron-withdrawing groups (such as trifluoromethyl) were introduced sequentially in a specific time sequence to initiate ring-opening reactions, successfully constructing a gradient, bipolar deep trap shell. The grafting of long-chain alkyl groups provided an initial hydrophobic environment and flexibility, while the subsequently introduced aniline derivatives introduced deep trap sites rich in π electrons and strongly polar groups. This gradient structure resulted in a region near the particle core exhibiting predominantly hydrophobic and compliant properties, facilitating dispersion and suppression of implantation; while the outer shell was enriched with deep-level traps. This structure can efficiently and stably capture implanted positive and negative charges at deep levels, preventing their migration and accumulation, thereby effectively mitigating local electric field distortion.

[0024] In summary, this solution, through the aforementioned structured and functionally partitioned surface modification design, enables the nanofiller to simultaneously exert a synergistic effect of migration barrier and deep trapping within the polymer matrix. Without significantly increasing the overall polarity and dielectric loss of the material, it significantly enhances the space charge suppression capability and electric field strength of the insulating material under a high-voltage DC electric field, while simultaneously improving the material's DC breakdown strength, volume resistivity, and long-term electrothermal stability. This provides an effective solution for developing DC cable insulation with higher voltage ratings and higher reliability. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only for this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a scanning electron microscope image of the modified nano-silica provided in Example 1 of the present invention; Figure 2 Infrared spectra of modified nano-silica provided in Examples 1 and 2 of this invention. Detailed Implementation

[0027] 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.

[0028] All raw materials used in the specific implementation method are commercially available or synthesized by the method described, and the listed suppliers and models can be found on their official websites: the cross-linked cable material is Borlink LS4258DCE (used for >320kV extruded DC insulation, with a melt flow rate of 2g / 10min at 190℃ / 2.16kg).

[0029] Example 1 S1: 20g of nano-silica was vacuum dried at 120℃ for 12h and then added to 500g of anhydrous toluene. The mixture was ultrasonically dispersed for 30min. Under nitrogen protection, 12g of 3-aminopropyltriethoxysilane was added and the mixture was heated to 80℃ and stirred under reflux for 6h. After the reaction was completed, the mixture was cooled and filtered. The filter cake was washed twice each with anhydrous toluene and anhydrous ethanol, and then vacuum dried at 80℃ for 10h to obtain aminated nano-silica. S2: The aminated nano-silica obtained in step S1 was added to 400g of anhydrous dichloromethane, ultrasonically dispersed for 30min, cooled to 0℃ and 25g of triethylamine was added and stirred for 10min. Then, 10g of α-bromoisobutyryl bromide was pre-dissolved in 50g of anhydrous dichloromethane and added dropwise at 0℃. After the addition was completed, the mixture was stirred at 0℃ for 1h and then raised to 25℃ and stirred for 6h. After the reaction was completed, the mixture was filtered and washed three times each with anhydrous dichloromethane and anhydrous ethanol. Then it was vacuum dried at 60℃ for 10h to obtain surface-initiated nano-silica. S3: The surface-initiated nano-silica obtained in step S2 was added to 250g of anisole and ultrasonically dispersed for 30min. Under nitrogen protection, 120g of lauryl methacrylate, 1g of cuprous bromide and 2g of N,N,N',N'',N''-pentamethyldiethylenetriamine were added and stirred at 60℃ for 4h. After the reaction, the mixture was rapidly cooled and exposed to air to terminate the reaction. After filtration, the mixture was washed three times each with acetone and methanol. The filter cake was then added to 500g of ethanol and 5g of disodium ethylenediaminetetraacetate was added and stirred for 30min to further complex and remove the copper salt. After filtration, the mixture was vacuum dried at 60℃ for 10h to obtain nano-silica grafted with an inner low-polarity long alkyl brush layer. S4: All the inner-layer long alkyl brush-grafted nano-silica obtained in step S3 was added to 220g of anisole and ultrasonically dispersed for 30min. Then, under nitrogen protection, 100g of glycidyl methacrylate, 1g of cuprous bromide and 2g of N,N,N',N'',N''-pentamethyldiethylenetriamine were added. The reaction was terminated after stirring and polymerization at 55℃ for 3h and filtered. The nano-silica was washed three times each with acetone and methanol. Then, it was complexed with 500g of ethanol and 5g of disodium ethylenediaminetetraacetate for 30min to remove copper. After filtration, it was vacuum dried at 60℃ for 10h to obtain brush-grafted nano-silica. S5: Add the brush-grafted nano-silica obtained in step S4 to 300g of anhydrous toluene, ultrasonically disperse for 30min, add 14g of dodecanethiol and 1g of triethylamine at 50℃ and stir for 6h. After the reaction is completed, filter and wash three times each with anhydrous ethanol and acetone, and then vacuum dry at 60℃ for 10h to obtain modified nano-silica. S6: After drying 1000g of cross-linked cable material at 70℃ for 6h, add it to a mixer and pre-melt mix it at 125℃ and 60rpm for 6min. Then add 10g of modified nano-silica and continue mixing for 6min. Discharge the material, cool and granulate it. Then pre-press it at 130℃ for 5min in a flat vulcanizing machine to remove air bubbles. Then transfer it to 180℃ and hold for 20min to complete the cross-linking. Then place the cross-linked sample in a vacuum degassing chamber at 80℃ for 48h to obtain the high voltage DC cable insulation material.

[0030] Example 2: The difference from Example 1 is that: S5: The brush-grafted nano-silica obtained in step S4 was added to 300g of anhydrous toluene and ultrasonically dispersed for 30min. 12g of dodecanethiol and 1g of triethylamine were added at 50℃ and stirred for 6h. The temperature was then raised to 90℃ and 2g of 3,5-bis(trifluoromethyl)aniline was added and the reaction continued for 8h. After the reaction was completed, the mixture was filtered and washed three times each with anhydrous ethanol and acetone. The mixture was then vacuum dried at 60℃ for 10h to obtain modified nano-silica. The remaining conditions were the same as in Example 1.

[0031] Example 3: The difference from Example 1 is that: S5: Add the brush-grafted nano-silica obtained in step S4 to 300g of anhydrous toluene, ultrasonically disperse for 30min, add 8g of n-decyl mercaptan and 1g of triethylamine at 50℃ and stir for 6h. After the reaction is completed, filter and wash three times each with anhydrous ethanol and acetone, and then vacuum dry at 60℃ for 10h to obtain modified nano-silica; the remaining conditions are the same as in Example 1.

[0032] Example 4: The difference from Example 1 is that: S5: Add the brush-grafted nano-silica obtained in step S4 to 300g of anhydrous toluene, ultrasonically disperse for 30min, add 14g of octadecyl mercaptan and 1g of triethylamine at 50℃ and stir for 6h. After the reaction is completed, filter and wash three times each with anhydrous ethanol and acetone, and then vacuum dry at 60℃ for 10h to obtain modified nano-silica; the remaining conditions are the same as in Example 1.

[0033] Example 5: The difference from Example 2 is that: S5: The brush-grafted nano-silica obtained in step S4 was added to 300g of anhydrous toluene and ultrasonically dispersed for 30min. 12g of dodecanethiol and 1g of triethylamine were added at 50℃ and stirred for 6h. The temperature was then raised to 90℃ and 1g of 3,5-bis(trifluoromethyl)aniline was added and the reaction continued for 8h. After the reaction was completed, the mixture was filtered and washed three times each with anhydrous ethanol and acetone. The mixture was then vacuum dried at 60℃ for 10h to obtain modified nano-silica. The remaining conditions were the same as in Example 1.

[0034] Example 6: The difference from Example 2 is that: S5: The brush-grafted nano-silica obtained in step S4 was added to 300g of anhydrous toluene and ultrasonically dispersed for 30min. 12g of dodecanethiol and 1g of triethylamine were added at 50℃ and stirred for 6h. The temperature was then raised to 90℃ and 3g of 3,5-bis(trifluoromethyl)aniline was added and the reaction continued for 8h. After the reaction was completed, the mixture was filtered and washed three times each with anhydrous ethanol and acetone. The mixture was then vacuum dried at 60℃ for 10h to obtain modified nano-silica. The remaining conditions were the same as in Example 1.

[0035] Example 7: The difference from Example 2 is that: S5: Add the brush-grafted nano-silica obtained in step S4 to 300g of anhydrous toluene, ultrasonically disperse for 30min, add 12g of dodecanethiol and 1g of triethylamine at 50℃ and stir for 6h, then raise the temperature to 90℃ and add 2g of p-trifluoromethylaniline to continue the reaction for 8h. After the reaction is completed, filter and wash three times each with anhydrous ethanol and acetone, then vacuum dry at 60℃ for 10h to obtain modified nano-silica; the remaining conditions are the same as in Example 1.

[0036] Comparative Example 1: The difference from Example 1 is that the 10g of modified nano-silica added in step S6 is replaced with 10g of nano-silica; the other conditions are the same as in Example 1.

[0037] Comparative Example 2: The difference from Example 1 is that the 10g of modified nano-silica added in step S6 is replaced with 10g of aminated nano-silica; the other conditions are the same as in Example 1.

[0038] Comparative Example 3: The difference from Example 1 is that the 10g of modified nano-silica added in step S6 is replaced with 10g of nano-silica grafted with an inner low-polarity long alkyl brush layer; the other conditions are the same as in Example 1.

[0039] Comparative Example 4: The difference from Example 1 is that step S5 is omitted, and the nano-silica grafted onto the brush layer obtained in step S4 is directly used in step S6; the other conditions are the same as in Example 1.

[0040] Comparative Example 5 The difference from Example 2 is that the amount of 3,5-bis(trifluoromethyl)aniline added in step S5 is 6g; the other conditions are the same as in Example 2.

[0041] Comparative Example 6: The difference from Example 1 is that the dodecanethiol in step S5 is replaced with an equimolar amount of n-butanethiol; the other conditions are the same as in Example 1.

[0042] Performance testing: Scanning electron microscopy: Quantum field emission scanning electron microscopy was used to image the nano-silica and modified nano-silica in Example 1, respectively. The results are as follows: Figure 1 As shown.

[0043] Fourier transform infrared spectroscopy: The modified nano-silica prepared in Examples 1 and 2 were scanned using a Fourier transform infrared spectrometer. The results are as follows: Figure 2 As shown.

[0044] Volume resistivity (room temperature): Volume resistivity was tested according to GB / T 31838.2-2019. The sample was a 1.00 mm thick cross-linked sheet. The electrode system adopted a three-electrode structure with a protective electrode. The main electrode diameter was 50 mm and the protective ring width was 5 mm. The electrode layers were formed by vacuum evaporation of aluminum on both sides of the sample. The sample was pretreated at 25℃ for 24 h and then placed in the electrode fixture. A DC voltage of 500 V was applied and the stable current I600 s after 600 s was recorded. The volume resistivity ρv,25℃ (Ω·m) was calculated. Five samples were tested for each sample and the arithmetic mean was taken.

[0045] Volume resistivity (high temperature): High temperature volume resistivity test was performed according to GB / T 31838.7-2021. The sample and electrode structure were the same as those for volume resistivity (room temperature) test. After the sample was clamped, it was placed in a temperature control chamber and kept at 90℃ for 30 min to reach thermal equilibrium. Then a DC voltage of 500V was applied and the stable current I600s was recorded 600s after the voltage was applied. The volume resistivity ρv,90℃ (Ω·m) was calculated. Five samples were tested for each sample and the arithmetic mean was taken.

[0046] Relative permittivity and dielectric loss factor: Dielectric properties were tested according to GB / T 31838.6-2021. The sample was a 1.00 mm cross-linked sheet. A parallel plate capacitor clamp with a protective electrode and an electrode diameter of 50 mm was used. Before the test, the sample was pretreated for 24 h at (23±2)℃ and (50±5)% relative humidity. An impedance analyzer was used to apply a test voltage of 1.0 Vrms at 50 Hz. The capacitance and loss tangent were read, and the relative permittivity εr and dielectric loss factor tanδ were calculated according to the standard. Five samples were tested for each sample and the arithmetic mean was taken.

[0047] DC breakdown strength: DC breakdown strength was tested according to GB / T 1408.2-2016. The sample was a 0.30mm cross-linked sheet. The electrode was a ball-to-ball electrode with a diameter of 25mm. The test medium was degassed transformer oil and the temperature was constant at (25±1)℃. The test adopted a uniform voltage increase method with a voltage increase rate of 1.0kV / s. The breakdown voltage Ubd of each sample was recorded and the breakdown strength Ebd (kV / mm) was calculated according to Ebd=Ubd / d. Ten pieces were tested for each sample. The breakdown data were analyzed by two-parameter Weibull statistical analysis according to GB / T 29310-2012 to obtain the characteristic breakdown strength E0 (kV / mm) and shape parameter β.

[0048] Mechanical properties (tensile strength and elongation at break): Mechanical properties were tested according to GB / T 2951.11-2008. The specimens were dumbbell-shaped specimens cut from 1.00mm cross-linked sheets with a gauge length of 20mm. The test temperature was (23±2)℃. The specimens were stretched to breakage at a tensile speed of 250mm / min using an electronic tensile testing machine. The tensile strength σt (MPa) and elongation at break εb (%) were recorded. Five tests were performed on each sample and the arithmetic mean was taken.

[0049] Table 1 Performance Test Results

[0050] Data Analysis: As can be seen from the data in Table 1, the high-voltage DC cable insulation material prepared by this invention exhibits high volume resistivity, low dielectric loss factor, and high DC characteristic breakdown strength at both room temperature and high temperature, while maintaining a low relative permittivity. Furthermore, with optimized matching of the amounts of 1-dodecylthiol and aniline introduced, the characteristic breakdown strength and shape parameters show a trend of first increasing and then stabilizing, indicating that the gradient bipolar deep trap shell can enhance charge trapping and migration blocking capabilities without significantly increasing bulk polarity. Different alkylthiol segment lengths alter the hydrophobic shielding and interface compliance of the outer layer of particles, thereby affecting dielectric loss and mechanical elongation performance; the introduction of appropriate amounts of 3,5-bis(trifluoromethyl)aniline or p-trifluoromethylaniline at epoxy sites further deepens the traps and improves breakdown stability, making the material less prone to field strength distortion and breakdown failure under DC fields.

[0051] As can be seen from the data in Table 1 for Example 5 and Comparative Examples 1 and 2, when the nano-silica is not directionally grafted or only subjected to 3-aminopropyltriethoxysilane amination treatment, the volume resistivity and DC breakdown stability of the material show a decreasing trend, while the dielectric loss increases. The main reason is that the exposed silanol groups or amination interfaces have strong polarity, which easily induces carrier injection and migration channels. At the same time, particle agglomeration causes local defects, making the traps mainly shallow traps and making it difficult to form a continuous deep trap network, resulting in more charge accumulation and field distortion under DC field.

[0052] As can be seen from the data in Table 1 for Examples 1 and 5 and Comparative Examples 3 and 4, when only an inner long alkyl brush layer is constructed (Comparative Example 3), although the dielectric loss is reduced and the volume resistivity is increased, the DC breakdown strength improvement is limited. When only an outer epoxy-enriched brush layer is present without subsequent ring-opening introduction (Comparative Example 4), the breakdown strength can be improved, but the dielectric loss is easily affected by the epoxy polarity. It is evident that the dispersion and hydrophobic shielding of the long alkyl brush layer, the trapping effect of the epoxy sites, and the gradient bipolar deep trap shell of "first introducing 1-dodecylthiol, then introducing 3,5-bis(trifluoromethyl)aniline or p-trifluoromethylaniline and retaining residual epoxy" need to be formed synergistically to simultaneously achieve migration blocking and deep trapping.

[0053] As can be seen from the data in Table 1 for Examples 2, 5, and 6 and Comparative Example 5, when the amount of 3,5-bis(trifluoromethyl)aniline introduced at the epoxy reaction sites is too high, although the aromatic ring and trifluoromethyl group are conducive to the formation of deeper traps, the excessive polar groups will increase the degree of interfacial polarization, resulting in unfavorable changes in volume resistivity and dielectric loss, and increasing the dispersion of breakdown data. This may be because excessive ring-opening and hydrogen bonding enhance the local interaction in the amorphous region, forming microscale polar enrichment regions, which in turn weakens the continuity of the deep trap network. Therefore, the amount of aniline introduced needs to be matched with the hydrophobic shielding ability of the long alkyl brush layer to obtain higher DC withstand capability and stability.

[0054] As can be seen from the data in Table 1 for Example 1 and Comparative Example 6, replacing 1-dodecylthiol with n-butanethiol resulted in a decrease in both the volume resistivity and DC breakdown strength of the material, while increasing dielectric loss. The main reason for this is that the short-chain thiols provide insufficient hydrophobic shielding after the epoxy ring opens, and the residual hydroxyl groups and polar groups at the interface are more likely to interact with trace amounts of moisture or impurities, thereby increasing the probability of carrier transitions and conductive channels. Simultaneously, the reduced interface compliance makes it easier for stress and electric fields to concentrate at local defects, weakening breakdown stability.

[0055] from Figure 1It can be seen that the modified nano-silica exhibits a relatively uniform near-spherical particle structure with clear particle outlines and particle sizes mainly distributed in the range of about 20-40 nm. A certain rough structure can be observed on the particle surface, which is related to the grafting of long alkyl brush layers and epoxy functional layers on the surface. This structure is beneficial to improving the interfacial compatibility and dispersion stability of nanoparticles in the polymer matrix, thus providing a good microstructure basis for the subsequent formation of a uniform interface region and the construction of a deep trap structure in the cross-linked polyethylene insulation system.

[0056] from Figure 2 It can be seen that the modified nano-silica prepared in both Example 1 and Example 2 has a density of approximately 1100 cm⁻¹. -1 800cm -1 and 460cm -1 The presence of distinct Si-O-Si framework characteristic absorption peaks at 2920 / 2850 cm⁻¹ indicates that the silica inorganic framework remains stable; simultaneously, both exhibit absorption peaks at 2920 / 2850 cm⁻¹. -1 An alkyl CH stretching vibration peak appears nearby, and it reaches 1730 cm⁻¹. -1 The presence of an absorption peak at the C=O group of the ester group indicates that the organic segments were successfully introduced into the surface brush layer. In contrast, Example 2 showed an absorption peak at 1600-1500 cm⁻¹. -1 Aromatic ring skeletal vibration characteristics are observed in the region, and at approximately 1320 cm⁻¹ -1 With 1160cm -1 Fingerprint absorption associated with -CF3 / CF was observed nearby, along with a 910 / 845cm fingerprint. -1 The relative weakening of the nearby epoxy characteristic peaks reflects the further introduction of 3,5-bis(trifluoromethyl)aniline and the consumption of some epoxy sites on the basis of dodecyl mercaptan ring opening, thus making the spectrum show more obvious fluorinated aromatic structural features.

[0057] 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, 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 high-voltage DC cable insulation material, characterized in that, include: Cross-linked cable material and modified nano-silica; The modified nano-silica includes nano-silica and a directional grafting layer coated on the surface of the nano-silica. The directional grafting layer includes an inner low-polarity long alkyl brush layer connected to the surface of the nano-silica, and an outer epoxy enrichment brush layer connected to the outside of the inner low-polarity long alkyl brush layer. The outer epoxy enrichment brush layer contains epoxy groups, and at least a portion of the epoxy groups are connected to long-chain alkyl thiols-derived alkyl sulfide groups through a ring-opening reaction, while retaining residual epoxy groups. The inner low-polarity long alkyl brush layer is formed by graft polymerization of lauryl methacrylate; the outer epoxy enrichment brush layer is formed by graft polymerization of glycidyl methacrylate.

2. The high-voltage DC cable insulation material according to claim 1, characterized in that, The mass ratio of the cross-linked cable material to the modified nano-silica is 100:

1.

3. The high-voltage DC cable insulation material according to claim 1, characterized in that, The average particle size of the nano-silica is 10-50 nm.

4. The high-voltage DC cable insulation material according to claim 1, characterized in that, The weight ratio of nano-silica, lauryl methacrylate, glycidyl methacrylate, and long-chain alkyl thiols in the raw materials for preparing the modified nano-silica is 20:120:100:8-14.

5. The high-voltage DC cable insulation material according to claim 1, characterized in that, The long-chain alkyl thiols are n-decyl thiols, dodecanethiol, or octadecethiol.

6. The high-voltage DC cable insulation material according to claim 1, characterized in that, Another portion of the epoxy group is further linked to an aniline derivative containing trifluoromethyl via a ring-opening reaction.

7. The high-voltage DC cable insulation material according to claim 6, characterized in that, The mass ratio of the long-chain alkyl thiol to the trifluoromethyl-containing aniline derivative is 12:1-3.

8. The high-voltage DC cable insulation material according to claim 6, characterized in that, The trifluoromethyl-containing aniline derivative is 3,5-bis(trifluoromethyl)aniline or p-trifluoromethylaniline.

9. The high-voltage DC cable insulation material according to claim 1, characterized in that, The cross-linked cable material has a melt flow rate of 2 g / 10 min at 190℃ / 2.16 kg.

10. A method for preparing a high-voltage DC cable insulation material according to any one of claims 1-9, characterized in that, Includes the following steps: (1) The cross-linked cable material and modified nano-silica are mixed at a mass ratio of 1000:10 and melt-kneaded to obtain a compound; (2) Granulate the mixture; (3) The granulation product is cross-linked and degassed to obtain a high voltage DC cable insulation material.

Images