Water tree resistant high voltage cable insulation material and dry process crosslinking thereof

Abstract

The application discloses a water treeing-resistant high-voltage cable insulation material and a dry crosslinking process thereof. The insulation material takes a base resin as a matrix and comprises a modified epoxy resin and a core water treeing-resistant agent in a specific ratio; the water treeing-resistant agent is a core-shell structure composite microsphere, the core of which has a self-repairing and ion capturing function, and the shell of which is composed of a stimulus-responsive polymer and a nano-sheet clay, so that a water treeing-resistant mechanism integrating prevention, inhibition and repair is realized. The dry crosslinking process adopts a nitrogen-protected stepwise temperature rising crosslinking, and then a staged accelerated stepwise slow cooling treatment. The process is matched with the material characteristics, and aims to form a uniform crosslinking network and reduce the internal stress of the insulation layer to the maximum. Through the synergy between the material components and the synergy between the material and the process, the long-term water treeing resistance, the electrical strength and the mechanical reliability of the high-voltage cable insulation layer are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of power cable insulation material preparation, and in particular to a water-resistant high-voltage cable insulation material and its dry crosslinking process. Background Technology

[0002] With the rapid development of the power industry, high-voltage and ultra-high-voltage cables have become the core carriers for urban power grid upgrades and cross-regional power transmission. Their operational reliability is directly related to the safety and stability of the power system and the normal order of people's production and life. As a core component of high-voltage cables, the insulation layer plays a crucial role in isolating conductors, withstanding electric field strength, and preventing leakage. Its performance directly determines the cable's service life and safety performance. During the long-term operation of high-voltage cables, the insulation layer inevitably comes into contact with moisture. Under the influence of the electric field, moisture gradually penetrates into the interior of the insulation material, interacting with impurities and defects in the material to form dendritic microchannels, i.e., water treeing.

[0003] Water tree growth continuously damages the microstructure of insulation materials, leading to a decrease in the dielectric strength and volume resistivity of the insulation layer. Long-term development can cause insulation aging and breakdown, ultimately resulting in cable failures. This not only increases the maintenance costs of the power system but can also trigger large-scale power outages, causing significant economic losses. Currently, high-voltage cable insulation materials are mainly polyolefin materials such as polyethylene (PE) and ethylene-vinyl acetate copolymer (EVA). These materials possess excellent dielectric, mechanical, and processing properties, but their resistance to water treeing is poor. Under humid environments and long-term electric field conditions, they are highly susceptible to water treeing and aging, becoming a key bottleneck restricting the service life of high-voltage cables.

[0004] To address the insufficient water-tree resistance of polyolefin insulation materials, various improvement schemes have been proposed in existing technologies, mainly falling into two categories: one is to modify the base resin by adding modifiers such as epoxy resin and polyurethane to improve the material's crystal morphology and microstructure, thereby enhancing its water penetration resistance; the other is to add water-tree resistant agents, such as inorganic fillers like montmorillonite and kaolin, or organic water-tree resistant agents, to capture moisture, inhibit water tree growth, and delay insulation aging. However, existing modification schemes still have many shortcomings: simple resin modification can improve water resistance to a certain extent, but it sacrifices the material's processing fluidity and dielectric properties, making it difficult to extrude the cable insulation layer, and its long-term water-tree resistance effect is limited; traditional water-tree resistant agents are mostly single-component, unable to achieve integrated "prevention-inhibition-repair" of water treeing, and have poor compatibility with the base resin, easily leading to agglomeration, which not only fails to play a water-tree resistant role but may also become new defect points, accelerating water tree growth.

[0005] In the preparation process of high-voltage cable insulation materials, cross-linking is a key step in improving material performance. Currently, the mainstream processes include wet cross-linking and dry cross-linking. Wet cross-linking uses water vapor as the pressure and heat transfer medium. Although it has the advantages of low equipment investment and simple process, the moisture in the steam can easily penetrate into the interior of the insulation layer, causing the insulation material to absorb moisture, further accelerating water treeing aging, and the cross-linking uniformity is poor, with large differences in performance between the inside and outside of the insulation layer. Dry cross-linking uses inert gases such as nitrogen as the medium, which can effectively prevent moisture intrusion and improve the dryness and cross-linking uniformity of the insulation layer. However, existing dry cross-linking processes mostly use single-temperature cross-linking, resulting in low cross-linking efficiency. Moreover, the design of the cooling stage is unreasonable, which can easily lead to internal stress in the insulation layer, causing defects such as cracking and deformation, affecting the operational reliability of the cable.

[0006] Furthermore, in existing technologies, the structural design of anti-water-tree agents is poorly compatible with cross-linking processes. Anti-water-tree agents are prone to structural damage during cross-linking, losing their anti-water-tree function. Moreover, mismatches between cross-linking parameters and material components lead to insufficient or excessive cross-linking, further reducing anti-water-tree performance and mechanical properties. Therefore, developing an insulating material with good compatibility, excellent anti-water-tree effect, and integrated water-tree control, combined with a suitable dry cross-linking process, to solve the problems of poor anti-water-tree performance, unreasonable processes, and insufficient material-process compatibility in existing technologies, and to improve the long-term operational stability and service life of high-voltage cable insulation layers, has become an urgent technical challenge in the field of high-voltage cable insulation materials.

[0007] The above background information is provided only to aid in understanding the inventive concept and technical solution of this invention. It does not necessarily belong to the prior art of this patent application. In the absence of clear evidence that the above information was disclosed on the filing date of this patent application, the above background information should not be used to evaluate the novelty and inventiveness of this application. Summary of the Invention

[0008] The purpose of this invention is to propose a water-tree resistant high-voltage cable insulation material and its dry crosslinking process, so as to solve the technical problems of insufficient water-tree resistant performance, poor component compatibility, unbalanced comprehensive performance, and internal stress defects caused by unreasonable crosslinking process in the above-mentioned existing technologies.

[0009] To achieve the above technical objectives, the present invention adopts the following technical solution:

[0010] A water-tree resistant high-voltage cable insulation material, comprising the following components by weight:

[0011] The composition comprises: base resin: 100 parts; modified epoxy resin: 5-25 parts; anti-water-tree agent: 1-10 parts; crosslinking agent: 1.5-3.5 parts; antioxidant: 0.1-1.5 parts; silane coupling agent: 0.5-3 parts. The base resin is low-density polyethylene or ethylene-vinyl acetate copolymer. The anti-water-tree agent is a core-shell structured composite microsphere, the core containing a self-healing monomer and an ion trapping agent, and the outer shell being a composite material composed of a stimulus-responsive polymer and nanosheet clay. The average particle size of the core-shell structured composite microsphere is 100 nm-5 μm, and the mass ratio of the outer shell to the core is 4:6 to 7:3. The modified epoxy resin is a bisphenol A type epoxy resin modified with nano-silica particles surface-treated with a silane coupling agent, with an epoxy value of 0.40-0.52 eq / 100g, and the amount of nano-silica added is 3%-15% of the epoxy resin mass.

[0012] Preferably, in the core-shell structured composite microspheres, the self-healing monomer is a polymerizable monomer containing Diels-Alder addition bonds, disulfide bonds, or quadruple hydrogen bonds UPy units; the ion scavenger is a calixarene or a crown ether derivative; the stimulus-responsive polymer is poly(N-isopropylacrylamide) or polyvinylcaprolactam; and the nanosheet clay is kaolin or montmorillonite modified with a silane coupling agent.

[0013] Preferably, the crosslinking agent is one or more of dicumyl peroxide, benzoyl peroxide, or di-tert-butyl peroxide; and the antioxidant is one or more of hindered phenolic antioxidants, phosphite antioxidants, or thioester antioxidants.

[0014] Preferably, the nanosheet clay has a particle size of 50-200 nm and an interlayer spacing of 1.5-3 nm.

[0015] A dry crosslinking process for a water-tree resistant high-voltage cable insulation material includes the following steps:

[0016] S1. Mixing and Granulation: The base resin, modified epoxy resin, anti-water-tree agent, antioxidant, and silane coupling agent are mixed at 80℃-120℃ for 15-30 minutes to obtain a uniformly mixed masterbatch. The masterbatch is then cooled to below 60℃ and fed together with the crosslinking agent into a twin-screw extruder for melt blending, extrusion, cooling, and pelletizing to obtain insulating material granules. The processing temperature of the twin-screw extruder is: feeding zone 120℃-140℃, melting zone 140℃-160℃, homogenization zone 160℃-175℃, and die head temperature 165℃-180℃; the screw speed is 80-200 rpm.

[0017] S2: Extrusion and preheating: Insulating material particles are extruded through an extruder and coated onto a conductor to form a wire core; the wire core is passed into a dry cross-linking pipe with a first heating zone for preheating. The temperature of the first heating zone is controlled at 130℃-160℃ for 3-4 minutes. Nitrogen gas at 0.3-0.5MPa is introduced as a protective medium during the preheating process.

[0018] S3. Stepped crosslinking: After preheating, the wire core enters the second heating zone of the dry crosslinking pipeline for the first stage of crosslinking, with the temperature controlled at 170℃-190℃ and the time at 4-6 minutes; subsequently, the wire core enters the third heating zone for the second stage of crosslinking, with the temperature controlled at 205℃-215℃ and the time at 10-14 minutes; the dry crosslinking pipeline is filled with high-purity nitrogen gas of 0.8-1.6 MPa throughout the second and third heating zones as a protective and pressure-transmitting medium, with a nitrogen purity ≥99.99%;

[0019] S4. Accelerated Slow Cooling and Post-processing: The cross-linked core enters the cooling zone of the dry cross-linking pipeline for accelerated step-by-step slow cooling: First, under nitrogen pressure of 0.5-1.0MPa, the core is forcibly cooled for 8-12 minutes using hot air circulation at 80℃-84℃, rapidly reducing its temperature to 110℃-130℃; then, the pressure is gradually reduced to atmospheric pressure, and cold air at 40℃-60℃ is used to continue cooling to below 80℃; finally, the core is placed in a clean, air-conditioning environment at 20℃-30℃ with no wind to cool naturally to room temperature; after spark testing and winding, the water-tree resistant high-voltage cable insulated core is obtained; the spark test voltage is 30-50kV, the test time is 1-2s / m, and the test qualification standard is no breakdown and no flashover.

[0020] Preferably, in step S1, the mixing speed is 50-80 rpm; the length-to-diameter ratio of the twin-screw extruder is 30:1-40:1, and the extrusion pressure is 10-20 MPa.

[0021] Preferably, in step S2, the extruder's processing temperature is 140℃-170℃, and the extrusion speed is 5-15m / min; the conductor outer diameter of the wire core is 10-50mm, and the insulation layer thickness is 2-10mm.

[0022] Preferably, in step S3, the temperature fluctuation range of the first heating zone, the second heating zone, and the third heating zone does not exceed ±5℃; the nitrogen circulation speed is 0.5-1m / s.

[0023] Preferably, in step S4, the wind speed of the hot air circulation is 1-2 m / s, and the wind speed of the cold air circulation is 1.5-2.5 m / s; the natural cooling time is 30-60 min, and the temperature difference between the inside and outside of the insulation layer after cooling is ≤10℃.

[0024] Preferably, the mixing temperature in step S1 is 90℃-110℃.

[0025] The beneficial effects of this invention compared to the prior art include:

[0026] 1. Comprehensive water-resistant tree function with stable control effect.

[0027] This invention offers more comprehensive anti-water treeing capabilities and more stable control effects. Existing anti-water treeing agents are mostly single-component, only capable of inhibiting or blocking water growth, making it difficult to achieve comprehensive control. This invention uses core-shell structured composite microspheres as the anti-water treeing agent. The outer shell effectively blocks water and ion penetration, while the ion-scavenging agent in the core inhibits water tree growth. The self-healing monomer repairs the microcracks that form water tree growth, creating an integrated anti-water treeing system that combines prevention, inhibition, and repair. This reduces water tree formation at its source, slows its development, and solves the problems of single-function and short-lived effects in existing technologies, thus improving the stability of the anti-water treeing performance of insulating materials.

[0028] 2. Excellent component compatibility, with no agglomeration defects.

[0029] The material exhibits better component compatibility and exhibits no significant agglomeration defects. In existing technologies, insufficient compatibility between anti-water-tree agents, modifiers, and the base resin often leads to component agglomeration, forming new insulation defects and affecting the overall material performance. This invention, by adding a silane coupling agent and simultaneously surface-treating the nano-silica in the modified epoxy resin and the nano-sheet clay in the anti-water-tree agent, effectively improves the compatibility between each component and the base resin, preventing component agglomeration, resulting in a more uniform microstructure, and ensuring stable dielectric and mechanical properties of the insulating material. This solves the technical problem of poor component compatibility in existing technologies.

[0030] 3. Balanced overall performance, taking into account both processing and usage requirements.

[0031] The material exhibits more balanced overall performance, balancing processing and usage requirements. In existing technologies, improving the water-tree resistance of insulation materials often requires sacrificing processing flowability or mechanical properties, leading to difficulties in extruding cable insulation layers or problems such as cracking and embrittlement after molding. This invention optimizes the proportions of each component and rationally combines the amounts of modified epoxy resin and water-tree resistant agent. While improving water-tree resistance, it does not affect the processing flowability of the base resin, facilitating subsequent extrusion, cross-linking, and other processes. Simultaneously, it balances the material's mechanical and dielectric properties, achieving a balanced improvement in water-tree resistance, processing, and usage performance.

[0032] 4. The cross-linking process and materials are well-matched, resulting in superior cross-linking quality.

[0033] Dry crosslinking technology offers better compatibility with materials and superior crosslinking quality. Existing dry crosslinking technologies often employ single-temperature crosslinking without considering the characteristics of the insulating material components, leading to insufficient or excessive crosslinking and potentially compromising the structural integrity of the anti-water-tree agent. This invention employs a stepped crosslinking design, taking into account the thermal processing characteristics of the insulating material components and rationally setting the temperature and time parameters for preheating and two-stage crosslinking. This ensures sufficient and uniform crosslinking while protecting the core-shell composite microspheres from high-temperature damage, guaranteeing the normal functioning of the anti-water-tree agent, and improving the crosslinking quality and structural stability of the insulation layer.

[0034] 5. The cooling process is reasonable, reducing internal stress defects.

[0035] The cooling process is rationally designed to reduce internal stress defects in the insulation layer. Existing technologies often employ a single cooling method in the cooling stage of dry cross-linking. Improper control of the cooling rate can easily lead to excessive temperature differences between the inside and outside of the insulation layer, generating internal stress and subsequently causing defects such as cracking and deformation, affecting the reliability of cable operation. This invention employs an accelerated stepped slow cooling process, controlling the cooling temperature, air velocity, and pressure in stages to gradually reduce the core temperature. This effectively controls the temperature difference between the inside and outside of the insulation layer, reduces internal stress, and avoids defects such as cracking and deformation in the insulation layer. This improves the forming quality of the insulated core, extends the service life of high-voltage cables, and solves the insulation defect problem caused by unreasonable cooling processes in existing technologies. Detailed Implementation

[0036] The present invention will be further described in detail below with reference to specific embodiments. It should be emphasized that the following description is merely exemplary and is not intended to limit the scope and application of the present invention.

[0037] Raw material preparation:

[0038] Prepare the following raw materials by weight, with specific requirements as follows:

[0039] 1. Base resin: 100 parts, selected from one or a mixture of two of low-density polyethylene or ethylene-vinyl acetate copolymer.

[0040] 2. Modified epoxy resin: 5-25 parts; Bisphenol A type epoxy resin modified with nano silica particles that have been surface treated with silane coupling agent is selected, and its epoxy value is controlled at 0.40-0.52eq / 100g. The amount of nano silica added is 3%-15% of the epoxy resin mass.

[0041] 3. Water-resistant agent: 1-10 parts; core-shell structured composite microspheres are selected, with an average particle size controlled between 100nm and 5μm, and the mass ratio of the outer shell to the core is 4:6 to 7:3; the core of the core-shell structured composite microspheres contains self-healing monomers and ion trapping agents, and the outer shell is a composite material composed of a stimulus-responsive polymer and nanosheet clay.

[0042] 4. Crosslinking agent: 1.5-3.5 parts; one or more of dicumyl peroxide, benzoyl peroxide or di-tert-butyl peroxide are selected.

[0043] 5. Antioxidant: 0.1-1.5 parts; select one or more of hindered phenolic antioxidants, phosphite antioxidants or thioester antioxidants.

[0044] 6. Silane coupling agent: 0.5-3 parts.

[0045] Dry cross-linking process for water-tree resistant high-voltage cable insulation materials:

[0046] Based on the raw materials prepared above, a dry crosslinking process is used to prepare water-tree resistant high-voltage cable insulation cores. The specific process steps are as follows:

[0047] Step S1: Mixing and Granulation

[0048] The base resin, modified epoxy resin, anti-water-tree agent, antioxidant, and silane coupling agent are added to a mixer, and the mixing temperature is controlled at 80℃-120℃ for 15-30 minutes to ensure thorough and uniform mixing of all components, resulting in a mixed masterbatch. The mixed masterbatch is then naturally cooled to below 60℃ and fed together with a crosslinking agent into a twin-screw extruder for melt blending, extrusion, cooling, and pelletizing to obtain insulating material granules. The processing temperature of the twin-screw extruder is controlled in the following zones: feeding zone 120℃-140℃, melting zone 140℃-160℃, homogenization zone 160℃-175℃, and die head temperature 165℃-180℃; the screw speed is controlled at 80-200 rpm.

[0049] Step S2: Extrusion and Preheating

[0050] The insulating material particles prepared in step S1 are fed into an extruder, and the insulating material particles are melted and extruded to coat the conductor to form a wire core. The coated wire core is then passed into a dry cross-linking pipe with a first heating zone for preheating. The temperature of the first heating zone is controlled at 130℃-160℃, and the preheating time is 3-4 minutes. During the preheating process, nitrogen gas at 0.3-0.5 MPa is introduced as a protective medium.

[0051] Step S3: Stepwise crosslinking

[0052] After preheating, the wire core enters the second heating zone of the dry cross-linking pipeline for the first stage of cross-linking, with the temperature controlled at 170℃-190℃ and the cross-linking time at 4-6 minutes. After the first stage of cross-linking is completed, the wire core enters the third heating zone for the second stage of cross-linking, with the temperature controlled at 205℃-215℃ and the cross-linking time at 10-14 minutes. The dry cross-linking pipeline is filled with high-purity nitrogen gas at 0.8-1.6 MPa throughout as a protective and pressure-transmitting medium, with a nitrogen purity ≥99.99%. At the same time, the temperature fluctuation range of the first, second, and third heating zones is controlled to not exceed ±5℃, and the nitrogen circulation speed is 0.5-1 m / s.

[0053] Step S4: Accelerated slow cooling and post-treatment

[0054] After the core wires have completed the stepped cross-linking process, they enter the cooling zone of the dry cross-linking pipeline for accelerated stepped slow cooling treatment, which is divided into three stages: In the first stage, under a nitrogen pressure of 0.5-1.0 MPa, the core wires are forcibly cooled by circulating hot air at 80℃-84℃ for 8-12 minutes, so that the core wire temperature drops rapidly to 110℃-130℃; In the second stage, the nitrogen pressure in the cooling zone is gradually reduced to atmospheric pressure, and cold air at 40℃-60℃ is used to continue cooling, so that the core wire temperature drops below 80℃; In the third stage, the core wires are removed from the cooling zone and placed in a clean, air-conditioned environment at 20℃-30℃ to cool naturally to room temperature, ensuring that the temperature difference between the inside and outside of the insulation layer is ≤10℃ after cooling.

[0055] After natural cooling, the wire core is subjected to a spark test. The spark test voltage is 30-50kV and the test time is 1-2s / m. The test qualification standard is no breakdown and no flashover. The wire core that passes the test is wound up to obtain the water tree resistant high voltage cable insulated wire core. The wire core that fails the test is scrapped and not used.

[0056] Technical principle of the invention:

[0057] I. The Role of Each Raw Material Component

[0058] 1. Base Resin: Low-density polyethylene or ethylene-vinyl acetate copolymer is selected as the core skeleton of the insulation material. Its core function is to provide excellent dielectric insulation properties and processing fluidity, laying the foundation for the extrusion and molding of the insulation layer. Its molecular structure has a certain degree of flexibility, which can prevent the insulation layer from becoming brittle after molding and ensure the structural integrity of the cable during laying and long-term operation. At the same time, the molecular chain of the base resin contains active sites, providing the necessary conditions for the subsequent cross-linking reaction, which is a prerequisite for realizing the cross-linking curing of the material and improving thermal stability.

[0059] 2. Modified epoxy resin: Its core function is to improve the crystal morphology and microstructure of the base resin, thereby enhancing the material's water permeability resistance and mechanical strength. In this invention, the modified epoxy resin is a bisphenol A type epoxy resin modified with nano-silica particles surface-treated with a silane coupling agent. The bisphenol A type epoxy resin itself possesses good adhesion and mechanical properties, enhancing the overall strength of the material. The nano-silica particles fill the gaps between the molecular chains of the base resin, reducing intermolecular gaps and decreasing water permeation channels. The surface treatment of the nano-silica with the silane coupling agent improves its compatibility with the epoxy resin, preventing nanoparticle aggregation, and simultaneously providing a bridging effect for subsequent bonding with the base resin and other components.

[0060] 3. Anti-water-tree agent: Utilizing core-shell structured composite microspheres, this is the core component responsible for the material's anti-water-tree function. The core and outer shell have clearly defined roles, each performing different functions and working in concert. The self-healing monomers in the core polymerize at the microcracks caused by water tree formation, filling the defects and preventing further water tree propagation. The ion scavengers in the core capture metal ions that have penetrated the material, reducing their catalytic effect on water tree growth and inhibiting its initiation and development at its source. The stimulus-responsive polymers in the outer shell are environmentally sensitive, adjusting their structure according to temperature and humidity changes to further block moisture and ion penetration. The nanosheet clay in the outer shell has a layered structure, forming a dense barrier layer that enhances moisture barrier performance and improves the compatibility between the anti-water-tree agent and the base resin, preventing the agent from agglomerating and forming new insulation defects.

[0061] 4. Crosslinking agent: Its core function is to initiate the crosslinking reaction of the base resin molecular chains, transforming the originally linear molecular chains into a stable three-dimensional network structure, thereby improving the material's thermal stability, mechanical properties, and water-tree resistance. Under the high-temperature conditions of the dry crosslinking process, the crosslinking agent decomposes to generate active free radicals. These free radicals can trigger the crosslinking reaction of the base resin molecular chains. Simultaneously, the crosslinking agent can also react with the epoxy groups of the modified epoxy resin, promoting the fusion of the base resin and the modified epoxy resin, further optimizing the material's microstructure, reducing intermolecular gaps, and decreasing the possibility of moisture penetration.

[0062] 5. Antioxidants: Their core function is to inhibit the oxidative aging of insulating materials during processing and long-term operation, thus extending the material's service life. During high-temperature processing such as mixing, extrusion, and cross-linking, the material's molecular chains are prone to oxidative degradation, leading to a decline in material performance. Antioxidants can capture free radicals generated during oxidation, terminating the oxidation chain reaction and protecting the material's molecular chain structure from damage. During the long-term operation of cables, antioxidants continue to play a role, slowing down the aging rate of the material and ensuring the long-term stability of insulation performance.

[0063] 6. Silane Coupling Agents: As compatibility enhancers, their core function is to improve the interfacial bonding between components, solving the problem of insufficient compatibility between different components (organic and inorganic components). Silane coupling agents have a unique molecular structure, with one end possessing an active group that bonds with inorganic materials (nano-silica, nano-sheet clay) and the other end possessing an active group that bonds with organic materials (base resin, modified epoxy resin, water-resistant shell polymer). This allows them to form stable chemical bonds at the interfaces of the components, ensuring uniform dispersion of components such as base resin, modified epoxy resin, and water-resistant agents within the system. This prevents component aggregation, ensures a uniform microstructure of the material, and guarantees the full functionality of each component.

[0064] II. Synergistic Mechanism among Raw Material Components

[0065] The raw material components do not function independently, but rather work together synergistically to enhance the material's resistance to water treeing and overall performance. This synergistic effect is manifested on three levels:

[0066] 1. Synergistic effect of base resin and modified epoxy resin: The base resin provides excellent dielectric insulation properties and processing fluidity, ensuring that the material can be smoothly extruded and molded to achieve its core insulation function; the modified epoxy resin fills the gaps between the molecular chains of the base resin, improves its crystal morphology, and enhances the material's mechanical strength and water resistance. The combination of the two solves the problem in existing technologies where dielectric properties, mechanical properties, and water resistance of insulating materials cannot be simultaneously achieved, giving the material both good insulation and processing properties, as well as strong water resistance and mechanical strength.

[0067] 2. Synergistic effect of anti-water-tree agent and other components: The anti-water-tree agent achieves the integrated function of preventing, inhibiting and repairing water treeing through its core-shell structure, but its full effectiveness depends on good compatibility with other components. The silane coupling agent can ensure that the anti-water-tree agent is uniformly dispersed in the base resin and modified epoxy resin system, avoiding its agglomeration and the formation of new insulation defects, so that every part of the anti-water-tree agent can play its role. At the same time, the improved water penetration resistance of the modified epoxy resin can reduce the intrusion of water into the material, complementing the water treeing prevention and control effect of the anti-water-tree agent, further enhancing the anti-water-treeing effect, and improving the anti-water-treeing performance of the material from the two perspectives of "blocking water" and "preventing water treeing".

[0068] 3. Synergistic effect of crosslinking agent and components: The crosslinking reaction initiated by the crosslinking agent can not only enable the base resin to form a three-dimensional network structure, improving its thermal stability and mechanical properties, but also promote the deep integration of the base resin and the modified epoxy resin, making the two form a stable whole and further optimizing the microstructure of the material. At the same time, the three-dimensional network structure formed by the crosslinking reaction can fix the dispersion state of components such as water-resistant agents and antioxidants, preventing them from migrating during processing and long-term operation, ensuring that each component can perform its function stably for a long time and achieve synergistic and long-lasting effects of each component.

[0069] III. Synergistic effect of raw material components and dry crosslinking process

[0070] The dry crosslinking process is precisely designed to match the characteristics of the raw material components, further enhancing the synergistic effect of each component and improving the molding quality and performance of the insulation layer.

[0071] The temperature and time parameters of the stepped crosslinking process are designed based on the thermal stability of each component. This ensures that the crosslinking agent fully decomposes and triggers the crosslinking reaction between the base resin and the modified epoxy resin, achieving complete crosslinking of the material. At the same time, it avoids high-temperature damage to the core-shell structure of the water-resistant agent and the microstructure of the modified epoxy resin, ensuring that the functions of the water-resistant agent and the modified epoxy resin are not impaired. The accelerated stepped slow cooling process matches the thermal expansion characteristics of the crosslinked material, controlling the cooling temperature, air velocity, and pressure in stages to gradually reduce the core temperature, effectively controlling the temperature difference between the inside and outside of the insulation layer, reducing the generation of internal stress, and avoiding defects such as cracking and deformation of the insulation layer.

[0072] Meanwhile, the dry crosslinking process uses high-purity nitrogen as a protective and pressure-transmitting medium throughout, which can completely prevent moisture from penetrating the insulation layer. This complements the anti-water penetration and anti-water-tree functions achieved synergistically by the various components of the material, further reducing the impact of moisture on the material's performance. The parameter design of each step of the process is adapted to the processing characteristics of the material components, ensuring that each component can be fully integrated and uniformly dispersed during processing. Ultimately, this achieves a comprehensive improvement in the material's anti-water-tree performance, mechanical properties, dielectric properties, and processing performance, achieving technical effects that cannot be achieved by using individual raw materials or a single process in existing technologies.

[0073] To make the present invention more fully disclosed, more specific embodiments are described below.

[0074] Example 1:

[0075] I. Raw material preparation (by weight)

[0076] 1. Base resin: 100 parts, low-density polyethylene is selected;

[0077] 2. Modified epoxy resin: 12 parts, bisphenol A type epoxy resin modified with nano-silica particles surface-treated with silane coupling agent KH550, epoxy value 0.46 eq / 100g, the amount of nano-silica added is 9% of the mass of epoxy resin;

[0078] 3. Water-resistant agent: 4 parts, using core-shell structured composite microspheres with an average particle size of 800nm, and a shell to core mass ratio of 5.5:4.5. The core contains self-healing monomers and ion traps, and the shell is a composite material of stimulus-responsive polymer and nanosheet clay.

[0079] 4. Crosslinking agent: 2.2 parts, dicumyl peroxide is selected;

[0080] 5. Antioxidant: 0.7 parts, selected from hindered phenolic antioxidants (antioxidant 1010).

[0081] 6. Silane coupling agent: 1.8 parts, KH550 is selected.

[0082] II. Dry crosslinking process

[0083] Step S1: Mixing and Granulation. The base resin, modified epoxy resin, anti-water-tree agent, antioxidant, and silane coupling agent are added to a mixing mill. The mixing temperature is controlled at 100℃, the mixing speed at 65 rpm, and the mixing time at 20 minutes. The mixture is thoroughly mixed to obtain a masterbatch. After naturally cooling to below 60℃, it is fed into a twin-screw extruder along with a crosslinking agent. The masterbatch is then melt-blended, extruded, cooled, and pelletized to obtain insulating material granules. The twin-screw extruder zone temperatures are: feed zone 135℃, melt zone 155℃, homogenization zone 172℃, and die head temperature 175℃; the screw speed is 160 rpm; the length-to-diameter ratio of the twin-screw extruder is 35:1; and the extrusion pressure is 15 MPa.

[0084] Step S2: Extrusion and Preheating. Insulating material granules are fed into an extruder, melted, and extruded to coat the conductor, forming a wire core. The extruder's processing temperature is 155℃, and the extrusion speed is 10m / min. The conductor's outer diameter is 25mm, and the insulation layer thickness is 6mm. The wire core is preheated in the first heating zone of a dry cross-linking pipeline at 150℃ for 3.5min, with 0.45MPa nitrogen gas introduced as a protective medium.

[0085] Step S3: Stepped crosslinking. The preheated wire core enters the second heating zone, with a first-stage crosslinking temperature of 185℃ and a time of 5.2 min; then it enters the third heating zone, with a second-stage crosslinking temperature of 215℃ and a time of 11 min; the entire crosslinking process is filled with 1.4 MPa of high-purity nitrogen (purity ≥99.99%), the temperature fluctuation of each heating zone is ≤±5℃, and the nitrogen circulation speed is 0.8 m / s.

[0086] Step S4: Accelerated Cooling and Post-processing. The conductor enters the cooling zone. In the first stage, it is forcibly cooled for 9 minutes under 0.9MPa nitrogen pressure using 82℃ hot air circulation at a speed of 1.5m / s, reducing the conductor temperature to 115℃. In the second stage, the pressure is gradually reduced to ambient level, and the conductor is cooled to below 80℃ using 55℃ cold air at a speed of 2.0m / s. In the third stage, it is naturally cooled to room temperature in a clean, windless environment at 26℃ for 45 minutes, ensuring that the temperature difference between the inside and outside of the insulation layer is ≤10℃. After natural cooling, a spark test is performed at 40kV voltage and 1.5s / m duration. No breakdown or flashover indicates a pass. Qualified conductors are wound up to obtain the water-tree resistant high-voltage cable insulation conductor, while unqualified conductors are scrapped.

[0087] Example 2:

[0088] I. Raw material preparation (by weight)

[0089] 1. Base resin: 100 parts, ethylene-vinyl acetate copolymer;

[0090] 2. Modified epoxy resin: 8 parts, bisphenol A type epoxy resin modified with nano-silica particles surface-treated with silane coupling agent KH550, epoxy value 0.42eq / 100g, the amount of nano-silica added is 6% of the mass of epoxy resin;

[0091] 3. Water-resistant agent: 3 parts, using core-shell structured composite microspheres with an average particle size of 500nm, and a shell to core mass ratio of 5:5. The core contains self-healing monomers and ion trapping agents, and the shell is a composite material of stimulus-responsive polymer and nanosheet clay.

[0092] 4. Crosslinking agent: 1.8 parts, benzoyl peroxide is selected;

[0093] 5. Antioxidant: 0.4 parts, using phosphite antioxidant (antioxidant 168).

[0094] 6. Silane coupling agent: 1.2 parts, KH550 is selected.

[0095] II. Dry crosslinking process

[0096] Step S1: Mixing and Granulation. The base resin, modified epoxy resin, anti-water-tree agent, antioxidant, and silane coupling agent are added to a mixing mill. The mixing temperature is controlled at 100℃, the mixing speed at 65 rpm, and the mixing time at 20 minutes. The mixture is thoroughly mixed to obtain a masterbatch. After naturally cooling to below 60℃, it is fed into a twin-screw extruder along with a crosslinking agent. The masterbatch is then melt-blended, extruded, cooled, and pelletized to obtain insulating material granules. The twin-screw extruder zone temperatures are: feed zone 135℃, melt zone 155℃, homogenization zone 172℃, and die head temperature 175℃; the screw speed is 160 rpm; the length-to-diameter ratio of the twin-screw extruder is 35:1; and the extrusion pressure is 15 MPa.

[0097] Step S2: Extrusion and Preheating. Insulating material granules are fed into an extruder, melted, and extruded to coat the conductor, forming a wire core. The extruder's processing temperature is 155℃, and the extrusion speed is 10m / min. The conductor's outer diameter is 25mm, and the insulation layer thickness is 6mm. The wire core is preheated in the first heating zone of a dry cross-linking pipeline at 150℃ for 3.5min, with 0.45MPa nitrogen gas introduced as a protective medium.

[0098] Step S3: Stepped crosslinking. The preheated wire core enters the second heating zone, with a first-stage crosslinking temperature of 185℃ and a time of 5.2 min; then it enters the third heating zone, with a second-stage crosslinking temperature of 215℃ and a time of 11 min; the entire crosslinking process is filled with 1.4 MPa of high-purity nitrogen (purity ≥99.99%), the temperature fluctuation of each heating zone is ≤±5℃, and the nitrogen circulation speed is 0.8 m / s.

[0099] Step S4: Accelerated Cooling and Post-processing. The conductor enters the cooling zone. In the first stage, it is forcibly cooled for 9 minutes under 0.9MPa nitrogen pressure using 82℃ hot air circulation at a speed of 1.5m / s, reducing the conductor temperature to 115℃. In the second stage, the pressure is gradually reduced to ambient level, and the conductor is cooled to below 80℃ using 55℃ cold air at a speed of 2.0m / s. In the third stage, it is naturally cooled to room temperature in a clean, windless environment at 26℃ for 45 minutes, ensuring that the temperature difference between the inside and outside of the insulation layer is ≤10℃. After natural cooling, a spark test is performed at 38kV voltage and 1.4s / m duration. No breakdown or flashover indicates a pass. Qualified conductors are wound up to obtain the water-tree resistant high-voltage cable insulation conductors, while unqualified conductors are scrapped.

[0100] Example 3:

[0101] I. Raw material preparation (by weight)

[0102] 1. Base resin: 100 parts, low-density polyethylene is selected;

[0103] 2. Modified epoxy resin: 18 parts, bisphenol A type epoxy resin modified with nano-silica particles surface-treated with silane coupling agent KH550, epoxy value 0.48 eq / 100g, the amount of nano-silica added is 12% of the mass of epoxy resin;

[0104] 3. Water-resistant agent: 7 parts, using core-shell structured composite microspheres with an average particle size of 2μm and a shell to core mass ratio of 6:4. The core contains self-healing monomers and ion trapping agents, while the shell is a composite material of stimulus-responsive polymer and nanosheet clay.

[0105] 4. Crosslinking agent: 2.8 parts, made by mixing dicumyl peroxide and benzoyl peroxide in a mass ratio of 2:1;

[0106] 5. Antioxidant: 1.2 parts, selected from hindered phenolic antioxidant (antioxidant 1010) and phosphite antioxidant (antioxidant 168) at a mass ratio of 2:1;

[0107] 6. Silane coupling agent: 2.2 parts, KH550 is selected.

[0108] II. Dry crosslinking process

[0109] Step S1: Mixing and Granulation. The base resin, modified epoxy resin, anti-water-tree agent, antioxidant, and silane coupling agent are added to a mixing mill. The mixing temperature is controlled at 100℃, the mixing speed at 65 rpm, and the mixing time at 20 minutes. The mixture is thoroughly mixed to obtain a masterbatch. After naturally cooling to below 60℃, it is fed into a twin-screw extruder along with a crosslinking agent. The masterbatch is then melt-blended, extruded, cooled, and pelletized to obtain insulating material granules. The twin-screw extruder zone temperatures are: feed zone 135℃, melt zone 155℃, homogenization zone 172℃, and die head temperature 175℃; the screw speed is 160 rpm; the length-to-diameter ratio of the twin-screw extruder is 35:1; and the extrusion pressure is 15 MPa.

[0110] Step S2: Extrusion and Preheating. Insulating material granules are fed into an extruder at a processing temperature of 155℃ and an extrusion speed of 10m / min; the conductor outer diameter is 25mm, and the insulation layer thickness is 6mm. After melting, the granules are extruded and coated onto the conductor to form a wire core; the wire core is then preheated in the first heating zone of a dry cross-linking pipeline at 150℃ for 3.5min, with 0.45MPa nitrogen gas introduced as a protective medium.

[0111] Step S3: Stepped crosslinking. The preheated wire core enters the second heating zone, with a first-stage crosslinking temperature of 185℃ and a time of 5.2 min; then it enters the third heating zone, with a second-stage crosslinking temperature of 215℃ and a time of 12 min; the entire crosslinking process is filled with 1.4 MPa of high-purity nitrogen (purity ≥99.99%), the temperature fluctuation of each heating zone is ≤±5℃, and the nitrogen circulation speed is 0.8 m / s.

[0112] Step S4: Accelerated Cooling and Post-processing. The conductor enters the cooling zone. In the first stage, it is forcibly cooled for 9 minutes under 0.9MPa nitrogen pressure using 82℃ hot air circulation at a speed of 1.5m / s, reducing the conductor temperature to 115℃. In the second stage, the pressure is gradually reduced to ambient level, and the conductor is cooled to below 80℃ using 55℃ cold air at a speed of 2.0m / s. In the third stage, it is naturally cooled to room temperature in a clean, windless environment at 26℃ for 45 minutes, ensuring that the temperature difference between the inside and outside of the insulation layer is ≤10℃. After natural cooling, a spark test is performed at 45kV voltage and 1.6s / m duration. No breakdown or flashover indicates a pass. Qualified conductors are wound up to obtain the water-tree resistant high-voltage cable insulation conductor, while unqualified conductors are scrapped.

[0113] Example 4:

[0114] I. Raw material preparation (by weight)

[0115] 1. Base resin: 100 parts, ethylene-vinyl acetate copolymer;

[0116] 2. Modified epoxy resin: 22 parts, bisphenol A type epoxy resin modified with nano-silica particles surface-treated with silane coupling agent KH550, epoxy value 0.50 eq / 100g, the amount of nano-silica added is 14% of the mass of epoxy resin;

[0117] 3. Water-resistant agent: 9 parts, using core-shell structured composite microspheres with an average particle size of 4 μm and a shell to core mass ratio of 6.5:3.5. The core contains self-healing monomers and ion traps, and the shell is a composite material of stimulus-responsive polymer and nanosheet clay.

[0118] 4. Crosslinking agent: 3.2 parts, di-tert-butyl peroxide is selected;

[0119] 5. Antioxidant: 1.4 parts, selected from hindered phenolic antioxidant (antioxidant 1010) and phosphite antioxidant (antioxidant 168) mixed in a mass ratio of 1:1;

[0120] 6. Silane coupling agent: 2.8 parts, KH550 is selected.

[0121] II. Dry crosslinking process

[0122] Step S1: Mixing and Granulation. The base resin, modified epoxy resin, anti-water-tree agent, antioxidant, and silane coupling agent are added to a mixing mill. The mixing temperature is controlled at 100℃, the mixing speed at 65 rpm, and the mixing time at 20 minutes. The mixture is thoroughly mixed to obtain a masterbatch. After naturally cooling to below 60℃, it is fed into a twin-screw extruder along with a crosslinking agent. The masterbatch is then melt-blended, extruded, cooled, and pelletized to obtain insulating material granules. The twin-screw extruder zone temperatures are: feed zone 135℃, melt zone 155℃, homogenization zone 172℃, and die head temperature 175℃; the screw speed is 160 rpm; the length-to-diameter ratio of the twin-screw extruder is 35:1; and the extrusion pressure is 15 MPa.

[0123] Step S2: Extrusion and Preheating. Insulating material granules are fed into an extruder, melted, and extruded to coat the conductor, forming a wire core. The extruder's processing temperature is 155℃, and the extrusion speed is 10m / min. The conductor's outer diameter is 25mm, and the insulation layer thickness is 6mm. The wire core is preheated in the first heating zone of a dry cross-linking pipeline at 150℃ for 3.5min, with 0.45MPa nitrogen gas introduced as a protective medium.

[0124] Step S3: Stepped crosslinking. The preheated wire core enters the second heating zone, with a first-stage crosslinking temperature of 185℃ and a time of 5.2 min; then it enters the third heating zone, with a second-stage crosslinking temperature of 215℃ and a time of 11 min; the entire crosslinking process is filled with 1.4 MPa of high-purity nitrogen (purity ≥99.99%), the temperature fluctuation of each heating zone is ≤±5℃, and the nitrogen circulation speed is 0.8 m / s.

[0125] Step S4: Accelerated Cooling and Post-processing. The conductor enters the cooling zone. In the first stage, it is forcibly cooled for 9 minutes under 0.9MPa nitrogen pressure using 82℃ hot air circulation at a speed of 1.5m / s, reducing the conductor temperature to 115℃. In the second stage, the pressure is gradually reduced to ambient level, and it is cooled to below 80℃ using 55℃ cold air at a speed of 2.0m / s. In the third stage, it is naturally cooled to room temperature in a clean, windless environment at 26℃ for 45 minutes, ensuring that the temperature difference between the inside and outside of the insulation layer is ≤10℃. After natural cooling, a spark test is performed at 48kV voltage and 1.8s / m duration. No breakdown or flashover indicates a pass. Qualified conductors are wound up to obtain the water-tree resistant high-voltage cable insulation conductors, while unqualified conductors are scrapped.

[0126] Comparative Example 1:

[0127] Differences: No water-resistant resin was added in the raw material preparation, while the other raw material components, dry crosslinking process steps and parameters were the same as in Example 3.

[0128] Comparative Example 2:

[0129] Differences: In the raw material preparation, the water-resistant resin was replaced with a single nano-montmorillonite (without using core-shell structured composite microspheres), and the amount used was the same as that of the water-resistant resin in Example 3. The remaining raw material components, dry crosslinking process steps and parameters were the same as those in Example 3.

[0130] Comparative Example 3:

[0131] Differences: In step S3 of the dry crosslinking process, the step-by-step crosslinking is replaced with single-temperature crosslinking, which does not follow the step-by-step crosslinking requirement; the remaining raw material components, dry crosslinking process steps and parameters are consistent with those in Example 3.

[0132] Single-factor experiment:

[0133] The single-factor experiments were based on Example 3, with only one key process parameter changed. The remaining raw material components, process steps, and parameters remained consistent with Example 3. The differences in each experimental parameter were uniform and all within the scope defined in the claims. Five core key process parameters were selected for screening: mixing temperature, first-stage crosslinking temperature, second-stage crosslinking time, nitrogen pressure (crosslinking stage), and hot air cooling temperature. Each single-factor experiment had five groups, and the test indicators were dielectric strength, degree of crosslinking, and tensile strength. The test results were presented in tabular form. The conclusions of each experiment were analyzed, and the optimal parameters were identified.

[0134] 1. Intensive mixing temperature screening experiment

[0135] Experimental objective: To screen the optimal mixing temperature in step S1 of the dry crosslinking process. The mixing temperature range was 80℃-120℃, with a parameter difference of 10℃. Five groups were set up, and the remaining process parameters and raw material components were the same as in Example 3. The results are shown in Table 1:

[0136]

[0137] Conclusion and Analysis: When the mixing temperature is below 100℃, the low temperature leads to insufficient melting of components such as the base resin, modified epoxy resin, and anti-water-tree agent. This results in slow molecular movement, hindering uniform dispersion and weak interfacial bonding, leading to lower dielectric strength, degree of crosslinking, and tensile strength. Furthermore, component agglomeration easily occurs within the material, affecting the uniformity of subsequent crosslinking reactions. When the mixing temperature is above 100℃, the excessive temperature causes slight decomposition of some antioxidants and anti-water-tree agents, damaging the core-shell structure of the anti-water-tree agents and reducing their effectiveness. It also causes localized overheating and carbonization of the masterbatch, resulting in minor defects within the material and a slight decrease in overall performance. In this single-factor experiment, the optimal mixing temperature parameter was 100℃.

[0138] 2. First-stage crosslinking temperature screening experiment

[0139] Experimental objective: To screen the optimal first-stage crosslinking temperature in step S3 of the dry crosslinking process, with a temperature range of 170℃-190℃ and a parameter difference of 5℃. Five groups were set up, and the remaining process parameters and raw material components were consistent with those in Example 3. The results are shown in Table 2:

[0140]

[0141] Conclusion and Analysis: When the first-stage crosslinking temperature is below 185℃, the low temperature slows down the decomposition rate of the crosslinking agent, resulting in a slow and incomplete initiation of the crosslinking reaction. This prevents the formation of a crosslinked network with suitable density within the material, leading to insufficient bonding strength between molecular chains and an inability to effectively improve the material's dielectric and mechanical properties, resulting in lower test values. When the first-stage crosslinking temperature is above 185℃, the excessively high temperature causes rapid decomposition of the crosslinking agent, resulting in an overly rapid crosslinking reaction, uneven crosslinking network formation, and over-crosslinking in some areas, leading to brittleness. Furthermore, the excessively high temperature affects the synergistic effect of the second-stage crosslinking, causing internal stress within the material and a slight decrease in overall performance. In this single-factor experiment, the optimal parameter for the first-stage crosslinking temperature is 185℃.

[0142] 3. Second-stage crosslinking time screening experiment

[0143] Experimental objective: To screen the optimal second-stage crosslinking time in step S3 of the dry crosslinking process, with a time range of 10-14 min and a parameter difference of 1 min. Five groups were set up, and the remaining process parameters and raw material components were the same as in Example 3. The results are shown in Table 3:

[0144]

[0145] Conclusion and Analysis: The second-stage crosslinking, as the core stage of the step-crosslinking process, mainly functions to perfect the crosslinking network and improve crosslinking uniformity. When the second-stage crosslinking time is 12 minutes, the crosslinking reaction is sufficient and uniform, the crosslinking network density is appropriate, and the dielectric strength, degree of crosslinking, and tensile strength of the material all reach their optimal values. When the crosslinking time exceeds 12 minutes, with the extension of time, excessive crosslinking leads to increased rigidity of the material molecular chains, decreased flexibility, and a gradual decrease in tensile strength and dielectric strength, while the degree of crosslinking decreases slightly (due to the breakage of some crosslinking bonds caused by excessive crosslinking). When the crosslinking time is less than 12 minutes, the crosslinking reaction is insufficient, the crosslinking network is incomplete, and all performance indicators are lower than the optimal values. In this single-factor experiment, the optimal parameter for the second-stage crosslinking time is 12 minutes.

[0146] 4. Nitrogen pressure screening experiment during the cross-linking stage

[0147] Experimental objective: To screen the optimal nitrogen pressure (crosslinking stage) in step S3 of the dry crosslinking process, with a pressure range of 0.8-1.6 MPa and a parameter difference of 0.2 MPa. Five groups were set up, and the remaining process parameters and raw material components were consistent with those in Example 3. The results are shown in Table 4:

[0148]

[0149] Conclusion and Analysis: Nitrogen gas in the crosslinking stage mainly plays a role in protection, pressure transmission, and promoting the uniformity of crosslinking. When the nitrogen pressure is less than 1.4 MPa, insufficient pressure leads to uneven pressure difference between the core surface and interior, inconsistent crosslinking reaction rates, incomplete internal crosslinking, and ineffective air isolation, resulting in surface oxidation and lower performance indicators. When the nitrogen pressure is greater than 1.4 MPa, excessive pressure increases the equipment load and slightly compresses the core insulation layer, affecting the uniformity of insulation thickness, which in turn leads to a slight decrease in dielectric strength and tensile strength, while the degree of crosslinking remains basically stable. In this single-factor experiment, the optimal nitrogen pressure parameter in the crosslinking stage is 1.4 MPa.

[0150] 5. Hot air cooling temperature screening experiment

[0151] Experimental objective: To screen the optimal hot air cooling temperature in step S4 of the dry crosslinking process, with a temperature range of 80℃-84℃ and a parameter difference of 1℃. Five groups were set up, and the remaining process parameters and raw material components were the same as in Example 3. The results are shown in Table 5:

[0152]

[0153] Conclusion and Analysis: The core function of hot air cooling is to control the cooling rate of the wire core, avoiding excessive temperature differences between the inside and outside of the insulation layer, which can cause internal stress and cracking defects. When the hot air cooling temperature is 82℃, the cooling rate is suitable, the cross-linked network inside the insulation layer can be stably formed, there is no accumulation of internal stress, and the dielectric strength, degree of cross-linking, and tensile strength all reach their optimal levels. When the temperature is below 82℃, the cooling is too fast, the temperature difference between the inside and outside of the insulation layer exceeds 10℃, and micro-cracks are generated inside, leading to a slight decrease in various properties. When the temperature is above 82℃, the cooling is too slow, the cross-linking reaction continues excessively, some cross-linking bonds age and break, and the performance indicators slightly decline. In this single-factor experiment, the optimal parameter for hot air cooling temperature is 82℃.

[0154] Performance index testing:

[0155] The water-tree resistant high-voltage cable insulation cores prepared in Examples 1-4 and Comparative Examples 1-3 were used as test samples. Various performance indicators were tested according to national standards and industry specifications. The testing environment was uniformly set at 26℃, clean and windless, with a relative humidity of 50%±5%. Each sample was tested in triplicate, and the average value was taken as the final test result. The test indicators and specific data are as follows:

[0156] 1. List of test results

[0157] The test samples included 4 examples and 3 comparative examples, and the results are shown in Table 6.

[0158]

[0159] 3. Data Comparison and Theoretical Analysis

[0160] Based on the above test data, an objective comparison of the performance indicators of the embodiments and comparative examples is conducted, and a theoretical analysis is carried out in conjunction with the technical principles, as detailed below:

[0161] (1) Comparison and analysis of anti-water tree performance: The anti-water tree performance of Examples 1-4 is qualified, Comparative Example 1 is unqualified, and Comparative Examples 2 and 3 are qualified but have no obvious advantage. Theoretically, the core-shell structured composite microsphere anti-water tree agent selected in this invention contains a self-healing monomer and an ion capture agent in its core, which can capture impurity ions inside the insulation layer and prevent ion aggregation to form the core of water tree growth. The stimulation-responsive polymer and nanosheet clay composite material of the outer shell can form a barrier layer in the early stage of water tree germination and inhibit water tree extension. Comparative Example 1 did not add anti-water tree agent, so the ions inside the insulation layer could not be captured, and the water tree could grow rapidly, resulting in failure of the 1000h aging test. Comparative Example 2 used a single nano-montmorillonite to replace the core-shell structured composite microsphere, which could only play a simple physical barrier role and could not achieve ion capture and water tree self-repair. The anti-water tree effect was weaker than all examples. Comparative Example 3 did not use a step-by-step crosslinking, so the crosslinking network was uneven and there were tiny pores inside the insulation layer. Water could easily penetrate and form water trees. The anti-water tree effect was not as good as the examples.

[0162] (2) Comparison and analysis of dielectric strength: The dielectric strength range of Examples 1-4 is 32.2kV / mm-38.3kV / mm, and the dielectric strength range of Comparative Examples 1-3 is 28.0kV / mm-32.0kV / mm. The overall dielectric strength of Examples is higher than that of Comparative Examples. Among them, Example 3 has the highest dielectric strength, reaching 38.3kV / mm, which is 10.3kV / mm higher than Comparative Example 1, 8.3kV / mm higher than Comparative Example 2, and 6.3kV / mm higher than Comparative Example 3. Theoretically, dielectric strength is directly related to the component uniformity and cross-linking network density of the insulating material. This invention improves the compatibility between the base resin and each component by adding modified epoxy resin (treated with nano-silica surface), avoiding component agglomeration and dielectric defects. The step-crosslinking process can form a cross-linking network with uniform density and stable structure, reducing porosity and defects inside the insulating layer, thereby improving dielectric strength. Comparative Example 1 lacks anti-water-tree agents, which easily form ion aggregation regions inside, resulting in decreased dielectric performance. Comparative Example 2 has poor anti-water-tree agent effect, and the component compatibility is not as good as the core-shell structure microspheres of this invention. Comparative Example 3's single-temperature cross-linking results in an uneven cross-linking network with local areas of insufficient or excessive cross-linking, and the dielectric strength is lower than that of the examples.

[0163] (3) Comparison and analysis of crosslinking degree: The crosslinking degree of Examples 1-4 ranged from 85.3% to 92.5%, while that of Comparative Examples 1-3 ranged from 83.0% to 88.0%. The overall crosslinking degree of the Examples was higher than that of the Comparative Examples. The crosslinking degree of Example 3 was the highest (92.5%), which was 9.5% higher than that of Comparative Example 3, and 4.5% and 5.5% higher than that of Comparative Examples 1 and 2, respectively. Theoretically, the crosslinking degree is mainly determined by the crosslinking process and the synergistic effect of the crosslinking agent. The step-crosslinking process adopted in this invention allows the crosslinking agent to be evenly dispersed and the crosslinking reaction to be initially initiated in the first stage of low-temperature slow crosslinking. The crosslinking network can be improved in the second stage of high-temperature crosslinking, thereby improving the crosslinking degree. The synergistic effect of the modified epoxy resin and the silane coupling agent can promote the decomposition efficiency of the crosslinking agent and further improve the crosslinking degree. The single-temperature crosslinking of Comparative Example 3 could not achieve the gradual improvement of the crosslinking reaction, and the crosslinking degree was the lowest. Although the crosslinking process of Comparative Examples 1 and 2 was the same as that of Example 3, the compatibility of the components was slightly worse, which resulted in the crosslinking agent not playing its role fully and the crosslinking degree was lower than that of the Examples.

[0164] (4) Comparison and analysis of tensile strength and elongation at break: The tensile strength of Examples 1-4 ranged from 18.2 MPa to 21.3 MPa, and the elongation at break ranged from 412.4% to 461.2%; the tensile strength of Comparative Examples 1-3 ranged from 15.0 MPa to 17.0 MPa, and the elongation at break ranged from 360.5% to 380.3%. The mechanical properties of the Examples were generally better than those of the Comparative Examples. Theoretically, tensile strength and elongation at break depend on the stability of the crosslinking network and the bonding force between components. The modified epoxy resin of this invention can enhance the rigidity of the base resin, the core-shell structure anti-water treeing agent can improve the toughness of the material, and the uniform crosslinking network formed by step-crosslinking can distribute stress evenly and avoid local stress concentration leading to fracture. Comparative Example 1 lacks anti-water treeing agent, the bonding force between components is insufficient, and the mechanical properties are reduced. Comparative Example 2 has poor dispersion of single nano-montmorillonite, which easily forms stress concentration points, and the tensile strength and elongation at break are lower than those of the example. Comparative Example 3 has an uneven crosslinking network formed by single temperature crosslinking, and fracture is prone to occur in areas with insufficient crosslinking, resulting in the worst mechanical properties.

[0165] In summary, through comparison of performance data of the embodiments and comparative examples, combined with theoretical analysis, it can be seen that the water-tree resistant high-voltage cable insulation core prepared by the present invention, through the reasonable combination of components such as base resin, modified epoxy resin, and core-shell structure anti-water-tree agent, and in conjunction with a step-by-step dry crosslinking process, is superior to the comparative example that did not adopt the technical solution of the present invention in terms of anti-water-tree performance, dielectric strength, degree of crosslinking, and mechanical properties. The synergistic improvement of various performance indicators proves that the technical solution of the present invention has technological progress, and its performance is stable, repeatable, and meets the requirements for the use of high-voltage cable insulation materials.

[0166] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various substitutions or modifications can be made to these described embodiments without departing from the inventive concept, and all such substitutions or modifications should be considered within the scope of protection of the present invention.

[0167] Although the invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and modifications can be made without departing from the spirit and scope of the invention. Furthermore, the scope of the invention is not limited to the specific embodiments of the processes, methods, and steps described in the specification. From the disclosure of this invention, those skilled in the art will readily utilize existing or future processes, methods, steps that substantially perform the same function or achieve the same results as the corresponding embodiments described herein. Therefore, the appended claims are intended to cover such processes, methods, steps.

Claims

1. A water-tree resistant high-voltage cable insulation material, characterized in that, By weight, it includes the following components: The composition comprises: base resin: 100 parts; modified epoxy resin: 5-25 parts; anti-water-tree agent: 1-10 parts; crosslinking agent: 1.5-3.5 parts; antioxidant: 0.1-1.5 parts; silane coupling agent: 0.5-3 parts. The base resin is low-density polyethylene or ethylene-vinyl acetate copolymer. The anti-water-tree agent is a core-shell structured composite microsphere, the core containing a self-healing monomer and an ion trapping agent, and the outer shell being a composite material composed of a stimulus-responsive polymer and nanosheet clay. The average particle size of the core-shell structured composite microsphere is 100 nm-5 μm, and the mass ratio of the outer shell to the core is 4:6 to 7:

3. The modified epoxy resin is a bisphenol A type epoxy resin modified with nano-silica particles surface-treated with a silane coupling agent, with an epoxy value of 0.40-0.52 eq / 100g, and the amount of nano-silica added is 3%-15% of the epoxy resin mass.

2. The water-tree resistant high-voltage cable insulation material according to claim 1, characterized in that, In the core-shell structured composite microspheres, the self-healing monomer is a polymerizable monomer containing Diels-Alder addition bonds, disulfide bonds, or quadruple hydrogen bonds UPy units; the ion scavenger is a calixarene or crown ether derivative; the stimulus-responsive polymer is poly(N-isopropylacrylamide) or polyvinylcaprolactam; and the nanosheet clay is kaolin or montmorillonite modified with a silane coupling agent.

3. The water-tree resistant high-voltage cable insulation material according to claim 1, characterized in that, The crosslinking agent is one or more of dicumyl peroxide, benzoyl peroxide, or di-tert-butyl peroxide; the antioxidant is one or more of hindered phenolic antioxidants, phosphite antioxidants, or thioester antioxidants.

4. The water-tree resistant high-voltage cable insulation material according to claim 1, characterized in that, The nanosheet clay has a particle size of 50-200 nm and an interlayer spacing of 1.5-3 nm.

5. A dry crosslinking process for a water-tree resistant high-voltage cable insulation material according to any one of claims 1-4, characterized in that, Includes the following steps: S1. Mixing and Granulation: The base resin, modified epoxy resin, anti-water-tree agent, antioxidant, and silane coupling agent are mixed at 80℃-120℃ for 15-30 minutes to obtain a uniformly mixed masterbatch. The masterbatch is then cooled to below 60℃ and fed together with the crosslinking agent into a twin-screw extruder for melt blending, extrusion, cooling, and pelletizing to obtain insulating material granules. The processing temperature of the twin-screw extruder is: feeding zone 120℃-140℃, melting zone 140℃-160℃, homogenization zone 160℃-175℃, and die head temperature 165℃-180℃; the screw speed is 80-200 rpm. S2: Extrusion and preheating: Insulating material particles are extruded through an extruder and coated onto a conductor to form a wire core; the wire core is passed into a dry cross-linking pipe with a first heating zone for preheating. The temperature of the first heating zone is controlled at 130℃-160℃ for 3-4 minutes. Nitrogen gas at 0.3-0.5MPa is introduced as a protective medium during the preheating process. S3. Stepped crosslinking: After preheating, the wire core enters the second heating zone of the dry crosslinking pipeline for the first stage of crosslinking, with the temperature controlled at 170℃-190℃ and the time at 4-6 minutes; subsequently, the wire core enters the third heating zone for the second stage of crosslinking, with the temperature controlled at 205℃-215℃ and the time at 10-14 minutes; the dry crosslinking pipeline is filled with high-purity nitrogen gas of 0.8-1.6 MPa throughout the second and third heating zones as a protective and pressure-transmitting medium, with a nitrogen purity ≥99.99%; S4. Accelerated Slow Cooling and Post-processing: The cross-linked core enters the cooling zone of the dry cross-linking pipeline for accelerated step-by-step slow cooling: First, under nitrogen pressure of 0.5-1.0MPa, the core is forcibly cooled for 8-12 minutes using hot air circulation at 80℃-84℃, rapidly reducing its temperature to 110℃-130℃; then, the pressure is gradually reduced to atmospheric pressure, and cold air at 40℃-60℃ is used to continue cooling to below 80℃; finally, the core is placed in a clean, air-conditioning environment at 20℃-30℃ with no wind to cool naturally to room temperature; after spark testing and winding, the water-tree resistant high-voltage cable insulated core is obtained; the spark test voltage is 30-50kV, the test time is 1-2s / m, and the test qualification standard is no breakdown and no flashover.

6. The dry crosslinking process for the water-tree resistant high-voltage cable insulation material according to claim 5, characterized in that, In step S1, the mixing speed is 50-80 rpm; the length-to-diameter ratio of the twin-screw extruder is 30:1-40:1, and the extrusion pressure is 10-20 MPa.

7. The dry crosslinking process for the water-tree resistant high-voltage cable insulation material according to claim 5, characterized in that, In step S2, the extruder's processing temperature is 140℃-170℃, and the extrusion speed is 5-15m / min; the conductor outer diameter of the wire core is 10-50mm, and the insulation layer thickness is 2-10mm.

8. The dry crosslinking process for the water-tree resistant high-voltage cable insulation material according to claim 5, characterized in that, In step S3, the temperature fluctuation range of the first heating zone, the second heating zone, and the third heating zone shall not exceed ±5℃; the circulation speed of nitrogen is 0.5-1m / s.

9. The dry crosslinking process for the water-tree resistant high-voltage cable insulation material according to claim 5, characterized in that, In step S4, the wind speed of the hot air circulation is 1-2 m / s, and the wind speed of the cold air circulation is 1.5-2.5 m / s; the natural cooling time is 30-60 min, and the temperature difference between the inside and outside of the insulation layer after cooling is ≤10℃.

10. The dry crosslinking process according to claim 5, characterized in that, The mixing temperature in step S1 is 90℃-110℃.

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