A method for constructing a cable joint interface having a non-homogeneous three-layer sandwich structure and applications thereof

Abstract

The application relates to a construction method and application of a cable joint interface with a non-homogeneous three-layer sandwich structure, and belongs to the technical field of cable joint materials. Solvent, grafting monomers, a photocatalyst and a chain transfer agent are layered and coated on the surface of crosslinked polyethylene, and grafting growth occurs under ultraviolet lamp irradiation; through the photocatalytic grafting technology, a transition layer composed of long chains of polar methyl methacrylate is grown in situ on the surface of the crosslinked polyethylene base, and secondary crosslinking is carried out with molten polyethylene, so that a non-homogeneous three-layer sandwich structure is formed. The structure is different from physical blending, one end of the acrylate segment is firmly anchored on the crosslinked polyethylene through a covalent bond, and the other end is extended through the chain transfer agent to realize length-controllable extension, so that a dense polar molecular network is formed at the interface. The cable joint with the non-homogeneous three-layer sandwich structure is used as high-voltage direct-current cable joint reinforced insulation performance.

Description

Technical Field

[0001] This invention belongs to the field of cable joint material technology, specifically relating to a method for constructing and applying a cable joint interface with a heterogeneous three-layer sandwich structure. Background Technology

[0002] Power cable joints are the core components of power transmission systems, and their insulation reliability directly determines the operational safety of the lines. In the production of cable joints, extrusion injection molding is typically used, where new insulating material is injected into the outer side of the main insulation after the shielding layer has been removed to restore the insulation structure. However, due to limitations imposed by temperature differences, viscosity variations, and interfacial compatibility during the melting and fusion process between the injected insulating material and the cable's main insulation, complete molecular-scale fusion is often difficult to achieve.

[0003] At the junction of the restored insulation formed by the injected insulation and the cable body insulation, a special region with discontinuous physical and electrical properties is generated, called the "transition insulation zone". This region is the weakest part of the entire joint structure: on the one hand, the transition insulation zone has microscopic pores and a large free volume, which easily leads to local electric field distortion and charge accumulation; on the other hand, the interfacial bonding force in this region is weak, and under the mechanical stress of thermal expansion and contraction, it is very easy to generate microcracks or delamination, thereby inducing interfacial breakdown.

[0004] Currently, conventional technologies mainly improve fusion effects by increasing injection pressure or extending heat treatment time, but these methods cannot fundamentally solve the problems of restricted molecular chain movement and abrupt performance changes at the interface. Modifying the microstructure to construct a buffer structure with high bonding strength and smooth performance transition in the transition insulation region, thereby eliminating weak points in the joint insulation, has become a key technical problem urgently needing to be solved in the field of high-voltage cable accessories. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for constructing and applying a cable joint interface with a heterogeneous three-layer sandwich structure.

[0006] The method of the present invention can optimize the space charge distribution and introduce deep traps to capture charges; it can improve the breakdown field strength and enhance the insulation performance of the joint of submarine cross-linked polyethylene cable; it can improve the adhesion at the interface and enhance the fusion effect of the interface.

[0007] This invention involves layering a solvent, grafting monomer, photocatalyst, and chain transfer agent onto the surface of cross-linked polyethylene, and then allowing grafting growth to occur under ultraviolet light irradiation. The solvent is 1,4-dioxane; the polar molecule is methyl methacrylate; the photocatalyst is benzophenone; and the chain transfer agent is 2-cyano-2-propyldodecyl trithiocarbonate.

[0008] This invention utilizes photocatalytic grafting technology to grow a transition layer composed of long polar methyl methacrylate chains in situ on the surface of a cross-linked polyethylene (PE) matrix, and then performs secondary cross-linking with molten polyethylene to form a heterogeneous three-layer sandwich structure (PE-g-PMMA-PE). Unlike physical blending, this structure features acrylate segments with one end firmly anchored to the PE matrix via covalent bonds, and the other end extended in a controllable length using a chain transfer agent, thus forming a dense polar molecular network at the interface.

[0009] A method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure is specifically completed according to the following steps: I. Preparation of cross-linked polyethylene: Di-tert-butyl peroxide isopropylbenzene is mixed evenly with polyethylene particles, then transferred into a mold. The mold is then moved into a flat vulcanizing machine, the mold is closed, and matching vulcanizing pressure and temperature parameters are set. One vulcanization is completed within a preset time to obtain cross-linked polyethylene. II. Preparation of surface-modified cross-linked polyethylene: ① Add methyl methacrylate dropwise at a constant rate to 1,4-dioxane under constant temperature and stirring to obtain mixed solution I; add benzophenone powder to mixed solution I under constant temperature and stirring to obtain mixed solution II; add 2-cyano-2-propyldodecyl trithiocarbonate dropwise at a constant rate to mixed solution II under constant temperature and stirring to obtain mixed solution III; ② Stir the mixed solution III for a period of time and place it in a dark environment to prevent premature reaction; ③ The mixed solution III is evenly coated on the surface of the cross-linked polyethylene, and then irradiated under a 365nm ultraviolet lamp. During the irradiation, a layer of mixed solution III is coated at intervals. Then it is placed in acetone and stirred for cleaning. Finally, it is placed in a vacuum oven for degassing to obtain surface-modified cross-linked polyethylene. III. Constructing a cable joint interface with a heterogeneous three-layer sandwich structure: ① Mix di-tert-butyl peroxide isopropylbenzene with polyethylene particles evenly to obtain a mixture; spread the mixture evenly on the modified surface of the surface-modified cross-linked polyethylene, then transfer it into a mold, move the mold into a flat vulcanizing machine, close the mold and set the matching vulcanizing pressure and temperature parameters, vulcanize within a preset time to form a three-layer cross-linked polyethylene with an intermediate layer modified. ② The modified three-layer cross-linked polyethylene in the middle layer is placed in a vacuum oven for degassing to obtain a cable joint with a heterogeneous three-layer sandwich structure, thus completing a method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure.

[0010] A cable joint with a heterogeneous three-layer sandwich structure is used as a high-voltage DC cable joint to enhance insulation performance.

[0011] The principle of this invention: This invention innovates on improving the interfacial insulation capacity of cross-linked polyethylene (XLPE) and controls the key parameter of chain transfer agent content to enable XLPE to meet the requirements under different conditions. The invention involves adding methyl methacrylate dropwise at a constant rate to 1,4-dioxane under constant temperature and stirring to obtain mixed solution I; adding benzophenone powder to mixed solution I under constant temperature and stirring to obtain mixed solution II; and adding 2-cyano-2-propyldodecyl trithiocarbonate dropwise at a constant rate to mixed solution II under constant temperature and stirring to obtain mixed solution III. Finally, the mixed solution III was layered and coated onto the surface of cross-linked polyethylene. Under ultraviolet irradiation, long chains of methyl methacrylate were grown. The specific mechanism was as follows: (1) Under the photocatalytic action of benzophenone and 2-cyano-2-propyldodecyl trithiocarbonate, methyl methacrylate grew into long chains of polymethyl methacrylate on the surface of polyethylene. The long chains of polymethyl methacrylate provided enough sites for linking with polyethylene. (2) At the microscopic level, the in-situ grown acrylate chains effectively filled the micropores and free volume at the polyethylene interface. Due to the volume filling effect of the acrylate monomer during the polymerization process and the compression of the local free volume by the grafted chain segments, the mean free path of electrons under the action of the electric field was significantly reduced. This free volume compression mechanism effectively suppressed the acceleration of charge carriers at the interface, thereby blocking the formation of electrical breakdown channels from a physical structure perspective. (3) In this preparation method, methyl methacrylate can provide a large number of cross-linking sites, enhance the interface stability, and generate strong polar-nonpolar intermolecular interactions between the polar acrylate chains grown on the surface and the polyethylene molecular chains in the molten state. This force acts as a molecular anchor, achieving a strong constraint and fixation effect on the movement of the polyethylene long chains. At high temperatures, this fixation inhibits the untangling of PE chain segments, improving the thermodynamic stability of the interface. Simultaneously, it provides numerous charge traps, modulates the space charge distribution, making it more stable, reducing electric field distortion, and increasing the difficulty of charge migration, thereby improving breakdown strength and insulation performance. Results show that the prepared surface-modified crosslinked polyethylene can achieve a breakdown voltage of 42 kV at 30 °C.

[0012] Compared with the prior art, the present invention has the following advantages: I. This invention constructs a heterogeneous three-layer integrated structure, significantly improving interfacial bonding strength. This invention utilizes in-situ surface grafting and thermally induced molecular diffusion technology to construct a heterogeneous three-layer sandwich structure of "polyethylene-acrylate-polyethylene". This structure breaks through the limitations of traditional surface modification, which is limited to physical adsorption. Through the deep entanglement and covalent bond anchoring of the long chains of methyl methacrylate and the molecular chains of the polyethylene matrix at the interface, the integrity and mechanical peel strength of the material are greatly enhanced, effectively solving the problem of easy delamination in multilayer composite insulating materials during long-term service.

[0013] Second, this invention achieves a gradient transition in dielectric constant, effectively smoothing the interfacial electric field distribution. Unlike the abrupt change in dielectric constant caused by traditional modified layers, this invention utilizes the concentration gradient distribution of acrylate components in the depth direction to form a buffer layer between two layers of polyethylene, resulting in a smooth evolution of the dielectric constant. This gradient impedance matching effect effectively suppresses Maxwell-Wagner polarization accumulation of charge carriers at the interface, significantly alleviating electric field stress distortion at the interface, thereby improving the overall breakdown field strength of the composite system.

[0014] Third, this invention introduces high-density deep trap energy levels to synergistically suppress space charge injection and migration. By introducing long chains of methyl methacrylate containing strongly polar ester groups at the interface, a high-density acceptor-type deep trap is constructed in the polyethylene interface region. These deep traps can effectively capture high-energy hot electrons, converting their kinetic energy into potential energy and forming a space charge shielding layer. This significantly suppresses further injection of electrode charge, reduces interfacial conductivity loss, and improves the insulation stability of the material under strong fields.

[0015] Fourth, the molecular anchoring effect significantly enhances the stability of high-temperature electrical properties. The long methyl methacrylate chains grown in this invention exert strong intermolecular forces and physical constraints on the molten polyethylene molecular chains. This molecular anchoring effect restricts the micro-Brownian motion of the polyethylene chain segments at high temperatures and inhibits free volume expansion under thermal stimulation. Experiments show that this structure maintains low dielectric loss and high breakdown stability even at high temperatures, thus broadening the operating temperature range of polyethylene insulation materials.

[0016] V. This invention achieves precise filling of free volume, improving discharge resistance at the interface. Utilizing the spatial confinement filling effect of methyl methacrylate monomers during in-situ polymerization, this invention effectively compresses the micropores and free volume at the polyethylene interface. This densification of the microstructure significantly shortens the mean free path of electrons under an electric field, increases the initiation discharge voltage, and physically constructs a dense barrier to prevent electrical tree growth and charge penetration. Attached Figure Description

[0017] Figure 1 The Weibull distribution diagrams of the interface breakdown field strength of the cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1 at 30°C are shown. Figure 2 The graph shows the change of electrical conductivity current with electric field intensity at 30°C for the cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1. Figure 3The graph shows the change of electrical conductivity current with electric field intensity at 50°C for the cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1. Figure 4 The graph shows the change of electrical conductivity current with electric field intensity at 70°C for the cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1. Figure 5 Infrared spectra of cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1; Figure 6 The XRD patterns of cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1 are shown. Figure 7 The diagram shows the structure of the cable connector with a heterogeneous three-layer sandwich structure in Examples 1-3. Detailed Implementation

[0018] Specific Implementation Method 1: This implementation method is a method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure, specifically completed according to the following steps: I. Preparation of cross-linked polyethylene: Di-tert-butyl peroxide isopropylbenzene is mixed evenly with polyethylene particles, then transferred into a mold. The mold is then moved into a flat vulcanizing machine, the mold is closed, and matching vulcanizing pressure and temperature parameters are set. One vulcanization is completed within a preset time to obtain cross-linked polyethylene. II. Preparation of surface-modified cross-linked polyethylene: ① Add methyl methacrylate dropwise at a constant rate to 1,4-dioxane under constant temperature and stirring to obtain mixed solution I; add benzophenone powder to mixed solution I under constant temperature and stirring to obtain mixed solution II; add 2-cyano-2-propyldodecyl trithiocarbonate dropwise at a constant rate to mixed solution II under constant temperature and stirring to obtain mixed solution III; ② Stir the mixed solution III for a period of time and place it in a dark environment to prevent premature reaction; ③ The mixed solution III is evenly coated on the surface of the cross-linked polyethylene, and then irradiated under a 365nm ultraviolet lamp. During the irradiation, a layer of mixed solution III is coated at intervals. Then it is placed in acetone and stirred for cleaning. Finally, it is placed in a vacuum oven for degassing to obtain surface-modified cross-linked polyethylene. III. Constructing a cable joint interface with a heterogeneous three-layer sandwich structure: ① Mix di-tert-butyl peroxide isopropylbenzene with polyethylene particles evenly to obtain a mixture; spread the mixture evenly on the modified surface of the surface-modified cross-linked polyethylene, then transfer it into a mold, move the mold into a flat vulcanizing machine, close the mold and set the matching vulcanizing pressure and temperature parameters, vulcanize within a preset time to form a three-layer cross-linked polyethylene with an intermediate layer modified. ② The modified three-layer cross-linked polyethylene in the middle layer is placed in a vacuum oven for degassing to obtain a cable joint with a heterogeneous three-layer sandwich structure, thus completing a method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure.

[0019] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that: the mass of di-tert-butylperoxyisopropylbenzene mentioned in step one is 1.5% to 2.5% of the mass of the polyethylene particles; the particle size of the polyethylene particles mentioned in step one is 0.5 mm; and the mold size mentioned in step one is 10 cm × 10 cm × 200 μm. Other steps are the same as in Specific Implementation Method One.

[0020] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that: the pressure of the flat vulcanizing machine in step one is 15 MPa; the vulcanization temperature in step one is 175°C; and the vulcanization time in step one is 30 minutes. Other steps are the same as in Specific Implementation Method One or Two.

[0021] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in the following ways: the mass ratio of 1,4-dioxane, methyl methacrylate, benzophenone, and 2-cyano-2-propyldodecyl trithiocarbonate in step two① is 1:100:1:(1~9); the dropping rate in step two① is 45~50 drops / min; the stirring speed in step two① is 2.5r / s~2.8r / s, and the stirring time is 10min~15min; the constant temperature in step two① is 25℃~30℃. Other steps are the same as in Specific Implementation Methods One to Three.

[0022] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that: the stirring speed in step two ② is 2.5 r / s to 2.8 r / s, and the stirring time is 25 min to 30 min; the mass ratio of the mixed solution III coated each time to the surface area of ​​the cross-linked polyethylene in step two ③ is 0.2 g: 100 cm². 2 The irradiation time described in step 2③ is 1h~1.5h; during the irradiation period, a layer of mixed solution Ⅲ is coated at 10min intervals. Other steps are the same as in specific embodiments one to four.

[0023] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in the following ways: the irradiation distance in step two ③ is 10 cm; the degassing time in step two ③ is 40-50 hours; the temperature of the vacuum oven in step two ③ is 70°C; the stirring and cleaning time in step two ③ is 30-40 minutes, and the stirring and cleaning speed is 2.5-2.8 r / s. The other steps are the same as in Specific Implementation Methods One to Five.

[0024] Specific Implementation Method Seven: The difference between this implementation method and Specific Implementation Methods One through Six is ​​that the mass ratio of the mixture mentioned in step three① to the surface area of ​​the surface-modified cross-linked polyethylene is 2.6g:100cm². 2 The mass of di-tert-butylperoxyisopropylbenzene mentioned in step 3① is 1.5% to 2.5% of the mass of the polyethylene particles; the particle size of the polyethylene particles mentioned in step 3① is 0.5 mm; the mold size mentioned in step 3① is 10 cm × 10 cm × 400 μm. Other steps are the same as in specific embodiments one to six.

[0025] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in the following ways: the pressure of the flat vulcanizing machine in step three ① is 15 MPa; the vulcanization temperature in step three ① is 175°C; the vulcanization time in step three ① is 30 min; the degassing time in step three ② is 40 h to 50 h; and the temperature of the vacuum oven in step three ② is 70°C. Other steps are the same as in Specific Implementation Methods One to Seven.

[0026] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that: the cable joint with the heterogeneous three-layer sandwich structure described in step three ② has a breakdown electric field strength of 126~155KV / mm at 30℃; 115~143KV / mm at 50℃; and 110~139KV / mm at 70℃; at room temperature, the tensile strength can reach 12.06MPa. Other steps are the same as in Specific Implementation Methods One to Eight.

[0027] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods One to Nine in that it uses a cable connector with a non-homogeneous three-layer sandwich structure as a high-voltage DC cable connector to enhance insulation performance. The other steps are the same as in Specific Implementation Methods One to Nine.

[0028] The beneficial effects of the present invention are verified using the following embodiments: Example 1: A method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure, specifically completed according to the following steps: I. Preparation of cross-linked polyethylene: Di-tert-butyl peroxide isopropylbenzene was mixed evenly with 2.6g of polyethylene granules and then transferred into a mold. The mold was then moved into a flat vulcanizing machine, the mold was closed, and the matching vulcanizing pressure and temperature parameters were set. One vulcanization was completed within a preset time. The vulcanization temperature was 175℃ and the vulcanization time was 30min to obtain cross-linked polyethylene (XLPE). The mass of di-tert-butylperoxyisopropylbenzene mentioned in step one is 2% of the mass of the polyethylene particles; The polyethylene particles mentioned in step one have a particle size of 0.5 mm; The mold size mentioned in step one is 10cm × 10cm × 200μm; The pressure of the flat vulcanizing machine described in step one is 15 MPa; II. Preparation of surface-modified cross-linked polyethylene: ① Add methyl methacrylate dropwise at a constant rate to 1,4-dioxane under constant temperature and stirring to obtain mixed solution I; add benzophenone powder to mixed solution I under constant temperature and stirring to obtain mixed solution II; add 2-cyano-2-propyldodecyl trithiocarbonate dropwise at a constant rate to mixed solution II under constant temperature and stirring to obtain mixed solution III; The mass ratio of 1,4-dioxane, methyl methacrylate, benzophenone, and 2-cyano-2-propyldodecyl trithiocarbonate in step 2① is 1:100:1:1; The dripping rate described in step 2① is 50 drops / min; The stirring speed mentioned in step 2① is 2.8 r / s, and the stirring time is 15 min; The constant temperature mentioned in step 2① is 25℃; ② Stir the mixed solution III for a period of time and place it in a dark environment to prevent premature reaction; The stirring speed mentioned in step 2② is 2.8 r / s, and the stirring time is 30 min; ③ Apply 0.2g of mixed solution III evenly to the surface of cross-linked polyethylene, and then irradiate it under a 365nm ultraviolet lamp for 10min; The irradiation distance mentioned in step 2③ is 10cm; ④ Repeat step 2. ③ 5 times, then place in acetone and stir and wash for 30 minutes. Finally, place in a vacuum oven at 70°C for 48 hours to degas, and obtain surface-modified cross-linked polyethylene. The stirring and cleaning speed described in step 2④ is 2.8 r / s; III. Constructing a cable joint interface with a heterogeneous three-layer sandwich structure: ① Mix di-tert-butylperoxyisopropylbenzene with polyethylene particles evenly to obtain a mixture; spread the mixture evenly on the modified surface of the surface-modified cross-linked polyethylene, then transfer it into a mold, move the mold into a flat vulcanizing machine, close the mold and set the matching vulcanizing pressure and temperature parameters, vulcanize within a preset time, the vulcanizing temperature is 175℃ and the vulcanizing time is 30min, forming a three-layer cross-linked polyethylene with an intermediate layer modified. The mass ratio of the mixture described in step 3① to the surface area of ​​the surface-modified cross-linked polyethylene is 2.6 g: 100 cm³. 2 ; The mass of di-tert-butylperoxyisopropylbenzene mentioned in step 3① is 2% of the mass of the polyethylene particles; The polyethylene particles mentioned in step 3① have a particle size of 0.5 mm; The mold size mentioned in step 3① is 10cm×10cm×400μm; Step 3① The pressure of the flat vulcanizing machine is 15MPa; ② The modified three-layer cross-linked polyethylene in the middle layer is placed in a vacuum oven at 70°C for 48 hours to degas, and a cable joint with a heterogeneous three-layer sandwich structure is obtained (denoted as 1%), thus completing a method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure.

[0029] Example 2: The difference between this example and Example 1 is that the mass ratio of 1,4-dioxane, methyl methacrylate, benzophenone, and 2-cyano-2-propyldodecyl trithiocarbonate in step 2① is 1:100:1:3; the cable connector with a heterogeneous three-layer sandwich structure obtained in step 3② is 3%. Other steps and parameters are the same as in Example 1.

[0030] Example 3: The difference between this example and Example 1 is that the mass ratio of 1,4-dioxane, methyl methacrylate, benzophenone, and 2-cyano-2-propyldodecyl trithiocarbonate in step 2① is 1:100:1:9; the cable connector with a heterogeneous three-layer sandwich structure obtained in step 3② is denoted as 9%. Other steps and parameters are the same as in Example 1.

[0031] Comparative Example 1: The difference between this example and Example 1 is that the mass ratio of 1,4-dioxane, methyl methacrylate, benzophenone, and 2-cyano-2-propyldodecyl trithiocarbonate in step two ① is 1:100:1:0; the cable connector with a heterogeneous three-layer sandwich structure obtained in step three ② is recorded as 0% (pure XLPE). All other steps and parameters are the same as in Example 1.

[0032] Figure 1The Weibull distribution diagrams of the interface breakdown field strength of the cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1 at 30°C are shown. Depend on Figure 1 It is evident that the interfacial breakdown field strength is improved for the 1%, 3%, and 9% fractions, with the 9% fraction showing a 3% increase, the 3% fraction a 39% increase, and the 1% fraction an 82% increase. These improvements are primarily due to the introduction of numerous "deep traps" by the polar groups at the XLPE interface. When high-energy electrons move under an electric field, these deep traps effectively capture them, converting their energy into heat or reducing their kinetic energy. Once trapped, electrons have difficulty gaining sufficient energy to collide with and ionize other molecules, thus inhibiting electron avalanche formation and directly increasing the breakdown field strength at the interface. Under a strong electric field, space charge easily accumulates at the interface. The introduction of acrylate, due to its electronegativity and trapping effect, forms a stable space charge layer near the electrode or at the interface. The local electric field generated by these accumulated charges is opposite in direction to the applied electric field, thereby weakening the actual electric field strength at the electrode tip or interface defects, making the electric field distribution more uniform and preventing localized breakdown. Acrylic esters possess a certain degree of fluidity and permeability, enabling them to fill microscopic pores or defects at the XLPE interface and reduce localized breakdown caused by air gap discharge. If a grafting reaction occurs during crosslinking, the acrylates will be covalently fixed to the polyethylene molecular chains. This robust bond enhances the mechanical strength and electrical continuity of the interface, reducing the probability of electrical treeing along the interface.

[0033] Figure 2 The graph shows the change of electrical conductivity current with electric field intensity at 30°C for the cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1. Figure 3 The graph shows the change of electrical conductivity current with electric field intensity at 50°C for the cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1. Figure 4 The graph shows the change of electrical conductivity current with electric field intensity at 70°C for the cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1. Depend on Figures 2-4It can be seen that at 30℃, the interfacial conductivity exhibits a non-linear relationship with the electric field strength. This is because acrylate molecules contain highly polar ester groups (-COOR). These polar groups act as efficient charge traps in the non-polar polyethylene matrix. Electrons or holes that were originally free at the interface are captured by deep traps during migration. Compared to pure XLPE, acrylate introduces deeper traps. Charge carriers falling into deep traps require extremely high energy to escape. This leads to a significant reduction in the concentration of free charge carriers participating in conduction, thus decreasing conductivity. When acrylate traps a large number of charges of the same polarity near the electrode / insulator interface, these bound space charges generate a reverse electric field opposite to the applied electric field. This reverse electric field weakens the effective field strength on the electrode surface, thereby increasing the energy barrier for charge injection from the metal electrode into the insulator and reducing the charge supply at the source. Acrylic substances can physically fill microscopic defects, free volumes, or air gaps on the XLPE interface. Filling these tiny spaces reduces the area available for electron acceleration in air gaps or low-density regions, decreasing carrier multiplication due to collisional ionization and further suppressing current growth. If acrylates undergo grafting with XLPE, polar groups are chemically bonded to the molecular chain. The dipole-dipole interactions between strongly polar groups enhance the bonding forces between molecular chains, restricting the relaxation and movement of polymer chain segments. Since carrier migration is often accompanied by the cooperative movement of chain segments, this segment confinement also indirectly inhibits conductivity.

[0034] Figure 5 Infrared spectra of cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1; Depend on Figure 5 It can be seen that, compared with the 0% component of the comparative example, 1735cm -1 The corresponding value is the C=O group. The 1%, 3%, and 9% components show significant absorption peaks in this range because the acrylate contains C=O groups, indicating that the acrylate is grafted onto the cross-linked polyethylene. The peak at 2150 cm⁻¹ is... -1 The characteristic absorption peaks nearby are caused by the cyano group of the terminal 2-cyano-2-propyldodecyl trithiocarbonate, indicating that 2-cyano-2-propyldodecyl trithiocarbonate, as a chain transfer agent, is also grafted onto the end of the acrylate chain.

[0035] Figure 6 The XRD patterns of cable joints with heterogeneous three-layer sandwich structures prepared in Examples 1-3 and Comparative Example 1 are shown. Depend on Figure 6It can be seen that, compared with the 0% component as a comparative example, the diffraction peak positions of the 1%, 3%, and 9% components in the examples shift to the right, from 2θ=21.35 to 21.37. This indicates that the interplanar spacing has shrunk, and the molecular chains are arranged more compactly in the crystalline region of cross-linked polyethylene. The introduction of acrylate or the surface modification process may produce a "compression" effect, or the modification may induce a small rearrangement of the crystal structure, resulting in a reduction in the distance between the crystal planes. The peak shift to a higher angle usually corresponds to the presence of compressive stress inside the material. During the surface modification process, acrylate molecules enter between polyethylene chains, or the difference in the coefficient of thermal expansion between the modified layer and the substrate may produce a compression effect in the crystalline region of the XLPE surface, resulting in a smaller lattice constant. For the orthorhombic crystal system of polyethylene, around 21.4° usually corresponds to the (110) crystal plane. The rightward shift of the peak position indicates that the packing density of the (110) crystal plane has increased. The introduction of polar groups may, through intermolecular forces, pull the surrounding polyethylene chains, making them more regularly or compactly arranged during the crystallization process. A decrease in interplanar spacing is often accompanied by a compression of the material's microscopic free volume. A more compact lattice arrangement and a smaller free volume mean greater resistance to carrier migration within the material, resulting in more distorted pathways, which in turn reduces electrical conductivity and increases breakdown strength.

[0036] Figure 7 These are schematic diagrams of cable joints with a heterogeneous three-layer sandwich structure as shown in Examples 1-3. Depend on Figure 7 As can be seen, the intermediate layer is an acrylate grafted modified layer grown in situ through photo-initiated growth, with its two sides connected to the cable body insulation and the injected recovery insulation, respectively. The figure illustrates the gradient distribution characteristics of the modified components within the transition insulation region. Through molecular chain entanglement and chemical anchoring at the interface, the physical interface is eliminated, achieving a smooth transition between electrical and mechanical properties.

[0037] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure, characterized in that... The method is specifically implemented according to the following steps: I. Preparation of cross-linked polyethylene: Di-tert-butyl peroxide isopropylbenzene is mixed evenly with polyethylene particles, then transferred into a mold. The mold is then moved into a flat vulcanizing machine, the mold is closed, and matching vulcanizing pressure and temperature parameters are set. One vulcanization is completed within a preset time to obtain cross-linked polyethylene. II. Preparation of surface-modified cross-linked polyethylene: ① Add methyl methacrylate dropwise at a constant rate to 1,4-dioxane under constant temperature and stirring to obtain mixed solution I; add benzophenone powder to mixed solution I under constant temperature and stirring to obtain mixed solution II; add 2-cyano-2-propyldodecyl trithiocarbonate dropwise at a constant rate to mixed solution II under constant temperature and stirring to obtain mixed solution III; ② Stir the mixed solution III for a period of time and place it in a dark environment to prevent premature reaction; ③ The mixed solution III is evenly coated on the surface of the cross-linked polyethylene, and then irradiated under a 365nm ultraviolet lamp. During the irradiation, a layer of mixed solution III is coated at intervals. Then it is placed in acetone and stirred for cleaning. Finally, it is placed in a vacuum oven for degassing to obtain surface-modified cross-linked polyethylene. III. Constructing a cable joint interface with a heterogeneous three-layer sandwich structure: ① Mix di-tert-butyl peroxide isopropylbenzene with polyethylene particles evenly to obtain a mixture; spread the mixture evenly on the modified surface of the surface-modified cross-linked polyethylene, then transfer it into a mold, move the mold into a flat vulcanizing machine, close the mold and set the matching vulcanizing pressure and temperature parameters, vulcanize within a preset time to form a three-layer cross-linked polyethylene with an intermediate layer modified. ② The modified three-layer cross-linked polyethylene in the middle layer is placed in a vacuum oven for degassing to obtain a cable joint with a heterogeneous three-layer sandwich structure, thus completing a method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure.

2. The method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure according to claim 1, characterized in that... The mass of di-tert-butylperoxyisopropylbenzene mentioned in step one is 1.5% to 2.5% of the mass of polyethylene particles; the particle size of polyethylene particles mentioned in step one is 0.5 mm; the mold size mentioned in step one is 10 cm × 10 cm × 200 μm.

3. The method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure according to claim 1, characterized in that... The pressure of the flat vulcanizing machine mentioned in step one is 15 MPa; the vulcanization temperature mentioned in step one is 175℃; and the vulcanization time mentioned in step one is 30 min.

4. The method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure according to claim 1, characterized in that... The mass ratio of 1,4-dioxane, methyl methacrylate, benzophenone, and 2-cyano-2-propyldodecyl trithiocarbonate in step 2① is 1:100:1:(1~9); the dropping rate in step 2① is 45 drops~50 drops / min; the stirring speed in step 2① is 2.5 r / s~2.8 r / s, and the stirring time is 10 min~15 min; the constant temperature in step 2① is 25℃~30℃.

5. The method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure according to claim 1, characterized in that... In step 2②, the stirring speed is 2.5 r / s to 2.8 r / s, and the stirring time is 25 min to 30 min; in step 2③, the mass ratio of each coating of mixed solution III to the surface area of ​​cross-linked polyethylene is 0.2 g: 100 cm². 2 The irradiation time mentioned in step 2③ is 1h~1.5h; during the irradiation period, a layer of mixed solution Ⅲ is coated at 10min intervals.

6. The method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure according to claim 1, characterized in that... The irradiation distance in step 2.3 is 10 cm; the degassing time in step 2.3 is 40 h to 50 h; the temperature of the vacuum oven in step 2.3 is 70 °C; the stirring and cleaning time in step 2.3 is 30 min to 40 min, and the stirring and cleaning speed is 2.5 r / s to 2.8 r / s.

7. The method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure according to claim 1, characterized in that... The mass ratio of the mixture described in step 3① to the surface area of ​​the surface-modified cross-linked polyethylene is 2.6 g: 100 cm³. 2 The mass of di-tert-butylperoxyisopropylbenzene mentioned in step 3① is 1.5%~2.5% of the mass of polyethylene particles; the particle size of the polyethylene particles mentioned in step 3① is 0.5mm; the mold size mentioned in step 3① is 10cm×10cm×400μm.

8. The method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure according to claim 1, characterized in that... The pressure of the flat vulcanizing machine in step 3① is 15MPa; the vulcanization temperature in step 3① is 175℃; the vulcanization time in step 3① is 30min; the degassing time in step 3② is 40h~50h; and the temperature of the vacuum oven in step 3② is 70℃.

9. A method for constructing a cable joint interface with a heterogeneous three-layer sandwich structure according to claim 1, characterized in that... The cable joint with the heterogeneous three-layer sandwich structure described in step 3② has a breakdown electric field strength of 126~155KV / mm at 30℃; 115~143KV / mm at 50℃; and 110~139KV / mm at 70℃. At room temperature, the tensile strength can reach 12.06MPa.

10. The application of the cable joint with a heterogeneous three-layer sandwich structure prepared by the method as described in claim 1, characterized in that... The cable joint with the heterogeneous three-layer sandwich structure is used as a high-voltage DC cable joint to enhance insulation performance.

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