A cross-linked polyethylene insulation material for medium-voltage DC cables and its preparation method

Abstract

This invention discloses a cross-linked polyethylene insulation material for medium-voltage DC cables and its preparation method. By weight, it comprises the following raw materials: 100-110 parts LDPE, 5-8 parts modified filler, 0.4-0.6 parts antioxidant, 0.2-0.3 parts cross-linking aid, and 0.4-0.7 parts initiator. The modified filler, prepared by a specific method, combined with an optimized cross-linking and antioxidant system, significantly improves the overall performance of the insulation material. This modified filler, through the synergistic effect of its deep trap energy levels in the core and the shallow trap energy levels of the outer organic molecules, increases the DC resistivity and breakdown field strength of the material. Simultaneously, the flexible long-chain structure grafted onto the filler surface improves its interfacial compatibility with the polyethylene matrix, giving the material excellent mechanical properties that meet the stringent requirements for long-term safe and stable operation of medium-voltage DC cables.

Description

Technical Field

[0001] This invention belongs to the technical field, specifically relating to a cross-linked polyethylene insulation material for medium-voltage DC cables and its preparation method. Background Technology

[0002] High-voltage and ultra-high-voltage cables are crucial components of power transmission networks, and polymer insulation materials, as the core materials determining the service life and operational reliability of cables, directly affect the safety and stability of the entire power system. Among numerous insulation materials, cross-linked polyethylene (XLPE) has become the preferred material for medium- and high-voltage AC power cable insulation due to its excellent dielectric properties, good heat resistance, and stable chemical properties. Through chemical or physical methods, linear polyethylene molecular chains form a three-dimensional network structure, transforming it from a thermoplastic material into a thermosetting material. This significantly improves the material's mechanical strength, resistance to environmental stress cracking, and short-term breakdown field strength, meeting the insulation requirements of AC cables.

[0003] However, with the rapid development of high-voltage direct current (HVDC) transmission technology, cross-linked polyethylene (XLPE) insulation materials traditionally used in AC applications have revealed serious limitations under DC conditions. The core technical challenge lies in the "space charge effect": under a continuous high DC electric field, charges are injected from the electrodes or generated within the insulating medium, slowly migrating and accumulating to form a stable space charge layer. These accumulated charges severely distort the original electric field distribution within the insulation, causing local field strengths in certain areas to far exceed design values, thus significantly accelerating material aging and even inducing premature insulation breakdown. To address this problem, the current mainstream technical approach is to add inorganic nanofillers, such as magnesium oxide, silicon oxide, and nanoclay, to the polyethylene matrix. These fillers can introduce traps, capturing charges to some extent and inhibiting their migration. However, this method has inherent technical defects: on the one hand, there is a natural interfacial incompatibility between inorganic fillers and non-polar polyethylene matrix, which leads to uneven dispersion of fillers in the polymer and easy agglomeration. This not only weakens the suppression effect on space charge, but also deteriorates the processing performance and mechanical properties of the material, making the insulation layer brittle and reducing its toughness. On the other hand, simple physical blending or surface treatment using general coupling agents cannot form a stable chemical bond between the filler and the matrix. Under long-term thermal and electrical stress, micro-defects are easily generated at the interface, which become new charge accumulation points and weak links for insulation breakdown. This method fails to fundamentally solve the contradiction between DC insulation performance and material mechanical properties.

[0004] Chinese patent application CN111704761A discloses a cross-linked polyethylene (XLPE) insulated cable material and its preparation method. By weight, it comprises the following components: 80-100 parts modified polyethylene, 5-10 parts nano-modified organosilicon microspheres, 0.5-2 parts cross-linking agent, 0.05-1 part cross-linking aid, 0.1-1 part stabilizer, 0.2-0.5 parts antioxidant, and 0.5-1 part lubricant. This invention solves the problems of high water permeability and poor cross-linking effect still present in current XLPE insulated cable materials. The XLPE cable material obtained by this invention has excellent mechanical properties, high high-temperature resistance, low water resistance, and excellent cross-linking properties, effectively expanding the application range of XLPE cable materials. However, the nano-modified organosilicon microspheres added in this patent mainly reduce the water permeability of polyethylene; their improvement on the electrical and mechanical properties of the cable material still needs further enhancement. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a cross-linked polyethylene insulation material for medium-voltage DC cables and its preparation method.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A cross-linked polyethylene insulation material for medium-voltage DC cables, comprising the following raw materials by weight:

[0008] 100-110 parts LDPE, 5-8 parts modified filler, 0.4-0.6 parts antioxidant, 0.2-0.3 parts crosslinking aid, and 0.4-0.7 parts initiator.

[0009] Preferably, a cross-linked polyethylene insulation material for medium-voltage DC cables comprises, by weight, the following raw materials:

[0010] 100-105 parts LDPE, 5-7 parts modified filler, 0.4-0.5 parts antioxidant, 0.2-0.25 parts crosslinking aid, and 0.4-0.6 parts initiator.

[0011] Preferably, the antioxidant is antioxidant TBM-6.

[0012] Preferably, the crosslinking agent is triallyl isocyanurate, and the initiator is dicumyl peroxide.

[0013] Preferably, the modified filler is prepared by the following method:

[0014] S1. Add zinc nitrate to deionized water, then add tetrabutyl titanate ethanol solution, stir well, add sodium hydroxide solution, adjust pH, carry out hydrothermal reaction, filter, wash, dry, and calcine to obtain zinc oxide / titanium oxide complex.

[0015] S2. Add the zinc oxide / titanium oxide complex to an aqueous ethanol solution, then add γ-glycidoxypropyltrimethoxysilane and stir to react, to obtain the pretreated zinc oxide / titanium oxide complex.

[0016] S3. Add the pretreated zinc oxide / titanium oxide complex to toluene, then add oleic acid and triethylamine, and heat to react to obtain the organic zinc oxide / titanium oxide complex.

[0017] S4. Add the organic zinc oxide / titanium oxide composite to toluene, then add 2-naphthyl thiol and azobisisobutyronitrile, and carry out a constant temperature reaction to obtain the modified filler.

[0018] Preferably, in step S1, the mass ratio of zinc nitrate, deionized water, and tetrabutyl titanate ethanol solution is 25-35:800-900:150-200, the concentration of the tetrabutyl titanate solution is 15-20 g / L, and the mass concentration of the ammonia water is 1-2%.

[0019] Preferably, in step S1, the pH is 9-10, the hydrothermal reaction temperature is 140-160℃ and the time is 6-8h, and the calcination temperature is 400-500℃ and the time is 2-3h.

[0020] In this invention, zinc oxide / titanium oxide composites are prepared by hydrothermal method. The unique band structure (heterojunction) that can be formed introduces a series of deep-level traps in its band gap. Under DC high voltage, these traps can strongly capture and bind high-energy electrons injected from the electrode, greatly restricting their movement. This effectively suppresses the injection and accumulation of space charge and improves the DC breakdown strength of the insulating material.

[0021] Preferably, in step S2, the mass ratio of the zinc oxide / titanium oxide complex and γ-glycidyl etheroxypropyltrimethoxysilane is 50-60:7-9, the stirring reaction temperature is 60-70℃, and the time is 3-4h.

[0022] In this invention, silane modification of the zinc oxide / titanium oxide composite introduces epoxy groups onto the composite, which is beneficial for subsequent reactions. Furthermore, a robust chemical bridge is established between the γ-glycidyl etheroxypropyltrimethoxysilane zinc oxide / titanium oxide composite and the organic functional layer, ensuring that the modified filler does not separate from the polymer matrix in the final insulation material, thereby avoiding a significant decrease in the mechanical properties of the material.

[0023] Preferably, in step S3, the mass ratio of the pretreated zinc oxide / titanium oxide complex, oleic acid, and triethylamine is 50-60:8-10:1-2, and the heating reaction temperature is 70-80℃ for 2-3 hours.

[0024] In this invention, by pretreating the epoxy groups on the zinc oxide / titanium oxide composite to react with the carboxyl groups on oleic acid, the flexible long chains of oleic acid can penetrate into the molecular chains of the polyethylene matrix to generate physical entanglement, forming a "flexible interface layer". This greatly improves the compatibility between the rigid inorganic filler and the LDPE matrix, alleviates stress concentration, and enables the insulation material to maintain high electrical performance while also having high tensile strength and elongation at break.

[0025] Preferably, in step S4, the mass ratio of the organic zinc oxide / titanium oxide complex, 2-naphthiol, and azobisisobutyronitrile is 60-70:5-7:0.2-0.3, and the isothermal reaction is carried out at a temperature of 80-90°C for 1-2 hours.

[0026] In this invention, free radicals are generated by the thermal decomposition of azobisisobutyronitrile (AIBN), triggering a highly efficient "thiol-ene" click chemistry reaction. Naphthiol molecules are introduced into the organic zinc oxide / titanium oxide composite. Due to its large π-conjugated system, the naphthyl group in 2-naphthiol introduces a large number of high-density, shallow-energy traps into the band gap of polyethylene. These shallow traps act like "buffer zones," effectively capturing and scattering low-energy charge carriers, reducing the carrier mobility, thereby suppressing local electric field distortion, homogenizing the internal electric field distribution of the material, and further reducing the material's conductivity. Through the synergistic effect of deep and shallow traps, the zinc oxide / titanium oxide composite and 2-naphthiol improve the material's all-round, multi-level suppression of space charge.

[0027] This invention also protects a method for preparing cross-linked polyethylene insulation material for medium-voltage DC cables as described above, comprising the following steps: weighing raw materials according to the formula, adding LDPE, modified filler, and antioxidant to a high-speed mixer, mixing for 10-15 minutes, and then feeding the mixture through the feed port of a twin-screw extruder. The temperatures of the extruder's first to tenth stages are set to 80-110℃, 130-140℃, 145-155℃, 155-165℃, 160-170℃, 165-175℃, 160-170℃, 155-165℃, 150-160℃, and 155-165℃, respectively, with the die temperature at 160-170℃. In the eighth stage of the extruder, a mixture of cross-linking agent and initiator is injected into the melt through a metering pump. After the material is melted, mixed, and dispersed evenly, it is extruded into strips through the extruder die, cut into uniform granules by a pelletizer, and dried to obtain the final product.

[0028] In this invention, the mixture of crosslinking agent and initiator is injected via a metering pump in the latter stage (eighth stage) of the twin-screw extruder. This ensures that all solid components are fully mixed and dispersed in the melt before the temperature-sensitive initiator is introduced, minimizing its residence time in the high-temperature zone. This process avoids the significant process risk of pre-crosslinking of materials during extrusion granulation, guaranteeing the stability of the production process and the excellent processability of the final product granules.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) The cross-linked polyethylene insulation material for medium-voltage DC cables provided by the present invention significantly improves the comprehensive performance of the insulation material by adding modified filler prepared by a specific method and an optimized cross-linking and antioxidant system. The modified filler effectively captures and scatters charge carriers through the synergistic effect of the deep trap energy level of its core and the shallow trap energy level of the outer organic molecules, thereby greatly reducing the space charge accumulation effect and conductivity of the material under DC high voltage field, and improving the DC resistivity and breakdown field strength of the material. At the same time, the flexible long chain structure grafted on the filler surface improves its interfacial compatibility with the polyethylene matrix, so that the insulation material has excellent DC electrical performance while maintaining good mechanical properties, and can meet the stringent requirements for long-term safe and stable operation of medium-voltage DC cables.

[0031] (2) The cross-linked polyethylene insulation material for medium-voltage DC cables provided by the present invention incorporates modified fillers that employ a multi-step chemical reaction to firmly anchor organic molecular layers (including flexible long chains and shallow-trap naphthyl groups) with specific functions onto inorganic nano-cores through stable covalent bonds. This ensures that the functional layers will not detach or migrate from the filler surface under extreme conditions such as melt processing, thus guaranteeing the stability and durability of the functional structure in the final insulation material. This overcomes the technical defects of physical coating method modified fillers, which are prone to phase separation in polymer matrix and lead to unstable performance. The zinc oxide / titanium oxide composite core, prepared first, possesses a unique band structure that provides deep-level traps to capture and fix high-energy injected charges, effectively preventing the formation of penetrating conductive channels. Subsequently, the zinc oxide / titanium oxide composite is modified with a silane coupling agent to introduce active epoxy groups. Then, through the reaction of epoxy groups with the carboxyl groups on oleic acid, the long oleic acid chain is introduced into the surface structure of the filler, significantly improving the interfacial compatibility between the inorganic filler and the non-polar polyethylene matrix. The flexible alkyl chain with up to 18 carbons in the oleic acid molecule can be physically entangled in the polyethylene matrix like a polymer chain, forming a "flexible interfacial transition layer". This transition layer effectively alleviates stress concentration between rigid inorganic particles and the polymer matrix, and improves the interfacial bonding strength. Thus, without sacrificing or even improving key mechanical properties such as tensile strength and elongation at break, it achieves significant optimization of electrical properties, avoiding the material embrittlement problem often caused by traditional inorganic filler filling. Finally, through the mercaptoene reaction, 2-naphthiols with a large π-conjugated system are introduced to construct a large number of shallow energy level traps in the polymer matrix. These shallow traps can effectively scatter and capture low-energy charge carriers, reducing their mobility, thereby macroscopically homogenizing the electric field and suppressing local field distortion. Through the reasonable configuration and synergistic effect of deep and shallow traps, the space charge suppression ability of the insulating material of this invention is far superior to that of material systems that only introduce a single type of trap.

[0032] (3) The cross-linked polyethylene insulation material for medium-voltage DC cables provided by the present invention adopts a process in which a mixture of liquid initiator and cross-linking agent is precisely injected into the molten polymer matrix through a metering pump at the rear end (eighth section) of a twin-screw extruder. This avoids the long-term contact between the initiator and polyethylene in the front hopper or high-speed mixer, and fundamentally eliminates the risk of premature decomposition of the initiator due to local frictional heat or shear heat during the mixing process, thereby causing the material to "pre-cross-link". This process ensures the uniformity of material mixing and dispersion in the extruder and the precise controllability of the cross-linking reaction, which significantly improves the quality uniformity of the final insulation material particles and the safety and stability of the production process. Detailed Implementation

[0033] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0034] Unless otherwise specified, all chemical reagents and materials in this invention are purchased from the market or synthesized from raw materials purchased from the market.

[0035] The LDPE is of the Borealis LE4201R grade; the titanium dioxide has a particle size of 20-30 μm.

[0036] Example 1

[0037] A cross-linked polyethylene insulation material for medium-voltage DC cables, comprising the following raw materials by weight:

[0038] 105 parts LDPE, 6 parts modified filler, 0.5 parts antioxidant TBM-6, 0.25 parts triallyl isocyanurate, and 0.5 parts dicumyl peroxide.

[0039] The modified filler is prepared by the following method:

[0040] S1. Add 30g of zinc nitrate to 850g of deionized water, then add 180g of tetrabutyl titanate ethanol solution with a concentration of 17g / L, stir well, then add sodium hydroxide solution with a mass concentration of 1.5% to adjust the pH to 95, and hydrothermally react at 150℃ for 7h. After the reaction is completed, filter, wash, dry, and calcine at 450℃ for 2.5h to obtain zinc oxide / titanium oxide composite.

[0041] S2. Add 55g of zinc oxide / titanium oxide complex to 900mL of ethanol aqueous solution (ethanol to ethanol volume ratio of 3:1), then add 8g of γ-glycidoxypropyltrimethoxysilane, and stir the reaction at 65℃ for 3.5h to obtain pretreated zinc oxide / titanium oxide complex.

[0042] S3. Add 55g of pretreated zinc oxide / titanium oxide composite to 1L of toluene, then add 9g of oleic acid and 1.5g of triethylamine, and react at 75℃ for 2.5h to obtain organic zinc oxide / titanium oxide composite.

[0043] S4. Add 65g of organic zinc oxide / titanium oxide composite to 1L of toluene, then add 6g of 2-naphthyl mercaptan and 0.25g of azobisisobutyronitrile, and react at 85℃ for 1.5h to obtain the modified filler.

[0044] A method for preparing cross-linked polyethylene insulation material for medium-voltage DC cables includes the following steps: weighing raw materials according to the formula, adding LDPE, modified filler, and antioxidant to a high-speed mixer, mixing for 15 minutes, and then feeding the mixture through the feed port of a twin-screw extruder. The temperatures of the extruder's first to tenth sections are set to 95℃, 135℃, 150℃, 160℃, 165℃, 170℃, 165℃, 160℃, 155℃, and 160℃, respectively, with the die temperature set at 160℃. In the eighth section of the extruder, a mixture of triallyl isocyanurate and dicumyl peroxide is injected into the melt through a metering pump. After the material is melted, mixed, and evenly dispersed, it is extruded into strips through the extruder die, cut into uniform granules by a pelletizer, and dried to obtain the final product.

[0045] Example 2

[0046] A cross-linked polyethylene insulation material for medium-voltage DC cables, comprising the following raw materials by weight:

[0047] 100 parts LDPE, 5 parts modified filler, 0.4 parts antioxidant TBM-6, 0.2 parts triallyl isocyanurate, and 0.4 parts dicumyl peroxide.

[0048] The modified filler is prepared by the following method:

[0049] S1. Add 35g of zinc nitrate to 900g of deionized water, then add 200g of tetrabutyl titanate ethanol solution with a concentration of 15g / L, stir well, add sodium hydroxide solution with a mass concentration of 1%, adjust the pH to 9, and hydrothermally react at 140℃ for 8h. After the reaction is completed, filter, wash, dry, and calcine at 400℃ for 3h to obtain zinc oxide / titanium oxide composite.

[0050] S2. Add 60g of zinc oxide / titanium oxide complex to 900mL of ethanol aqueous solution (ethanol to ethanol volume ratio of 3:1), then add 9g of γ-glycidoxypropyltrimethoxysilane, and stir at 60℃ for 4h to obtain pretreated zinc oxide / titanium oxide complex.

[0051] S3. Add 60g of pretreated zinc oxide / titanium oxide composite to 1L of toluene, then add 10g of oleic acid and 2g of triethylamine, and react at 70℃ for 3h to obtain organic zinc oxide / titanium oxide composite.

[0052] S4. Add 70g of organic zinc oxide / titanium oxide composite to 1L of toluene, then add 7g of 2-naphthyl mercaptan and 0.3g of azobisisobutyronitrile, and react at 80℃ for 2h to obtain the modified filler.

[0053] A method for preparing cross-linked polyethylene insulation material for medium-voltage DC cables includes the following steps: weighing raw materials according to the formula, adding LDPE, modified filler, and antioxidant to a high-speed mixer, mixing for 15 minutes, and then feeding the mixture through the feed port of a twin-screw extruder. The temperatures of the extruder's first to tenth sections are set to 95℃, 135℃, 150℃, 160℃, 165℃, 170℃, 165℃, 160℃, 155℃, and 160℃, respectively, with the die temperature set at 160℃. In the eighth section of the extruder, a mixture of triallyl isocyanurate and dicumyl peroxide is injected into the melt through a metering pump. After the material is melted, mixed, and evenly dispersed, it is extruded into strips through the extruder die, cut into uniform granules by a pelletizer, and dried to obtain the final product.

[0054] Example 3

[0055] A cross-linked polyethylene insulation material for medium-voltage DC cables, comprising the following raw materials by weight:

[0056] 110 parts LDPE, 8 parts modified filler, 0.6 parts antioxidant TBM-6, 0.3 parts triallyl isocyanurate, and 0.7 parts dicumyl peroxide.

[0057] The modified filler is prepared by the following method:

[0058] S1. Add 25g of zinc nitrate to 800g of deionized water, then add 150g of 20g / L tetrabutyl titanate ethanol solution, stir well, add 2% sodium hydroxide solution, adjust the pH to 10, and hydrothermally react at 160℃ for 6h. After the reaction is completed, filter, wash, dry, and calcine at 500℃ for 2h to obtain zinc oxide / titanium oxide composite.

[0059] S2. Add 50g of zinc oxide / titanium oxide complex to 900mL of ethanol aqueous solution (ethanol to titanium oxide volume ratio of 3:1), then add 7g of γ-glycidoxypropyltrimethoxysilane, stir and react at 70℃ for 3h to obtain pretreated zinc oxide / titanium oxide complex.

[0060] S3. Add 50g of pretreated zinc oxide / titanium oxide composite to 1L of toluene, then add 8g of oleic acid and 1g of triethylamine, and react at 80℃ for 2h to obtain organic zinc oxide / titanium oxide composite.

[0061] S4. Add 60g of organic zinc oxide / titanium oxide composite to 1L of toluene, then add 5g of 2-naphthyl mercaptan and 0.2g of azobisisobutyronitrile, and react at 90℃ for 1h to obtain the modified filler.

[0062] A method for preparing cross-linked polyethylene insulation material for medium-voltage DC cables includes the following steps: weighing raw materials according to the formula, adding LDPE, modified filler, and antioxidant to a high-speed mixer, mixing for 15 minutes, and then feeding the mixture through the feed port of a twin-screw extruder. The temperatures of the extruder's first to tenth sections are set to 95℃, 135℃, 150℃, 160℃, 165℃, 170℃, 165℃, 160℃, 155℃, and 160℃, respectively, with the die temperature set at 160℃. In the eighth section of the extruder, a mixture of triallyl isocyanurate and dicumyl peroxide is injected into the melt through a metering pump. After the material is melted, mixed, and evenly dispersed, it is extruded into strips through the extruder die, cut into uniform granules by a pelletizer, and dried to obtain the final product.

[0063] Comparative Example 1

[0064] A cross-linked polyethylene insulation material for medium-voltage DC cables, comprising the following raw materials by weight:

[0065] 105 parts LDPE, 6 parts titanium dioxide, 0.5 parts antioxidant TBM-6, 0.25 parts triallyl isocyanurate, and 0.5 parts dicumyl peroxide.

[0066] A method for preparing cross-linked polyethylene insulation material for medium-voltage DC cables includes the following steps: weighing raw materials according to the formula, adding LDPE, titanium dioxide, and antioxidant to a high-speed mixer, mixing for 15 minutes, and then feeding the mixture through the feed port of a twin-screw extruder. The temperatures of the extruder's first to tenth stages are set to 95℃, 135℃, 150℃, 160℃, 165℃, 170℃, 165℃, 160℃, 155℃, and 160℃, respectively, with the die temperature set at 160℃. In the eighth stage of the extruder, a mixture of triallyl isocyanurate and dicumyl peroxide is injected into the melt through a metering pump. After the material is melted, mixed, and evenly dispersed, it is extruded into strips through the extruder die, cut into uniform granules by a pelletizer, and dried to obtain the final product.

[0067] Compared to Example 1, this comparative example replaces the modified filler with titanium dioxide.

[0068] Comparative Example 2

[0069] A cross-linked polyethylene insulation material for medium-voltage DC cables, comprising the following raw materials by weight:

[0070] 105 parts LDPE, 6 parts modified filler, 0.5 parts antioxidant TBM-6, 0.25 parts triallyl isocyanurate, and 0.5 parts dicumyl peroxide.

[0071] The modified filler is prepared by the following method:

[0072] S1. Add 30g of zinc nitrate to 850g of deionized water, then add 180g of tetrabutyl titanate ethanol solution with a concentration of 17g / L, stir well, then add sodium hydroxide solution with a mass concentration of 1.5% to adjust the pH to 95, and hydrothermally react at 150℃ for 7h. After the reaction is completed, filter, wash, dry, and calcine at 450℃ for 2.5h to obtain zinc oxide / titanium oxide composite.

[0073] S2. Add 55g of zinc oxide / titanium oxide composite to 900mL of ethanol aqueous solution (ethanol to titanium oxide volume ratio of 3:1), then add 8g of γ-glycidoxypropyltrimethoxysilane, and stir the reaction at 65℃ for 3.5h to obtain the modified filler.

[0074] A method for preparing cross-linked polyethylene insulation material for medium-voltage DC cables includes the following steps: weighing raw materials according to the formula, adding LDPE, modified filler, and antioxidant to a high-speed mixer, mixing for 15 minutes, and then feeding the mixture through the feed port of a twin-screw extruder. The temperatures of the extruder's first to tenth sections are set to 95℃, 135℃, 150℃, 160℃, 165℃, 170℃, 165℃, 160℃, 155℃, and 160℃, respectively, with the die temperature set at 160℃. In the eighth section of the extruder, a mixture of triallyl isocyanurate and dicumyl peroxide is injected into the melt through a metering pump. After the material is melted, mixed, and evenly dispersed, it is extruded into strips through the extruder die, cut into uniform granules by a pelletizer, and dried to obtain the final product.

[0075] Compared to Example 1, this comparative example did not introduce oleic acid and 2-naphthiol into the modified filler.

[0076] Comparative Example 3

[0077] A cross-linked polyethylene insulation material for medium-voltage DC cables, comprising the following raw materials by weight:

[0078] 105 parts LDPE, 6 parts modified filler, 0.5 parts antioxidant TBM-6, 0.25 parts triallyl isocyanurate, and 0.5 parts dicumyl peroxide.

[0079] The modified filler is prepared by the following method:

[0080] S1. Add 30g of zinc nitrate to 850g of deionized water, then add 180g of tetrabutyl titanate ethanol solution with a concentration of 17g / L, stir well, then add sodium hydroxide solution with a mass concentration of 1.5% to adjust the pH to 95, and hydrothermally react at 150℃ for 7h. After the reaction is completed, filter, wash, dry, and calcine at 450℃ for 2.5h to obtain zinc oxide / titanium oxide composite.

[0081] S2. Add 55g of zinc oxide / titanium oxide complex to 900mL of ethanol aqueous solution (ethanol to ethanol volume ratio of 3:1), then add 8g of γ-glycidoxypropyltrimethoxysilane, and stir the reaction at 65℃ for 3.5h to obtain pretreated zinc oxide / titanium oxide complex.

[0082] S3. Add 55g of pretreated zinc oxide / titanium oxide composite to 1L of toluene, then add 6g of 2-naphthyl mercaptan and 1.5g of triethylamine, and react at 75℃ for 2.5h to obtain the modified filler.

[0083] A method for preparing cross-linked polyethylene insulation material for medium-voltage DC cables includes the following steps: weighing raw materials according to the formula, adding LDPE, modified filler, and antioxidant to a high-speed mixer, mixing for 15 minutes, and then feeding the mixture through the feed port of a twin-screw extruder. The temperatures of the extruder's first to tenth sections are set to 95℃, 135℃, 150℃, 160℃, 165℃, 170℃, 165℃, 160℃, 155℃, and 160℃, respectively, with the die temperature set at 160℃. In the eighth section of the extruder, a mixture of triallyl isocyanurate and dicumyl peroxide is injected into the melt through a metering pump. After the material is melted, mixed, and evenly dispersed, it is extruded into strips through the extruder die, cut into uniform granules by a pelletizer, and dried to obtain the final product.

[0084] Compared to Example 1, this comparative example did not introduce oleic acid into the modified filler.

[0085] Comparative Example 4

[0086] A cross-linked polyethylene insulation material for medium-voltage DC cables, comprising the following raw materials by weight:

[0087] 105 parts LDPE, 6 parts modified filler, 0.5 parts antioxidant TBM-6, 0.25 parts triallyl isocyanurate, and 0.5 parts dicumyl peroxide.

[0088] The modified filler is prepared by the following method:

[0089] S1. Add 30g of zinc nitrate to 850g of deionized water, then add 180g of tetrabutyl titanate ethanol solution with a concentration of 17g / L, stir well, then add sodium hydroxide solution with a mass concentration of 1.5% to adjust the pH to 95, and hydrothermally react at 150℃ for 7h. After the reaction is completed, filter, wash, dry, and calcine at 450℃ for 2.5h to obtain zinc oxide / titanium oxide composite.

[0090] S2. Add 55g of zinc oxide / titanium oxide complex to 900mL of ethanol aqueous solution (ethanol to ethanol volume ratio of 3:1), then add 8g of γ-glycidoxypropyltrimethoxysilane, and stir the reaction at 65℃ for 3.5h to obtain pretreated zinc oxide / titanium oxide complex.

[0091] S3. Add 55g of pretreated zinc oxide / titanium oxide composite to 1L of toluene, then add 9g of oleic acid and 1.5g of triethylamine, and react at 75℃ for 2.5h to obtain the modified filler.

[0092] A method for preparing cross-linked polyethylene insulation material for medium-voltage DC cables includes the following steps: weighing raw materials according to the formula, adding LDPE, modified filler, and antioxidant to a high-speed mixer, mixing for 15 minutes, and then feeding the mixture through the feed port of a twin-screw extruder. The temperatures of the extruder's first to tenth sections are set to 95℃, 135℃, 150℃, 160℃, 165℃, 170℃, 165℃, 160℃, 155℃, and 160℃, respectively, with the die temperature set at 160℃. In the eighth section of the extruder, a mixture of triallyl isocyanurate and dicumyl peroxide is injected into the melt through a metering pump. After the material is melted, mixed, and evenly dispersed, it is extruded into strips through the extruder die, cut into uniform granules by a pelletizer, and dried to obtain the final product.

[0093] Compared to Example 1, this comparative example did not introduce 2-naphthiol into the modified filler.

[0094] The cross-linked polyethylene insulation materials for medium-voltage DC cables obtained in Examples 1-3 and Comparative Examples 1-4 were subjected to performance tests as follows: The cross-linked polyethylene insulation materials for medium-voltage DC cables prepared in each group were molded at 10 MPa and 135°C for 5 min, and then held at 15 MPa and 175°C for 15 min in a flat vulcanizing machine to obtain test samples. Tensile strength and elongation at break were tested according to standard GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets"; volume resistivity was tested according to standard GB / T 31838.2-2019 "Dielectric and resistive properties of solid insulating materials - Part 2: Resistive properties (DC method) - Volume resistivity and volume resistivity"; dielectric strength and DC breakdown strength were tested according to standard GB / T 1408.1-2016 "Electrical strength test methods for insulating materials - Part 1: Tests at power frequency"; electric field distortion was tested according to Appendix B of standard GB / T31489.1-2015 "Extruded insulated power cable systems for DC transmission with rated voltage of 500kV and below - Part 1: Test methods and requirements"; the test results are shown in Table 1 below.

[0095] Table 1

[0096]

[0097] As can be seen from Table 1 above, the cross-linked polyethylene insulation material for medium-voltage DC cables prepared by this invention has excellent electrical properties and good mechanical properties, which can meet the stringent requirements for long-term safe and stable operation of medium-voltage DC cables.

[0098] The above description is a further detailed explanation of the present invention in conjunction with specific implementation examples. It should not be considered that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered to fall within the protection scope of the present invention.

[0099] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A crosslinked polyethylene insulation compound for medium voltage DC cables, characterized in that, By weight, it includes the following raw materials: 100 - 110 parts of LDPE, 5 - 8 parts of modified filler, 0.4 - 0.6 parts of antioxidant, 0.2 - 0.3 parts of crosslinking agent, 0.4 - 0.7 parts of initiator; The preparation method of the modified filler is as follows: S1. Add zinc nitrate into deionized water, then add tetrabutyl titanate ethanol solution. After stirring evenly, add sodium hydroxide solution, adjust the pH, carry out hydrothermal reaction, filter, wash, dry and calcine to obtain zinc oxide / titanium oxide composite; S2. Add the zinc oxide / titanium oxide composite into an ethanol - water solution, then add γ - glycidyl ether oxypropyltrimethoxysilane, and carry out stirring reaction to obtain a pretreated zinc oxide / titanium oxide composite; S3. Add the pretreated zinc oxide / titanium oxide composite into toluene, then add oleic acid and triethylamine, and carry out heating reaction to obtain an organic zinc oxide / titanium oxide composite; S4. Add the organic zinc oxide / titanium oxide composite into toluene, then add 2 - naphthalenethiol and azobisisobutyronitrile, and carry out a constant - temperature reaction to obtain the modified filler.

2. The crosslinked polyethylene insulation compound for medium voltage DC cables according to claim 1, characterized in that, By weight, it includes the following raw materials: 100 - 105 parts of LDPE, 5 - 7 parts of modified filler, 0.4 - 0.5 parts of antioxidant, 0.2 - 0.25 parts of crosslinking agent, 0.4 - 0.6 parts of initiator.

3. The crosslinked polyethylene insulation compound for medium voltage DC cables according to claim 1, characterized in that, The antioxidant is antioxidant TBM - 6, the crosslinking agent is triallyl isocyanurate, and the initiator is dicumyl peroxide.

4. The crosslinked polyethylene insulation compound for medium voltage DC cables according to claim 1, characterized in that, In step S1, the mass ratio of zinc nitrate, deionized water, and tetrabutyl titanate ethanol solution is 25 - 35:800 - 900:150 - 200, the concentration of the tetrabutyl titanate ethanol solution is 15 - 20 g / L, and the mass concentration of the sodium hydroxide solution is 1 - 2%.

5. The crosslinked polyethylene insulating material for medium-voltage DC cables according to claim 1, wherein In step S1, the pH is 9 - 10, the temperature of the hydrothermal reaction is 140 - 160 °C, the time is 6 - 8 h, the temperature of the calcination is 400 - 500 °C, and the time is 2 - 3 h.

6. The crosslinked polyethylene insulating material for medium-voltage DC cables according to claim 1, wherein In step S2, the mass ratio of the zinc oxide / titanium oxide composite and γ - glycidyl ether oxypropyltrimethoxysilane is 50 - 60:7 - 9, the temperature of the stirring reaction is 60 - 70 °C, and the time is 3 - 4 h.

7. The crosslinked polyethylene insulating material for medium-voltage DC cables according to claim 1, wherein In step S3, the mass ratio of the pretreated zinc oxide / titanium oxide composite, oleic acid, and triethylamine is 50 - 60:8 - 10:1 - 2, the temperature of the heating reaction is 70 - 80 °C, and the time is 2 - 3 h.

8. The crosslinked polyethylene insulating material for medium-voltage DC cables according to claim 1, wherein In step S4, the mass ratio of the organic zinc oxide / titanium oxide composite, 2 - naphthalenethiol, and azobisisobutyronitrile is 60 - 70:5 - 7:0.2 - 0.3, the temperature of the constant - temperature reaction is 80 - 90 °C, and the time is 1 - 2 h.

9. A preparation method of the crosslinked polyethylene insulating material for medium-voltage DC cables according to any one of claims 1-8, characterized in that, It includes the following steps: Weigh the raw materials according to the formula, add LDPE, modified filler, and antioxidant into a high - speed mixer, mix for 10 - 15 min, then through the feeding port of a twin - screw extruder, at the eighth section of the extruder, inject the mixture of crosslinking agent and initiator into the melt through a metering pump. After the material is melted, mixed, and dispersed evenly, it is extruded into strips through the die head of the extruder, cut into uniform granular materials by a granulator, and dried to obtain the product.

Images