A composite dielectric based on in-situ growth of insulating layer modified silicon rubber / polypropylene interface, and a preparation method and application thereof

Abstract

The application relates to a composite dielectric based on in-situ growth of an insulating layer modified silicon rubber / polypropylene interface, a preparation method and application thereof, and belongs to the technical field of cable accessory insulation material and polymer interface modification. The composite dielectric based on in-situ growth of an insulating layer modified silicon rubber / polypropylene interface is characterized in that a SiO2 insulating layer is grown at the interface of the silicon rubber and the polypropylene by using a sol-gel method; the sol-gel method is to prepare a mixed system with different proportions by adjusting the volume ratio of tetraethyl orthosilicate as a precursor and anhydrous ethanol, and then to use ammonia as a catalyst and deionized water to promote the hydrolysis reaction. The application is aimed at the problem that the PP has excellent insulation performance but the PP / SiR interface has insufficient electrical performance, and a uniform and dense inorganic transition layer is constructed to effectively inhibit the interface charge accumulation and electric field distortion, improve the bulk breakdown field strength of the modified silicon rubber and the PP / modified silicon rubber interface breakdown voltage, and has good insulation stability and engineering application prospect.

Description

Technical Field

[0001] This invention belongs to the field of cable accessory insulation materials and polymer interface modification technology, specifically relating to a composite medium based on in-situ grown insulation layer modified silicone rubber / polypropylene interface, its preparation method and application. Background Technology

[0002] Currently, cross-linked polyethylene (XLPE) remains the primary insulation material for high-voltage cables, while silicone rubber is commonly used as reinforcing insulation material for related cable accessories. XLPE / silicone rubber interfaces are widely used in composite insulation structures such as cable joints and terminals due to their mature technology and well-developed supporting systems. With the development of green manufacturing and recyclable insulation materials, polypropylene (PP), due to its advantages such as low conductivity, superior dielectric properties, high heat resistance, and recyclability, is considered a potential next-generation cable insulation material to replace XLPE.

[0003] However, when PP is used as the main insulation material in cable accessories, the key problem is not its inherent insulation performance, but rather that the interfacial electrical properties after it forms an interface with silicone rubber are often lower than those of the traditional XLPE / silicone rubber interface. Existing research has shown that in composite insulation structures, the interface is a sensitive region for space charge accumulation, electric field distortion, and local breakdown. Although PP itself has better electrical properties than XLPE, there are unfavorable factors between PP and silicone rubber in terms of conductivity matching, interfacial polarization, trap distribution, and interfacial charge evolution. These factors make the PP / silicone rubber interface more prone to charge accumulation and localized field strength enhancement, resulting in low interfacial breakdown strength and insufficient long-term operational reliability.

[0004] Furthermore, silicone rubber and polypropylene (PP) are polymer materials with significantly different phase properties, exhibiting differences in surface energy, polarity, microscopic contact state, and interfacial charge transport paths. If the interface lacks an effective transition layer, the two phases are prone to significant interfacial mismatch under an electric field, thus limiting the widespread application of PP in high-voltage cable accessories. Therefore, improving the silicone rubber / PP interfacial matching relationship, suppressing interfacial charge accumulation, and optimizing the local electric field distribution without weakening the insulating advantages of PP itself has become a key technical challenge in the design of this type of composite dielectric.

[0005] Existing interface modification methods mostly focus on filler blending and surface roughening. However, these methods often suffer from poor uniformity of the interface transition layer, insufficient structural controllability, or significant impact on the flexibility and processing performance of the material itself. In contrast, the sol-gel method for in-situ growth of an inorganic insulating layer on the surface of silicone rubber can construct a controllable interface transition layer while maintaining the flexible matrix characteristics of silicone rubber, thus providing a new technical path for improving the electrical properties of the silicone rubber / PP interface. Summary of the Invention

[0006] One of the objectives of this invention is to address the insufficient interfacial electrical properties in existing PP / silicone rubber interfacial composite insulation systems. The second objective of this invention is to leverage the superior electrical properties and recyclability of polypropylene compared to cross-linked polyethylene to promote the application of environmentally friendly polypropylene insulation materials in the fields of heat-resistant cables and high-performance electrical insulation. The third objective of this invention is to improve the adaptability of composite insulation structures in different environments and enhance their stability in engineering applications.

[0007] To address the aforementioned technical problems, this invention provides a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface, its preparation method, and its application.

[0008] A composite medium based on in-situ grown insulating layer modified silicone rubber / polypropylene interface, wherein the composite medium is grown by using sol-gel method to grow SiO2 insulating layer at the interface of silicone rubber and polypropylene. The sol-gel method uses tetraethyl orthosilicate as a precursor, and prepares mixed systems with different ratios by adjusting the volume ratio of tetraethyl orthosilicate to anhydrous ethanol. Ammonia is then used as a catalyst, and deionized water is used to promote the hydrolysis reaction. The silicone rubber is a two-component liquid adhesive for cable accessories, comprising component A and component B. Component A is model BLUESIL™ 550-A, and component B is model BLUESIL™ 550-B; it was purchased from Wacker Silicones.

[0009] A method for preparing a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface is specifically carried out according to the following steps: 1. Add equal volumes of component A and component B of liquid silicone rubber to two reactors respectively; stir them separately, then mix component A and component B of the silicone rubber under vacuum conditions, and stir again under vacuum conditions to obtain a silicone rubber mixture. 2. Apply the silicone rubber mixture evenly into the mold, and then place the mold into a flat vulcanizing machine for a first vulcanization to obtain the composite medium after the first vulcanization. 3. Place the composite medium after primary vulcanization in a high-temperature forced-air drying oven for secondary vulcanization to obtain the composite medium after secondary vulcanization; let the composite medium after secondary vulcanization stand at room temperature for a period of time to obtain a silicone rubber film. 4. Mix tetraethyl orthosilicate with anhydrous ethanol to obtain a mixed solution; add deionized water dropwise to the mixed solution, continue stirring, then add ammonia water and continue stirring to obtain a mixed sol; 5. Let the mixed sol stand, then pour it into a petri dish, and immerse the silicone rubber film in the mixed sol. After immersion, slowly lift the film to form a uniform gel film on its surface, thus obtaining the composite film. 6. The composite film is placed in a vacuum oven and dried at gradient temperatures to obtain silicone rubber modified with in-situ grown SiO2 insulating layer; 7. Spread the polypropylene granules evenly into the mold, then place the mold into a flat vulcanizing machine, melt them separately under gradient pressure, and then cool them with water to room temperature to obtain a polypropylene film. 8. A polypropylene film is placed on the insulating layer of silicone rubber modified by in-situ growth of SiO2 insulating layer to obtain a composite medium based on the interface of silicone rubber / polypropylene modified by in-situ growth of insulating layer.

[0010] A composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface is used as an insulating material or interface insulating material in high-voltage cable accessories, heat-resistant wires and cables, temperature measuring wires, or power transmission and distribution cables for high-temperature conditions.

[0011] The principle of this invention: This invention addresses the hydrolysis of tetraethyl orthosilicate (TEOS) under ammonia catalysis to generate silanol, followed by a condensation reaction to construct a Si-O-Si network structure in situ on the surface of silicone rubber, forming a silica insulating layer. This insulating layer serves two purposes: firstly, it acts as an interfacial transition layer between silicone rubber and polypropylene, mitigating mismatches in conductivity, polarity, and interfacial trap distribution between the two phases; secondly, by introducing an inorganic layer with high insulation, a high potential barrier, and a stable surface state, it effectively suppresses the injection, accumulation, and migration of interfacial charges, reducing interfacial polarization and space charge modulation electric field effects, thereby improving the electrical properties of the silicone rubber / polypropylene interface. Furthermore, when the thickness and surface coverage of the silica layer are within a suitable range, the conductivity of the modified silicone rubber itself and the PP / modified silicone rubber interface can be significantly reduced, the interfacial breakdown strength can be improved, and the uniformity of space charge distribution can be enhanced. When the silica content is too low, the interfacial coverage is insufficient, making it difficult to form an effective transition layer, and the interfacial charge suppression effect is limited. When the silica content is too high, it is easy to cause excessive accumulation of the surface layer, uneven structure, or the generation of new defect regions, which is not conducive to the homogenization of the interfacial electric field and the improvement of electrical performance. Therefore, a medium content of in-situ grown silica layer is more conducive to optimizing the interfacial insulation performance. The results show that the composite dielectric based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface can reach an interfacial breakdown voltage of 20.76 kV at 0.3 MPa, 23.05 kV at 0.4 MPa, and 24.61 kV at 0.5 MPa.

[0012] Compared with the prior art, the present invention has the following advantages: I. Polypropylene itself has better electrical properties than the existing mainstream cross-linked polyethylene, and has the advantages of being recyclable and environmentally friendly. This invention solves the key shortcomings of PP in cable accessory applications through interface modification, and provides a new interface engineering solution for PP to replace XLPE. II. This invention constructs a stable and controllable silicone rubber / polypropylene transition interface by growing a silica insulating layer in situ on the surface of silicone rubber, which improves the conductivity matching and interface trap characteristics of the two-phase interface and effectively suppresses interface charge accumulation and electric field distortion. Third, the sol-gel method used in this invention is simple and suitable for continuous and large-scale preparation. The resulting composite medium can be used as an insulating material or interface insulating material in high-voltage cable accessories, heat-resistant wires and cables, temperature measuring wires, or power transmission and distribution cables for high-temperature conditions, and has good engineering application prospects. Attached Figure Description

[0013] Figure 1 (a) shows the XRD characterization of the in-situ grown SiO2 insulating layer modified silicone rubber prepared in step six of Examples 1-4 and the pure silicone rubber film prepared in Control Example 1; (b) shows the infrared characterization; and (c) shows the scanning electron microscope image. Figure 2 The stress-strain curves at room temperature are for the in-situ grown SiO2 insulating layer modified silicone rubber prepared in step six of Examples 1-4 and the pure silicone rubber film prepared in Control Example 1. Figure 3 (a) The dielectric constant and dielectric loss diagrams of the in-situ grown SiO2 insulating layer modified silicone rubber prepared in step six of Examples 1-4 and the pure silicone rubber film prepared in Comparative Example 1 at 30°C; (b) The dielectric constant and dielectric loss diagram at 50°C; (c) The dielectric constant and dielectric loss diagram at 70°C; (d) The dielectric constant and dielectric loss diagram at 90°C. Figure 4 (a) Conductivity diagrams of the in-situ grown SiO2 insulating layer modified silicone rubber prepared in step six of Examples 1-4 and the pure silicone rubber film prepared in Comparative Example 1 at 30°C; (b) Conductivity diagram at 50°C; (c) Conductivity diagram at 70°C; (d) Conductivity diagram at 90°C. Figure 5 (a) is a charge accumulation density diagram of the composite medium based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface prepared in step eight of Examples 1-4, and the composite medium based on the silicone rubber film / polypropylene interface prepared in Comparative Example 1; (b)-(e) are charge accumulation density diagrams of different modified silicone rubber / polypropylene interfaces. Figure 6(a) Breakdown field strength diagrams of the in-situ grown SiO2 insulating layer modified silicone rubber prepared in step six of Examples 1-4 and the pure silicone rubber film prepared in Comparative Example 1 at 30°C; (b) Breakdown field strength diagram at 50°C; (c) Breakdown field strength diagram at 70°C. Figure 7 (a) is the breakdown voltage diagram of the composite medium based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface prepared in step eight of Examples 1-4, and the silicone rubber film / polypropylene prepared in Comparative Example 1 at 0.3 MPa; (b) is the breakdown voltage diagram at 0.4 MPa; and (c) is the breakdown voltage diagram at 0.5 MPa. Detailed Implementation

[0014] Specific Implementation Method 1: This implementation method is a composite medium based on in-situ grown insulating layer modified silicone rubber / polypropylene interface. The composite medium is grown by using the sol-gel method to grow a SiO2 insulating layer at the interface of silicone rubber and polypropylene. The sol-gel method uses tetraethyl orthosilicate as a precursor, and prepares mixed systems with different ratios by adjusting the volume ratio of tetraethyl orthosilicate to anhydrous ethanol. Ammonia is then used as a catalyst, and deionized water is used to promote the hydrolysis reaction. The silicone rubber is a two-component liquid adhesive for cable accessories, comprising component A and component B. Component A is model BLUESIL™ 550-A and component B is model BLUESIL™ 550-B; it was purchased from Wacker Silicones. The polypropylene mentioned is selected from Sinopec Yangzi Petrochemical Co., Ltd., and its model is PPB-MO2-G.

[0015] This embodiment addresses the problem that while PP itself has excellent insulation properties, the electrical properties of the PP / SiR interface are insufficient. By constructing a uniform and dense inorganic transition layer, it effectively suppresses the accumulation of interfacial charge and electric field distortion, improves the bulk breakdown field strength of the modified silicone rubber and the breakdown voltage of the PP / modified silicone rubber interface, and has good insulation stability and promising engineering application prospects.

[0016] Specific Implementation Method Two: The difference between this implementation method and Specific Implementation Method One is that the total volume ratio of components A and B of the silicone rubber to the amounts of polypropylene, tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia is 120mL:2.6g:10mL:(40~80mL):8mL:(1~2mL). The other steps are the same as in Specific Implementation Method One.

[0017] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that it involves a method for preparing a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface, specifically completed according to the following steps: 1. Add equal volumes of component A and component B of liquid silicone rubber to two reactors respectively; stir them separately, then mix component A and component B of the silicone rubber under vacuum conditions, and stir again under vacuum conditions to obtain a silicone rubber mixture. 2. Apply the silicone rubber mixture evenly into the mold, and then place the mold into a flat vulcanizing machine for a first vulcanization to obtain the composite medium after the first vulcanization. 3. Place the composite medium after primary vulcanization in a high-temperature forced-air drying oven for secondary vulcanization to obtain the composite medium after secondary vulcanization; let the composite medium after secondary vulcanization stand at room temperature for a period of time to obtain a silicone rubber film. 4. Mix tetraethyl orthosilicate with anhydrous ethanol to obtain a mixed solution; add deionized water dropwise to the mixed solution, continue stirring, then add ammonia water and continue stirring to obtain a mixed sol; 5. Let the mixed sol stand, then pour it into a petri dish, and immerse the silicone rubber film in the mixed sol. After immersion, slowly lift the film to form a uniform gel film on its surface, thus obtaining the composite film. 6. The composite film is placed in a vacuum oven and dried at gradient temperatures to obtain silicone rubber modified with in-situ grown SiO2 insulating layer; 7. Spread the polypropylene granules evenly into the mold, then place the mold into a flat vulcanizing machine, melt them separately under gradient pressure, and then cool them with water to room temperature to obtain a polypropylene film. 8. Place the polypropylene film on the insulating layer of the silicone rubber modified by in-situ growth of the insulating layer, to obtain a composite medium based on the silicone rubber / polypropylene interface modified by in-situ growth of the insulating layer. Other steps are the same as in specific implementation method one or two.

[0018] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that: the stirring speed in step one is 2.4 r / s, and the stirring time is 1 h to 1.5 h; the stirring speed under vacuum conditions in step one is 3.0 r / s, and the stirring time is 1.5 h to 2 h. Other steps are the same as in Specific Implementation Methods One to Three.

[0019] Specific Implementation Method Five: The difference between this implementation method and Specific Implementation Methods One to Four is that the mold mentioned in step two is a polyester film mold with a size of 100mm. 100mm 0.175mm; the temperature of the first vulcanization in step two is 160℃~170℃, the time of the first vulcanization is 10min~20min, and the pressure of the flat vulcanizing machine is 15MPa. Other steps are the same as in specific embodiments one to four.

[0020] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in the following ways: the temperature for secondary vulcanization in step three is 200℃, and the time for secondary vulcanization is 4 hours; the settling time in step three is 0.5 hours to 1 hour; in step four, tetraethyl orthosilicate and anhydrous ethanol are mixed and stirred at a stirring speed of 3 r / s for 10 minutes to obtain a mixed solution; deionized water is added dropwise to the mixed solution, stirring is continued for 10 minutes, then ammonia is added, and stirring is continued for 2 to 2.5 hours to obtain a mixed sol. Other steps are the same as in Specific Implementation Methods One to Five.

[0021] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in the following ways: the volume ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia in step four is 10 mL:(40~80 mL):8 mL:(1~2 mL); the total volume ratio of the silicone rubber components A and B in step one to the tetraethyl orthosilicate in step four is 120 mL:10 mL; the standing time in step five is 0.5 h, and the lifting speed is 0.5 cm / s. Other steps are the same as in Specific Implementation Methods One to Six.

[0022] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One through Seven in the following ways: the gradient temperatures mentioned in step six are 25℃, 50℃, and 75℃; drying is performed at 25℃, 50℃, and 75℃ for 1 hour to 1.5 hours respectively; the total volume ratio of the silicone rubber components A and B mentioned in step one to the mass ratio of the polypropylene mentioned in step seven is 120mL:2.6g; the mold mentioned in step seven is a polyester film mold with a size of 100mm. 100mm 0.175mm; the gradient pressure mentioned in step seven is 0MPa, 5MPa, 10MPa, and 15MPa; melting is carried out at 0MPa, 5MPa, 10MPa, and 15MPa for 10min, 5min, 5min, and 5min respectively. Other steps are the same as in specific embodiments one to seven.

[0023] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that: the composite dielectric based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface described in step seven has a breakdown field strength of 128.93~141.77 kV / mm at 30°C, 129.44~144.36 kV / mm at 50°C, and 130.22~147.95 kV / mm at 70°C; at room temperature, the composite dielectric based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface described in step seven has an interface breakdown voltage of 18.69~20.76 kV at 0.3 MPa, 20.03~23.05 kV at 0.4 MPa, and 21.70~24.61 kV at 0.5 MPa. Other steps are the same as in Specific Implementation Methods One to Eight.

[0024] Specific Implementation Method 10: This implementation method is based on a composite medium of in-situ grown insulating layer modified silicone rubber / polypropylene interface as an insulating material or interface insulating material in high-voltage cable accessories, heat-resistant wires and cables, temperature measuring wires, or power transmission and distribution cables for high-temperature conditions.

[0025] The beneficial effects of the present invention are verified using the following embodiments: Example 1: A method for preparing a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface, specifically completed according to the following steps: 1. Add equal volumes of component A and component B of liquid silicone rubber to two reactors respectively; stir them separately, then mix component A and component B of the silicone rubber under vacuum conditions, and stir again under vacuum conditions to obtain a silicone rubber mixture. The silicone rubber mentioned in step one is a two-component liquid adhesive for cable accessories, comprising component A and component B. Component A is model BLUESIL™ 550-A and component B is model BLUESIL™ 550-B; purchased from Wacker Silicones. The stirring speed described in step one is 2.4 r / s, and the stirring time is 1.5 h. In step one, the stirring speed under vacuum conditions is 3.0 r / s, and the stirring time is 1.5 h; 2. Apply the silicone rubber mixture evenly into the mold, and then place the mold into a flat vulcanizing machine for a first vulcanization to obtain the composite medium after the first vulcanization. The mold mentioned in step two is a polyester film mold with a size of 100mm. 100mm 0.175mm; The temperature of the first vulcanization in step two is 165℃, the time of the first vulcanization is 15min, and the pressure of the flat vulcanizing machine is 15MPa. 3. Place the composite medium after primary vulcanization in a high-temperature forced-air drying oven for secondary vulcanization to obtain the composite medium after secondary vulcanization; let the composite medium after secondary vulcanization stand at room temperature for 1 hour to obtain a silicone rubber film. The temperature for the secondary vulcanization in step three is 200℃, and the time for the secondary vulcanization is 4 hours. 4. Mix tetraethyl orthosilicate and anhydrous ethanol at a stirring speed of 3 r / s for 10 min to obtain a mixed solution; add deionized water dropwise to the mixed solution, continue stirring for 10 min, then add 25% ammonia water by mass, and continue stirring for 2.5 h to obtain a mixed sol; The volume ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia in step four is 10 mL: 80 mL: 8 mL: 1 mL. The total volume ratio of the silicone rubber components A and B mentioned in step one to the volume ratio of the tetraethyl orthosilicate mentioned in step four is 120 mL: 10 mL. 5. Let the mixed sol stand, then pour it into a petri dish, and immerse the silicone rubber film in the mixed sol. After immersion, slowly lift the film to form a uniform gel film on its surface, thus obtaining the composite film. The settling time mentioned in step five is 0.5 hours, and the lifting speed is 0.5 cm / s; 6. Place the composite film in a vacuum oven and dry it at a gradient temperature to obtain silicone rubber modified with in-situ grown SiO2 insulating layer (denoted as SiR-1 or 1:8). The gradient temperatures mentioned in step six are 25℃, 50℃, and 75℃; drying is carried out at 25℃, 50℃, and 75℃ for 1.5 hours respectively. 7. Spread the polypropylene granules evenly into the mold, then place the mold into a flat vulcanizing machine, melt them separately under gradient pressure, and then cool them with water to room temperature to obtain a polypropylene film. The polypropylene mentioned in step seven is selected from Sinopec Yangzi Petrochemical Co., Ltd., and its model is PPB-MO2-G; The total volume of the silicone rubber components A and B mentioned in step one and the mass ratio of the polypropylene particles mentioned in step seven are 120 mL: 2.6 g. The mold mentioned in step seven is a polyester film mold with a size of 100mm. 100mm 0.175mm; The gradient pressure mentioned in step seven is 0 MPa, 5 MPa, 10 MPa, and 15 MPa; melting is carried out at 0 MPa, 5 MPa, 10 MPa, and 15 MPa for 10 min, 5 min, 5 min, and 5 min, respectively. 8. Place the polypropylene film on the insulating layer of the silicone rubber modified by in-situ growth of SiO2 insulating layer to obtain a composite medium based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface (denoted as PP / 1:8 or PP / SiR-1).

[0026] Example 2: The difference between this example and Example 1 is that the volume ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia in step four is 10mL:60mL:8mL:1.6mL; the silicone rubber modified with the in-situ grown SiO2 insulating layer obtained in step six is ​​denoted as SiR-2 or 1:6; the composite medium based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface obtained in step eight is denoted as PP / 1:6 or PP / SiR-2. All other steps and parameters are the same as in Example 1.

[0027] Example 3: The difference between this example and Example 1 is that the volume ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia in step four is 10mL:50mL:8mL:1.8mL; the silicone rubber modified with the in-situ grown SiO2 insulating layer obtained in step six is ​​designated as SiR-3 or 1:5; the composite medium based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface obtained in step eight is designated as PP / 1:5 or PP / SiR-3. All other steps and parameters are the same as in Example 1.

[0028] Example 4: The difference between this example and Example 1 is that the volume ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia in step four is 10mL:40mL:8mL:2mL; the silicone rubber modified with the in-situ grown SiO2 insulating layer obtained in step six is ​​denoted as SiR-4 or 1:4; the composite medium based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface obtained in step eight is denoted as PP / 1:4 or PP / SiR-4. All other steps and parameters are the same as in Example 1.

[0029] Comparative Example 1: A method for preparing pure silicone rubber film (SiR) and PP / SiR from existing two-component silicone rubber, specifically completed according to the following steps: 1. Add equal volumes of component A and component B of liquid silicone rubber to two reactors respectively; stir them separately, then mix component A and component B of the silicone rubber under vacuum conditions, and stir again under vacuum conditions to obtain a silicone rubber mixture. The liquid silicone rubber component A mentioned in step one is model number BLUESIL™ 550-A; the liquid silicone rubber component B is model number BLUESIL™ 550-B; both were purchased from Wacker Silicones. The stirring speed described in step one is 2.4 r / s, and the stirring time is 1.5 h. In step one, the stirring speed under vacuum is 3.0 / s, and the stirring time is 2 hours. 2. Apply the silicone rubber mixture evenly into the mold, and then place the mold into a flat vulcanizing machine for one vulcanization to obtain a composite medium with a thickness of 130μm after one vulcanization. The mold mentioned in step two is a polyester film mold with a size of 100mm. 100mm 0.175mm; The temperature of the first vulcanization in step two is 165℃, the time of the first vulcanization is 15min, and the pressure of the flat vulcanizing machine is 15MPa. 3. The composite medium after primary vulcanization is placed in a high-temperature forced-air drying oven for secondary vulcanization to obtain the composite medium after secondary vulcanization; the composite medium after secondary vulcanization is left to stand at room temperature for 1 hour to obtain pure silicone rubber film (SiR). The temperature for the secondary vulcanization in step three is 200℃, and the time for the secondary vulcanization is 4 hours. 4. Spread the polypropylene granules evenly into the mold, then place the mold into a flat vulcanizing machine, melt them separately under gradient pressure, and then cool them with water to room temperature to obtain a polypropylene film. The polypropylene mentioned in step four is selected from Sinopec Yangzi Petrochemical Co., Ltd., and its model is PPB-MO2-G; The total volume of the silicone rubber components A and B mentioned in step one and the mass ratio of the polypropylene particles mentioned in step four are 120 mL: 2.6 g. The mold mentioned in step four is a polyester film mold with a size of 100mm. 100mm 0.175mm; The gradient pressure mentioned in step four is 0 MPa, 5 MPa, 10 MPa, and 15 MPa; melting is carried out at 0 MPa, 5 MPa, 10 MPa, and 15 MPa for 10 min, 5 min, 5 min, and 5 min, respectively. 5. Place the polypropylene film on the pure silicone rubber film to obtain a composite medium (PP / SiR) at the silicone rubber film / polypropylene interface.

[0030] Figure 1(a) shows the XRD characterization of the in-situ grown SiO2 insulating layer modified silicone rubber prepared in step six of Examples 1-4 and the pure silicone rubber film prepared in Control Example 1; (b) shows the infrared characterization; and (c) shows the scanning electron microscope image. Figure 1 (a) shows the diffraction peak intensity as a function of angle for in-situ grown SiO2 insulating layer modified silicone rubber prepared in different embodiments using X-ray diffraction analysis. The XRD curves show that the crystallinity of the silica layer on the silicone rubber surface changes significantly with the ratio of TEOS to anhydrous ethanol. In the SiR-4 sample, the diffraction peaks are significantly enhanced, indicating a high degree of crystallinity in the silica; while the diffraction peaks of the SiR-1 sample are relatively flat, indicating that the SiO2 layer formed on its surface is relatively amorphous and the surface structure is relatively loose. Figure 1 (b) The spectrum shows that the characteristic absorption peaks of the Si-O-Si bonds are more pronounced in the SiR-4 sample, indicating that the formation of silica in this sample is more complete and uniform. Figure 1 (c) It can be seen that the surface of SiR is relatively smooth and there is almost no obvious particle structure; while in the SiR-4 sample, uniformly distributed silica particles can be seen, indicating that a stable SiO2 particle layer is formed on the surface of the modified sample.

[0031] Figure 2 The stress-strain curves at room temperature are for the in-situ grown SiO2 insulating layer modified silicone rubber prepared in step six of Examples 1-4 and the pure silicone rubber film prepared in Control Example 1. Depend on Figure 2 The stress-strain curves show that, overall, the mechanical properties of silicone rubber modified with different ratios of TEOS and anhydrous ethanol gradually increase with the increase of SiO2 formation. The unmodified silicone rubber exhibits the lowest stress at 6.43 MPa; the SiR-4 sample shows the highest curve and the largest fracture stress at 8.85 MPa, but the lowest elongation at break. The SiR-1 and SiR-2 samples are next, exhibiting a balance of strength and ductility. This indicates that insufficient SiO2 formation at low ratios makes it difficult to form an effective inorganic reinforcing network, resulting in weak support and stress transfer for the silicone rubber chains. The mechanism is that at lower SiO2 contents, the inorganic phase distribution is sparse, the interfacial effect is limited, and the reinforcing effect is not significant. Appropriate amounts of SiO2 can improve stress transfer efficiency and resistance to deformation by forming a more uniform rigid network and interfacial constraints. However, when the SiO2 content further increases, although it can significantly improve strength and modulus, it can also excessively restrict the movement of silicone rubber molecular chains, leading to a decrease in material flexibility.

[0032] Figure 3(a) The dielectric constant and dielectric loss diagrams of the in-situ grown SiO2 insulating layer modified silicone rubber prepared in step six of Examples 1-4 and the pure silicone rubber film prepared in Comparative Example 1 at 30°C; (b) The dielectric constant and dielectric loss diagram at 50°C; (c) The dielectric constant and dielectric loss diagram at 70°C; (d) The dielectric constant and dielectric loss diagram at 90°C. Depend on Figure 3 It is evident that the dielectric constant and dielectric loss of modified silicone rubbers with different ratios exhibit significant ratio-dependent and temperature-dependent characteristics at 30℃, 50℃, 70℃, and 90℃. Overall, the dielectric constant of each sample does not change significantly with increasing frequency within the test frequency range, fluctuating only slightly at the low-frequency end, indicating that its polarization response is generally stable. Simultaneously, the dielectric loss decreases rapidly with increasing frequency and approaches zero in the mid-to-high frequency region, indicating certain interfacial polarization and dipole orientation loss at low frequencies, while at high frequencies, this slow polarization process gradually becomes difficult to follow changes in the applied electric field. Regarding different samples, SiR-2 and SiR-3 generally have higher dielectric constants, with SiR-2 exhibiting the highest dielectric constant at most temperatures, indicating that a moderate amount of SiO2 formation is beneficial for improving the polarization ability of the system and enhancing the interfacial response; SiR-4 is next, while SiR and SiR-1 have relatively lower dielectric constants. The mechanism is that when the TEOS content is low, less SiO2 is generated in situ on the surface, making it difficult to form a sufficient interface regulation layer. This has a limited impact on the polarization of silicone rubber molecular chains and the polarization of the two-phase interface, resulting in a minimal improvement in dielectric properties. However, at a medium ratio, the SiO2 distribution is more uniform, and the interface layer is more complete. This increases the polarization centers at the interface and optimizes the two-phase interface interaction between the inorganic and organic phases, allowing the system to achieve a higher and more stable dielectric constant while maintaining low losses. When the TEOS ratio is further increased to 1:4, although the dielectric constant can still be improved, excessive SiO2 may lead to local agglomeration or enhanced interface constraint, restricting dipole orientation and increasing interface inhomogeneity. Therefore, the overall performance is not as good as the medium-ratio sample. As the temperature increases from 30℃ to 90℃, the overall dielectric constant of each sample decreases slightly or tends to level off, while low-frequency loss fluctuations increase. This indicates that increased temperature promotes molecular chain segment movement and interfacial charge migration, but the modified layer can still maintain good dielectric stability at high frequencies. Overall, in-situ growth of SiO2 does improve the dielectric response of silicone rubber, with SiR-2 showing better performance in loss control and two-phase interface matching.

[0033] Figure 4 (a) Conductivity diagrams of the in-situ grown SiO2 insulating layer modified silicone rubber prepared in step six of Examples 1-4 and the pure silicone rubber film prepared in Comparative Example 1 at 30°C; (b) Conductivity diagram at 50°C; (c) Conductivity diagram at 70°C; (d) Conductivity diagram at 90°C. Depend on Figure 4 As can be seen, at 30℃, 50℃, 70℃, and 90℃, the conductivity of all samples increased with increasing electric field, exhibiting obvious field strength dependence and temperature dependence, but significant differences existed between different formulations. Overall, pure SiR consistently exhibited the highest conductivity, and its conductivity increased most significantly in the high-field region with increasing temperature, indicating that unmodified silicone rubber is more prone to carrier migration and conductive channel expansion under high temperature and high field conditions. In contrast, the conductivity of the modified samples decreased significantly, indicating that the in-situ grown SiO2 layer effectively suppressed charge transport. Among them, the SiR-2 sample had the lowest overall conductivity at all four temperatures and the most gradual change with electric field, indicating that it had the best insulation performance and optimal conductivity stability. SiR-1 was second, but its conductivity increased more significantly at medium and high temperatures and high fields. The conductivity of SiR-3 and SiR-4 was generally higher than that of SiR-2, especially with faster growth under high field conditions, indicating that their ability to suppress carrier transport was weakened. The mechanism is that, with a low ratio such as 1:8, the TEOS content is low, resulting in a thinner or discontinuous SiO2 layer, making it difficult to form a complete and dense insulating barrier layer. This limits the suppression of carrier injection and migration, so although the conductivity is lower than that of pure SiR, it is still not as good as the optimal ratio. With a medium ratio of 1:6, the amount and distribution of the SiO2 layer are most suitable. It can form a relatively uniform and dense inorganic insulating layer, effectively improving the interface barrier and limiting charge injection, without introducing new interface defects due to excessive inorganic phase. Therefore, it exhibits the lowest conductivity and the best insulation performance. When the ratio is further increased to 1:5 and 1:4, excessive SiO2 may lead to local agglomeration, increased phase interfaces, or structural inhomogeneity, which in turn provides more interface channels for carrier accumulation and transition. At the same time, excessively strong interface constraints may also bring new local defects, leading to a rebound in conductivity. Overall, the figure shows that in-situ growth of SiO2 on the surface of silicone rubber via the sol-gel method can significantly reduce the electrical conductivity of the material, and a moderate TEOS / ethanol ratio (1:6) is most conducive to obtaining an insulating layer with density, uniformity and interface stability, thereby achieving the best electrical insulation performance.

[0034] Figure 5 (a) is a charge accumulation density diagram of the composite medium based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface prepared in step eight of Examples 1-4, and the composite medium based on the silicone rubber film / polypropylene interface prepared in Comparative Example 1; (b)-(e) are charge accumulation density diagrams of different modified silicone rubber / polypropylene interfaces. Depend on Figure 5It is evident that after different modified silicone rubbers form an interface with PP, the interfacial charge accumulation density is predominantly negative, and its absolute value increases significantly with increasing electric field. However, the response to temperature varies greatly among different formulations. For unmodified PP / SiR, the interfacial charge remains essentially stable and negative between 25-70℃, for example, approximately -5 to -4 × 10⁻⁵ at 1 kV / mm. -5 C / m 2 At 5kV / mm, the value is approximately -25 to -27 × 10⁻⁶. -5 C / m 2 At 10kV / mm, the value is approximately -37 to -39 × 10⁻⁶. -5 C / m 2 At 12.5 kV / mm, it can reach -40 to -44 × 10⁻⁴. -5 C / m 2 At 90℃, all curves shifted upwards, indicating some release of interfacial charge at high temperatures, but overall, a large accumulation of negative charge remained. The absolute value of PP / 1:8 at low temperatures was significantly lower than that of pure SiR, indicating that a small amount of SiO2 could weaken interfacial charge accumulation; however, at 90℃, most curves rapidly approached zero, even turning positive at 12.5 kV / mm to approximately 8 × 10⁻⁶. -5 C / m 2 This indicates that the SiO2 layer formed by low ratios is discontinuous and lacks sufficient interface constraint, resulting in poor interfacial charge stability under high temperature and high field conditions. PP / 1:6 performs best, exhibiting the smallest absolute value of interfacial charge across all electric fields from 25-50℃, indicating the most stable interface. PP / 1:5 is second best, suggesting that moderately high SiO2 content can improve the interface, but it is not as stable as 1:6. PP / 1:4 shows further deterioration, maintaining a large negative value under high field conditions from 25-50℃, such as approximately -27 to -28 × 10⁻² at 7.5 kV / mm. -5 C / m 2 10kV / mm is approximately -33×10 -5 C / m 2 At 70℃, the low-to-medium field slightly rebounded, but the high field remained very large. At 90℃, it almost returned to its maximum negative value, indicating that excessive SiO2 at high ratios actually led to interface defects, agglomeration, or an increase in the two-phase interface, making it easier to form deep traps and local charge accumulation under high temperature and high field conditions. The unmodified PP / SiR interface showed the most significant negative charge accumulation, while the interface charge was effectively weakened after the introduction of SiO2. Among them, the 1:6 sample showed the smallest interface charge accumulation density and the best temperature stability under most temperatures and electric fields, indicating that the SiO2 layer formed was the most uniform and dense, which could more effectively optimize the two-phase interface matching between PP and SiR and suppress interface polarization and charge injection. When the ratio was too low (1:8), the SiO2 layer was discontinuous and the blocking effect was insufficient, while when the ratio was too high (1:4), the excessive inorganic phase may introduce new heterogeneous interfaces and local defects.

[0035] Figure 6 (a) Breakdown field strength diagrams of the in-situ grown SiO2 insulating layer modified silicone rubber prepared in step six of Examples 1-4 and the pure silicone rubber film prepared in Comparative Example 1 at 30°C; (b) Breakdown field strength diagram at 50°C; (c) Breakdown field strength diagram at 70°C. Depend on Figure 6 It can be seen that the characteristic breakdown field strength of pure SiR is 124.85 kV / mm, while that of modified SiR-2 increases to 147.95 kV / mm, an increase of approximately 18.5%, which is the highest among all groups. SiR-1, SiR-3, and SiR-4 have characteristics of approximately 130.22, 139.53, and 139.83 kV / mm, respectively, which are higher than pure SiR, but still lower than SiR-2. With increasing temperature, the breakdown field strength of all samples decreases to some extent, but the modified samples still maintain a significant advantage. This indicates that in-situ growth of SiO2 on the surface of silicone rubber via the sol-gel method effectively improves the intrinsic insulation strength of the material. The mechanism mainly lies in the following: the in-situ generated SiO2 layer has high bond energy and intrinsic insulation, which can construct a denser and more stable inorganic / organic synergistic network on the surface of silicone rubber. On the one hand, it hinders the injection and migration of charge carriers, increases the energy required for electrons to cross local weak regions, and thus reduces the probability of conductive channel formation. On the other hand, the introduction of SiO2 will form more deep traps in silicone rubber, which can capture high-energy electrons and inhibit electron migration, delaying the expansion of electrical trees and local breakdown channels. In addition, an appropriate amount of SiO2 can also improve the uniformity of the internal microstructure of the material, reduce local defects and stress concentration, and make the electric field distribution more uniform, thus significantly improving the breakdown field strength. The 1:6 ratio is optimal because it provides the most suitable amount and distribution of SiO2, forming a continuous, uniform, and dense insulating barrier layer without causing agglomeration or new interface defects due to excessive inorganic phase. If the ratio is too low, insufficient SiO2 formation makes it difficult to form a complete protective layer, limiting the reinforcing effect. If the ratio is too high, local agglomeration, interface inhomogeneity, and excessive restriction of chain segment movement may introduce new electrical weaknesses. Therefore, although the breakdown performance is still better than pure SiR, it is not as good as the medium-ratio sample. Overall, the improvement in the breakdown strength of modified silicone rubber is essentially the result of the combined effects of deep trap trapping and defect suppression brought about by in-situ SiO2 growth.

[0036] Figure 7 (a) is the breakdown voltage diagram of the composite medium based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface prepared in step eight of Examples 1-4, and the silicone rubber film / polypropylene prepared in Control Example 1 at 0.3 MPa; (b) is the breakdown voltage diagram at 0.4 MPa; (c) is the breakdown voltage diagram at 0.5 MPa. Depend on Figure 7It can be seen that under pressures of 0.3 MPa, 0.4 MPa, and 0.5 MPa, the breakdown voltage of the PP / SiR-2 interface is significantly higher than that of the unmodified PP / SiR. Specifically, at 0.3 MPa, the breakdown voltage of PP / SiR is 15.47 kV, while that of PP / SiR-2 increases to 20.76 kV, an increase of approximately 34.2%; at 0.4 MPa, the breakdown voltage of PP / SiR is 16.41 kV, while that of PP / SiR-2 increases to 23.05 kV, an increase of approximately 40.5%; and at 0.5 MPa, the breakdown voltage of PP / SiR is 18.94 kV, while that of PP / SiR-2 increases to 24.61 kV, an increase of approximately 29.9%. Furthermore, the modification not only improves the breakdown voltage but also reduces the interfacial breakdown dispersion and enhances reliability. The mechanism lies in the fact that the SiO2 insulating layer grown in situ on the surface of SiR-2 constructs a more uniform and dense interface transition layer between PP and SiR, which can effectively improve the interface matching of the two phases, reduce the interface micro-gap and local defects, suppress the accumulation of interface charge and the resulting electric field distortion, thereby improving the interface breakdown strength. At the same time, as the pressure increases from 0.3MPa to 0.5MPa, the breakdown voltage of both PP / SiR and PP / SiR-2 further increases, which also indicates that the applied pressure helps to compact the interface and enhance the contact tightness. The modified SiR-2 exhibits better interface stability and insulation reliability in this process.

[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 composite dielectric based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface, characterized in that... The composite medium is formed by growing a SiO2 insulating layer at the interface between silicone rubber and polypropylene using the sol-gel method. The sol-gel method uses tetraethyl orthosilicate as a precursor, and prepares mixed systems with different ratios by adjusting the volume ratio of tetraethyl orthosilicate to anhydrous ethanol. Ammonia is then used as a catalyst, and deionized water is used to promote the hydrolysis reaction. The silicone rubber is a two-component liquid adhesive for cable accessories, comprising component A and component B.

2. The composite medium based on in-situ grown insulating layer modified silicone rubber / polypropylene interface according to claim 1, characterized in that... The ratio of the total volume of components A and B of the silicone rubber to the amount of polypropylene, tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia is 120mL:2.6g:10mL:(40~80mL):8mL:(1~2mL).

3. The method for preparing a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface as described in claim 1, characterized in that... The preparation method described herein is specifically carried out according to the following steps:

1. Add equal volumes of component A and component B of liquid silicone rubber to two reactors respectively; stir them separately, then mix component A and component B of the silicone rubber under vacuum conditions, and stir again under vacuum conditions to obtain a silicone rubber mixture.

2. Apply the silicone rubber mixture evenly into the mold, and then place the mold into a flat vulcanizing machine for a first vulcanization to obtain the composite medium after the first vulcanization.

3. Place the composite medium after primary vulcanization in a high-temperature forced-air drying oven for secondary vulcanization to obtain the composite medium after secondary vulcanization; let the composite medium after secondary vulcanization stand at room temperature for a period of time to obtain a silicone rubber film.

4. Mix tetraethyl orthosilicate with anhydrous ethanol to obtain a mixed solution; add deionized water dropwise to the mixed solution, continue stirring, then add ammonia water and continue stirring to obtain a mixed sol; 5. Let the mixed sol stand, then pour it into a petri dish, and immerse the silicone rubber film in the mixed sol. After immersion, slowly lift the film to form a uniform gel film on its surface, thus obtaining the composite film.

6. The composite film is placed in a vacuum oven and dried at gradient temperatures to obtain silicone rubber modified with in-situ grown SiO2 insulating layer; 7. Spread the polypropylene granules evenly into the mold, then place the mold into a flat vulcanizing machine, melt them separately under gradient pressure, and then cool them with water to room temperature to obtain a polypropylene film.

8. A polypropylene film is placed on the insulating layer of silicone rubber modified by in-situ growth of SiO2 insulating layer to obtain a composite medium based on the interface of silicone rubber / polypropylene modified by in-situ growth of insulating layer.

4. The method for preparing a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface according to claim 3, characterized in that... The stirring speed in step one is 2.4 r / s, and the stirring time is 1 h to 1.5 h; the stirring speed under vacuum conditions in step one is 3.0 r / s, and the stirring time is 1.5 h to 2 h.

5. The method for preparing a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface according to claim 3, characterized in that... The mold mentioned in step two is a polyester film mold with a size of 100mm. 100mm 0.175mm; the temperature of the first vulcanization in step two is 160℃~170℃, the time of the first vulcanization is 10min~20min, and the pressure of the flat vulcanizing machine is 15MPa.

6. The method for preparing a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface according to claim 3, characterized in that... The temperature of the secondary vulcanization in step three is 200℃, and the time of the secondary vulcanization is 4h; the standing time in step three is 0.5h~1h; in step four, tetraethyl orthosilicate and anhydrous ethanol are mixed and stirred at a stirring speed of 3r / s for 10min to obtain a mixed solution; deionized water is added dropwise to the mixed solution, stirring is continued for 10min, then ammonia is added, and stirring is continued for 2~2.5h to obtain a mixed sol.

7. The method for preparing a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface according to claim 3, characterized in that... In step four, the volume ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia is 10 mL:(40~80 mL):8 mL:(1~2 mL); the total volume ratio of the silicone rubber components A and B in step one to the tetraethyl orthosilicate in step four is 120 mL:10 mL; the standing time in step five is 0.5 h, and the lifting speed is 0.5 cm / s.

8. The method for preparing a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface according to claim 3, characterized in that... The gradient temperatures mentioned in step six are 25℃, 50℃, and 75℃; drying is carried out at 25℃, 50℃, and 75℃ for 1h to 1.5h respectively; the total volume ratio of the silicone rubber components A and B mentioned in step one to the mass ratio of the polypropylene mentioned in step seven is 120mL:2.6g; the mold mentioned in step seven is a polyester film mold with a size of 100mm. 100mm 0.175mm; the gradient pressure mentioned in step seven is 0MPa, 5MPa, 10MPa, 15MPa; melting for 10min, 5min, 5min, 5min respectively at 0MPa, 5MPa, 10MPa, 15MPa.

9. The method for preparing a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface according to claim 3, characterized in that... The composite dielectric based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface described in step seven has a breakdown field strength of 128.93~141.77 kV / mm at 30°C, 129.44~144.36 kV / mm at 50°C, and 130.22~147.95 kV / mm at 70°C. At room temperature, the composite dielectric based on the in-situ grown insulating layer modified silicone rubber / polypropylene interface described in step seven has an interface breakdown voltage of 18.69~20.76 kV at 0.3 MPa, 20.03~23.05 kV at 0.4 MPa, and 21.70~24.61 kV at 0.5 MPa.

10. The application of a composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface as described in claim 1, characterized in that... A composite medium based on an in-situ grown insulating layer modified silicone rubber / polypropylene interface is used as an insulating material or interface insulating material in high-voltage cable accessories, heat-resistant wires and cables, temperature measuring wires, or power transmission and distribution cables for high-temperature conditions.

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