A back protective film for photovoltaic module and a preparation process thereof

Abstract

The application discloses a back protective film for photovoltaic module and a preparation process thereof. The protective film comprises a weather-resistant layer, a substrate layer and a heat-sealing layer, and the thickness ratio is (10-25):(50-80):(10-25). The weather-resistant layer and the heat-sealing layer are made of a modified polyvinyl chloride composition, which comprises a PVC resin, a polyvinylidene fluoride-hexafluoropropylene copolymer, a polyhexafluorobutyl methacrylate grafted nano cerium dioxide, an alumina-coated hindered amine light stabilizer, an epoxy soybean oil and a methyl tin mercapto thermal stabilizer. The substrate layer is made of a reinforced polyvinyl chloride composition, which comprises a titanate coupling agent modified basalt flake, an ASA impact modifier, a chlorinated polyethylene and a lubricant. The process comprises double-screw melt blending granulation of two types of granules, three-layer co-extrusion film forming, wind ring cooling, traction, corona treatment and winding to obtain the protective film. The film has the properties of ultraviolet resistance, low migration and mechanical reinforcement, and is suitable for back protection of photovoltaic modules.

Description

Technical Field

[0001] This invention belongs to the field of photovoltaic module protective film preparation technology, specifically relating to a photovoltaic module back protective film and its preparation process. Background Technology

[0002] With the continued application of photovoltaic modules in outdoor power plants, commercial and industrial rooftops, and distributed systems, the back-side protective material needs to withstand the combined effects of various external factors under long-term exposure conditions, including ultraviolet radiation, alternating temperature and humidity, damp heat corrosion, electrical stress, and mechanical stress. The back-side protective film not only provides electrical insulation and mechanical protection for the internal structure of the module but also directly relates to moisture barrier properties, interfacial adhesion, dimensional stability, and long-term service reliability. Considering that photovoltaic modules typically need to meet usage requirements of over 25 years, the back-side protective material, in addition to basic processing and molding properties, should maintain high levels of weather resistance, UV aging resistance, water and oxygen barrier properties, mechanical strength, and interlayer bonding stability to meet the actual needs of module encapsulation, lamination, and subsequent long-term operation.

[0003] For example, Chinese patent application CN 115000217A discloses a photovoltaic module using an isolation protective film. By adding an isolation film consisting of a base film and an adhesive layer to the upper and lower sides of the cell layer, the traditional five-layer structure is expanded to a seven-layer structure. Combined with UV adhesive curing interconnect technology, this aims to solve the problems of high breakage rate and easy loosening of conductors in traditional welding processes for thin-film cells, thereby improving the encapsulation protection effect and reliability of the module. Another example is Chinese patent application CN112635599 B, which provides a composite film for the backsheet of a photovoltaic module. This technology laminates a white film layer and a black film layer with a specific formulation. The black film layer uses carbon black or metallic black to give the module an all-black appearance, while the white film layer uses the high reflectivity of titanium dioxide to reflect light that has not been absorbed by the cell back into the cell to improve power generation. Although this patent does not use polyvinyl chloride (PVC) as the substrate, its composite film layer structure constructed with fluorocarbon resin achieves a balance between appearance and efficiency while significantly improving the weather resistance of the backsheet.

[0004] Therefore, current backsheet materials are still mainly composite and coated structures. The former typically relies on multi-layer base film composites to achieve weather resistance, barrier properties, and mechanical properties, while the latter relies more on surface coatings to improve weather resistance and insulation performance. Although these solutions can meet the requirements of photovoltaic modules to a certain extent, they often suffer from problems such as complex material systems, long preparation processes, and high manufacturing costs. Especially when using fluorinated resins or other highly weather-resistant materials, material costs and supply stability can also affect product promotion. In contrast, polyvinyl chloride (PVC) materials have advantages such as wide availability, lower cost, and better processing adaptability, and have a certain application foundation in film preparation. However, conventional PVC systems are prone to insufficient UV aging resistance, yellowing, brittleness, and unsatisfactory moisture barrier properties under long-term outdoor use. Especially under the combined effects of humidity and heat and photothermal effects, problems such as additive migration, interface degradation, and accumulation of microscopic defects can further affect its long-term stability.

[0005] Therefore, how to further improve the weather resistance, barrier properties, insulation and interlayer stability of PVC materials while retaining their ease of processing and cost advantages, and at the same time adapt to the requirements of industrial continuous co-extrusion molding, remains a technical problem that needs to be solved in this field. Summary of the Invention

[0006] To address the aforementioned shortcomings, this invention provides a protective film for the back of a photovoltaic module and its preparation process.

[0007] The present invention provides the following technical solution: a protective film on the back of a photovoltaic module, comprising a weather-resistant layer, a substrate layer and a heat-sealing layer arranged sequentially from top to bottom, wherein the thickness ratio of the weather-resistant layer, the substrate layer and the heat-sealing layer is (10-25):(50-80):(10-25); the weather-resistant layer and the heat-sealing layer are made of modified polyvinyl chloride composition, and the substrate layer is made of reinforced polyvinyl chloride composition; The raw materials for preparing the modified polyvinyl chloride composition, by weight, include: 80-95 parts of polyvinyl chloride resin, 5-15 parts of polyvinylidene fluoride-hexafluoropropylene copolymer, 2-4 parts of modified nano-cerium dioxide, 1-3 parts of modified hindered amine light stabilizer, 2-5 parts of epoxidized soybean oil, and 1-2 parts of methyltin mercapto heat stabilizer. The raw materials for preparing the reinforced polyvinyl chloride composition, by weight, include: 100 parts of polyvinyl chloride resin, 5-10 parts of basalt flakes modified with titanate coupling agent, 3-8 parts of acrylonitrile-styrene-acrylate terpolymer as an impact modifier, 4-6 parts of chlorinated polyethylene, and 0.5-1.5 parts of lubricant.

[0008] Furthermore, the preparation method of the modified nano-cerium dioxide includes the following steps: A1: Based on 100 parts by weight of nano-cerium dioxide (CeO2) with a particle size D50 of 10-50 nm, the nano-cerium dioxide (CeO2) is dispersed in ethanol, with the amount of ethanol being 800-2000 parts by weight; 3-10 parts by weight of silane coupling agent are added, and the reaction is carried out at 60-70°C with stirring for 4-6 h; the silane coupling agent is KH-570; A2: After the reaction is complete, centrifuge at 6000-10000 rpm for 5-15 min to separate the solid. Wash the separated solid with 75% ethanol aqueous solution 1-2 times, and then wash with deionized water 1-3 times. A3: Subsequently, the nano-cerium dioxide was vacuum dried at a temperature of 60-80 °C for 6-12 h to obtain double-bond functionalized nano-cerium dioxide with carbon-carbon double bonds introduced on the surface by introducing olefin bonds on the methacryloyl group. A4: The double-bond functionalized nano-cerium dioxide obtained in step A3 is added to 600-3000 parts by weight of ethyl acetate to form a dispersion; then 30-120 parts by weight of hexafluorobutyl methacrylate and 0.5-2.5 parts by weight of azobisisobutyronitrile are added, and the mixture is reacted at 75-85°C for 6-8 h under nitrogen protection. A5: After the reaction is complete and cooled to room temperature, add 3 to 8 times the volume of the reaction product in n-hexane. Let stand for 1 to 6 hours to complete the induced precipitation. Then, vacuum dry the precipitated solid at 50 to 70°C for 6 to 12 hours to obtain modified cerium dioxide nanoparticles with a particle size D50 of 20 to 80 nm. In this application, the nanoparticles obtained through steps A1-A5 are specifically poly(hexafluorobutyl methacrylate) grafted and modified cerium dioxide.

[0009] Furthermore, the modified hindered amine light stabilizer is an alumina-coated hindered amine light stabilizer, and its preparation method includes the following steps: B1: Add the hindered amine light stabilizer GW-944 to an ethanol-water dispersion with a volume ratio of ethanol to deionized water of 60:40 to 95:5 to obtain a pre-dispersion, wherein the solid content of GW-944 in the pre-dispersion is 10 to 80 g / L. B2: Disperse ultrasonically at 150-400 W for 10-30 min, then stir at 300-800 rpm for 10-30 min at 20-35℃ to obtain GW-944 dispersion; B3: Add the aluminum source precursor aqueous solution dropwise to the GW-944 dispersion for 10–60 min, and adjust the pH of the GW-944 dispersion with the added aluminum source precursor to 8.5–10.5 in real time. After the addition is completed, continue stirring and reacting at 25–45°C for 2–8 h. In step B3, the aluminum source precursor hydrolyzes and deposits in situ on the surface of GW-944 to form an alumina coating layer. The mass fraction concentration of the aluminum source precursor in the aqueous solution is 5%–10%. The amount of the aluminum source precursor aqueous solution added is 2%–15% (w / w) of the mass of the added GW-944. The pH is adjusted by ammonia or organic amine. B4: After the reaction is complete, the solid is collected by centrifugation at 6000-12000 rpm for 5-15 min, washed with deionized water 3-5 times, and then dried under vacuum at 50-70℃ for 6-12 h to obtain alumina-coated hindered amine light stabilizer.

[0010] Furthermore, the aluminum source precursor is at least one of aluminum isopropoxide, aluminum sec-butoxide, or aluminum chloride.

[0011] Furthermore, the preparation method of the titanate coupling agent modified basalt flakes includes the following steps: C1: Basalt flakes with a particle size of 5–20 μm and a thickness of 0.5–2 μm are added to acetone, ultrasonically cleaned for 10–20 min, filtered, and dried at 60–80 °C for 1–3 h; the dried basalt flakes are dispersed in isopropanol to obtain a basalt flake isopropanol dispersion; the solid content of the basalt flakes in the basalt flake isopropanol dispersion is 50–200 g / L; C2: Add titanate coupling agent to the isopropanol dispersion of basalt flakes, and stir the mixture at 150-180 rpm for 0.5-2 h at 25-45°C; the mass ratio of titanate coupling agent to basalt flakes is (0.3-2.0):1. C3: After the reaction is complete, filter and wash twice with the anhydrous ethanol, then vacuum dry at 60-90°C for 2-6 h to obtain titanate coupling agent modified basalt flakes.

[0012] Furthermore, the titanate coupling agent is one of isopropyltriisostearoyl titanate, tetraisopropyl titanate, or isopropyltris(dicepanoylphosphoyloxy) titanate.

[0013] Further, the lubricant is at least one of stearic acid, calcium stearate, or polyethylene wax; the methyltin mercapto heat stabilizer is one or more of methyltin mercaptoethanol, methyltin mercaptoacetate, methyltin mercaptopropionate, methyltin isooctyl mercaptoacetate, or methyltin isooctyl mercaptopropionate.

[0014] This application also provides a process for preparing the protective film on the back of a photovoltaic module as described above, comprising the following steps: S1: The modified polyvinyl chloride composition raw material and the reinforced polyvinyl chloride composition raw material are respectively added to a high-speed mixer and mixed. Then, they are respectively passed through a twin-screw extruder at a temperature of 155℃~185℃, a speed of 150~450 rpm, a main machine torque of 45%~80% and a melt pressure of 8 MPa~16 MPa to melt blend and granulate, respectively to obtain the corresponding first granules and second granules. S2: The first granules are fed into the first and third extruders of the three-layer co-extrusion blown film unit, and the second granules are fed into the second extruder. After the three extruders melt and plasticize, they are extruded through the three-layer co-extrusion die to form a film bubble. The resulting film bubble forms a weather-resistant layer, a substrate layer, and a heat-sealing layer from top to bottom. The weather-resistant layer and the heat-sealing layer are both made from the first granules, and the substrate layer is made from the second granules. The ratio of the feed rates of the three extruders corresponds to the ratio of the thicknesses of the three-layer structure. S3: The membrane bubble is cooled by the air ring, flattened by the herringbone plate, pulled, corona treated, and then wound up to obtain the protective film on the back of the photovoltaic module.

[0015] Furthermore, during the melting and plasticizing process of the three extruders in step S2, the melt pressure at the extruder die head is stably controlled at 12 MPa to 18 MPa; and the actual melt temperature when the material reaches the die head is controlled at 185℃ to 195℃.

[0016] Furthermore, the air ring cooling in step S3 adopts a dual-outlet air ring, with the main air ring airflow controlled at 2500 m³ / h. 3 / h~3500 m 3 / h, air temperature is 8℃~15℃; the angle of the herringbone clamp is controlled at 15°~25°; the draw ratio of traction speed to extrusion speed is controlled between 1:3 and 1:6; the power of corona treatment is 2.0 kW~3.5 kW, so that the surface tension of the film after corona treatment reaches 52 dyn / cm~56 dyn / cm; the winding tension is controlled at 80 N~120 N, and the taper tension is controlled at 30%~50% to avoid problems such as crushing, wrinkling, excessive internal stress, core collapse, and telescope roll caused by maintaining high tension after the roll diameter increases.

[0017] The beneficial effects of this invention are as follows: 1. This application introduces grafted modified nano-cerium dioxide into the modified polyvinyl chloride composition of the weather-resistant layer and the heat-sealing layer, and combines it with an alumina-coated hindered amine light stabilizer, which can form a synergy in terms of ultraviolet absorption / quenching and free radical inhibition, thereby improving the efficiency of photothermal aging inhibition. At the same time, the coating structure and grafting structure help reduce the risk of small molecule migration and precipitation, and improve the long-term stability of the weather-resistant layer and the heat-sealing layer.

[0018] 2. In this application, basalt flakes are modified with titanate coupling agent in the substrate layer, and work together with ASA impact modifier and chlorinated polyethylene to form a more stable interfacial bond and stress transmission path in the PVC matrix, thereby improving the film's impact resistance, tear resistance and dimensional stability, and forming a "sheet barrier" effect to prevent water vapor / oxygen penetration, thus enhancing the comprehensive mechanical and barrier properties of the back protective film.

[0019] 3. This application uses two types of granules to granulate separately, and three extruders feed the material according to the corresponding layer position during the three-layer co-extrusion blown film process, so as to achieve one-time molding of the three-layer structure. This allows the material system of the weather-resistant layer, substrate layer and heat-sealing layer to be fed in the corresponding way and matched with the layer structure on the equipment, and the thickness ratio is controlled by the feeding amount. With the setting of air ring cooling, traction and corona treatment parameters in the post-processing, a finished film with controllable surface tension and stable winding quality can be obtained. The process repeatability and batch consistency are better, which is convenient for large-scale manufacturing and component packaging applications. Detailed Implementation

[0020] The technical solutions of the embodiments 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.

[0021] All the reagents used in this application can be purchased commercially. For example, the polyvinylidene fluoride-hexafluoropropylene copolymer used in this application, CAS number 9011-17-0, was purchased from Hubei Maidehao Biotechnology Co., Ltd.

[0022] Example 1 This embodiment provides a protective film for the back of a photovoltaic module, comprising a weather-resistant layer, a substrate layer, and a heat-sealing layer arranged sequentially from top to bottom, wherein the thickness ratio of the weather-resistant layer, the substrate layer, and the heat-sealing layer is 10:80:10; the weather-resistant layer and the heat-sealing layer are made of a modified polyvinyl chloride composition, and the substrate layer is made of a reinforced polyvinyl chloride composition; The raw materials for preparing the modified polyvinyl chloride composition, by weight, include: 80 parts of polyvinyl chloride resin, 10 parts of polyvinylidene fluoride-hexafluoropropylene copolymer, 2 parts of modified nano-cerium dioxide, 2 parts of modified hindered amine light stabilizer, 5 parts of epoxidized soybean oil, and 1 part of methyl tin mercaptohydride as a methyl tin mercaptohydride heat stabilizer. The raw materials for preparing the reinforced polyvinyl chloride composition, by weight, include: 100 parts of polyvinyl chloride resin, 7 parts of basalt flakes modified with titanate coupling agent, 3 parts of acrylonitrile-styrene-acrylate terpolymer as an impact modifier, 5 parts of chlorinated polyethylene (CPE), and 0.5 parts of polyethylene wax as a lubricant.

[0023] The preparation method of modified nano-cerium dioxide in this embodiment includes the following steps: A1: Based on 100 parts by weight of nano-cerium dioxide (CeO2) with a particle size D50 of 10 nm, the nano-cerium dioxide (CeO2) is dispersed in ethanol, and the amount of ethanol used is 2000 parts by weight; 3 parts by weight of silane coupling agent KH-570 are added, and the mixture is stirred at 60 °C for 5 h. A2: After the reaction is complete, centrifuge at 8000 rpm for 5 min to separate the solid. Wash the separated solid once with 75% ethanol aqueous solution and then wash it three times with deionized water. A3: Subsequently, the nano-cerium dioxide was vacuum dried at 80°C for 6 h to obtain olefin bonds introduced on the surface of the methacryloyl group, thus possessing carbon-carbon double bonds. A4: The double-bond functionalized nano-cerium dioxide obtained in step A3 was added to 600 parts by weight of ethyl acetate to form a dispersion; then 120 parts by weight of hexafluorobutyl methacrylate and 1.5 parts by weight of azobisisobutyronitrile were added, and the mixture was reacted at 85°C for 7 h under nitrogen protection. A5: After the reaction was completed and cooled to room temperature, 8 times the volume of the reaction product was added to n-hexane. After standing for 2 hours to complete the induced precipitation, the solid obtained by precipitation was vacuum dried at 50°C for 12 hours to obtain modified nano-cerium dioxide with a particle size D50 of 40 nm.

[0024] The modified hindered amine light stabilizer used in this embodiment is an alumina-coated hindered amine light stabilizer, and its preparation method includes the following steps: B1: The hindered amine light stabilizer GW-944 was added to an ethanol-water dispersion with a volume ratio of 60:40 for ethanol and deionized water to obtain a pre-dispersion, wherein the solid content of GW-944 in the pre-dispersion was 80 g / L. B2: Ultrasonic dispersion at 150 W for 30 min, followed by stirring at 300 rpm for 10 min at 35℃ to obtain GW-944 dispersion; B3: Add an aqueous solution of aluminum isopropoxide, serving as an aluminum source precursor, dropwise to the GW-944 dispersion for 60 min, while adjusting the pH of the GW-944 dispersion containing the added aluminum isopropoxide solution to 8.5 in real time. After the addition is complete, continue stirring and reacting at 45°C for 2 h. In step B3, the aluminum source precursor hydrolyzes and deposits an alumina coating layer in situ on the surface of GW-944. The mass fraction concentration of aluminum isopropoxide in the aluminum isopropoxide aqueous solution is 6.0% (w / w), and the amount of aluminum isopropoxide aqueous solution added is 2% (w / w) of the mass of the added GW-944. In this embodiment, the pH is adjusted using ammonia. B4: After the reaction was completed, the solid was collected by centrifugation at 12,000 rpm for 15 min, washed three times with deionized water, and then dried under vacuum at 70 °C for 9 h to obtain alumina-coated hindered amine light stabilizer.

[0025] The preparation method of basalt flakes modified with titanate coupling agent used in this embodiment includes the following steps: C1: Basalt flakes with a particle size of 5 μm and a thickness of 1.2 μm were ultrasonically cleaned in acetone for 10 min, filtered, and dried at 70 °C for 3 h; the dried basalt flakes were dispersed in isopropanol to obtain a basalt flake isopropanol dispersion; the solid content of the basalt flakes in the basalt flake isopropanol dispersion was 200 g / L. C2: Tetraisopropyl titanate as a titanate coupling agent was added to the isopropanol dispersion of basalt flakes, and the mixture was stirred at 180 rpm for 1.0 h at 25 °C; the mass ratio of tetraisopropyl titanate to basalt flakes was 0.3:1. C3: After the reaction is complete, filter and wash twice with the anhydrous ethanol, then vacuum dry at 60°C for 4 hours to obtain basalt flakes modified with titanate coupling agent, more specifically tetraisopropyl titanate modified basalt flakes.

[0026] The preparation process of the protective film on the back of the photovoltaic module in this embodiment includes the following steps: S1: The modified polyvinyl chloride composition raw material and the reinforced polyvinyl chloride composition raw material are added to a high-speed mixer and mixed. Then, they are melt-blended and granulated by a twin-screw extruder at 155°C, 300 rpm, 45% of the main machine torque and 16 MPa melt pressure to obtain the corresponding first granules and second granules. S2: The first granules are fed into the first and third extruders of the three-layer co-extrusion blown film unit, and the second granules are fed into the second extruder. After melting and plasticizing in the three extruders, the material is extruded through the three-layer co-extrusion die to form a film bubble. The resulting film bubble forms a weather-resistant layer, a substrate layer, and a heat-sealing layer from top to bottom. The weather-resistant layer and the heat-sealing layer are both made from the first granules, and the substrate layer is made from the second granules. The ratio of the feed rates of the three extruders corresponds to the ratio of the thicknesses of the three-layer structure. During the melting and plasticizing process, the melt pressure at the extruder die head is stably controlled at 12MPa. The actual melt temperature when the material reaches the die head is controlled at 185℃. S3: The membrane bubble is passed through a dual-outlet air ring at a speed of 2500 m. 3The main airflow rate is / h, and the air temperature is 15℃ for cooling the air ring. After the herringbone plate is flattened at a 20° angle, it is pulled at a speed with a traction speed to extrusion speed ratio of 1:3. Then, it is corona treated with a power of 2.0 kW to make the surface tension of the corona-treated film reach 52 dyn / cm. Finally, it is wound with a winding tension of 100N and a taper tension of 30% to obtain a photovoltaic module back protective film with a thickness ratio of 10:80:10 arranged from top to bottom: a weather-resistant layer, a substrate layer, and a heat-sealing layer.

[0027] Example 2 This embodiment provides a protective film for the back of a photovoltaic module, comprising a weather-resistant layer, a substrate layer, and a heat-sealing layer arranged sequentially from top to bottom, wherein the thickness ratio of the weather-resistant layer, the substrate layer, and the heat-sealing layer is 20:50:25; the weather-resistant layer and the heat-sealing layer are made of a modified polyvinyl chloride composition, and the substrate layer is made of a reinforced polyvinyl chloride composition; The raw materials for preparing the modified polyvinyl chloride composition, by weight, include: 90 parts of polyvinyl chloride resin, 5 parts of polyvinylidene fluoride-hexafluoropropylene copolymer, 3 parts of modified nano-cerium dioxide, 1 part of modified hindered amine light stabilizer, 3 parts of epoxidized soybean oil, and 2 parts of methyltin isooctyl mercaptoacetate as a methyltin mercaptoacetate heat stabilizer. The raw materials for preparing the reinforced polyvinyl chloride composition, by weight, include: 100 parts of polyvinyl chloride resin, 5 parts of basalt flakes modified with titanate coupling agent, 6 parts of acrylonitrile-styrene-acrylate terpolymer as an impact modifier, 4 parts of chlorinated polyethylene (CPE), and 1.5 parts of calcium stearate as a lubricant.

[0028] The preparation method of modified nano-cerium dioxide in this embodiment includes the following steps: A1: Based on 100 parts by weight of nano-cerium dioxide (CeO2) with a particle size D50 of 30 nm, the nano-cerium dioxide (CeO2) is dispersed in ethanol, and the amount of ethanol used is 1200 parts by weight; 7 parts by weight of silane coupling agent KH-570 are added, and the mixture is stirred at 65 °C for 4 h. A2: After the reaction is complete, centrifuge at 6000 rpm for 10 min to separate the solid. Wash the separated solid twice with 75% ethanol aqueous solution and then twice with deionized water. A3: Subsequently, the nano-cerium dioxide was vacuum dried at 70°C for 9 hours to obtain olefin bonds introduced on the surface of the methacryloyl group, thus possessing carbon-carbon double bonds. A4: The double-bond functionalized nano-cerium dioxide obtained in step A3 is added to 1800 parts by weight of ethyl acetate to form a dispersion; then 60 parts by weight of hexafluorobutyl methacrylate and 0.5 parts by weight of azobisisobutyronitrile are added, and the mixture is reacted at 80°C for 6 hours under nitrogen protection. A5: After the reaction was completed and cooled to room temperature, 5 times the volume of the reaction product was added to n-hexane. After standing for 1 hour to complete the induced precipitation, the solid obtained by precipitation was vacuum dried at 60℃ for 8 hours to obtain modified nano-cerium dioxide with a particle size D50 of 20nm.

[0029] The modified hindered amine light stabilizer used in this embodiment is an alumina-coated hindered amine light stabilizer, and its preparation method includes the following steps: B1: Hindered amine light stabilizer GW-944 was added to an ethanol-water dispersion with a volume ratio of 80:20 for ethanol and deionized water to obtain a pre-dispersion, wherein the solid content of GW-944 in the pre-dispersion was 40 g / L. B2: Ultrasonic dispersion at 300 W for 20 min, followed by stirring at 600 rpm for 20 min at 25℃ to obtain GW-944 dispersion; B3: Add an aqueous solution of aluminum sec-butoxide, serving as the aluminum source precursor, dropwise to the GW-944 dispersion for 30 min, while adjusting the pH of the GW-944 dispersion containing the added aluminum sec-butoxide aqueous solution to 9.5 in real time. After the addition is complete, continue stirring and reacting at 35°C for 5 h. In step B3, the aluminum source precursor hydrolyzes and deposits in situ on the surface of GW-944 to form an alumina coating layer. The mass fraction concentration of aluminum sec-butoxide in the aluminum sec-butoxide aqueous solution is 5.0% (w / w), and the amount of aluminum sec-butoxide aqueous solution added is 8% (w / w) of the mass of the added GW-944. In this embodiment, the pH is adjusted by triethylamine. B4: After the reaction was completed, the solid was collected by centrifugation at 9000 rpm for 10 min, washed 4 times with deionized water, and then dried under vacuum at 60℃ for 6 h to obtain alumina-coated hindered amine light stabilizer.

[0030] The preparation method of basalt flakes modified with titanate coupling agent used in this embodiment includes the following steps: C1: Basalt flakes with a particle size of 12 μm and a thickness of 0.5 μm were ultrasonically cleaned in acetone for 15 min, filtered, and dried at 60 °C for 2 h; the dried basalt flakes were dispersed in isopropanol to obtain a basalt flake isopropanol dispersion; the solid content of the basalt flakes in the basalt flake isopropanol dispersion was 120 g / L; C2: Isopropyltriisostearoyl titanate as a titanate coupling agent was added to the isopropanol dispersion of basalt flakes, and the mixture was stirred at 160 rpm for 0.5 h at 35 °C; the mass ratio of the added isopropyltriisostearoyl titanate to the mass of the basalt flakes was 1.0:1. C3: After the reaction is complete, filter and wash twice with the anhydrous ethanol, then vacuum dry at 75°C for 2 hours to obtain basalt flakes modified with titanate coupling agent, more specifically, basalt flakes modified with isopropyl triisostearoyl titanate.

[0031] The preparation process of the protective film on the back of the photovoltaic module in this embodiment includes the following steps: S1: The modified polyvinyl chloride composition raw material and the reinforced polyvinyl chloride composition raw material are added to a high-speed mixer and mixed. Then, they are melt-blended and granulated by a twin-screw extruder at a temperature of 170°C, a speed of 150 rpm, 65% of the main machine torque, and a melt pressure of 12 MPa to obtain the corresponding first granules and second granules. S2: The first granules are fed into the first and third extruders of the three-layer co-extrusion blown film unit, and the second granules are fed into the second extruder. After melting and plasticizing in the three extruders, the material is extruded through the three-layer co-extrusion die to form a film bubble. The resulting film bubble forms a weather-resistant layer, a substrate layer, and a heat-sealing layer from top to bottom. The weather-resistant layer and the heat-sealing layer are both made from the first granules, and the substrate layer is made from the second granules. The ratio of the feed rates of the three extruders corresponds to the ratio of the thicknesses of the three-layer structure. During the melting and plasticizing process, the melt pressure at the extruder die head is stably controlled at 15MPa. The actual melt temperature when the material reaches the die head is controlled at 190℃. S3: The membrane bubble is passed through a dual-outlet air ring at a speed of 3000 m. 3 The main airflow rate is / h, and the air temperature is 12℃ for cooling the air ring. After the herringbone plate is flattened at a 15° angle, it is pulled at a speed with a traction speed to extrusion speed ratio of 1:5. Then, it is corona treated with a power of 2.8 kW to make the surface tension of the corona-treated film reach 54 dyn / cm. Finally, it is wound up with a winding tension of 80N and a taper tension of 40% to obtain the photovoltaic module back protective film of this embodiment, which has a weather-resistant layer, a substrate layer and a heat-sealing layer arranged from top to bottom with a thickness ratio of 20:50:25.

[0032] Example 3 This embodiment provides a protective film for the back of a photovoltaic module, comprising a weather-resistant layer, a substrate layer, and a heat-sealing layer arranged sequentially from top to bottom, wherein the thickness ratio of the weather-resistant layer, the substrate layer, and the heat-sealing layer is 18:64:18; the weather-resistant layer and the heat-sealing layer are made of a modified polyvinyl chloride composition, and the substrate layer is made of a reinforced polyvinyl chloride composition; The raw materials for preparing the modified polyvinyl chloride composition, by weight, include: 95 parts of polyvinyl chloride resin, 15 parts of polyvinylidene fluoride-hexafluoropropylene copolymer, 4 parts of modified nano-cerium dioxide, 3 parts of modified hindered amine light stabilizer, 2 parts of epoxidized soybean oil, and 1.5 parts of methyltin mercaptopropionate as a methyltin mercapto-type heat stabilizer. The raw materials for preparing the reinforced polyvinyl chloride composition, by weight, include: 100 parts of polyvinyl chloride resin, 10 parts of basalt flakes modified with titanate coupling agent, 8 parts of acrylonitrile-styrene-acrylate terpolymer as an impact modifier, 6 parts of chlorinated polyethylene (CPE), and 1.0 part of stearic acid as a lubricant.

[0033] The preparation method of modified nano-cerium dioxide in this embodiment includes the following steps: A1: Based on 100 parts by weight of nano-cerium dioxide (CeO2) with a particle size D50 of 50 nm, the nano-cerium dioxide (CeO2) is dispersed in ethanol, and the amount of ethanol used is 800 parts by weight; 10 parts by weight of silane coupling agent KH-570 are added, and the mixture is stirred at 70 °C for 6 h. A2: After the reaction is complete, centrifuge at 10,000 rpm for 15 min to separate the solid. Wash the separated solid once with a 75% ethanol aqueous solution and then wash it once with deionized water. A3: Subsequently, the nano-cerium dioxide was vacuum dried at 60°C for 12 hours to obtain olefin bonds introduced on the surface of the methacryloyl group, thus possessing carbon-carbon double bonds. A4: The double-bond functionalized nano-cerium dioxide obtained in step A3 was added to 3000 parts by weight of ethyl acetate to form a dispersion; then 30 parts by weight of hexafluorobutyl methacrylate and 2.5 parts by weight of azobisisobutyronitrile were added, and the mixture was reacted at 75°C for 8 hours under nitrogen protection. A5: After the reaction was completed and cooled to room temperature, three times the volume of the reaction product was added to n-hexane. After standing for 6 hours to complete the induced precipitation, the solid obtained by precipitation was vacuum dried at 70°C for 6 hours to obtain modified nano-cerium dioxide with a particle size D50 of 80 nm.

[0034] The modified hindered amine light stabilizer used in this embodiment is an alumina-coated hindered amine light stabilizer, and its preparation method includes the following steps: B1: Add the hindered amine light stabilizer GW-944 to an ethanol-water dispersion with a volume ratio of 95:5 to ethanol and deionized water to obtain a pre-dispersion, wherein the solid content of GW-944 in the pre-dispersion is 10 g / L. B2: Ultrasonic dispersion at 400 W for 10 min, followed by stirring at 800 rpm for 30 min at 20℃ to obtain GW-944 dispersion; B3: Add aluminum chloride aqueous solution, which serves as the aluminum source precursor, dropwise to the GW-944 dispersion for 10 min, and adjust the pH of the GW-944 dispersion with added aluminum chloride aqueous solution to 10.5 in real time. After the addition is completed, continue stirring and reacting at 25°C for 8 h. In step B3, the aluminum source precursor hydrolyzes and deposits in situ on the surface of GW-944 to form an alumina coating layer. The mass fraction concentration of aluminum chloride in the aluminum chloride aqueous solution is 10.0% (w / w), and the amount of aluminum chloride aqueous solution added is 15% (w / w) of the mass of GW-944 added. In this embodiment, the pH is adjusted by ammonia. B4: After the reaction was completed, the solid was collected by centrifugation at 6000 rpm for 5 min, washed 5 times with deionized water, and then dried under vacuum at 50℃ for 12 h to obtain alumina-coated hindered amine light stabilizer.

[0035] The preparation method of basalt flakes modified with titanate coupling agent used in this embodiment includes the following steps: C1: Basalt flakes with a particle size of 20 μm and a thickness of 2.0 μm were ultrasonically cleaned in acetone for 20 min, filtered, and dried at 80 °C for 1 h; the dried basalt flakes were dispersed in isopropanol to obtain a basalt flake isopropanol dispersion; the solid content of the basalt flakes in the basalt flake isopropanol dispersion was 50 g / L; C2: Isopropyl tris(dicepylphosphonooxy)titanate, as a titanate coupling agent, was added to the isopropanol dispersion of basalt flakes, and the mixture was stirred at 150 rpm for 2.0 h at 45 °C; the mass ratio of the added isopropyl tris(dicepylphosphonooxy)titanate to the mass of the basalt flakes was 2.0:1. C3: After the reaction is complete, filter and wash twice with the anhydrous ethanol, then vacuum dry at 90°C for 6 hours to obtain basalt flakes modified with titanate coupling agent, more specifically isopropyl tris(dicepylphosphonooxy) titanate modified basalt flakes.

[0036] The preparation process of the protective film on the back of the photovoltaic module in this embodiment includes the following steps: S1: The modified polyvinyl chloride composition raw material and the reinforced polyvinyl chloride composition raw material are added to a high-speed mixer and mixed. Then, they are melt-blended and granulated by a twin-screw extruder at 185°C, 450 rpm, 80% of the main machine torque and 8 MPa melt pressure to obtain the corresponding first granules and second granules. S2: The first granules are fed into the first and third extruders of the three-layer co-extrusion blown film unit, and the second granules are fed into the second extruder. After melting and plasticizing in the three extruders, the material is extruded through the three-layer co-extrusion die to form a film bubble. The resulting film bubble forms a weather-resistant layer, a substrate layer, and a heat-sealing layer from top to bottom. The weather-resistant layer and the heat-sealing layer are both made from the first granules, and the substrate layer is made from the second granules. The ratio of the feed rates of the three extruders corresponds to the ratio of the thicknesses of the three-layer structure. During the melting and plasticizing process, the melt pressure at the extruder die head is stably controlled at 18MPa. The actual melt temperature when the material reaches the die head is controlled at 195℃. S3: The membrane bubble is passed through a dual-outlet air ring at a speed of 3500 m. 3 The main airflow rate is / h, and the air temperature is 8℃ for cooling the air ring. After being flattened by the herringbone plate at a 25° angle, it is pulled at a speed with a traction speed to extrusion speed ratio of 1:6. Then, it is corona treated with a power of 3.5kW to make the surface tension of the corona-treated film reach 56 dyn / cm. Finally, it is wound up with a winding tension of 120N and a taper tension of 50% to obtain the photovoltaic module back protective film of this embodiment, which has a weather-resistant layer, a substrate layer and a heat-sealing layer arranged from top to bottom with a thickness ratio of 18:64:18.

[0037] Comparative Example 1 Compared to Example 2, this comparative example uses commercially available unmodified cerium dioxide with a D50 particle size of 30 nm instead of the modified nano-cerium dioxide in Example 2, and uncoated hindered amine light stabilizer GW-944 instead of the modified hindered amine light stabilizer in Example 2. The other raw materials, corresponding components, and preparation processes are the same as in Example 2.

[0038] Comparative Example 2 Compared to Example 2, this comparative example directly uses unmodified basalt flakes with a particle size of 12 μm and a thickness of 0.5 μm to replace the titanate coupling agent modified basalt flakes in Example 2. The other raw materials, corresponding components, and preparation processes are the same as in Example 2.

[0039] To demonstrate the weather resistance and barrier properties of the technical solution of this application, the back protective films of Examples 1 to 3 and Comparative Examples 1 to 2 were selected as samples. Samples of specified sizes were cut and conditioned to stable quality in a standard environment. In the UV weathering evaluation, the samples were placed in a fluorescent UV lamp aging device, and irradiation and condensation cycles were set according to GB / T 16422.3-2002 "Laboratory Light Source Exposure Test Methods for Plastics - Part 3 Fluorescent UV Lamps", with a cumulative aging time of 2000 h. Before and after aging, the yellowing index was measured according to GB / T 39822-2021 "Determination of Yellow Index and its Change Value of Plastics" and ΔYI was calculated. Simultaneously, tensile tests were conducted according to GB / T 1040.3-2006 "Determination of Tensile Properties of Plastics - Part 3 Test Conditions for Films and Sheets" to obtain the elongation at break, and the elongation at break retention rate was calculated as the ratio of elongation at break after aging to that before aging. In the barrier performance evaluation, the water vapor transmission rate (WVTR) of the samples was determined using the cup method according to GB / T 1037-2021 "Determination of Water Vapor Transmission Performance of Plastic Films and Sheets - Cup Method for Weight Gain and Loss". In the damp heat aging evaluation, the samples were aged for 1000 hours under constant damp heat conditions according to GB / T 2423.3-2016 "Environmental Testing - Part 2: Test Methods - Cab - Constant Damp Heat Test". The WVTR was measured before and after aging according to GB / T 1037-2021, and the WVTR change rate was calculated based on the change after aging relative to before aging. Each test was performed in at least three parallel measurements, and the average value was used for comparative analysis. The test results are shown in Table 1.

[0040] The aging conditions for the yellowing index ΔYI and elongation at break retention rate in Table 1 were determined using the aforementioned UV aging conditions, supplemented by their respective yellowing index and elongation at break retention rate measurement standards. The third indicator, water vapor transmission rate (WVTR), was measured solely using GB / T 1037-2021. The fourth indicator, the WVTR change rate after damp heat aging, was measured using both GB / T 2423.3-2016 and GB / T 1037-2021 standards before and after aging.

[0041] Table 1. Test results of weather resistance and barrier properties The yellowing index ΔYI for Examples 1 to 3 were 1.8, 1.5, and 2.2, respectively, significantly lower than 8.5 for Comparative Example 1, and also lower than or close to 2.1 for Comparative Example 2. This result indicates that the structure and formulation of this application can effectively inhibit color degradation under long-term UV exposure, maintaining a more stable weather-resistant appearance. Comparative Example 1, in particular, demonstrates a significant advantage in resisting yellowing.

[0042] In terms of elongation at break retention, Examples 1 to 3 were 92%, 94%, and 89%, respectively, which are higher than Comparative Example 1's 55% and still maintain an advantage over Comparative Example 2's 88%. The improved elongation retention indicates that the material can still maintain good ductility and resistance to embrittlement after UV aging, meaning that the back protective film is less likely to experience early cracking or mechanical failure under thermal cycling and wind stress during outdoor service.

[0043] Looking at the WVTR values, Examples 1 to 3 are 1.2, 1.1, and 1.5, respectively, which are significantly lower than Comparative Example 1's 2.8 and Comparative Example 2's 1.6. This indicates that the film of this application has a stronger water vapor barrier capability, which can reduce the migration rate of water vapor into the component and help mitigate the risk of degradation of the encapsulation interface and electrical performance under humid and hot conditions.

[0044] Further analysis of the WVTR change rate after damp heat aging shows that Examples 1 to 3 had rates of 5%, 4%, and 7%, respectively, significantly lower than the 25% of Comparative Example 1 and 15% of Comparative Example 2. This indicator reflects the ability to maintain barrier performance under damp heat stress. The lower change rate in this application indicates that the microstructure and interface state of the film are more stable during damp heat aging, resulting in less degradation of barrier performance. Consequently, it can better maintain its effective shielding effect against water vapor under long-term constant damp heat service conditions.

[0045] In summary, the protective film on the back of this application not only exhibits lower yellowing and higher elongation retention under ultraviolet aging, but also shows better water vapor barrier level and lower barrier attenuation after normal and humid heat aging, demonstrating a synergistic improvement in weather resistance and barrier stability.

[0046] To demonstrate the electrical insulation and mechanical properties of this application, the back protective films of Examples 1 to 3 and Comparative Examples 1 to 2 were selected as samples. After conditioning under standard conditions, electrical insulation and mechanical property tests were conducted. Volume resistivity testing was performed according to GB / T 31838.2-2019 "Dielectric and resistive properties of solid insulating materials - Part 2: Resistive characteristics - DC method - Volume resistivity and volume resistivity", using the DC method to determine the sample volume resistivity and convert it to volume resistivity. Dielectric strength testing was performed according to GB / T 1408.1-2016 "Electrical strength test methods for insulating materials - Part 1: Tests at power frequency", using a short-time power frequency voltage boost method to measure the breakdown voltage and convert it to dielectric strength. Tensile strength testing was also performed according to GB / T1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets", preparing film samples and obtaining tensile strength data under specified test conditions. The interlayer peel strength was tested according to GB / T 8808-1988 "Test Method for Peel Strength of Flexible Composite Plastic Materials". Peel tests were performed on the composite layers, and the peel strength was characterized by the peel force per unit width. At least three parallel tests were conducted for each item, and the average value was used for comparative analysis. The test results are shown in Table 2.

[0047] Table 2 Test Results of Electrical Insulation and Mechanical Performance In terms of volume resistivity, Examples 1 to 3 are 2.5 × 10⁻⁶ respectively. 15 3.2×10 15 1.8×10 15 Ω·cm, significantly higher than 4.5×10 in Comparative Example 1. 14 The Ω·cm of Example 2 is approximately seven times higher than that of Comparative Example 1, and Example 1 is more than five times higher, indicating that the protective film on the back of this application has fewer volumetric conductive channels under a DC electric field, resulting in higher insulation reliability. Compared to Comparative Example 2's 2.2 × 10⁻⁶ Ω·cm, the Ω·cm of Example 2 is significantly higher than that of Comparative Example 1. 15 Compared to Ω·cm, Example 2 still maintains a higher level, indicating that the solution of this application has a stable advantage in improving insulation performance.

[0048] In terms of dielectric strength, Examples 1 to 3 are 110 kV / mm, 115 kV / mm, and 105 kV / mm, respectively, which are significantly higher than 78 kV / mm of Comparative Example 1, with an improvement of more than 30%. They are also higher than 102 kV / mm of Comparative Example 2, indicating that the film of this application has a stronger ability to withstand electric field stress and a lower risk of breakdown, which can improve the electrical safety margin of the back protective layer during long-term service.

[0049] In terms of tensile strength, Examples 1 to 3 showed strengths of 52 MPa, 55 MPa, and 48 MPa, respectively, which are generally higher than the 45 MPa of Comparative Example 1 and the 43 MPa of Comparative Example 2. Example 2, reaching 55 MPa, demonstrated better load-bearing capacity and crack resistance. These results indicate that the substrate reinforcement system and layer structure design can effectively improve the overall strength level of the film, providing more reliable mechanical support for the back of the component under transportation, installation, and thermal expansion and contraction stress.

[0050] In terms of interlayer peel strength, Examples 1 to 3 showed strengths of 6.5 N / cm, 6.8 N / cm, and 5.8 N / cm, respectively, all higher than Comparative Example 1's 5.2 N / cm and significantly higher than Comparative Example 2's 3.5 N / cm, with Example 2 nearly doubling the strength compared to Comparative Example 2. These results indicate that the interlayer interface of this application exhibits stronger bonding and greater resistance to delamination, which helps reduce the probability of interface warping and interlayer failure under thermal cycling and humid conditions.

[0051] Based on the comparative analysis of the four test results in Table 2, the protective film on the back of this application shows that it simultaneously improves in volume resistivity and dielectric strength, resulting in stronger electrical insulation and breakdown resistance; and simultaneously improves in tensile strength and interlayer peel strength, resulting in better load-bearing stability and interface durability.

[0052] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A protective film for the back of a photovoltaic module, characterized in that, It includes a weather-resistant layer, a substrate layer and a heat-sealing layer arranged sequentially from top to bottom, with the thickness ratio of the weather-resistant layer, the substrate layer and the heat-sealing layer being (10-25):(50-80):(10-25); the weather-resistant layer and the heat-sealing layer are made of modified polyvinyl chloride composition, and the substrate layer is made of reinforced polyvinyl chloride composition; The raw materials for preparing the modified polyvinyl chloride composition, by weight, include: 80-95 parts of polyvinyl chloride resin, 5-15 parts of polyvinylidene fluoride-hexafluoropropylene copolymer, 2-4 parts of modified nano-cerium dioxide, 1-3 parts of modified hindered amine light stabilizer, 2-5 parts of epoxidized soybean oil, and 1-2 parts of methyltin mercapto heat stabilizer. The raw materials for preparing the reinforced polyvinyl chloride composition, by weight, include: 100 parts of polyvinyl chloride resin, 5-10 parts of basalt flakes modified with titanate coupling agent, 3-8 parts of acrylonitrile-styrene-acrylate terpolymer as an impact modifier, 4-6 parts of chlorinated polyethylene, and 0.5-1.5 parts of lubricant.

2. The photovoltaic module back protective film according to claim 1, characterized in that, The method for preparing the modified nano-cerium dioxide includes the following steps: A1: Based on 100 parts by weight of nano-cerium dioxide with a particle size D50 of 10-50 nm, the nano-cerium dioxide is dispersed in ethanol, with the amount of ethanol being 800-2000 parts by weight; 3-10 parts by weight of silane coupling agent are added, and the mixture is stirred at 60-70°C for 4-6 h; the silane coupling agent is KH-570; A2: After the reaction is complete, centrifuge at 6000-10000 rpm for 5-15 min to separate the solid. Wash the separated solid with 75% ethanol aqueous solution 1-2 times, and then wash with deionized water 1-3 times. A3: Subsequently, the nano-cerium dioxide was vacuum dried at a temperature of 60-80 °C for 6-12 h to obtain double-bond functionalized nano-cerium dioxide with carbon-carbon double bonds introduced on the surface by introducing olefin bonds on the methacryloyl group. A4: The double-bond functionalized nano-cerium dioxide obtained in step A3 is added to 600-3000 parts by weight of ethyl acetate to form a dispersion; then 30-120 parts by weight of hexafluorobutyl methacrylate and 0.5-2.5 parts by weight of azobisisobutyronitrile are added, and the mixture is reacted at 75-85°C for 6-8 h under nitrogen protection. A5: After the reaction is completed and cooled to room temperature, add 3 to 8 times the volume of the reaction product in n-hexane, let stand for 1 to 6 hours to complete the induced precipitation, and then vacuum dry the precipitated solid at 50 to 70°C for 6 to 12 hours to obtain modified nano-cerium dioxide with a particle size D50 of 20 to 80 nm.

3. The photovoltaic module back protective film according to claim 1, characterized in that, The modified hindered amine light stabilizer is an alumina-coated hindered amine light stabilizer, and its preparation method includes the following steps: B1: Add the hindered amine light stabilizer GW-944 to an ethanol-water dispersion with a volume ratio of ethanol to deionized water of 60:40 to 95:5 to obtain a pre-dispersion, wherein the solid content of GW-944 in the pre-dispersion is 10 to 80 g / L. B2: Disperse ultrasonically at 150-400 W for 10-30 min, then stir at 300-800 rpm for 10-30 min at 20-35℃ to obtain GW-944 dispersion; B3: Add the aluminum source precursor aqueous solution dropwise to the GW-944 dispersion for 10–60 min, and adjust the pH of the GW-944 dispersion with the added aluminum source precursor to 8.5–10.5 in real time. After the addition is complete, continue stirring and reacting at 25–45℃ for 2–8 h; the mass fraction concentration of the aluminum source precursor in the aqueous solution is 5%–10%; the amount of the aluminum source precursor aqueous solution added is 2%–15% of the mass of the added GW-944. B4: After the reaction is complete, the solid is collected by centrifugation at 6000-12000 rpm for 5-15 min, washed with deionized water 3-5 times, and then dried under vacuum at 50-70℃ for 6-12 h to obtain alumina-coated hindered amine light stabilizer.

4. The photovoltaic module back protective film according to claim 3, characterized in that, The aluminum source precursor is at least one of aluminum isopropoxide, aluminum sec-butoxide, or aluminum chloride.

5. The photovoltaic module back protective film according to claim 1, characterized in that, The preparation method of the titanate coupling agent modified basalt flakes includes the following steps: C1: Basalt flakes with a particle size of 5–20 μm and a thickness of 0.5–2 μm are ultrasonically cleaned in acetone for 10–20 min, filtered, and dried at 60–80 °C for 1–3 h; the dried basalt flakes are dispersed in isopropanol to obtain a basalt flake isopropanol dispersion; the solid content of the basalt flakes in the basalt flake isopropanol dispersion is 50–200 g / L; C2: Add titanate coupling agent to the isopropanol dispersion of basalt flakes, and stir the mixture at 150-180 rpm for 0.5-2 h at 25-45°C; the mass ratio of titanate coupling agent to basalt flakes is (0.3-2.0):

1. C3: After the reaction is complete, filter and wash twice with the anhydrous ethanol, then vacuum dry at 60-90°C for 2-6 h to obtain titanate coupling agent modified basalt flakes.

6. The photovoltaic module back protective film according to claim 5, characterized in that, The titanate coupling agent is one of isopropyltriisostearoyl titanate, tetraisopropyl titanate, or isopropyltris(dicepylphosphoyloxy) titanate.

7. The photovoltaic module back protective film according to claim 1, characterized in that, The lubricant is at least one of stearic acid, calcium stearate, or polyethylene wax; the methyltin mercapto heat stabilizer is one or more of methyltin mercaptoethanol, methyltin mercaptoacetate, methyltin mercaptopropionate, methyltin isooctyl mercaptoacetate, or methyltin isooctyl mercaptopropionate.

8. A process for preparing a protective film on the back of a photovoltaic module as described in any one of claims 1-7, characterized in that, Includes the following steps: S1: The modified polyvinyl chloride composition raw material and the reinforced polyvinyl chloride composition raw material are respectively added to a high-speed mixer and mixed. Then, they are respectively passed through a twin-screw extruder at a temperature of 155℃~185℃, a speed of 150~450 rpm, a main machine torque of 45%~80% and a melt pressure of 8 MPa~16 MPa to melt blend and granulate, respectively to obtain the corresponding first granules and second granules. S2: The first granules are fed into the first and third extruders of the three-layer co-extrusion blown film unit, and the second granules are fed into the second extruder. After the three extruders melt and plasticize, they are extruded through the three-layer co-extrusion die to form a film bubble. The resulting film bubble forms a weather-resistant layer, a substrate layer, and a heat-sealing layer from top to bottom. The weather-resistant layer and the heat-sealing layer are both made from the first granules, and the substrate layer is made from the second granules. The ratio of the feed rates of the three extruders corresponds to the ratio of the thicknesses of the three-layer structure. S3: The membrane bubble is cooled by the air ring, flattened by the herringbone plate, pulled, corona treated, and then wound up to obtain the protective film on the back of the photovoltaic module.

9. The preparation process of the photovoltaic module back protective film according to claim 8, characterized in that, In step S2, during the melting and plasticizing process of the three extruders, the melt pressure at the extruder die head is stably controlled between 12 MPa and 18 MPa; the actual melt temperature when the material reaches the die head is controlled between 185℃ and 195℃.

10. The preparation process of the photovoltaic module back protective film according to claim 8, characterized in that, The air ring cooling in step S3 adopts a dual-outlet air ring, with the main air ring airflow controlled at 2500 m³ / h. 3 / h~3500 m 3 / h, air temperature is 8℃~15℃; the angle of the herringbone clamp is controlled at 15°~25°; the draw ratio of traction speed to extrusion speed is controlled between 1:3 and 1:6; the power of corona treatment is 2.0 kW~3.5 kW, thereby making the surface tension of the film after corona treatment reach 52 dyn / cm~56 dyn / cm; the winding tension is controlled at 80 N~120 N, and the taper tension is controlled at 30%~50%.

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