A base film for a photovoltaic back sheet with low titanium white dosage and high reflective white enhancement and a preparation method thereof

Abstract

The application discloses a base film for a photovoltaic back plate with low titanium white dosage and high reflective white enhancement and a preparation method thereof. The base film has a skin layer-core layer-skin layer three-layer structure, the core layer raw material comprises a reflectant master batch, a first master batch, a second master batch, PET, PETG and an antioxidant, the reflectant master batch contains modified TiO2, the first master batch is prepared from PET, POE and POE-g-MA, and the second master batch contains nano CaCO3, PET and a dispersing agent. The core layer is subjected to bidirectional stretching and step heat setting to form micro cavities with a D50 of 0.6-1.2 microns. The application cooperates light scattering through the micro cavities and the modified TiO2, greatly reduces the titanium white dosage, and is suitable for industrial application of the photovoltaic back plate due to the up-to-standard reflectivity of the base film, excellent mechanical properties and weather resistance.

Description

Technical Field

[0001] This application relates to the field of photovoltaic cells, specifically to a base film for photovoltaic backsheets with low titanium dioxide content and high reflectivity white enhancement, and its preparation method. Background Technology

[0002] Photovoltaic modules are the core energy conversion units of photovoltaic power generation systems. As a key peripheral encapsulation material for crystalline silicon photovoltaic modules, the photovoltaic backsheet is laid on the back of the module and plays an important role in protecting the photovoltaic cells, blocking external moisture and oxygen erosion, and maintaining the structural mechanical stability of the module. Its material properties directly determine the outdoor service life and power generation reliability of photovoltaic modules, and it is one of the core materials to ensure that photovoltaic modules can achieve long-term stable operation for 25 years.

[0003] The base film used in photovoltaic backsheets serves as the core functional carrier of the backsheet and must possess two key characteristics: high reflectivity and long-term weather resistance. High reflectivity allows light transmitted through the solar cells to be reflected back to the cells, improving the photoelectric conversion efficiency of the module. Industry standards generally require the base film to have a reflectivity of ≥90% at a wavelength of 550 nm. Long-term weather resistance ensures that the base film does not experience significant performance degradation within 25 years under complex outdoor environments such as humidity, heat, and ultraviolet radiation. Currently, the mainstream method for preparing high-reflectivity photovoltaic backsheet base films involves highly filling a biaxially oriented polyester (BOPET) matrix with rutile titanium dioxide (TiO2), relying on the light scattering effect of TiO2 to enhance reflectivity.

[0004] High TiO2 filler solutions have insurmountable drawbacks. A TiO2 mass fraction of 15%–25% is required to meet reflectivity requirements, significantly increasing the production cost of the base film. Excessive rigid inorganic fillers disrupt the continuity of the polyester matrix, causing internal stress concentration, leading to a significant decrease in the elongation at break of the base film, material embrittlement, and poor processing adaptability. Simultaneously, TiO2's photocatalytic activity accelerates the aging and yellowing of the polyester matrix, drastically reducing the weather resistance of the base film. Improvement solutions such as inorganic filler replacement suffer from insufficient reflectivity and deterioration of mechanical properties. Currently, the industry lacks a low-cost, low-TiO2 base film for photovoltaic backsheets that simultaneously offers high reflectivity and long-term weather resistance. Summary of the Invention

[0005] The purpose of this application is to provide a base film for photovoltaic backsheets that maintains high reflectivity while having a low titanium dioxide content.

[0006] To achieve the above objectives, the technical solution adopted in this application is as follows: a base film for photovoltaic backsheets with low titanium dioxide content and high reflectivity white enhancement is provided, comprising two skin layers and a core layer disposed between the skin layers. The raw materials for preparing the core layer include reflectant masterbatch, first masterbatch, second masterbatch, PET, PETG and antioxidant. The raw materials for preparing the reflectant masterbatch include modified TiO2, wherein the modified TiO2 is silicon-aluminum coated TiO2.

[0007] As a preferred embodiment, the raw materials for preparing the first masterbatch include PET, POE, and POE-g-MA.

[0008] As a preferred embodiment, the raw materials for preparing the second masterbatch include nano-CaCO3, PET, and a dispersant.

[0009] As a preferred embodiment, the core layer is formed into microcavities after longitudinal stretching, transverse stretching and shaping, and the microcavity D50 is 0.6~1.2 μm.

[0010] As a preferred embodiment, the modified TiO2 accounts for 5% to 15% of the total mass of the reflector masterbatch, and the reflector masterbatch accounts for 5% to 40% of the total mass of the core layer.

[0011] As a preferred embodiment, the core layer thickness is 80% to 90% of the total thickness.

[0012] A method for preparing a low-titanium dioxide-content, high-reflectivity, white-enhanced photovoltaic backsheet base film is provided, comprising the following preparation steps: S1: preparing modified TiO2, and preparing a reflective masterbatch using the modified TiO2, coupling agent, and PET as raw materials; preparing a first masterbatch using PET, POE, and POE-g-MA as raw materials; and preparing a second masterbatch using nano-CaCO3, PET, and dispersant as raw materials; S2: drying and mixing PET, PETG, the reflective masterbatch, the first masterbatch, the second masterbatch, and an antioxidant as core layer raw materials, and drying and mixing PET and an antioxidant as skin layer raw materials, and performing segmented melt extrusion through an extruder to form a cast sheet with the skin-core-skin structure; S3: stretching the cast sheet longitudinally and laterally and performing stepped heat setting to obtain the low-titanium dioxide-content, high-reflectivity, white-enhanced photovoltaic backsheet base film.

[0013] As a preferred embodiment, the casting is subjected to longitudinal stretching and transverse stretching, wherein the longitudinal stretching temperature is 85~100 ℃, the longitudinal stretching ratio is 3~3.5, and the longitudinal stretching rate is 100~150 % / s; the transverse stretching temperature is 95~110 ℃, the transverse stretching ratio is 3.5~4, and the transverse stretching rate is 80~120 % / s.

[0014] As another preferred embodiment, the stepped heat setting includes a pre-setting stage, a main setting stage, and a cooling setting stage. The pre-setting stage is set at a temperature of 160~205 ℃ and a dwell time of 2~3 s. The main setting stage is set at a temperature of 210~230 ℃ and a dwell time of 2~4 s. The cooling setting stage is protected by inert gas, with a temperature of 120~150 ℃ and a dwell time of 1~3 s.

[0015] Further preferred, the modified TiO2 is prepared by: preparing a nano-TiO2 dispersion and adjusting the pH, adding a silicate-containing solution to obtain silicon-coated TiO2, adjusting the pH and heating the reaction system, adding an aluminum-containing solution and aging for a period of time, and washing and drying the product to obtain the modified TiO2.

[0016] Compared with the prior art, the beneficial effects of this application are as follows: (1) This application achieves a stable reflectance of ≥90% at 550 nm wavelength by means of the synergistic effect of micro-hole main scattering and modified TiO2 auxiliary scattering, under the premise of significantly reducing the amount of titanium dioxide added, thus solving the industry problem of high cost and difficulty in balancing reflectance and cost in traditional high-filling titanium dioxide systems. (2) This application uses silicon-aluminum double-layer coating to modify TiO2, which isolates its photocatalytic active sites and avoids aging and yellowing of the polyester matrix and performance degradation; at the same time, it optimizes the dispersibility of inorganic fillers, eliminates matrix stress concentration, and ensures excellent mechanical properties such as elongation at break and tensile strength of the base film, thus solving the defects of easy embrittlement and mechanical property deterioration of traditional high filler system. (3) This application relies on stretching process and stepped heat setting process to achieve uniform micro-cavity size, stable structure without collapse, and precise locking of cavity volume fraction and morphology; it not only ensures long-term stable light scattering efficiency, but also improves the adaptability and dimensional stability of base film processing, taking into account the comprehensive performance of high reflectivity, weather resistance, mechanical properties and mass production processing. Detailed Implementation

[0017] The present application will be further described below with reference to specific embodiments. It should be noted that, without conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0018] The terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion, for example, a process, method, system, product, or device that includes a series of steps or units is not necessarily limited to those steps or units that are explicitly listed, but may include other steps or units that are not explicitly listed or that are inherent to such process, method, product, or device.

[0019] This application provides a base film for photovoltaic backsheets with low titanium dioxide content and high reflectivity white enhancement, comprising two skin layers and a core layer disposed between the skin layers. The raw materials for preparing the core layer include reflectant masterbatch, first masterbatch, second masterbatch, PET, PETG and antioxidant. The raw materials for preparing the reflectant masterbatch include modified TiO2, which is silicon-aluminum coated TiO2. The raw materials for preparing the first masterbatch include PET, POE and POE-g-MA. The raw materials for preparing the second masterbatch include nano-CaCO3, PET and dispersant.

[0020] This application achieves controllable micro-cavity generation and synergistic light scattering stability through the deep coupling of a core layer-specific functionalized raw material formulation with precise biaxial stretching and stepped heat setting processes. This results in quaternary optimization of weather resistance, mechanical properties, reflection, and cost, breaking through traditional technological bottlenecks in three dimensions: product performance, mass production, and cost control. The core layer employs a functional compound formulation specifically tailored for stretching cavity formation, low titanium dioxide scattering, and weather stability, achieving multiple technical effects such as low dosage, high weather resistance, and excellent dispersibility.

[0021] This application also provides a method for preparing a base film for photovoltaic backsheets with low titanium dioxide content and high reflectivity white enhancement, comprising the following steps: S1: Prepare modified TiO2, which is silicon-aluminum coated TiO2. Prepare reflective masterbatch using modified TiO2, coupling agent and PET as raw materials; prepare first masterbatch using PET, POE and POE-g-MA as raw materials; prepare second masterbatch using nano CaCO3, PET and dispersant as raw materials. S2: PET, PETG, reflective masterbatch, first masterbatch, second masterbatch and antioxidant are dried and mixed as core layer raw materials, and PET and antioxidant are dried and mixed as skin layer raw materials. The extruder performs segmented melt extrusion, and the upper and lower skin layers wrap the core layer in the die head to form a skin-core-skin layer structure of the cast sheet. S3: The casting is stretched longitudinally and laterally and then heat-set in a stepped manner to obtain the low titanium dioxide content, high reflectance white enhanced photovoltaic backsheet base film of this application.

[0022] The modified TiO2 in this application is a silicon-aluminum double-layer coated TiO2. Unmodified TiO2 will generate strong oxidizing OH free radicals under ultraviolet light, which will directly attack the ester bonds of PET, causing chain breakage, yellowing and embrittlement of the material. The silicon-aluminum double-layer coating forms a dense physical barrier, which can block the contact between the photocatalytic active sites of TiO2 and the polymer matrix, overcoming the fatal defects of traditional TiO2 system such as accelerated aging and yellowing and rapid decay of reflectivity.

[0023] The aluminum coating helps eliminate electrostatic agglomeration between TiO2 particles, while the silicon coating enhances the nucleophilicity between the material and the organic phase. This allows for the use of coupling agents and formulation into reflective masterbatches, effectively addressing the problems of inorganic fillers easily agglomerating and unevenly dispersing in polyester matrices, leading to unstable reflectivity and deteriorated mechanical properties. The double-layer coating also prevents direct contact between the PET melt and TiO2, avoiding TiO2-catalyzed thermal oxidative degradation of PET during melt extrusion, biaxial stretching, and heat setting. This solves the process compatibility defects of TiO2-filled systems, such as yellowing and matrix degradation during high-temperature processing.

[0024] This application induces the generation of micro-voids through a stretching process, which serve as the main light scattering unit, while low-content reflective masterbatch serves as the auxiliary light scattering unit. Through the synergistic scattering effect of the two scattering units, high reflectivity with low titanium dioxide content is achieved, breaking the contradiction between cost and reflectivity in the prior art.

[0025] In some embodiments, the preparation method of modified TiO2 is a prior art, which can be used to prepare silicon-aluminum double-coated TiO2. This application provides a method for preparing modified TiO2: preparing a nano-TiO2 dispersion and adjusting the pH, adding a silicate-containing solution to obtain silicon-coated TiO2, adjusting the pH and heating the reaction system, adding an aluminum-containing solution and aging for a period of time, and washing and drying the product to obtain modified TiO2.

[0026] In some embodiments, the modified TiO2 accounts for 5% to 15% of the total mass of the reflector masterbatch, and the amount of reflector masterbatch added accounts for 5% to 40% of the total mass of the core layer. Excessive modified TiO2 may disrupt the continuity of the PET matrix, forming a large number of stress concentration points, causing a sharp drop in elongation at break, and excessive TiO2 may crowd out nucleation space, resulting in too low a content of microvoids and a significant decrease in scattering efficiency.

[0027] In some embodiments, a coupling agent is added to the raw materials for preparing the reflective masterbatch to improve the dispersibility of modified TiO2, inhibit its powder agglomeration, and build a strong interface layer in PET. Good interface bonding can ensure uniform generation of voids and dimensional stability during stretching, and avoid local interface debonding that could cause film breakage. This can both utilize TiO2 to improve the rigidity and tensile strength of the film and retain the elongation at break and impact resistance of the matrix.

[0028] In a preferred embodiment, the coupling agent is titanate TTS, silane KH550, or a mixture of the two.

[0029] The raw materials for preparing the first masterbatch of this application include POE, POE-g-MA, and PET, with the maleic anhydride grafting rate of POE-g-MA being 0.8%~1.2%. POE is a non-polar polyolefin, while PET is a highly polar polyester. Direct blending would result in severe phase separation and POE agglomeration. Adding POE-g-MA to the first masterbatch and adjusting the POE content can alleviate the interfacial incompatibility problem between non-polar POE and polar PET to a certain extent, stabilize the stretched pore structure, and ensure toughness and processing stability.

[0030] During compounding, under the stabilization of the POE-g-MA compatibilizer, POE is dispersed in the PET matrix as uniform microspheres of 1~2 μm. The POE microspheres and the PET matrix undergo controllable debonding at the interface, forming a large number of uniform, ellipsoidal, non-connected microcavities. These regular microcavities and the low content of coated modified TiO2 form synergistic light scattering, which improves the reflectivity of the base film under the premise of low TiO2 content, and achieves an innovative solution of low addition and high reflectivity.

[0031] After adding reflective masterbatch and first masterbatch to the core layer, it was found that the size of the micro-voids formed by relying solely on the first masterbatch was uneven, resulting in a large number of ineffective macropores. This caused the scattering efficiency to fail to reach the expected value. Furthermore, during the core layer stretching process, the voids rapidly merged and enlarged, and after heat setting, a large number of voids collapsed, making it impossible to achieve a high reflectivity.

[0032] This application adds a second masterbatch to the raw materials for preparing the core layer. The raw materials for preparing the second masterbatch include nano-CaCO3, PET, and a dispersant. The uniform dispersion of nano-CaCO3 in the PET matrix provides numerous rigid nucleation sites for stretching and debinding, reducing the density of micro-voids. Furthermore, the rigid nanoparticles can limit excessive void expansion, stabilizing the void size at D50 = 0.6~1.2 μm and D90 < 3 μm, achieving the most efficient light scattering and significantly reducing reflectivity fluctuations.

[0033] CaCO3 particles anchor themselves to the cavity walls, preventing cavities from merging during stretching and supporting them from collapsing during heat setting, thus stabilizing the cavity volume fraction between 35% and 42% and locking in the synergistic reflection effect. CaCO3 also has a slight rigidity-reinforcing effect, which can eliminate stress concentration, stabilize elongation at break, and reduce tensile breakage rate.

[0034] The preparation methods of reflective masterbatch, first masterbatch and second masterbatch are existing technologies in the industry for extrusion granulation. Those skilled in the art can understand the melting temperature of PET and adjust the temperature of each section of the twin-screw extruder according to the state of the raw materials and products, so they will not be described in detail here.

[0035] The core layer is also prepared using PETG, specifically a copolymerized PET containing 30 mol% 1,4-cyclohexanediethanol (CHDM). By combining PETG with PET, the rigid ring structure of CHDM is inserted into the PET molecular chain, disrupting the chain segment regularity and preventing premature crystallization of pure PET during casting quenching or stretching preheating. This ensures that the initial crystallinity of the cast sheet is <10%, providing a foundation for subsequent uniform stretching and pore formation. It does not hinder molecular orientation during stretching and allows for stable crystal formation during heat setting, preventing pore collapse.

[0036] This application utilizes a three-layer co-extrusion casting process to form a cast sheet with a skin-core-skin structure. Preferably, the upper and lower skin layers have the same thickness. More preferably, the thickness of a single skin layer is approximately 5% to 10% of the total base film thickness, and the thickness of the core layer is 80% to 90% of the total base film thickness. Preferably, the thickness of the base film used in three-layer photovoltaic backsheets in the industry is approximately 50 μm to 350 μm, and the flow rates of the skin extruder and core extruder can be controlled according to actual needs.

[0037] This application employs longitudinal stretching, transverse stretching, and stepped heat setting processes to ensure the formation of dimensionally stable microcavities in the core layer, creating a synergistic light scattering effect with the low-content coated modified TiO2. In some embodiments, the longitudinal stretching temperature is 85~100 ℃; the stretching ratio is 3~3.5, and the stretching rate is 100~150 % / s; the transverse stretching temperature is 95~110 ℃, the stretching ratio is 3.5~4, and the stretching rate is 80~120 % / s.

[0038] The stepped heat setting process is divided into three stages: pre-setting, main setting, and cooling setting. Pre-set type: temperature 160~205 ℃, residence time 2~3 s, achieving MD relaxation of 1%~2% and TD relaxation of 1%~2%; Main shaping: temperature 210~230 ℃, residence time 2~4 s, achieving MD relaxation of 2%~4% and TD relaxation of 2%~4%; Cooling and shaping: Temperature 120~150 ℃, residence time 1~3 s, inert gas protection, to achieve product shaping.

[0039] Longitudinal stretching completes the initial longitudinal orientation of the molecular chains. Combined with the nucleation and pinning effect of nano-CaCO3 in the second masterbatch, it initially limits the excessive expansion of cavities, locks in the morphological prototype of cavities, prevents local interface debonding and loss of control during stretching, lays the structural foundation for the uniform development of cavities during transverse stretching, reduces ineffective macropores, and improves the uniformity of cavity morphology.

[0040] Lateral stretching enhances the synergistic light scattering of micro-voids and low-content modified TiO2, achieving bidirectional balanced orientation of molecular chains, eliminating the anisotropy of unidirectional stretching, improving the lateral mechanical properties of the base film, ensuring overall mechanical stability, and achieving high reflectivity at 550 nm wavelength with a significant reduction in titanium dioxide usage, thus resolving the contradiction between cost and reflectivity.

[0041] The pre-setting stage gently releases the instantaneous internal stress of the stretching, avoiding stress concentration that could lead to embrittlement and warping. It also serves to initially fix the cavity structure, preventing rapid cavity shrinkage and merging, and maintaining uniform distribution. The main setting stage eliminates residual tensile stress, improves the crystallinity of PET, enhances the heat resistance and long-term dimensional stability of the base film, locks in the cavity volume fraction and morphology, prevents cavity collapse during heat setting, and ensures long-term non-decay of reflectivity. The cooling setting stage is suitable for rapid cooling and solidification of molecular chains and cavity structures, locking in the synergistic scattering system and mechanical properties. Combined with an inert atmosphere, it avoids high-temperature thermal oxidation and yellowing, protects the modified TiO2 coating layer and the PET matrix, and improves weather resistance.

[0042] It is worth noting that various existing additives, such as UV absorbers, hydrolysis resistant agents, compatibilizers, stabilizers, and coupling agents, can be added to the skin or core layer to achieve different functions. For the purpose of simplifying the solution, only one minimum solution is disclosed here, and the description of the above additives is omitted. The actual additives can be adjusted according to actual production needs.

[0043] Example 1 A high-reflectance white enhanced base film for photovoltaic backsheets with low titanium dioxide content is prepared according to the following steps: Preparation of a modified TiO2: 0.1 g of (NaPO3)6 was prepared into a 200 mL solution and added to a flask, followed by 20 g of TiO2 nanoparticles. The oil bath temperature was pre-set to 80 ℃. After dispersion by magnetic stirring, the pH of the system was adjusted to 9-10. During the coating process, Na2SiO3 solution (based on SiO2 being 3 wt% of TiO2) and H2SO4 solution were slowly added dropwise using a co-current titration method, while simultaneously controlling the pH of the reaction system within the range of 9-10. After the addition was complete, the mixture was aged for 2 hours to complete the silicon-coated TiO2. First, adjust the pH of the reaction system to 9 and the oil bath temperature to 80 °C. Then, slowly add Al2(SO4)3 solution (based on Al2O3 being 2 wt% of TiO2) and NaOH solution using a co-current titration method, while simultaneously controlling the pH of the reaction system to 9 and the temperature to 80 °C. The titration time is controlled to approximately 1.5 h. After titration, age the mixture for 1 h, then adjust the pH to 7 and continue aging for another 1 h. Finally, the resulting slurry is vacuum filtered, washed, dried, and ground to obtain modified TiO2.

[0044] Preparation of reflective masterbatch: Weigh 15% of dried modified TiO2 by mass fraction and add it to a high-speed mixer and heat it. Then add 2% coupling agent and stir for a period of time. Then add 83% PET carrier to form a uniform premix. Extrude it in a twin-screw extruder, cool it and cut it to obtain reflective masterbatch. The coupling agent is a mixture of titanate TTS and silane KH550 in a mass ratio of 2:1. Preparation of the first masterbatch: PET, POE and POE-g-MA are dried and sieved. By mass fraction, 10% POE, 10% POE-g-MA and 80% PET are added to a high-speed mixer to achieve initial uniform dispersion of the raw materials. Then, the raw materials are melt-blended in a twin-screw extruder and extruded and granulated to obtain the first masterbatch. Preparation of the second masterbatch: The raw materials are dried and sieved. By mass fraction, 10% nano CaCO3, 88% PET and 2% dispersant are added to a high-speed mixer to achieve initial uniform dispersion of the raw materials. Then, the raw materials are melt-blended in a twin-screw extruder and extruded and granulated to obtain the second masterbatch. The nano CaCO3 particle size is 80 nm. Three-layer co-extrusion: The core layer raw material and the skin layer raw material are deeply dehumidified and dried. The core layer raw material, by weight, includes 50% PET, 10% PETG, 20% reflective masterbatch, 9.7% first masterbatch, 10% second masterbatch, and 0.3% antioxidant. The skin layer raw material, by weight, includes 99.7% PET and 0.3% antioxidant, which is a mixture of Irganox 1010 and Irgafos 168 in a 1:1 weight ratio. The various raw materials for the core layer are mixed at low speed in a side mixer at 100 r / min for 3 min to avoid stratification; the raw materials for the skin layer are not premixed and are fed directly; three extruders, A, B and C, are fed independently, with extruders A and C each accounting for 10% of the total flow rate for the skin layer and extruder B accounting for 80% of the total flow rate for the core layer. The extruders perform segmented melt extrusion, and the three melt streams enter the die distributor through independent channels. Inside the die, the upper and lower skin layers wrap around the core layer to form a skin-core-skin layer structure. The composite melt is discharged at a constant speed through the die lip to form a stable melt curtain, which is then tightly attached to the mirror-finished cooling roller through electrostatic adsorption for rapid quenching, resulting in a three-layer structure casting with an initial crystallinity of <10% and a thickness of 900 μm. Stretching and Shaping: The casting is fed into a longitudinal stretching machine, preheated to 100 ℃ for 3 s; stretched at 95 ℃, with a stretching ratio of 3.2 times and a stretching rate of 120 % / s; and then set in a cooling roller at 70 ℃ for 3 s. Subsequently, the casting is fed into a transverse stretching machine, preheated to 100 ℃ for 4 s; stretched at 105 ℃, with a stretching ratio of 3.8 times and a stretching rate of 100 % / s. The inlet of the heat setting oven is pre-filled with high-purity nitrogen. The film is directly fed into the three-stage heat setting zone from the TD stretching zone. Pre-setting: temperature 180 ℃, residence time 2 s, achieving MD relaxation of 1% and TD relaxation of 1%; Main setting: temperature 200 ℃, residence time 3 s, achieving MD relaxation of 3% and TD relaxation of 2%; Cooling setting: temperature 130 ℃, residence time 1 s, ensuring the warpage of the finished product is <3mm / m. 2 The low titanium dioxide content, high reflectance white enhanced photovoltaic backsheet base film of this application was obtained, with a finished thickness of about 74 µm; POE is an ethylene-octene copolymer with a density of 0.870 g / cm³. 3 The grafting rate of POE-g-MA maleic anhydride was 1.2%; the TiO2 nanopowder was unmodified rutile TiO2; and PETG was copolymerized PET containing 30 mol% 1,4-cyclohexanediethanol (CHDM).

[0045] It is worth mentioning that other additives and the set temperature of the twin-screw extruder, or other non-essential data that are not disclosed in detail, are not discussed further here. The temperature setting of the twin-screw extruder is adjusted according to the actual PET material used and can be adjusted based on the subjective experience of those skilled in the art.

[0046] Example 2 The amount of modified TiO2 in the reflective masterbatch was adjusted to 5%, while the other preparation steps remained the same as those in Example 1.

[0047] Example 3 The amount of modified TiO2 in the reflective masterbatch was adjusted to 10%, while the other preparation steps remained the same as those in Example 1.

[0048] Example 4 The flow rates of the skin extruders A and C were adjusted to 8%, and the flow rate of the core extruder B was adjusted to 84%. Other preparation steps remained the same as those in Example 1.

[0049] Example 5 Adjust the flow rates of the skin extruders A and C to 5%, and the flow rate of the core extruder B to 90%. Other preparation steps are consistent with the preparation steps in Example 1.

[0050] Example 6 The longitudinal stretching ratio was adjusted to 3, and the transverse stretching ratio was adjusted to 3.6. Other preparation steps were consistent with those in Example 1.

[0051] Example 7 The longitudinal stretching ratio was adjusted to 3.4, the transverse stretching ratio to 4, and the other preparation steps were consistent with those in Example 1.

[0052] Comparative Example 1 Unmodified rutile TiO2 was selected for the reflective masterbatch, and the other preparation steps were consistent with those in Example 1.

[0053] Comparative Example 2 The reflector masterbatch uses silicon-coated TiO2, and the other preparation steps are consistent with those in Example 1.

[0054] Comparative Example 3 No first masterbatch was added to the core layer raw material, the PET mass was adjusted to 59.7%, and the other preparation steps were consistent with the preparation steps in Example 1.

[0055] Comparative Example 4 No second masterbatch was added to the core layer raw material, the PET mass was adjusted to 60%, and the other preparation steps were consistent with the preparation steps in Example 1.

[0056] Comparative Example 5 The raw materials for the preparation of the first masterbatch were adjusted to 10% POE-g-MA and 90% PET, while the other preparation steps remained the same as those in Example 1.

[0057] Comparative Example 6 After longitudinal and transverse stretching, the castings were heat-set at 200 °C for 5 seconds. Other preparation steps were consistent with those in Example 1.

[0058] Performance testing Reflectivity was measured using GB / T 2410-2008 "Determination of transmittance and haze of transparent plastics" and GB / T 31034-2024 "Insulating backsheet for crystalline silicon solar cell modules", with a reflectivity (%) at 550 nm wavelength. Tensile strength and elongation at break were measured using GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets". Hole volume fraction was measured using GB / T 1033.1-2008 "Determination of density of non-foamed plastics - Part 1: Impregnation method, liquid pyrometer method and titration method". Hole median size was measured using GB / T 33835-2017 "Determination of pore structure parameters of polymer microporous membranes - Scanning electron microscopy".

[0059] The photovoltaic backsheet base films prepared in the above embodiments and comparative examples were subjected to performance tests according to the above method, and the test results are recorded in Table 1 below.

[0060] Table 1 Performance test results of each embodiment and each comparative example

[0061] The base films of all embodiments in this application achieve high reflectivity, high mechanical properties, and high long-term stability while reducing the amount of titanium dioxide used. Their performance is superior to that of the base films prepared in the comparative example. They achieve controllable generation of micro-vacuum cells and stable synergistic light scattering, maintaining reflectivity above 90%.

[0062] In Examples 1-3, the variable was the amount of modified TiO2 in the reflectant masterbatch. Within the range of 5% to 15%, the 550 nm reflectivity of the base film gradually increased with the increase of modified TiO2 content, while the tensile strength, elongation at break, and other mechanical properties remained stable without significant deterioration. Adding excessive modified TiO2 would lead to a sharp drop in elongation at break, so it is necessary to control the appropriate addition of modified TiO2.

[0063] Examples 1, 4, and 5 involved adjusting the flow ratio of different skin layers to the core layer. As the skin layer thickness ratio decreased and the core layer thickness ratio increased, the reflectivity of the base film showed a continuous upward trend, while the mechanical properties remained stable. Examples 1, 6, and 7 involved adjusting the longitudinal and transverse stretching ratios. As the stretching ratio increased, the hole volume fraction of the base film gradually increased, and the hole D50 size stabilized within the optimal scattering range of 0.6~1.0 μm, with a simultaneous increase in reflectivity. Simultaneously, the balanced increase in the bidirectional stretching ratio eliminated the anisotropy of the base film, ensuring balanced and stable mechanical properties in the MD / TD directions.

[0064] The silicon-aluminum double-layer coating modified TiO2 in this application effectively blocks the photocatalytic activity of TiO2 and improves the dispersibility of inorganic fillers in the polyester matrix, which is the core of achieving high reflectivity with low dosage while avoiding the degradation of the matrix's mechanical properties. The POE and POE-g-MA composite system in the first masterbatch is beneficial for stretching and forming pores and main light scattering units; without this component, the high reflectivity effect with low titanium dioxide addition cannot be achieved. The nano-CaCO3 in the second masterbatch has nucleation and supporting functions, enabling controllable hole size, structural stability, and scattering efficiency. The stepped heat setting process in this application is the core process feature for locking the hole structure, ensuring long-term stability of reflectivity, and improving the dimensional stability of the base film; conventional heat setting cannot achieve the technical effects of this application.

[0065] In summary, this application employs a three-layer structure design: skin-core-skin. Through the synergistic combination of a functionalized compound formulation in the core layer and precise biaxial stretching and stepped heat setting processes, relying on a silicon-aluminum double-layer coated modified TiO2, a POE composite compatibility system, and a nano-calcium carbonate nucleation support system, it achieves controllable micro-hole generation and synergistic light scattering, significantly reducing the amount of titanium dioxide used. This application effectively solves industry problems such as high cost, deteriorated mechanical properties, and easy aging and yellowing of traditional high-filler titanium dioxide systems. While reducing filler addition, it simultaneously ensures high reflectivity, excellent mechanical properties, and long-term weather resistance of the base film, achieving comprehensive optimization of performance, processing, and cost. It breaks through existing technical bottlenecks and possesses significant technical advantages and industrial application value.

[0066] The basic principles, main features, and advantages of this application have been described above. Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely the principles of this application. Various changes and modifications can be made to this application without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claims. The scope of protection claimed by this application is defined by the appended claims and their equivalents.

Claims

1. A base film for photovoltaic backsheets with low titanium dioxide content and high reflectivity white enhancement, characterized in that, It includes two skin layers and a core layer disposed between the skin layers. The raw materials for preparing the core layer include reflector masterbatch, first masterbatch, second masterbatch, PET, PETG and antioxidant. The raw materials for preparing the reflector masterbatch include modified TiO2, which is silicon-aluminum coated TiO2.

2. The low titanium dioxide content, high reflectance white enhanced photovoltaic backsheet base film as described in claim 1, characterized in that, The raw materials for preparing the first masterbatch include PET, POE, and POE-g-MA.

3. The low titanium dioxide content, high reflectance white enhanced photovoltaic backsheet base film as described in claim 1, characterized in that, The raw materials for preparing the second masterbatch include nano-CaCO3, PET, and dispersant.

4. The low titanium dioxide content, high reflectance white enhanced photovoltaic backsheet base film as described in claim 1, characterized in that, The core layer is stretched longitudinally, stretched laterally, and shaped to form microcavities, and the D50 of the microcavities is 0.6~1.2 μm.

5. The low titanium dioxide content, high reflectance white enhanced photovoltaic backsheet base film as described in claim 1, characterized in that, The modified TiO2 accounts for 5% to 15% of the total mass of the reflector masterbatch, and the reflector masterbatch accounts for 5% to 40% of the total mass of the core layer.

6. The low titanium dioxide content, high reflectance white enhanced photovoltaic backsheet base film as described in claim 1, characterized in that, The core layer thickness is 80% to 90% of the total thickness.

7. A method for preparing a base film for photovoltaic backsheets with low titanium dioxide content and high reflectance white enhancement, characterized in that, The preparation steps include the following: S1: Prepare modified TiO2, and prepare reflective masterbatch using the modified TiO2, coupling agent and PET as raw materials; prepare first masterbatch using PET, POE and POE-g-MA as raw materials; prepare second masterbatch using nano CaCO3, PET and dispersant as raw materials; S2: PET, PETG, the reflective masterbatch, the first masterbatch, the second masterbatch, and the antioxidant are dried and mixed as core layer raw materials, and PET and the antioxidant are dried and mixed as skin layer raw materials. The mixture is then melt-extruded in stages through an extruder to form a cast sheet with the skin layer-core layer-skin layer structure. S3: The cast sheet is stretched longitudinally and laterally and then subjected to stepped heat setting to obtain the low titanium dioxide content, high reflectivity white enhanced photovoltaic backsheet base film.

8. The preparation method according to claim 7, characterized in that, The casting is subjected to longitudinal stretching and transverse stretching. The longitudinal stretching temperature is 85~100 ℃, the longitudinal stretching ratio is 3~3.5, and the longitudinal stretching rate is 100~150% / s. The transverse stretching temperature is 95~110 ℃, the transverse stretching ratio is 3.5~4, and the transverse stretching rate is 80~120% / s.

9. The preparation method according to claim 7, characterized in that, The stepped heat setting includes a pre-setting stage, a main setting stage, and a cooling setting stage. The pre-setting stage is set at a temperature of 160~205 ℃ and a dwell time of 2~3 s. The main setting stage is set at a temperature of 210~230 ℃ and a dwell time of 2~4 s. The cooling setting stage is protected by inert gas, with a temperature of 120~150 ℃ and a dwell time of 1~3 s.

10. The preparation method according to claim 7, characterized in that, The modified TiO2 is prepared by: preparing a nano-TiO2 dispersion and adjusting the pH, adding a silicate-containing solution to obtain silicon-coated TiO2, adjusting the pH and heating the reaction system, adding an aluminum-containing solution and aging for a period of time, and washing and drying the product to obtain the modified TiO2.

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