A double-sided weatherable multilayer polyester film and a method of making the same

Abstract

This invention relates to the field of multilayer polyester film technology, specifically to a double-sided weather-resistant multilayer polyester film and its preparation method. This invention overcomes the problem in existing multilayer polyester films where adhesion and weather resistance cannot be simultaneously achieved. The polyester film of this invention has an asymmetric three-layer co-extrusion structure, wherein the outer layer is an in-situ polymerized modified PET-PEN-UV copolyester, compounded with a polycarbodiimide anti-hydrolysis agent to construct a highly dense weather-resistant barrier layer; the middle layer is a CHDM-modified reinforced polyester, utilizing recycled materials and chemically bonding between layers through epoxy chain extenders and toughening compatibilizers; the inner layer is a low-melting-point copolyester, incorporating functional masterbatches to provide chemically bonded adhesion. This invention, through in-situ polymerization dispersion technology, reactive co-extrusion process, and gradient heat setting technology, yields a film with excellent weather resistance and high adhesion, suitable for photovoltaic module backsheets and outdoor high-weather-resistant protection applications.

Description

Technical Field

[0001] This invention relates to the field of multilayer polyester film technology, specifically to a double-sided weather-resistant multilayer polyester film and its preparation method. Background Technology

[0002] Currently, polyester film is widely used in solar photovoltaic backsheets, outdoor electronic displays, and architectural films due to its excellent mechanical and insulation properties. However, conventional PET film contains ester bonds in its molecular chain, which are prone to hydrolysis and chain breakage under high temperature and humidity conditions. It is also sensitive to ultraviolet light, and long-term outdoor use can lead to yellowing, powdering, and brittleness. To address these issues, existing technologies often employ multilayer co-extrusion to prepare composite films. However, in practical applications, there is often an irreconcilable contradiction between long-term weather resistance and interfacial adhesion.

[0003] To impart excellent UV aging resistance to multilayer polyester films, the conventional approach is to add high concentrations of UV absorbers and light stabilizers to the surface matrix. However, due to cost and process limitations, these additives are mostly small-molecule compounds with limited compatibility with the polyester matrix. Under prolonged high temperature, high humidity, and light exposure cycling, these small-molecule additives readily migrate, diffuse, and accumulate on the film surface, forming a microscopic blooming layer or weak boundary layer. This precipitate not only causes a sharp decrease in the surface tension of the film but also hinders effective contact between the encapsulating film and the PET substrate, leading to a significant reduction in peel strength after lamination and ultimately causing component delamination failure.

[0004] In addition, those skilled in the art typically use corona treatment or online chemical primer coating to activate the surface. However, corona treatment has a short effective period, and the polar groups after treatment are prone to photochemical degradation under ultraviolet light attack, resulting in surface powdering and loss of adhesion as the aging time increases. While online coating can improve initial adhesion, it introduces an additional coating interface, and the coating material is usually acrylic or polyurethane, which is often less weather-resistant than the PET body. Under long-term ultraviolet radiation, it is prone to yellowing and cracking before the substrate, which leads to the delamination of the coating interface with the substrate. Even if the substrate itself is intact, the component will still be scrapped due to interface failure.

[0005] In conclusion, how to completely solve the migration problem of weather-resistant additives without sacrificing mechanical properties, construct a stable chemical bonding interface, and achieve a balance between long-lasting weather resistance of the bulk material and durable surface adhesion remains a problem that the high-performance polyester film industry urgently needs to overcome.

[0006] Therefore, this invention proposes a double-sided weather-resistant multilayer polyester film and its preparation method. Summary of the Invention

[0007] The purpose of this invention is to provide a double-sided weather-resistant multilayer polyester film and its preparation method. The polyester film of this invention has an asymmetric three-layer co-extrusion structure. The outer layer is an in-situ polymerized modified PET-PEN-UV copolyester, compounded with a polycarbodiimide anti-hydrolysis agent to construct a highly dense weather-resistant barrier layer. The middle layer is a CHDM-modified reinforced polyester, utilizing recycled materials and chemically bonding between layers through epoxy chain extenders and toughening compatibilizers. The inner layer is a low-melting-point copolyester with added functional masterbatch to provide chemically bonded adhesion. This invention, through in-situ polymerization dispersion technology, reactive co-extrusion process, and gradient heat setting technology, yields a film with excellent weather resistance and high adhesion, suitable for photovoltaic module backsheets and outdoor high-weather-resistant protection applications.

[0008] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing a double-sided weather-resistant multilayer polyester film. The outer layer is obtained by mixing 100 parts of an outer layer matrix, 2.0-4.0 parts of polycarbodiimide masterbatch, and 0.6 parts of anti-blocking masterbatch. The middle layer is obtained by mixing 65 parts of a middle layer matrix, 30-40 parts of mixed recycled material, 0.8-2 parts of epoxy chain extender, and 3-5 parts of toughening compatibilizer. The inner layer is obtained by mixing 100 parts of an inner layer matrix and 4-6 parts of mixed inner layer functional masterbatch. Co-extrusion is performed using three single-screw extruders, wherein the screw speed of the outer layer extruder is 40-60 rpm, and a split-type screw is used. The temperature of each zone is the feeding zone temperature. The temperatures are as follows: 255℃ in the compression zone, 265℃ in the metering zone, 275℃ in the melt tube, and 282℃ in the melt tube; the melt temperature is strictly controlled to be below 285℃. The screw speed of the intermediate layer extruder is 60-80 rpm, with high vacuum exhaust at -0.08 MPa. The temperatures in each zone are: 260℃ in the feeding zone, 270℃ in the compression zone, 275℃ in the metering zone, and 280℃ in the melt tube. The screw speed of the inner layer extruder is 30-50 rpm. The temperatures in each zone are: 230℃ in the feeding zone, 250℃ in the compression zone, 255℃ in the metering zone, and 260℃ in the melt tube. The three layers of melt converge in a composite die at 275℃, with a die lip opening of 1.2- The resulting melt, at a thickness of 1.5mm, is cast onto a quenching roller at 20-25℃ with a roller speed of 15-20m / min. An electrostatic edge-bonding device with a voltage set to 7-10kV is used, along with air knife edge pressing, to ensure the inner layer adheres to the quenching roller surface. Next, a stretching process is performed. For longitudinal stretching, the preheating roller temperature is 90℃, the stretching roller temperature is 105-110℃, and the stretching ratio is 3.1-3.4 times. For transverse stretching, the preheating zone temperature is 110℃, the air nozzle speed is 25m / s, the stretching zone temperature is 120-130℃, and the stretching ratio is 3.8-4.0 times to obtain a stretched film. The stretched film then undergoes heat setting treatment. The first shaping zone is heated to 225-232℃, with a residence time of 12s and a fan speed of 1800rpm to promote crystallization. The second shaping zone is cooled to 175-185℃ for thermal relaxation, with a transverse relaxation rate of 3%-5% to obtain the post-treated film. The post-treated film is cooled to below 40℃ by a cooling roller, and the inner layer surface is subjected to corona treatment with a discharge power density of 20W·min / m² to make its surface tension ≥54dyn / cm. Finally, it is wound up to obtain a multilayer polyester film with a thickness of 50μm, in which the thickness ratio of the outer layer, middle layer and inner layer is 10-15:70-80:10-15.

[0009] The preparation steps of polycarbodiimide masterbatch in this invention are as follows: 85 parts of PET chips (intrinsic viscosity of 0.60-0.64 dL / g) are vacuum dried at 160℃ for 4-5 hours, controlling the moisture content to be less than 20 ppm. 15 parts of polycarbodiimide powder are dried. PET chips and 0.2 parts of antioxidant are added to a co-rotating twin-screw extruder with a length-to-diameter ratio of 40:1. Polycarbodiimide powder is added through a side feed port. The temperature of zone one (feeding zone) is 230-240℃, and the temperature of zone two (melting zone) is... The temperature is 250-255℃, the temperature in zones 3-5 (dispersion zone) is 255-260℃, the temperature in zones 6-8 (metering zone) is 250-255℃, and the die head temperature is 250-255℃. After water-cooled strip pelletizing, it must be immediately centrifuged for dehydration and then subjected to secondary low-temperature drying, vacuum drying at 90℃ for 2 hours to prevent moisture from consuming the active groups of PCDI during storage. The polycarbodiimide masterbatch is obtained by vacuum sealing in aluminum foil bags, in which the effective content of PCDI is 15%.

[0010] Preferably, the preparation of the outer substrate includes the following steps: Add 85 parts of purified terephthalic acid, 12-18 parts of dimethyl 2,6-naphthalenedicarboxylate, and 65 parts of ethylene glycol to a reactor equipped with an anchor stirrer and a fractionating column. Start stirring at 40-60 rpm, and purge the system three times with high-purity nitrogen. Add 0.04 parts of manganese acetate as a transesterification catalyst. Heat the system to 200-230℃, control the pressure inside the reactor at 0.2-0.3 MPa, and maintain the top temperature of the fractionating column at 65-75℃. Methanol was separated at ℃, and then the temperature was raised to the column top temperature of 105-115℃ to remove esterification water. The reaction continued for 3-4 hours to obtain the reaction system. At this point, the theoretical distillation rate of methanol in the system reached over 95%, and the amount of esterification water generated reached 95% of the theoretical value. Subsequently, the reaction system was depressurized to atmospheric pressure, and 0.03 parts of antimony glycol catalyst were added to enter the polycondensation stage. The system temperature was linearly raised to 280-285℃ within 45 minutes, and the vacuum system was turned on. Most of the free ethylene glycol was discharged through the pre-polycondensation stage of 10-20 kPa. Then, the pressure was gradually reduced linearly from atmospheric pressure to below 100 Pa within 60 minutes. The reaction was continued for 3 hours at a speed of 40 r / min. The intrinsic viscosity of the system was measured to be 0.50 dL / g. The feeding pressure was 0.3-0.5 MPa. The UV absorber slurry was injected through the high-pressure feeding system. The vacuum degree was maintained at <50 Pa. The reaction temperature was strictly controlled not to exceed 285℃, and the stirring speed was 40-60 r / min. The reaction was carried out at pm for 40-60 min, and the torque was monitored until the intrinsic viscosity reached 0.68-0.70 dL / g. The reaction was then stopped, the vacuum was broken with nitrogen, and the mixture was extruded and pelletized to obtain an outer matrix with a melting point of 245-250℃. The UV absorber slurry was obtained by pre-dispersing 2-4 parts of 4-[4,6-bis(2,4-dimethylyl)-1,3,5-triazin-2-yl]-1,3-phenylene glycol in ethylene glycol at 70℃ at 3000 rpm for 30 min under high shear.

[0011] Preferably, the preparation of the intermediate layer matrix includes the following steps: 0.8 parts of KH-560 modified nano-silica powder are added to 5 parts of ethylene glycol and dispersed at 3000 rpm for 30 min using a high-shear disperser to obtain a nano-slurry; 100 parts of PTA, 58 parts of EG, 3-5 parts of 1,4-cyclohexanediethanol and the nano-slurry are added to an esterification reactor, and then 0.02 parts of tetrabutyl titanate are added. The mixture is stirred at 50 rpm and co-esterified at 0.25 MPa pressure and 250-260℃. During this process, the temperature at the top of the distillation column is strictly controlled at 105-110℃ to prevent CHDM from volatilizing with water vapor. The reaction time is 2-3 h to obtain the esterification system; the esterification system is transferred to a polycondensation reactor and polycondensed at 280-285℃. Polyester chips with an intrinsic viscosity of 0.65 dL / g were obtained by polycondensation for 2-2.5 h under a vacuum of less than 40 Pa. The polyester chips were then subjected to solid-state polycondensation and added to a rotary drum solid-state thickening reactor. They were first pre-crystallized at 170 °C and 5 rpm for 2 h to prevent sticking. Then, high-purity nitrogen was introduced, the temperature was raised to 215 °C, and the reaction was carried out at 2 rpm for 12 h. The intrinsic viscosity of the final chips was monitored and increased to 0.82-0.85 dL / g to obtain the intermediate layer matrix.

[0012] Preferably, the preparation of the inner layer matrix includes the following steps: 85 parts of PTA, 10-15 parts of isophthalic acid, and 60 parts of EG are added to a reaction vessel, the stirring speed is 50 rpm, and the esterification reaction is carried out at 240-250℃ and 0.2 MPa pressure for 3 hours until the water output reaches the theoretical value of 96%; then 0.03 parts of antimony glycolate are added, the temperature is raised to 265-270℃, the vacuum degree is reduced to below 50 Pa within 50 minutes, and the polycondensation reaction is carried out for 2-2.5 hours to obtain an inner layer matrix with a melting point of 210-215℃ and IV of 0.65-0.68 dL / g.

[0013] Preferably, the preparation of the inner layer functional masterbatch includes the following steps: 35 parts of ethylene-butyl acrylate-glycidyl methacrylate terpolymer were mixed evenly with 65 parts of inner layer matrix and fed into a twin-screw extruder with a length-to-diameter ratio of L / D=40:1. The screw speed was set to 250-300 rpm, and the temperatures of each zone of the extruder were set as follows: feeding zone 200℃, conveying and mixing zone 215℃, metering zone 220℃, and die head 220℃. After melt blending, water cooling and stretching, pelletizing, and centrifugal drying, the inner layer functional masterbatch was obtained.

[0014] The present invention also provides a double-sided weather-resistant multilayer polyester film, the raw materials for which include purified terephthalic acid, ethylene glycol, dimethyl 2,6-naphthalenedicarboxylate, 1,4-cyclohexanediethanol, isophthalic acid, polycarbodiimide masterbatch, ultraviolet absorber, epoxy chain extender and toughening compatibilizer.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Existing technologies mostly use physical blending to add UV absorbers, which are prone to precipitation and uneven dispersion. In this invention, a reactive triazine UV absorber and dimethyl 2,6-naphthalenedicarboxylate are introduced in the outer layer synthesis stage to achieve molecular-level grafting of weather-resistant functional groups and the introduction of rigid naphthalene ring structure. The planar structure of the naphthalene ring restricts the movement of molecular chain segments, significantly improving gas barrier properties. Combined with the end-capping effect of PCDI, the path of hydrolysis and photo-oxidative aging is cut off from the chemical structure, which is significantly better than traditional physical blending modified films.

[0016] 2. To address the risk of interlayer delamination caused by differences in added components during multilayer co-extrusion, this invention introduces an epoxy chain extender and an E-BA-GMA toughening compatibilizer into the intermediate layer. During melt extrusion, the epoxy groups not only repair the degraded molecular chains of recycled materials in situ and restore the mechanical skeleton, but also react chemically with polar groups at the interface between layers to form strong molecular sutures, thereby improving the interlayer delamination force and achieving a balance between low cost and high strength and toughness.

[0017] 3. The inner layer of this invention introduces a functional masterbatch containing glycidyl methacrylate. Utilizing the low crystallinity of the IPA-modified matrix, under the high-temperature conditions of module lamination, it induces the ring-opening of GMA epoxy groups, which then undergo a covalent cross-linking reaction with the EVA film. This chemical bonding mechanism ensures improved film adhesion and completely solves the risk of delamination throughout the entire life cycle of photovoltaic modules.

[0018] 4. This invention uses alcoholysis to prepare nano-silica slurry and participates in in-situ polymerization. The nanoparticles are coated with polymer chains in the early stage of polyester synthesis, achieving monodispersity in the matrix and effectively avoiding the formation of secondary agglomerates. Combined with precise control of the refractive index of the three-layer matrix and process control of surface flatness, the film has high rigidity and high barrier properties while meeting the stringent requirements of high light transmittance of the backsheet for bifacial photovoltaic modules.

[0019] 5. This invention employs a gradient thermal setting process. The high temperature in the first setting zone fully induces the perfect crystallization of the naphthalene ring segments in the outer layer, maximizing the barrier performance and promoting the solid-phase reaction between PCDI and the end group. The cooling relaxation in the second setting zone effectively releases the internal stress generated by biaxial stretching, especially controlling the excessive shrinkage of the low-melting-point components in the inner layer. The mechanical properties of the film are maintained, ensuring the dimensional stability of the photovoltaic module during lamination and improving the module's encapsulation yield. Attached Figure Description

[0020] Figure 1 The graphs show the changes in tensile strength of the multilayer polyester films obtained in Examples 1-5 and Comparative Examples 3-7 of this invention before and after aging. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] The PTA in this invention is purified terephthalic acid with an acid value of 675±2 mg KOH / g and metal impurities <5 ppm; EG is ethylene glycol with a purity ≥99.9% and moisture content ≤0.05%; NDC is dimethyl 2,6-naphthalenedicarboxylate with a purity ≥99.8%, a melting point of 190℃, and an acid value ≤0.1 mg KOH / g; CHDM is 1,4-cyclohexanediethanol with a cis-trans isomer ratio of 30:70 and a purity ≥99.5%; IPA is isophthalic acid with a purity ≥99.8% and an acid value of 675±2 mg KOH / g; and nano-silica is synthesized by a gas-phase method with a particle size of 20-30 nm and a specific surface area of ​​200±25 m² / g. g; the silane coupling agent is KH-560, specifically γ-glycidyl etheroxypropyltrimethoxysilane, with a boiling point of 290℃ and CAS number 2530-83-8; PCDI is polycarbodiimide, with NCN content >13%, softening point of 100-120℃, and thermal decomposition temperature >300℃; the toughening compatibilizer is E-BA-GMA, specifically ethylene-butyl acrylate-glycidyl methacrylate, with GMA content of 8%, butyl acrylate content of 24%, and melt flow rate of 6g / 10min (190℃ / 2.16kg); the epoxy chain extender is a styrene-acrylic acid-epoxy oligomer, specifically Joncryl ADR 4468, a product of BASF, has a molecular weight of 6800 and an epoxy equivalent of 285 g / mol. The anti-blocking masterbatch is Sylysia series silicone powder masterbatch in PET, with an average particle size of 2.7 μm. The UV absorber is 4-[4,6-bis(2,4-xylyl)-1,3,5-triazine-2-yl]-1,3-phenylene glycol, CAS number 1668-53-7, with an effective absorption band of 290-360 nm and a thermogravimetric temperature (TGA5%) greater than 350 °C. The recycled material used in this invention comes from the scraps or defective products of the film production line itself, with an inner layer / middle layer and outer layer ratio of 1:8:1. However, it has undergone multiple thermal history processes before being reused, and its intrinsic viscosity is 0.55-0.62 dL / g. Through the synergistic effect of epoxy chain extender and toughening compatibilizer, the waste material is recycled.

[0023] Please see Figure 1 This invention provides a double-sided weather-resistant multilayer polyester film and its preparation method, the technical solution of which is as follows: Example 1 85 parts of purified terephthalic acid, 15 parts of dimethyl 2,6-naphthalenedicarboxylate, and 65 parts of ethylene glycol were added to a reactor equipped with an anchor stirrer and a fractionating column. Stirring was started at 50 rpm, and the system was purged three times with high-purity nitrogen. 0.04 parts of manganese acetate were added as an ester exchange catalyst. The system was heated to 220°C, and the pressure inside the reactor was controlled at 0.2 MPa. At this point, the top temperature of the fractionating column was controlled at 70°C to separate methanol. The temperature was then raised to the top temperature of the column to 110°C to remove esterified water. The reaction continued for 4 hours to obtain the reaction system. At this point, the theoretical methanol distillation rate reached over 95%, and the amount of esterified water generated reached 95% of the theoretical value. The reaction system was then depressurized to atmospheric pressure, and 0.03 parts of antimony glycol catalyst were added to initiate the polycondensation stage. The system temperature was linearly raised to 285°C within 45 minutes, and the reaction was started simultaneously. A vacuum system was used to linearly reduce the absolute pressure inside the reactor from atmospheric pressure to below 100 Pa within 60 minutes. The reaction was continued for 3 hours at a speed of 40 r / min. The intrinsic viscosity of the system was measured to reach 0.50 dL / g. The feeding pressure was 0.4 MPa. The UV absorber slurry was then injected through a high-pressure feeding system while maintaining a vacuum of <50 Pa. The stirring speed was 50 rpm, and the reaction temperature was strictly controlled to not exceed 285℃. The reaction was continued for 40 minutes. The intrinsic viscosity was monitored until it reached 0.69 dL / g. The reaction was then stopped, and the vacuum was broken with nitrogen. The reaction yielded the outer matrix. The UV absorber slurry was obtained by pre-dispersing 2.5 parts of 4-[4,6-bis(2,4-xylyl)-1,3,5-triazin-2-yl]-1,3-phenylene glycol in 10 parts of ethylene glycol at 70℃ under high shear for 30 minutes at 3000 rpm. 0.8 parts of KH-560 modified nano-silica powder were added to 5 parts of ethylene glycol and dispersed at 3000 rpm for 30 min using a high-shear disperser to obtain a nano-slurry. 100 parts of PTA, 58 parts of EG, 4 parts of 1,4-cyclohexanediethanol and the nano-slurry were added to an esterification reactor, followed by 0.02 parts of tetrabutyl titanate. Stirring was started at 50 rpm, and a co-esterification reaction was carried out at 0.25 MPa pressure and 250℃. The distillation column top temperature was strictly controlled at 110℃ to prevent CHDM from volatilizing with water vapor. The reaction was carried out for 2 h to obtain the esterification system. The esterification system was transferred to a polycondensation reactor and polycondensed at 280℃ and a vacuum degree <40 Pa for 2.5 h to obtain an intrinsic viscosity of 0.65. Polyester chips with dL / g were subjected to solid-state polycondensation and added to a rotary drum solid-state viscosity-enhancing reactor. They were first pre-crystallized at 170℃ and 5 rpm for 2 hours to prevent adhesion. Then, high-purity nitrogen was introduced and the temperature was raised to 215℃. The reaction was carried out at 2 rpm for 12 hours. The intrinsic viscosity of the final chips was monitored and increased to 0.84 dL / g to obtain the intermediate layer matrix. 85 parts of PTA, 12 parts of isophthalic acid, and 60 parts of EG were added to a reactor and stirred at 50 rpm. The coesterification reaction was carried out at 245℃ and 0.2 MPa pressure for 3.0 h until the water output reached the theoretical value of 96%. Then, 0.03 parts of antimony glycol were added, the temperature was raised to 270℃, the vacuum degree was reduced to below 50 Pa within 50 min, and the polycondensation reaction was carried out for 2 h to obtain the inner layer matrix with an intrinsic viscosity of 0.66 dL / g. 35 parts of ethylene-butyl acrylate-glycidyl methacrylate copolymer and 65 parts of inner layer matrix were mixed evenly and fed into a twin-screw extruder with an L / D ratio of 40:1 and a screw speed of 300 rpm. The extruder temperatures were set as follows: feeding zone 200℃, conveying and mixing zone 215℃, metering zone 220℃, and die head 220℃. After melt blending, water-cooled stretching, pelletizing, and centrifugal drying, the inner layer functional masterbatch was obtained. 100 parts of outer layer matrix, 3 parts of polycarbodiimide masterbatch, and 0.6 parts of anti-blocking masterbatch were added to a crystallization drying bed and continuously dried at 170℃ for 5 hours, with the drying dew point ≤ -50℃ to obtain the outer layer material; 65 parts of intermediate layer matrix, 35 parts of mixed recycled material, 1.5 parts of epoxy chain extender, and 4 parts of toughening compatibilizer were added to a crystallization drying bed and dried at 160℃ for 5 hours to obtain the intermediate layer material; 100 parts of inner layer matrix and 5 parts of mixed inner layer functional masterbatch were dried at 135℃ for 6 hours using a dehumidifying dryer to obtain the inner layer material. Three single-screw extruders were used for co-extrusion. The outer extruder had a screw speed of 50 rpm and used a split screw design. The temperatures in each zone were: feeding zone 255℃, compression zone 265℃, metering zone 275℃, and melt tube 282℃, with the melt temperature strictly controlled below 285℃. The middle extruder had a screw speed of 80 rpm and used high-vacuum exhaust with a vacuum level of -0.08 MPa. The temperatures in each zone were: feeding zone 260℃, compression zone 275℃, and melt tube 282℃, with the melt temperature strictly controlled below 285℃. The temperature zones are: 70℃ (feeding zone), 275℃ (compression zone), and 280℃ (melt tube); the inner extruder screw speed is 40 rpm, and the temperatures in each zone are: 230℃ (feeding zone), 250℃ (compression zone), 255℃ (metering zone), and 260℃ (melt tube); the three layers of melt converge in a 275℃ composite die head with a die lip opening of 1.5 mm, and the resulting melt is cast onto a 25℃ quench roller at a speed of 18 m / min, using an electrostatic edge-attaching device with a voltage set to 8 kV, in conjunction with gas... The inner layer is pressed against the cold roller surface by a knife; then it undergoes a stretching process, in which the preheating roller temperature for longitudinal stretching is 90℃, the stretching roller temperature is 108℃, and the stretching ratio is 3.3 times; the preheating zone temperature for transverse stretching is 110℃, the nozzle speed is 25m / s, the stretching zone temperature is 125℃, and the stretching ratio is 3.9 times to obtain a stretched film; the stretched film is then heat-set, in which the temperature of the first setting zone is 228℃, the residence time is 12s, and the fan speed is 1800rpm to promote crystallization; the temperature of the second setting zone is reduced to 180℃ for thermal relaxation, and the transverse relaxation rate is set to 4% to obtain a post-treated film; the post-treated film is cooled to below 40℃ by a cooling roller, and the inner layer surface is corona treated with a discharge power density of 20W·min / m² to make its surface tension 54dyn / cm; finally, it is wound up to obtain a multilayer polyester film with a thickness of 50μm, of which the outer layer is 6μm, the middle layer is 38μm, and the inner layer is 6μm.

[0024] Examples 2-5 follow the same preparation method and parameter conditions as Example 1, with differences shown in Table 1.

[0025] Table 1. Parameter variations in Examples 1-5 Key process parameters Example 1 Example 2 Example 3 Example 4 Example 5 NDC dosage / part 15 18 12 14 16 UV absorber mass dosage / part 2.5 3 2 2.5 2.8 Polycarbodiimide masterbatch dosage / part 3 4 2 3 3.5 Maximum temperature of outer extruder / °C 282 284 278 280 283 CHDM dosage / part 4 3 5 4 3.5 Recycled material quantity / part 35 32 30 40 35 Epoxy chain extender mass dosage / part 1.5 0.8 1.2 2 1 Compatibilizer dosage (per part) 4 3 5 5 3.5 Maximum temperature of intermediate layer extruder / °C 280 278 275 278 280 IPA dosage / part 12 10 15 12 14 Inner layer functional masterbatch mass usage / part 5 4 6 5 5.5 Maximum temperature of inner extruder / °C 260 265 255 260 258 Inner / Middle / Outer Layer Thickness Ratio / % 12 / 76 / 12 15 / 70 / 15 10 / 80 / 10 12 / 76 / 12 14 / 72 / 14 Total film thickness / μm 50 50 50 50 50 Longitudinal tensile temperature / ℃ 108 110 105 106 109 Longitudinal stretch ratio 3.3 3.4 3.1 3.2 3.3 Transverse tensile temperature / ℃ 125 130 120 125 128 Lateral stretch ratio 3.9 4 3.7 3.8 3.9 First heat setting temperature / ℃ 228 232 225 226 230 Second heat setting temperature / °C 180 185 175 180 180

[0026] Comparative Example 1 is the same as Example 1, except that dimethyl 2,6-naphthalenedicarboxylate is not added in the synthesis of the outer matrix, and only PTA is used.

[0027] Comparative Example 2 is the same as Example 1, except that the outer matrix synthesis does not involve in-situ polymerization of UV absorbers, but instead uses physical blending to add an equal amount of ordinary UV absorbers.

[0028] Comparative Example 3 is the same as Example 1, except that polycarbodiimide masterbatch is not added to the outer layer of the film during melting, while the amounts of other components remain unchanged.

[0029] Comparative Example 4 is the same as Example 1, except that no epoxy chain extender and compatibilizer are added to the raw material of the film interlayer, and 35 parts of recycled material are added directly.

[0030] Comparative Example 5 is the same as Example 1, except that no inner layer functional masterbatch is added to the inner layer raw material of the film, while the amount of other components remains unchanged.

[0031] Comparative Example 6 is the same as Example 1, except that the outer matrix is ​​prepared by physical blending and the matrix resin is only pure PET. Then, the NDC polymer, UV absorber and PET chips are melt-blended and granulated by a twin-screw extruder. The rest of the process remains the same.

[0032] Comparative Example 7 is the same as Example 1, except that the intermediate layer matrix is ​​prepared by physical blending and the matrix resin is only pure PET. Then, high-content CHDM copolyester chips, modified nano-silica and PET chips are melt-blended and granulated by a twin-screw extruder. The rest of the process remains unchanged.

[0033] Comparative Example 8 is the same as Example 1, except that the flow channel of the die head is reversed so that the outer layer is attached to the quenching roller and the inner layer is located on the air side, while the rest of the process remains unchanged.

[0034] Comparative Example 9 is the same as Example 1, except that it uses a single-temperature low-temperature setting, with a heat setting temperature of 200°C, while the rest of the process remains unchanged.

[0035] Experiment Example 1 Mechanical Property Testing The multilayer polyester films obtained in Examples 1-5 and Comparative Examples 1-5 were subjected to mechanical property tests. Following GB / T1040.3-2006, the tensile speed was set to 100 mm / min, and the gauge length (clamp spacing) was set to 100 mm. Tensile strength and elongation at break were tested. Following GB / T 9639.1-2008, Method A was used, suitable for materials with low impact resistance. The dart head diameter was 38 mm, the drop height was 0.66 m, and the step method was used. The mass of the weights was increased or decreased based on the breakage of the previous sample until the statistical requirements were met. The mass and energy of the falling object that caused 50% of the samples to break were calculated. The test results are shown in Table 2.

[0036] Table 2 Test results of the examples and comparative examples Example Tensile strength / MPa Elongation at break / % Dart impact intensity / J Example 1 210.5 135.2 2.9 Example 2 218.4 120.7 2.4 Example 3 195.8 150.3 3.6 Example 4 202.1 132.6 3.1 Example 5 208.9 140.4 3 Comparative Example 1 190.2 145.8 2.5 Comparative Example 2 205.3 130.5 2.7 Comparative Example 3 198.7 125.1 2.3 Comparative Example 4 155.6 60.9 0.9 Comparative Example 5 212 128.2 2.2

[0037] As shown in Table 2, the mechanical properties of the multilayer polyester films obtained in the comparative examples, through adjustments to the component system and process, are significantly different from those in the examples. Because the naphthalene ring in the NDC molecular structure is a rigid bicyclic structure, it has greater steric hindrance and higher molecular chain rigidity compared to the benzene ring structure of PTA. In Example 1, the introduction of NDC copolymerization with PET, with the rigid naphthalene ring unit embedded in the polyester backbone, significantly improves the polymer's modulus and strength. In Comparative Example 1, the absence of NDC increases molecular chain flexibility, macroscopically manifested as a decrease in rigidity, and the impact strength is also slightly reduced due to the decrease in modulus. In Comparative Example 2, the UV absorber is physically blended. Due to the limited compatibility between small molecule additives and the polyester matrix, they easily form micro-aggregates during film stretching, becoming stress concentration points, leading to a decrease in elongation at break and impact strength. PCDI acts as a chain extender and end-capping agent during high-temperature extrusion, reacting with the end carboxyl groups generated by the thermal degradation of polyester to repair broken molecular chains and inhibit degradation due to hydrolysis and heat. The degradation caused a decrease in molecular weight. In Comparative Example 3, the lack of PCDI led to a certain degree of degradation of the matrix resin during processing, resulting in a decrease in intrinsic viscosity and thus weakening the mechanical properties of the film. Due to the repeated thermal history processes of the recycled material, the molecular weight has been severely degraded, and the source of the recycled material is complex, with compatibility issues with the new material. The epoxy chain extender introduced in the examples can repair the molecular weight of the recycled material in situ, while the epoxy groups in the toughening compatibilizer can undergo chemical reactions at the interface of the new and old materials, playing a molecular stitching role. In Comparative Example 4, the lack of these additives resulted in a large number of phase separation defects and low molecular weight weak regions forming inside the intermediate layer, which quickly became crack propagation sources under stress, making the film extremely brittle. In Comparative Example 5, no inner layer functional masterbatch was added. Although the tensile strength did not change much, the impact strength decreased significantly. The ethylene-butyl acrylate flexible segments in the functional masterbatch played a toughening role in the inner layer and could absorb impact energy. The lack of this component made the inner layer relatively rigid, lacking a rubber phase to absorb impact, thus reducing the overall impact resistance of the film.

[0038] Experiment Example 2: UV Resistance and Adhesion Performance Test The multilayer polyester films prepared in Examples 1-5 and Comparative Examples 6-9 were tested for UV resistance and adhesion. UV aging tests were conducted on the films according to GB / T 16422.3-2022 using a UVB-313 fluorescent UV lamp with an irradiance of 0.71 W / m². 2@310nm, 60℃ light exposure for 8 hours, then transferred to 50℃ light-free condensation for 4 hours, exposed for 2000 hours. Tensile strength tests were conducted before and after aging according to the test standards of Experiment Example 1, and the strength retention rate was calculated. Color difference before and after aging was determined using a spectrophotometer according to GB / T 7921-2008. According to GB / T 2791-1995 and GB / T 31034-2014, using a glass / EVA film / test film (with EVA inner layer), a vacuum laminator was used at 145℃ for 5 minutes of vacuuming and 10 minutes of pressure. After lamination and curing, strips 15mm wide were cut, and T-shaped peeling was performed at a tensile speed of 100mm / min. The average force during peeling was recorded, and the peel strength was calculated. According to GB / T 8808-1988, the sample ends were immersed in ethyl acetate or induced with adhesive tape to attempt to separate the interlayers. If separation was possible, a T-type peel test was performed at a speed of 300 mm / min. In this invention, "material fracture" occurred, and the interlayers could not be separated, which was recorded as "non-peelable / substrate fracture," indicating that the interlayer bonding force was greater than the substrate strength; the test results are shown in Table 3; the changes in tensile strength of the multilayer polyester films obtained in Examples 1-5 and Comparative Examples 3-7 under UV conditions are shown in Table 3. Figure 1 As shown.

[0039] Table 3. Test results of the examples and comparative examples Example Tensile strength retention rate after aging / % UV aging color difference / ΔE Peel strength of EVA film / N / cm Interlayer adhesion / N / 15 mm Example 1 92.5 0.52 72.4 Non-removable Example 2 94.1 0.48 68.9 Non-removable Example 3 89.3 0.65 75.2 Non-removable Example 4 90.8 0.59 71.6 Non-removable Example 5 93.4 0.55 81.3 Non-removable Comparative Example 3 65.2 2.84 64.1 5.8 Comparative Example 4 78.6 1.12 60.5 1.2 Comparative Example 5 91.9 0.56 63.4 Non-removable Comparative Example 6 82.3 2.15 66.8 4.5 Comparative Example 7 85.7 0.94 69.2 3.1

[0040] Through Table 3, Figure 1 The results show that, by adjusting the key functional components and preparation process, the multilayer polyester film obtained in the comparative example exhibits significant differences in weather resistance and adhesion compared to the example. In the example, the outer matrix is ​​polymerized in situ to introduce a reactive UV absorber containing dihydroxyl groups, which is then firmly bonded to the polyester molecular chain to prevent precipitation. Simultaneously, polycarbodiimide is introduced as an anti-hydrolysis end-capping agent to effectively capture the end carboxyl groups generated by polyester degradation. In the middle layer, an epoxy chain extender and a toughening compatibilizer are introduced, utilizing the high reactivity of epoxy groups to achieve the repair of the molecular weight of the recycled material and the chemical stitching of the three-layer interface during melt extrusion. The functional masterbatch of the inner layer provides abundant covalent bonding sites for subsequent bonding with the EVA film.

[0041] Because polyester materials are prone to hydrolysis and breakage of ester bonds under high temperature, high humidity, and ultraviolet radiation, producing terminal carboxyl groups, which can further catalyze hydrolysis reactions, polycarbodiimide masterbatch can undergo addition reactions with terminal carboxyl groups to form acylurea structures, thereby blocking the source of degradation. In Comparative Example 3, the lack of polycarbodiimide caused the outer matrix to rapidly degrade and pulverize during aging, resulting in not only a loss of mechanical strength but also significant yellowing of the film due to the accumulation of degradation products. At the same time, the severe degradation of the outer layer damaged the integrity of the overall structure, leading to a reduction in overall adhesion. The recycled material of this invention has undergone multiple thermal histories, has a low molecular weight and wide distribution, and a low melt viscosity. The chain extender in the examples can improve melt strength by coupling low molecular weight segments with multifunctional groups. The epoxy groups in the compatibilizer can chemically react with the end groups at the outer / intermediate layer and intermediate / inner layer interfaces to form cross-interfacial molecular entanglement and chemical bonding. In Comparative Example 4, the lack of these components resulted in the intermediate layer having low strength and its bonding with the surface layer relying only on weak physical diffusion and mechanical bonding. Interlocking leads to rapid brittle fracture of the interface under peeling force, resulting in delamination. In Comparative Example 5, after the functional masterbatch is missing, the inner layer becomes a common inert copolyester surface with low surface energy. There are only weak van der Waals forces between it and EVA, which cannot resist the damage of peeling force and completely fails to meet the requirements of photovoltaic backsheet encapsulation. The results of Comparative Examples 6-7 show that in-situ polymerization can graft UV absorbers onto the polymer chain at the molecular level, and the NDC units and modified nano-silica are evenly distributed in the chain. Physical blending cannot achieve such uniform dispersion at the molecular level. The UV absorbers are free in the matrix in the form of small molecules, which are easy to migrate to the surface or be lost during aging, resulting in the degradation of protective effectiveness over time. At the same time, physical blending has poor compatibility. Nano-silica is very easy to agglomerate, forming micron-sized agglomerates. When these agglomerates are distributed near the interlayer interface, they become stress concentration points, destroying the continuity of the interlayer interface, hindering the mutual diffusion and entanglement of molecular chains on both sides of the interface, and thus significantly weakening the bonding strength between layers.

[0042] Experiment Example 3: Barrier Performance and Light Transmission Performance Test

[0043] The barrier properties and light transmittance of the multilayer polyester films prepared in Examples 1-5 and Comparative Examples 6-9 were tested. Following GB / T 2410-2008, an integrating sphere haze meter was used with a D65 light source. Flat samples of 50mm × 50mm were cut, and the total transmittance (Tt) and scattered light transmittance (Td) were measured to calculate the haze. Following GB / T 26253-2010, at a temperature of 38℃ and a relative humidity of 90%, the samples were placed in the permeation chamber, with one side being a high-humidity chamber and the other side containing dry nitrogen. Because this invention has an asymmetric structure, the outer side was oriented towards the high-humidity side during testing to simulate the scenario where the outer side of the backplate blocks moisture in actual applications. After the transmittance stabilized, the water vapor transmittance was recorded. The test results are shown in Table 4.

[0044] Table 4. Test results of the examples and comparative examples Example transmittance / % Haze / % Water vapor transmission rate / g / (m²·24h) Example 1 91.5 2.8 1.3 Example 2 91.1 3.1 0.9 Example 3 92.2 2.4 1.6 Example 4 90.7 3.5 1.4 Example 5 91.3 2.9 1.2 Comparative Example 6 86.4 12.7 1.8 Comparative Example 7 87.2 9.6 1.9 Comparative Example 8 82.5 18.3 2.1 Comparative Example 9 90.9 3.2 2.3

[0045] As shown in Table 4, the barrier and optical properties of the polyester multilayer film obtained in the comparative example, obtained by adjusting the component preparation method and process, are significantly different from those of the example. In the embodiment of the present invention, in-situ polymerization is used to introduce modified components. The rigid naphthalene ring introduced by copolymerization in the outer layer forms a dense crystalline structure during high-temperature heat setting, which effectively extends the diffusion path of water molecules and significantly reduces water vapor transmission rate. At the same time, the nano-silica in the middle layer is uniformly dispersed in the polyester matrix through in-situ polymerization. Its particle size is much smaller than the wavelength of visible light, which greatly reduces Rayleigh scattering. Combined with the precise matching of the refractive indices of the three-layer matrix, the film is endowed with excellent high transmittance and low haze.

[0046] In Comparative Examples 6-7, the outer matrix was prepared using a physical blending method, directly melt-blending NDC polymers and PET chips. Due to insufficient transesterification and short blending time, a uniform random copolymer could not be formed, resulting in a microscopic phase-separated structure. The presence of this microscopic phase interface caused strong refraction and scattering of light when passing through the film, leading to a significant reduction in optical performance. Furthermore, the formed phase-separated interface is often a weak point for gas permeation, resulting in reduced barrier properties. The nanoparticles introduced by physical blending are extremely difficult to disperse in the high-viscosity polyester melt, forming micron-sized agglomerates. These agglomerates not only become light scattering centers, increasing haze, but also have weak bonding between the agglomerates and the matrix, easily generating micropores during biaxial stretching. These micropores readily become tunnels for water vapor to pass through rapidly, thus... This leads to increased water vapor permeability and decreased barrier performance. In Comparative Example 8, the change in the casting roll order caused the high-melting-point outer layer to adhere to the quenching roll, while the low-melting-point inner layer was exposed to the air. Due to the introduction of isophthalic acid, which is a slow-crystallizing component, its slow cooling rate in air resulted in uncontrollable crystallinity and the formation of large spherulites, increasing surface roughness. In addition, it caused the film to stick together at the stretching preheating roll. After this adhesion was peeled off, it left dense micro-defects and rough textures on the film surface, causing severe diffuse reflection and significantly reducing the overall performance. In Comparative Example 9, a low-temperature heat setting process was used. The ethylene naphthalate component in the outer layer could not form a high-density crystalline region, and the molecular chain arrangement was relatively loose, which could not effectively block the penetration of water molecules, resulting in a significant decrease in barrier performance.

[0047] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a double-sided weather-resistant multilayer polyester film, characterized in that, The three layers—outer, middle, and inner—are melt-co-extruded and cooled to form a multilayer polyester film. This film is then stretched and heat-set. Finally, it is cooled, corona-treated, and wound to obtain the multilayer polyester film. The outer layer is a mixture of an outer matrix, polycarbodiimide masterbatch, and anti-blocking masterbatch. The middle layer is a mixture of a middle matrix, recycled material, epoxy chain extender, and toughening compatibilizer. The inner layer is a mixture of an inner matrix and inner functional masterbatch. The outer matrix is ​​obtained by esterification and polycondensation of purified terephthalic acid, dimethyl 2,6-naphthalenedicarboxylate, ethylene glycol, and a UV absorber. The middle matrix is ​​obtained by esterification and polycondensation of nano-slurry, purified terephthalic acid, ethylene glycol, and 1,4-cyclohexanediethanol. The inner matrix is ​​obtained by esterification and polycondensation of purified terephthalic acid, ethylene glycol, and isophthalic acid.

2. The method for preparing a double-sided weather-resistant multilayer polyester film according to claim 1, characterized in that, The preparation of the outer substrate includes the following steps: The purified terephthalic acid, the dimethyl 2,6-naphthalenedicarboxylate, and the ethylene glycol were added to a reaction vessel, stirred, and purged with nitrogen. Then, manganese acetate was added to induce an esterification reaction to obtain a reaction system. Antimony glycolate was added to the reaction system and stirred to disperse it. The ultraviolet absorber was then added to react and obtain the outer matrix.

3. The method for preparing a double-sided weather-resistant multilayer polyester film according to claim 1, characterized in that, The preparation of the intermediate layer matrix includes the following steps: adding purified terephthalic acid, ethylene glycol, 1,4-cyclohexanediethanol and nano-slurry into an esterification reactor, then adding tetrabutyl titanate, and performing a co-esterification reaction to obtain an esterification system; performing a polycondensation reaction on the esterification system to obtain polyester chips; and performing solid-state polycondensation on the polyester chips to obtain the intermediate layer matrix.

4. The method for preparing a double-sided weather-resistant multilayer polyester film according to claim 1, characterized in that, The preparation of the inner layer matrix includes the following steps: adding the purified terephthalic acid, the ethylene glycol, and the isophthalic acid into a reaction vessel for co-esterification reaction; then adding antimony glycolate and performing a polycondensation reaction to obtain the inner layer matrix.

5. The method for preparing a double-sided weather-resistant multilayer polyester film according to claim 1, characterized in that, The preparation of the inner layer functional masterbatch includes the following steps: mixing the ethylene-butyl acrylate-glycidyl methacrylate copolymer with the inner layer matrix evenly, followed by melt blending, water-cooled stretching, pelletizing, and centrifugal drying to obtain the inner layer functional masterbatch.

6. The method for preparing a double-sided weather-resistant multilayer polyester film according to claim 1, characterized in that, The stretching process includes the following steps: the multilayer polyester casting is subjected to the stretching process, with the longitudinal stretching roller temperature at 105-110℃ and the stretching ratio at 3.1-3.4 times; the transverse stretching temperature at 120-135℃ and the stretching ratio at 3.7-4.0 times to obtain a stretched film.

7. A double-sided weather-resistant multilayer polyester film, characterized in that, The multilayer polyester film comprises an inner layer, a middle layer, and an outer layer; the raw materials include purified terephthalic acid, ethylene glycol, dimethyl 2,6-naphthalenedicarboxylate, 1,4-cyclohexanediethanol, isophthalic acid, polycarbodiimide masterbatch, ultraviolet absorber, epoxy chain extender, and toughening compatibilizer; the multilayer polyester film is prepared by the preparation method according to any one of claims 1-6.

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