Optical-grade high-brightness polyester film and preparation method thereof

Abstract

The invention discloses an optical-grade high-brightness polyester film and a preparation method thereof, the optical-grade high-brightness polyester film comprises a nanoscale multi-layer structure formed by PET layers and PEN layers which are alternately stacked, the total layer number is 350-700, and the total thickness is 30-60 [mu] m; wherein the refractive index of the PET layer ranges from 1.58 to 1.62, and the refractive index of the PEN layer ranges from 1.64 to 1.68; the average reflectivity of the film under the wavelength of 550 nm is larger than or equal to 98.5%, the light transmittance is larger than or equal to 91.5%, the haze is smaller than or equal to 0.5%, and the interlayer peel strength is larger than or equal to 3.0 N / cm. According to the invention, collaborative optimization of optical performance, mechanical strength and thermal stability is realized through a nanoscale multi-layer structural design in which PET and PEN are alternately stacked. Excellent optical performance is obtained through hundreds of layers of nano-scale alternating structures; the interlayer binding force is obviously improved through molecular modification of the laminated layers; and a uniaxial stretching process is adopted, so that the flexibility is kept while the layer thickness uniformity is ensured, and breakthrough is realized in the aspects of reflection efficiency, interface stability and processing yield.

Description

Technical Field

[0001] This invention relates to high-brightness polyester films used in liquid crystal displays, lighting, and other fields, and particularly to an optical-grade high-brightness polyester film and its preparation method. Background Technology

[0002] Optical-grade high-brightness polyester film is widely used in various high-tech and industrial fields due to its excellent optical properties (such as high transmittance, low haze, and high reflectivity) and physicochemical stability. For example, in the display and electronics industry, high-brightness polyester film can be used in backlight modules of devices such as LCD displays as a brightness enhancement film or reflective film to improve screen brightness and uniformity. In the lighting field, it can be used as a reflective layer in LED lamps to improve luminous efficiency and heat dissipation performance, reducing light loss. In the solar cell field, it can be used as a backsheet for photovoltaic modules, possessing high reflectivity to improve photoelectric conversion efficiency. In the packaging and printing field, it can be used as a high-transparency label to enhance the product's appearance and texture.

[0003] The applicant previously disclosed a high-brightness DOP composite film in Chinese patent CN 112946794 B, which achieves greater brightness by altering the surface physical structure of the film. However, this composite film is only suitable for specialized liquid crystal display applications, as its complex structure and excessive thickness make it difficult to apply widely to other fields.

[0004] CN 106526726 B discloses a polyester film that achieves high luminance, high light transmittance, and low haze by coating a substrate surface with a coating containing inorganic particles. These inorganic particles can be alumina, aluminum hydroxide, silicon oxide, titanium oxide, zirconium oxide, calcium carbonate, magnesium carbonate, and barium sulfate. However, the refractive index of the inorganic particles themselves is fixed, and there are compatibility and dispersibility issues with the coating adhesive. Too low an addition amount is insufficient to change the luminance value of the polyester film, while too high an addition amount reduces light transmittance and deteriorates the mechanical properties of the coating.

[0005] There are many existing technologies that use optical coatings to change the surface luminance of polyester films. For example, CN114196052A, CN 102108173 B and CN 106707373 B. These existing technologies all have problems with the compatibility and dispersion of optical particles in the coating. Moreover, the coating only adheres to the surface of the film and fails to change the luminance of the film itself. Once the coating peels off or becomes uneven, it will have a significant impact on the overall performance of the film. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide an optical-grade high-brightness polyester film and a method for preparing the same, so as to reduce or avoid the problems mentioned above.

[0007] To address the aforementioned technical problems, this invention proposes an optical-grade high-brightness polyester film, comprising a nanoscale multilayer structure formed by alternating layers of PET and PEN, with a total of 350-700 layers and a total thickness of 30-60 μm; wherein the refractive index of the PET layer is 1.58-1.62, and the refractive index of the PEN layer is 1.64-1.68; the film exhibits an average reflectivity ≥98.5%, a transmittance ≥91.5%, a haze ≤0.5%, and an interlayer peel strength ≥3.0 N / cm at a wavelength of 550 nm.

[0008] Preferably, the refractive index of the PET layer is 1.58, and the refractive index of the PEN layer is 1.64.

[0009] Preferably, the PET layer is prepared from 50-55 parts by weight of terephthalic acid, 25-28 parts by weight of ethylene glycol, 8-12 parts by weight of cyclohexanediol, 0.4-0.6 parts by weight of tetrafluoroterephthalic acid, 0.1-0.3 parts by weight of antioxidant and 0.05-0.1 parts by weight of catalyst.

[0010] Preferably, the PEN layer is prepared from 48-52 parts by weight of naphthalic acid, 30-32 parts by weight of ethylene glycol, 5-8 parts by weight of terephthalic acid, 1.0-1.5 parts by weight of polyester elastomer and 0.05-0.1 parts by weight of catalyst.

[0011] This invention also proposes a method for preparing the aforementioned optical-grade high-brightness polyester film, comprising the following steps: preparing modified PET chips and modified PEN chips respectively; feeding the prepared modified PET chips into a PET twin-screw extruder and the prepared modified PEN chips into a PEN twin-screw extruder; feeding the melt extruded from the PET twin-screw extruder and the PEN twin-screw extruder into a multilayer co-extrusion die, and layering and multiplying the extruded PET melt and PEN melt through the multilayer co-extrusion die to form a thick sheet with a nanoscale multilayer structure of 350-700 layers; feeding the thick sheet extruded from the multilayer co-extrusion die into a unidirectional stretching machine for longitudinal stretching, with a stretching ratio of 3.5-4.0 times; then performing heat setting treatment on the longitudinally stretched film, with a heat setting temperature of 210-220℃, a heat setting time of 5-8 seconds, and controlling the heat shrinkage rate to be <1.0%; finally, drying the film and slitting and winding it.

[0012] Preferably, the steps for preparing modified PET chips include: adding terephthalic acid, ethylene glycol, and cyclohexanediol to a reaction vessel in a specific ratio, heating to 240-250°C, purging with nitrogen, and stirring at 200-300 rpm to carry out an esterification and dehydration reaction until the esterification rate is >95%; in the later stage of the esterification process, i.e., after 1.5 hours of esterification reaction, adding tetrafluoroterephthalic acid and continuing the reaction for another 0.5 hours to ensure that the fluorinated monomer is embedded in the main chain; after the esterification rate reaches 95%, heating to 270°C. At -280℃, the pressure is reduced to 5-10 kPa, a catalyst is added, and pre-polymerization is carried out for 1 hour until the intrinsic viscosity rises to 0.4-0.5 dL / g. The pressure is then reduced to 50-100 Pa, the temperature is increased to 285-295℃, and polycondensation is carried out for 2-3 hours until the intrinsic viscosity reaches 0.8-0.9 dL / g. The melt generated in the reactor is then pelletized into modified PET chips with a particle size of 3-5 mm using a pelletizer. The chips are then vacuum dried at 120℃ for 4-6 hours until the moisture content is <50 ppm and stored for later use.

[0013] Preferably, the steps for preparing the modified PEN resin include: vacuum drying naphthalene dicarboxylic acid at 80°C for 4 hours to achieve a moisture content of <100ppm; drying terephthalic acid at 100°C for 6 hours to prevent agglomeration; pulverizing the polyester elastomer to a particle size of <500μm, pre-dispersing it with ethylene glycol at a mass ratio of 1:5 (ultrasonic treatment for 30 minutes) to form an elastomer slurry; then adding the raw materials other than the elastomer slurry to a stainless steel reaction vessel in proportion, heating to 240-250°C under nitrogen protection, maintaining a pressure of 0.2-0.3MPa, and stirring at 200rpm to carry out an esterification and dehydration reaction; the reaction time is 2.5-3 hours, until the water content reaches more than 95% of the theoretical value, completing the esterification; after esterification, cooling is performed. Add the pre-dispersed elastomer slurry to 220℃, and stir at 500 rpm for 30 minutes to ensure uniform dispersion of the elastomer. Then, raise the temperature to 250℃ and vacuum to 50 kPa to remove residual EG. Continue to raise the temperature to 270-280℃ and reduce the pressure to 1-5 kPa, and perform a pre-polymerization reaction for 1 hour until the intrinsic viscosity reaches 0.5-0.6 dL / g. Finally, raise the temperature to 290-300℃ and reduce the pressure to <100 Pa, and react for 2-3 hours until the intrinsic viscosity reaches 0.9-1.0 dL / g. The melt generated in the stainless steel reaction vessel is pelletized into modified PEN chips with a particle size of 3-5 mm using a pelletizer, and vacuum dried at 120℃ for 8 hours until the moisture content is <50 ppm. Store for later use.

[0014] Preferably, the temperature of the multi-layer co-extrusion die is controlled at 265-300℃.

[0015] Preferably, the preheating temperature for the unidirectional stretching is 85-100℃, the stretching temperature is 100-120℃, and the stretching direction is longitudinal.

[0016] Preferably, after heat setting, the film is dried at 120-150°C for 4-8 hours until the moisture content is ≤50ppm.

[0017] This invention achieves synergistic optimization of optical performance, mechanical strength, and thermal stability through a nanoscale multilayer structure design of alternating layers of PET and PEN. It achieves excellent optical performance through hundreds of alternating nanoscale layers; significantly enhances interlayer bonding through molecular modification of the stacked layers; and employs a unidirectional stretching process to ensure uniform layer thickness while maintaining flexibility. Breakthroughs are achieved in reflection efficiency, interface stability, and processing yield, making it widely applicable in Mini / MicroLED backlight modules, flexible displays, and high-precision optical sensors, combining high performance with industrial feasibility. Detailed Implementation

[0018] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, the specific embodiments of the present invention will now be described in detail.

[0019] In view of the shortcomings of existing high-brightness polyester films, such as complex structure and unstable performance, this invention proposes an optical-grade high-brightness polyester film. This optical-grade high-brightness polyester film comprises a nanoscale multilayer structure formed by alternating layers of PET and PEN. Since PET and PEN have different refractive indices (PEN has a higher refractive index and PET has a lower refractive index), this difference in refractive index creates multiple optical interfaces between the layers. When light is incident on these interfaces, partial reflection and transmission occur. If the thickness of each layer is properly designed to be close to one-quarter of the incident light wavelength (i.e., one-quarter wavelength), coherent stacking of reflected light occurs, resulting in enhanced reflection of light of specific wavelengths, thereby increasing the brightness of the film. By increasing the number of layers, high reflectivity can be achieved over a wider spectral range.

[0020] Based on the above principles, this invention designs and proposes an optical-grade high-brightness polyester film with a nanoscale multilayer structure consisting of alternating layers of PET and PEN. By precisely designing the thickness and number of each layer, an average reflectivity of over 98.5% can be achieved in the visible light range with hundreds of layers.

[0021] For example, in the visible light range (center wavelength 550nm), the refractive index of PET is approximately 1.58-1.62, and that of PEN is approximately 1.64-1.68. Using visible light as the target, the minimum optical thickness of a single layer, representing one-quarter of the visible light wavelength, is calculated. The minimum optical thickness for PET is approximately 87nm (taking a refractive index of 1.58 as an example, 550nm divided by 4 and then by 1.58), and the minimum optical thickness for PEN is approximately 84nm (taking a refractive index of 1.64 as an example, 550nm divided by 4 and then by 1.64). The difference between the two is minimal, and in industrial production, their thicknesses can be considered equal to achieve the desired interface reflection effect.

[0022] Furthermore, for a pair of structures consisting of PET and PEN layers, the total thickness is approximately 0.17 μm (87 nm + 84 nm). Since multilayer interference reflectivity increases with the number of layers, but the marginal effect decreases, considering both cost and brightness increases, when the number of layers is below 100, the increase in reflectivity due to the stacking effect is significantly less than 98.5%. However, when the number of layers exceeds 1000, the absorption and scattering losses of the material offset the gain, and the reflectivity actually decreases below the theoretical limit. Therefore, within the range of 100 to 1000 layers, considering the thickness limitations of materials used in common applications, taking 30 μm to 60 μm as an example, the total number of layers can be calculated: at a thickness of 30 μm, 30 μm divided by 0.17 μm equals 176.5 pairs of layers, and multiplying the total by 2 gives approximately 353 layers. Similarly, at a thickness of 60 μm, the total number of layers is approximately 706. Therefore, the preferred total number of film layers in this invention is 350-700 layers, the preferred refractive index of the PET layer is 1.58, and the preferred refractive index of the PEN layer is 1.64.

[0023] In a preferred embodiment, the PET layer is prepared from 50-55 parts by weight of terephthalic acid (PTA), 25-28 parts by weight of ethylene glycol (EG), 8-12 parts by weight of cyclohexanediol (CHDM), 0.4-0.6 parts by weight of tetrafluoroterephthalic acid (TFTA), 0.1-0.3 parts by weight of antioxidant, and 0.05-0.1 parts by weight of catalyst. PTA and EG are the basic monomers for preparing the PET layer. CHDM is used to replace part of the EG to reduce crystallinity and adjust the refractive index to a stable 1.58, while improving melt flowability. TFTA is used to introduce CF bonds, reduce material polarity, reduce interlayer friction (dynamic friction coefficient COF < 0.25), and improve co-extrusion uniformity. The antioxidant and catalyst are functional additives and can be conventional PET additives. For example, the antioxidant can be BASF's Irganox 1010 antioxidant, which reduces yellowing by inhibiting oxidative degradation during high-temperature processing; the catalyst can be tetrabutyl titanate.

[0024] Specifically, the components constituting the PET layer can be prepared into modified PET chips by the following method, which are then used as raw materials for extruding to form the PET layer.

[0025] For example, PTA, EG, and CHDM can be added to a reactor in a specific ratio, heated to 240-250℃, protected with nitrogen, and stirred at 200-300 rpm for esterification and dehydration until the esterification rate is >95%. In the later stages of esterification, after 1.5 hours, TFTA is added, and the reaction continues for another 0.5 hours to ensure the fluorinated monomer is embedded in the main chain. After the esterification rate reaches 95%, the temperature is raised to 270-280℃, the pressure is reduced to 5-10 kPa, a catalyst is added, and pre-polymerization is carried out for 1 hour until the intrinsic viscosity reaches 0.4-0.5 dL / g. The pressure is then reduced to 50-100 Pa, the temperature is raised to 285-295℃, and polycondensation is carried out for 2-3 hours until the intrinsic viscosity reaches 0.8-0.9 dL / g. The melt generated in the reactor is then pelletized into modified PET chips with a particle size of 3-5 mm using a pelletizer, vacuum dried at 120℃ for 4-6 hours until the moisture content is <50 ppm, and stored for later use.

[0026] In another preferred embodiment, the PEN layer is prepared from 48-52 parts by weight of naphthalenedicarboxylic acid (NDA), 30-32 parts by weight of ethylene glycol (EG), 5-8 parts by weight of terephthalic acid (PTA), 1.0-1.5 parts by weight of polyester elastomer, and 0.05-0.1 parts by weight of catalyst. NDA and EG are the basic monomers for PEN preparation, PTA is used to adjust the refractive index to a stable 1.64, and the polyester elastomer is used to enhance the adhesion to the PET layer. The polyester elastomer is preferably a copolymer elastomer of PBT and PEG, such as DuPont's Hytrel 3078, or WanElast 4556 from Wanhua Chemical, or other PBT / PEG copolymer elastomers suitable for PET and PEN processing from other companies. The catalyst can be tetrabutyl titanate or other commonly used catalysts.

[0027] Specifically, the components constituting the PEN layer can be prepared into modified PEN chips by the following method, which are then used as raw materials for extruding to form the PEN layer.

[0028] For example, naphthalene dicarboxylic acid (NDA) can be vacuum dried at 80°C for 4 hours to achieve a moisture content of <100ppm. Terephthalic acid (PTA) can be dried at 100°C for 6 hours to prevent agglomeration. Polyester elastomer (such as Hytrel 3078) can be pulverized to a particle size of <500μm and pre-dispersed with EG at a mass ratio of 1:5 (ultrasonic treatment for 30 minutes) to form an elastomer slurry. Then, raw materials other than the elastomer slurry are added to a stainless steel reaction vessel in proportion. Under nitrogen protection, the temperature is raised to 240-250°C, the pressure is 0.2-0.3MPa, and the stirring speed is 200rpm to carry out the esterification and dehydration reaction. The reaction time is 2.5-3 hours, until the water output reaches more than 95% of the theoretical value, completing the esterification. After esterification, the temperature is lowered to 220°C, the pre-dispersed elastomer slurry is added, and the mixture is stirred at 500rpm for 30 minutes to ensure uniform dispersion of the elastomer. Then, the temperature is raised to 250℃, and a vacuum of 50 kPa is applied to remove residual EG. The temperature is further raised to 270-280℃, and the pressure is reduced to 1-5 kPa for pre-polymerization reaction for 1 hour, until the intrinsic viscosity reaches 0.5-0.6 dL / g. Finally, the temperature is raised to 290-300℃, and the pressure is <100 Pa, and the reaction is continued for 2-3 hours until the intrinsic viscosity reaches 0.9-1.0 dL / g. The melt generated in the stainless steel reaction vessel is pelletized into modified PEN chips with a particle size of 3-5 mm using a pelletizer, vacuum dried at 120℃ for 8 hours until the moisture content is <50 ppm, and stored for later use.

[0029] The preparation method of the optical-grade high-brightness polyester film of the present invention is further described in detail below. For example, a PET twin-screw extruder for modified PET chips and a PEN twin-screw extruder for modified PEN chips can be set up. The prepared modified PET chips are fed into the PET twin-screw extruder, and the prepared modified PEN chips are fed into the PEN twin-screw extruder. The temperature range of the PET twin-screw extruder is set to 265-280°C, and the temperature range of the PEN twin-screw extruder is set to 280-300°C. The melt viscosity of the modified PET is adjusted to 1500 Pa·s by CHDM and fluorinated monomers. The melt viscosity of the modified PEN is adjusted to 1600 Pa·s by PTA and elastomers. The viscosity difference between the two melts is <7%, ensuring stable co-extrusion laminar flow.

[0030] The molten materials extruded from PET and PEN twin-screw extruders are fed into a multilayer co-extrusion die. The die distributes the extruded PET and PEN melts, multiplying them to form a multilayered sheet with hundreds of nanoscale layers. The temperature of the multilayer co-extrusion die is controlled between 265-300℃. Producing multilayered nanoscale multilayered structures with hundreds of layers typically requires a multi-channel melt distributor in conjunction with a layer multiplier. Multiple melt distributors and layer multipliers can be configured as needed. Equipment and processes for preparing multilayered nanoscale multilayered structures with hundreds of layers are existing technologies; those skilled in the art can refer to CN 101987499 B, CN 103338847 B, and CN 114474672 B for further understanding. The film thickness can be precisely controlled using the multilayer co-extrusion die in conjunction with a melt pump installed in the pipeline.

[0031] Next, the thick sheet extruded from the multi-layer co-extrusion die is fed into a uniaxial stretching machine for longitudinal stretching. The preheating temperature for uniaxial stretching is 85-100℃, the stretching temperature is 100-120℃, the stretching direction is longitudinal (MD), and the longitudinal stretching ratio is set to 3.5-4 times.

[0032] It is particularly important to note that conventional biaxial stretching is not recommended for the high-brightness polyester film of this invention. This is because the reflectivity of the multilayer film of this invention depends on the optical thickness (physical thickness × refractive index) of each pair of PET / PEN layers being precisely matched to one-quarter of the target wavelength. Longitudinal stretching, which only stretches along the MD direction, allows for precise control of the uniformity of each layer's thickness (error < ±2%), avoiding layer thickness fluctuations caused by anisotropic deformation (difference between MD and TD stretching ratios) resulting from biaxial stretching. Furthermore, uniaxial stretching, through high-ratio stretching in a single direction (3.5-4 times), ensures highly aligned molecular chains, guaranteeing strict alignment of the optical path lengths of each layer, resulting in reflectivity approaching the theoretical limit (>98.5%). In contrast, biaxial stretching introduces local thickness gradients due to lateral (TD) stretching, leading to a shift in the optical path difference and reducing the sharpness and intensity of the reflection peak.

[0033] On the other hand, PET and PEN have significantly different glass transition temperatures (Tg) (PET≈80℃, PEN≈125℃). Biaxial stretching requires staged heating along the MD and TD directions, which can lead to a mismatch in interlayer thermal expansion coefficients, causing interfacial shear stress and increasing the risk of delamination. Uniaxial stretching, however, only requires a gradient heating along the MD direction (PET 85-95℃, PEN 100-110℃), reducing interlayer stress concentration. Combined with the in-situ compatibilization effect of the copolyester elastomer, this ensures a peel strength >3N / cm. Furthermore, the polyester elastomer added to the PEN layer may become unevenly distributed due to anisotropic stress during biaxial stretching, weakening interfacial bonding. During uniaxial stretching, the elastomer is orderly arranged along the MD direction, enhancing interlayer bonding through molecular chain entanglement and avoiding the dispersion and disorder caused by transverse stretching.

[0034] The longitudinally stretched film is then heat-set at a temperature of 210-220℃ (higher than PEN's Tg≈110℃) for 5-8 seconds, with the heat shrinkage rate controlled to be <1.0%. Finally, the film is dried at 120-150℃ for 4-8 hours until the moisture content is ≤50ppm, and then slit and wound up.

[0035] The following detailed description of the parameters of the optical-grade high-brightness polyester film of the present invention is provided through specific embodiments.

[0036] Example 1

[0037] PET layer composition: PTA 50 parts by weight, EG 28 parts by weight, CHDM 10 parts by weight, TFTA 0.5 parts by weight, antioxidant 0.1 parts by weight, catalyst 0.05 parts by weight.

[0038] PEN layer composition: 50 parts by weight of NDA, 30 parts by weight of EG, 5 parts by weight of PTA, 1.2 parts by weight of elastomer (Hytrel 3078), and 0.05 parts by weight of catalyst.

[0039] Total number of layers: 350; total thickness: 30μm; longitudinal tensile ratio: 3.5.

[0040] Performance parameters: In the visible light range (550nm), the average reflectance is 98.5%, the transmittance is 92.3%, the haze is 0.35%, the tensile strength (MD) is 225MPa, the heat shrinkage rate (150℃) is 0.9%, and the interlayer peel strength is 3.0N / cm.

[0041] Example 2

[0042] PET layer composition: PTA 54 parts by weight, EG 27 parts by weight, CHDM 8 parts by weight, TFTA 0.6 parts by weight, antioxidant 0.3 parts by weight, catalyst 0.1 parts by weight.

[0043] PEN layer composition: 51 parts by weight of NDA, 32 parts by weight of EG, 8 parts by weight of PTA, 1.0 part by weight of elastomer, and 0.1 part by weight of catalyst.

[0044] The total number of layers is 468, and the total thickness is 40μm. The longitudinal stretch ratio is 4.0.

[0045] Performance parameters: In the visible light range (550nm), the average reflectance is 98.7%, the transmittance is 92.0%, the haze is 0.32%, the tensile strength (MD) is 230MPa, the heat shrinkage rate (150℃) is 0.8%, and the interlayer peel strength is 3.3N / cm.

[0046] Example 3

[0047] PET layer composition: PTA 53 parts by weight, EG 26 parts by weight, CHDM 11 parts by weight, TFTA 0.55 parts by weight, antioxidant 0.2 parts by weight, catalyst 0.08 parts by weight.

[0048] PEN layer composition: 48 parts by weight of NDA, 31 parts by weight of EG, 6 parts by weight of PTA, 1.5 parts by weight of elastomer, and 0.06 parts by weight of catalyst.

[0049] The total number of layers is 585, and the total thickness is 50μm. The longitudinal tensile ratio is 3.8 times.

[0050] Performance parameters: In the visible light range (550nm), the average reflectance is 99.0%, the transmittance is 91.8%, the haze is 0.30%, the tensile strength (MD) is 235MPa, the heat shrinkage rate (150℃) is 0.7%, and the interlayer peel strength is 3.5N / cm.

[0051] Example 4

[0052] PET layer composition: PTA 55 parts by weight, EG 25 parts by weight, CHDM 12 parts by weight, TFTA 0.4 parts by weight, antioxidant 0.1 parts by weight, catalyst 0.07 parts by weight.

[0053] PEN layer composition: 52 parts by weight of NDA, 30 parts by weight of EG, 7 parts by weight of PTA, 1.3 parts by weight of elastomer, and 0.07 parts by weight of catalyst.

[0054] It has a total of 700 layers and a total thickness of 60μm. The longitudinal stretch ratio is 3.6.

[0055] Performance parameters: In the visible light range (550nm), the average reflectance is 99.2%, the transmittance is 91.5%, the haze is 0.28%, the tensile strength (MD) is 240MPa, the heat shrinkage rate (150℃) is 0.6%, and the interlayer peel strength is 3.8N / cm.

[0056] The key performance comparison of the four embodiments is shown in the table below.

[0057]

[0058] Corresponding to the four embodiments described above, this application further provides four comparative examples to verify the technical effects of the preferred embodiments of the present invention.

[0059] Comparative Example 1

[0060] Based on Example 1, by changing the number of melt distributors and layer multipliers in the multilayer co-extrusion die, the total number of layers in the prepared polyester film was controlled to 200, and the total thickness was maintained at 30 μm. All other parameter values ​​remained the same as in Example 1. The relevant performance parameters are shown in the table below.

[0061]

[0062] Comparative Example 2

[0063] Based on Example 2, CHDM and TFTA were removed from the PET layer, and EG was replenished to 36 parts by weight, while the remaining parameters were the same as in Example 2.

[0064]

[0065] Comparative Example 3

[0066] Based on Example 3, the elastomer was removed from the PEN layer, while the remaining parameters were the same as in Example 3.

[0067]

[0068] Comparative Example 4

[0069] Based on Example 4, the stretching process was changed to biaxial stretching, with a stretching ratio of MD 3.5 times and TD 2.0 times, and the remaining parameters were the same as in Example 4.

[0070]

[0071] By comparing Examples 1-4, the core advantages of the present invention are summarized in the table below.

[0072]

[0073] In summary, the comparison demonstrates that this invention, through an alternating nano-multilayer structure design of PET and PEN, combined with molecular modification and precise process control, achieves synergistic optimization of optical performance, mechanical strength, and thermal stability. Its core advantages lie in: utilizing the principle of interfacial reflection, through hundreds of alternating nano-scale structures (single-layer thickness precisely matched to λ / 4 optical interference conditions), achieving superior optical performance in the visible light band (550nm) with an average reflectivity >98.5%, transmittance >91.5%, and haze <0.35%; introducing components such as cyclohexanediol (CHDM), tetrafluoroterephthalic acid (TFTA), and polyester elastomers significantly improves interlayer bonding (peel strength >3N / cm) and bending resistance (>50,000 cycles), while suppressing crystallization scattering and thermal shrinkage (<1.0%); and employing a unidirectional stretching process ensures layer thickness uniformity (error <±2%) while maintaining flexibility. Compared to traditional biaxially oriented or unmodified polyester films, this solution achieves breakthroughs in reflectivity, interface stability, and processing yield (>95%). It can be widely used in Mini / Micro LED backlight modules, flexible displays, and high-precision optical sensors, combining high performance with industrial feasibility.

[0074] Those skilled in the art should understand that although the present invention has been described with reference to multiple embodiments, not every embodiment contains only one independent technical solution. This description is provided merely for clarity; those skilled in the art should understand the specification as a whole and consider the technical solutions involved in each embodiment as being able to be combined with each other to form different embodiments to understand the scope of protection of the present invention.

[0075] The above description is merely an illustrative embodiment of the present invention and is not intended to limit the scope of the invention. Any equivalent changes, modifications, and combinations made by those skilled in the art without departing from the concept and principles of the present invention should fall within the scope of protection of the present invention.

Claims

1. An optical-grade high-brightness polyester film, characterized in that, The film comprises a nanoscale multilayer structure formed by alternating layers of PET and PEN, with a total of 350-700 layers and a total thickness of 30-60 μm. The refractive index of the PET layer is 1.58-1.62, and the refractive index of the PEN layer is 1.64-1.

68. The film has an average reflectivity ≥98.5%, a transmittance ≥91.5%, a haze ≤0.5%, and an interlayer peel strength ≥3.0 N / cm at a wavelength of 550 nm.

2. The polyester film as described in claim 1, characterized in that, The refractive index of the PET layer is 1.58, and the refractive index of the PEN layer is 1.

64.

3. The polyester film as described in claim 1, characterized in that, The PET layer is prepared from 50-55 parts by weight of terephthalic acid, 25-28 parts by weight of ethylene glycol, 8-12 parts by weight of cyclohexanediol, 0.4-0.6 parts by weight of tetrafluoroterephthalic acid, 0.1-0.3 parts by weight of antioxidant and 0.05-0.1 parts by weight of catalyst.

4. The polyester film as described in claim 1, characterized in that, The PEN layer is prepared from 48-52 parts by weight of naphthalic acid, 30-32 parts by weight of ethylene glycol, 5-8 parts by weight of terephthalic acid, 1.0-1.5 parts by weight of polyester elastomer and 0.05-0.1 parts by weight of catalyst.

5. A method for preparing an optical-grade high-brightness polyester film as described in any one of claims 1-4, characterized in that, Includes the following steps: Modified PET chips and modified PEN chips were prepared separately. The modified PET chips were fed into a PET twin-screw extruder, and the modified PEN chips were fed into a PEN twin-screw extruder. The melt extruded from the PET and PEN twin-screw extruders was fed into a multilayer co-extrusion die. The extruded PET and PEN melts were distributed and layered through the multilayer co-extrusion die to form a thick sheet with a nanoscale multilayer structure of 350-700 layers. The thick sheet extruded from the multilayer co-extrusion die was fed into a uniaxial stretching machine for longitudinal stretching at a stretch ratio of 3.5-4.0 times. The longitudinally stretched film was then heat-set at a temperature of 210-220℃ for 5-8 seconds, with the heat shrinkage rate controlled to be <1.0%. Finally, the film was dried, slit, and wound up.

6. The method as described in claim 5, characterized in that, The steps for preparing modified PET chips include: adding terephthalic acid, ethylene glycol, and cyclohexanediol to a reaction vessel in a specific ratio, heating to 240-250℃, purging with nitrogen, and stirring at 200-300 rpm to carry out an esterification and dehydration reaction until the esterification rate is >95%; in the later stage of the esterification process, i.e., after 1.5 hours of esterification reaction, adding tetrafluoroterephthalic acid and continuing the reaction for another 0.5 hours to ensure that the fluorinated monomer is embedded in the main chain; after the esterification rate reaches 95%, heating to 270-250℃. At 80℃, the pressure is reduced to 5-10 kPa, a catalyst is added, and pre-polymerization is carried out for 1 hour until the intrinsic viscosity rises to 0.4-0.5 dL / g. The pressure is then reduced to 50-100 Pa, the temperature is increased to 285-295℃, and polycondensation is carried out for 2-3 hours until the intrinsic viscosity reaches 0.8-0.9 dL / g. The melt generated in the reactor is then pelletized into modified PET chips with a particle size of 3-5 mm using a pelletizer. The chips are then vacuum dried at 120℃ for 4-6 hours until the moisture content is <50 ppm and stored for later use.

7. The method as described in claim 5, characterized in that, The steps for preparing modified PEN resin include: drying naphthalene dicarboxylic acid under vacuum at 80℃ for 4 hours to achieve a moisture content <100ppm; drying terephthalic acid at 100℃ for 6 hours to prevent agglomeration; pulverizing polyester elastomer to a particle size <500μm and pre-dispersing it with ethylene glycol at a mass ratio of 1:5 (ultrasonic treatment for 30 minutes) to form an elastomer slurry; then adding all raw materials except the elastomer slurry to a stainless steel reaction vessel according to the specified proportions, heating to 240-250℃ under nitrogen protection, maintaining a pressure of 0.2-0.3MPa, and a stirring speed of 200rpm to carry out an esterification and dehydration reaction; the reaction time is 2.5-3 hours, until the water content reaches more than 95% of the theoretical value, completing the esterification; after esterification, cooling to 2... At 20℃, add the pre-dispersed elastomer slurry and stir at 500 rpm for 30 minutes to ensure uniform dispersion of the elastomer. Then, raise the temperature to 250℃ and vacuum to 50 kPa to remove residual EG. Continue to raise the temperature to 270-280℃ and reduce the pressure to 1-5 kPa for pre-polymerization reaction for 1 hour, until the intrinsic viscosity reaches 0.5-0.6 dL / g. Finally, raise the temperature to 290-300℃ and reduce the pressure to <100 Pa for 2-3 hours until the intrinsic viscosity reaches 0.9-1.0 dL / g. The melt generated in the stainless steel reaction vessel is pelletized into modified PEN chips with a particle size of 3-5 mm using a pelletizer, vacuum dried at 120℃ for 8 hours until the moisture content is <50 ppm, and stored for later use.

8. The method as described in claim 5, characterized in that, The temperature of the multi-layer co-extrusion die is controlled at 265-300℃.

9. The method as described in claim 5, characterized in that, The preheating temperature for the uniaxial stretching is 85-100℃, the stretching temperature is 100-120℃, and the stretching direction is longitudinal.

10. The method as described in claim 5, characterized in that, After heat setting, the film is dried at 120-150℃ for 4-8 hours until the moisture content is ≤50ppm.

Images