Low-light-transmittance white polyester film and preparation method thereof

Abstract

The invention discloses a low-light-transmittance white polyester film and a preparation method thereof, the film is of a three-layer gradient composite structure, and comprises a functional surface layer A, a functional surface layer B and a functional surface layer C, c, an interface transition layer; b layer main light blocking layer; wherein the layer A, the layer C and the layer B are sequentially stacked, the total thickness of the film is 19-75 microns, and the light transmittance is 12%-20%. The low-light-transmittance white polyester film adopts a three-layer gradient composite structure, and the layer A directly faces the external environment and has the functions of weather resistance, scratch resistance and the like; the B layer serves as a light blocking main body to achieve light scattering and hiding effects, anatase type TiO2 hollow microspheres are used for replacing stretching to generate foam holes, and the sensitive influence of process fluctuation on the porosity is eliminated. And the C layer is used as a transition layer to match thermal expansion coefficients of the A / B layers and enhance the stability of the whole structure. The layer C plays a role in buffering and bonding between the layer A and the layer B, so that the influence of the performance difference of the two materials is reduced to the minimum, the internal stress is reduced, and the warping or layering risk is reduced.

Description

Technical Field

[0001] This invention relates to an optical film, particularly a low-transmittance optical film that can be used in the fields of solar photovoltaic backsheets, light source shielding materials, and electronic tapes, and especially to a low-transmittance white polyester film and its preparation method. Background Technology

[0002] Low-transmittance optical films have wide applications in solar photovoltaic backsheets, light source shielding materials, and electronic tapes. For example, in solar photovoltaic backsheets, low-transmittance optical films provide excellent light-blocking effects, preventing interference between solar cells and improving the power generation efficiency of the module. In optical light sources, low-transmittance optical films can be used to create decorative wallpapers with special effects, such as blackout wallpaper and glow-in-the-dark wallpaper. They can also be used as the inner layer material of lightbox fabrics, providing uniform backlighting and good light-blocking properties. In electronic tapes, low-transmittance optical films can serve as insulating or shielding layers; their low light transmittance allows them to block light, preventing interference with sensitive electronic components, making them particularly suitable for applications requiring light shading.

[0003] CN 101605841 B discloses a white polyester film and a reflective sheet. This prior art white polyester film is suitable for optical applications, particularly for liquid crystal display devices, backlight devices, and reflectors. Its white polyester film possesses high reflectivity and high opacity, making it suitable for optical films requiring light stability and anti-fouling properties. The basic design of this prior art white polyester film is a structure containing internal voids, and the resin constituting the film is composed of polyester resin and cyclic olefin copolymer resin. The void ratio of this film needs to be greater than 25% and less than 75% to ensure high brightness and high opacity. However, the void structure of this prior art easily absorbs moisture, and may expand and deform under long-term humid conditions, resulting in decreased uniformity.

[0004] CN 110837142 B discloses a white polyester reflective film and its application in liquid crystal display backlights. This prior art, through an ABA three-layer structure design (outer layer A containing polyester, inorganic particles, and a toughening agent, and middle layer B containing an incompatible resin to form pores), achieves a reflectivity exceeding 96% and a light uniformity exceeding 84.3%, making it suitable for side-lit and direct-lit backlights in ultra-thin electronic devices. The outer layer A of this prior art contains inorganic particles (such as silica) and a toughening agent, improving surface hardness and scratch resistance, while also initially scattering light. The middle layer B, by adding an incompatible resin (such as polymethylpentene), forms a micron-scale pore structure after stretching, enhancing light scattering and improving reflectivity. This prior art, through a layered isolation design, avoids direct mixing of incompatible resins and surface materials, preventing interface defects and ensuring that each component maximizes its performance. However, the pore structure of this prior art is also prone to absorbing moisture, and may expand and deform under long-term humidity conditions, leading to a decrease in uniformity. In addition, the thermal expansion coefficients of the A and B layers differ significantly, which can easily generate internal stress under temperature cycling, leading to warping or delamination. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a low-transmittance white polyester film and its preparation method, so as to reduce or avoid the problems mentioned above.

[0006] To address the aforementioned technical problems, this invention proposes a low-transmittance white polyester film, wherein the film has a three-layer gradient composite structure, including a functional surface layer (A layer), an interface transition layer (C layer), and a main light-blocking layer (B layer); wherein the A layer, C layer, and B layer are stacked sequentially, the total thickness of the film is 19 to 75 μm, and the transmittance is 12% to 20%.

[0007] Preferably, the thickness of layer A accounts for 10%-15% of the total thickness of the film, and is based on a copolymer modified polyester containing 3%-5% isophthalate by mass of layer A, and contains 1%-3% nano-barium sulfate, 0.5%-1.0% boron nitride nanosheets, and 3%-5% polyester elastomer TPEE by mass of layer A.

[0008] Preferably, the thickness of layer B accounts for 70%-80% of the total film thickness, and is made of film-grade polyester with an intrinsic viscosity of 0.62 to 0.65 dL / g as the substrate, and contains 20%-25% anatase titanium dioxide, 8%-10% porous silica hollow microspheres, and 0.3%-0.5% silane coupling agent KH-570 by weight of layer B.

[0009] Preferably, the thickness of the C layer accounts for 10%-15% of the total thickness of the film, and is based on an epoxy-modified copolyester containing 8%-10% epoxidation components, with 0.1% to 0.3% of benzotriazole UV absorber and 0.5% of lubricant added by weight of the C layer.

[0010] Preferably, layer A is the outer layer, layer B is the inner layer, and layer C is located between layer A and layer B.

[0011] This invention also proposes a method for preparing the aforementioned low-transmittance white polyester film, comprising the following steps: pre-mixing, granulating, and drying the raw materials for layers A, B, and C to prepare raw material masterbatches; using at least three independent extruders to melt-extrude each layer of the masterbatch to form a three-layer melt of A, B, and C; obtaining a three-layer composite film through die co-extrusion stretching; stretching the film longitudinally and laterally after die co-extrusion; and subjecting the stretched film to plasma surface modification treatment.

[0012] Preferably, the raw materials for layers A, B, and C are dried at a temperature of 120°C to 150°C before melt extrusion, and the drying time is 4 to 6 hours.

[0013] Preferably, the temperature range for melt extrusion is 310-315℃ for layer A; 305-310℃ for layer B; and 295-300℃ for layer C.

[0014] Preferably, the longitudinal stretch ratio is 3.0-3.5, and the longitudinal stretching temperature is 100-110℃; the transverse stretch ratio is 3.2-3.8, and the transverse stretching temperature is 115-120℃.

[0015] Preferably, the temperature for plasma surface modification treatment is 200°C to 230°C, and the treatment time is 3-5 seconds.

[0016] The low-transmittance white polyester film of this invention adopts a three-layer gradient composite structure. Layer A directly faces the external environment and has functions such as weather resistance and scratch resistance. Layer B acts as the main light-blocking element to achieve light scattering and concealment effects. Anatase TiO2 hollow microspheres replace the stretched-formed pores, eliminating the sensitive influence of process fluctuations on porosity. Layer C serves as a transition layer, matching the thermal expansion coefficients of layers A and B, while enhancing the overall structural stability. Layer C acts as a "buffer" and "bonding" layer between layers A and B, minimizing the impact of the performance differences between the two materials, thereby reducing internal stress and lowering the risk of warping or delamination. Attached Figure Description

[0017] The accompanying drawings are intended only to illustrate and explain this application and do not limit the scope of the invention.

[0018] Figure 1The diagram shown is a cross-sectional schematic of a low-transmittance white polyester film according to a specific embodiment of the present invention. Detailed Implementation

[0019] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments are now described with reference to the accompanying drawings. Identical components are denoted by the same reference numerals.

[0020] In view of the various problems existing in the prior art, the present invention proposes a low-transmittance white polyester film with a three-layer gradient composite structure, such as... Figure 1 As shown in the figure. The low-transmittance white polyester film of this invention differs from the traditional three-layer sandwich structure. This invention adopts a gradient composite structure in the form of ACB, where layer A is a functional surface layer with a thickness of 10-15%; layer B is the main light-blocking layer with a thickness of 70-80%; and layer C is an interface transition layer with a thickness of 10-15%. The total thickness of the low-transmittance white polyester film of the three-layer gradient composite structure is 19-75 μm.

[0021] In one specific embodiment, the functional surface layer of layer A is based on copolymerized modified polyester chips containing 3-5 wt% isophthalate, and contains 1-3 wt% (by weight of layer A) of nano-barium sulfate to improve surface smoothness and mechanical properties, 0.5-1.0 wt% (by weight of layer A) of boron nitride nanosheets to enhance thermal conductivity, and 3-5 wt% (by weight of layer A) of polyester elastomer TPEE as a toughening agent. Preferably, the nano-barium sulfate has a particle size of 200-500 nm; the boron nitride nanosheets have a transverse dimension of 500 nm to 2 μm and a thickness of 10-50 nm. The boron nitride nanosheets can be domestically produced Tianyuan New Materials' TX-BN10 (transverse dimension 1.2 μm ± 0.3 μm, thickness 20 ± 5 nm, hydroxylated surface) or imported Saint-Gobain's BNX-20 (transverse dimension 2.0 μm ± 0.5 μm, thickness 30 ± 10 nm, silane coated).

[0022] Specifically, the copolymer-modified polyester in the functional surface layer A is prepared from monomers in the following weight ratio: dimethyl isophthalate (DMI), dimethyl terephthalate (DMT), and ethylene glycol (EG) in a weight ratio of 1.0:(24.0-24.5):(25.0-25.5).

[0023] Furthermore, the copolymerized modified polyester can be prepared by the following process: DMI with a purity of ≥99%, DMT with a purity of ≥99.8%, and EG with a purity of 99.5% are weighed according to a set weight ratio and added to an esterification tank for esterification. The reaction temperature in the esterification tank is 160-190℃, and the reaction is carried out at atmospheric pressure under nitrogen protection for 2-4 hours. During the reaction, 0.02-0.05 wt% of tetrabutyl titanate or 0.03-0.06 wt% of antimony acetate is added as a catalyst, and 0.05-0.1 wt% of triphenyl phosphate (TPP) is added as a heat stabilizer. During the reaction, DMT / DMI undergoes transesterification with EG under the action of a catalyst, generating corresponding oligomers (such as BHET and BMIT) and releasing methanol (which needs to be condensed and recovered). The esterification reaction is stopped when the methanol distillation reaches more than 90% of the theoretical value. The esterification product is then transferred to a polycondensation tank for polycondensation. The reaction temperature in the polycondensation tank is 270-290℃, and the reaction is carried out for 2-3 hours under a vacuum of less than 100 Pa. The reaction is terminated when the intrinsic viscosity reaches 0.65-0.75 dL / g. The polycondensed melt is then discharged from the polycondensation tank to a granulator, granulated to a size of 2-4 mm, and dried at 180℃ for 4 hours. The performance parameters of the prepared copolymerized modified polyester are as follows: DSC: glass transition temperature (Tg) and melting temperature (Tm). The introduction of isophthalic acid esters leads to a decrease in Tm of about 10-20℃ (typical values: pure PET Tm = 250℃, 4wt% DMI modified PET Tm ≈ 235℃); FTIR: characteristic peak of isophthalic acid (inter-substituted peak of benzene ring, 1150-1170cm-1); NMR: DMI content 3-5wt%. As an A-layer substrate, the introduction of isophthalic acid weakens crystallinity and reduces haze, making it suitable for low-transmittance films (transmittance <20%).

[0024] In another specific embodiment, the B-layer main light-blocking layer is made of film-grade polyester chips with an intrinsic viscosity of 0.62-0.65 dL / g as the substrate, and contains 20-25 wt% anatase TiO2 of the total B-layer mass to obtain optimal light scattering efficiency, 8-10 wt% porous SiO2 hollow microspheres of the total B-layer mass to replace the traditional pore structure, and 0.3-0.5 wt% silane coupling agent KH-570 of the total B-layer mass.

[0025] The preferred anatase TiO2 particles have a diameter of 400-600 nm; the porous SiO2 hollow microspheres have a diameter of 5-8 μm and a wall thickness of 0.2 μm. The anatase TiO2 can be LR-988 from Longbai Group, R-2498 from CNNC Titanium Dioxide, Ti-Pure R-960 from Chemours, or TIPAQUECR-60 from Ishihara Sangyo. The porous SiO2 hollow microspheres can be SiliCellMF-7 from JGC Catalysts and Chemicals; K37 K-series hollow glass microspheres from 3M; or NHS-820 from Nantong Nahong Nanomaterials.

[0026] In another specific embodiment, the C-layer interface transition layer is based on epoxy-modified copolyester chips containing 8-10% epoxidation components, and contains 0.1-0.3 wt% of benzotriazole UV absorber and 0.5 wt% of fatty acid ester lubricant by weight of the total C-layer.

[0027] Specifically, the epoxy-modified copolyester can be prepared by the following process: Terephthalic acid (PTA) with a purity of ≥99.5% and ethylene glycol (EG) with a purity of ≥99.5% are weighed and added to a reaction vessel at a set molar ratio of 1:2. Then, 4-8% molar fraction of epoxycyclohexane adipate (ACHD) with a purity of ≥99% is added to the reaction vessel for esterification. The esterification reaction in the reaction vessel is carried out at 180℃ under normal pressure for 3-6 hours. When the esterification rate reaches 85%, the reaction system in the reaction vessel changes from turbid to transparent, and the esterification is complete. During the reaction, 0.02-0.05 wt% tetrabutyl titanate is added to the esterification vessel as a catalyst, and 0.05-0.1 wt% triphenyl phosphate (TPP) is added as a heat stabilizer. After esterification, the temperature is raised to 220℃ for pre-polymerization for 2-5 hours, until the EG content in the reaction vessel is removed by 90%. Then, the temperature is raised to 250℃ for final polymerization. Final polymerization takes 2-6 hours under a vacuum of less than 100 Pa. The reaction is terminated when the intrinsic viscosity reaches 0.62-0.65 dL / g. The polymerized melt is then transferred from the reaction vessel to a granulator, granulated to a size of 2-4 mm, and dried at 180℃ for 4 hours. The fatty acid ester lubricant can be a fatty acid ester, such as LOXIOL G12-V40 (fatty acid ester) from Henkel, Germany.

[0028] The low-transmittance white polyester film of this invention adopts a gradient composite structure in the form of ACB. Layer A directly faces the external environment and possesses surface functions such as weather resistance and scratch resistance. Layer B, as the main light-blocking layer, requires a high filler content to achieve light scattering and concealment effects. Anatase TiO2 hollow microspheres are used instead of stretched-formed bubbles, eliminating the sensitive influence of process fluctuations on porosity. Layer C, as a transition layer, harmonizes the interface between the two layers, matches the thermal expansion coefficients of layers A and B, and enhances the overall structural stability. Layer C acts as a "buffer" and "bond" between layers A and B, minimizing the impact of differences in the properties of the two materials (such as thermal expansion coefficients, mechanical properties, and surface properties), thereby reducing internal stress and lowering the risk of warping or delamination.

[0029] Furthermore, the low-transmittance white polyester film of the present invention can be prepared by the following method: First, a raw material masterbatch is prepared and pretreated.

[0030] To prepare the masterbatch for the functional surface layer A, 3–5 wt% isophthalate-modified polyester chips, 1–3 wt% nano-barium sulfate, 0.5–1.0 wt% boron nitride nanosheets, and 3–5 wt% polyester elastomer (TPEE) were premixed uniformly according to a specified ratio, granulated, and then dried to ensure that the material moisture content was controlled at a low level to prevent hydrolysis or bubble formation during melting. The drying temperature was 120°C to 150°C, and the drying time was 4 to 6 hours.

[0031] To prepare the masterbatch for the B-layer main light-blocking layer, premixed polyester chips with an intrinsic viscosity of 0.62–0.65 dL / g were mixed uniformly with 20–25 wt% anatase TiO2, 8–10 wt% porous SiO2 hollow microspheres, and 0.3–0.5 wt% silane coupling agent KH-570, followed by granulation and drying. The drying temperature was 120℃ to 150℃, and the drying time was 4 to 6 hours.

[0032] To prepare the C-layer interface transition layer masterbatch, epoxy-modified copolyester chips containing 8–10% epoxidized components were premixed with 0.1–0.3 wt% benzotriazole UV absorber and 0.5 wt% fatty acid ester lubricant, then granulated and dried. The drying temperature was 120°C to 150°C, and the drying time was 4 to 6 hours.

[0033] Then, at least three independent extruders are used to melt-extrude each layer of masterbatch. The pre-mixed masterbatch for layers A, B, and C is fed into its respective extruder. Each extruder is set with an appropriate temperature range based on the material characteristics (layer A 310-315℃; layer B 305-310℃; layer C 295-300℃) to ensure complete melting of the raw materials without thermal degradation. Each extruder is equipped with a high-speed shear press to further ensure the uniform dispersion of additives (such as nano-barium sulfate, boron nitride nanosheets, etc.).

[0034] A three-layer composite film is obtained by co-extrusion stretching using a die. A multi-channel co-extrusion die is used to simultaneously feed the molten material of each layer into the die. An A–C–B structure is adopted, that is: layer A serves as the outer surface layer (functional surface layer); layer C serves as the interface transition layer, sandwiched between layers A and B; layer B serves as the inner layer, providing the main light-blocking function.

[0035] After co-extrusion by the die head, the film is biaxially stretched. First, it is stretched longitudinally (MD) with a ratio of 3.0-3.5 and a longitudinal stretching temperature of 100-110℃. Then, it is stretched transversely (TD) with a ratio of 3.2-3.8 and a transverse stretching temperature of 115-120℃.

[0036] Finally, the prepared three-layer composite film can be subjected to plasma surface modification treatment. The surface is treated with an Ar / O2 mixed gas at a temperature of 200℃ to 230℃ for 3-5 seconds to improve printing / coating adhesion.

[0037] The low-transmittance white polyester film prepared by the method of the present invention has an overall transmittance of 13.5-18.9%, and a pencil hardness of ≥2H for the surface A layer. In terms of damp heat stability, the transmittance fluctuates by ±0.3% after 500h at 80℃ / 85% RH. In terms of bending resistance, the MIT withstands >800 folds (load 500g). The processing yield is ≥98% (for 75μm thick film).

[0038] Nine specific embodiments are given below (all using an ACB layer structure, i.e., layer A is the functional surface layer, layer C is the interface transition layer, layer B is the main light-blocking layer, and layer C is located between layers A and B). By controlling the proportion of additives in each layer within the raw material formulation range, low-transmittance white polyester films with a total thickness of 19–75 μm and a transmittance controlled at 12–20% are prepared. The specific formulations and key performance parameters of each embodiment are as follows.

[0039] Example 1

[0040] Layer A is a functional surface layer, accounting for 10% of the total layer thickness. Substrate: Copolymer modified polyester chips containing 3 wt% isophthalate, barium sulfate nanoparticles: 3 wt%, boron nitride nanosheets: 0.5 wt%, polyester elastomer (TPEE): 5.5 wt%.

[0041] Layer C is an interface transition layer, accounting for 10% of the total layer thickness. Substrate: epoxy-modified copolyester chips containing 8% epoxidation component, benzotriazole UV absorber: 0.3wt%, lubricant: 0.3wt%.

[0042] Layer B is the main light-blocking layer, accounting for 80% of the total layer thickness. Substrate: polyester chips for film (intrinsic viscosity 0.62 dL / g), anatase TiO2: 25wt%, porous SiO2 hollow microspheres: 4.5wt%, silane coupling agent KH-570: 0.5wt%.

[0043] Overall parameters: Total thickness: 50μm, light transmittance: 15%, reflectance: 85%, tensile strength: 120 MPa, thermal stability: approximately 200℃.

[0044] Example 2

[0045] Layer A, accounting for 15% of the total layer thickness. Substrate: Copolymer modified polyester chips containing 4 wt% isophthalate, barium sulfate nanoparticles: 4 wt%, boron nitride nanosheets: 0.7 wt%, TPEE: 5.3 wt%.

[0046] Layer C, comprising 15% of the total layer thickness. Substrate: Modified copolyester chips containing approximately 9 wt% epoxidized components, 0.5 wt% benzotriazole UV absorber, and 0.3 wt% lubricant.

[0047] Layer B, accounting for 70% of the total layer thickness. Substrate: Polyester chips for film (intrinsic viscosity 0.63 dL / g), TiO2: 24 wt%, porous SiO2 hollow microspheres: 7.5 wt%, KH-570: 0.5 wt%.

[0048] Overall parameters: Total thickness: 45μm, light transmittance: 14%, reflectance: 86%, tensile strength: 125 MPa, thermal stability: approximately 205℃.

[0049] Example 3

[0050] Layer A, accounting for 10% of the total layer thickness. Substrate: Copolymer modified polyester chips containing 5 wt% isophthalate, barium sulfate nanoparticles: 5 wt%, boron nitride nanosheets: 1.0 wt%, TPEE: 5.0 wt%.

[0051] Layer C, accounting for 15% of the total layer thickness. Substrate: epoxy-modified copolyester chips containing 10wt% epoxidizing components, benzotriazole UV absorber: 0.7wt%, lubricant: 0.3wt%.

[0052] Layer B, accounting for 75% of the total layer thickness. Substrate: Polyester chips for film (intrinsic viscosity 0.64 dL / g), TiO2: 23 wt%, porous SiO2 hollow microspheres: 10 wt%, KH-570: 1.0 wt%.

[0053] Overall parameters: Total thickness: 55μm, light transmittance: 13%, reflectance: 87%, tensile strength: 130 MPa, thermal stability: approximately 210℃.

[0054] Example 4

[0055] Layer A, accounting for 15% of the total layer thickness. Substrate: Copolymer modified polyester chips containing 3.5 wt% isophthalate, barium sulfate nanoparticles: 4 wt%, boron nitride nanosheets: 0.7 wt%, TPEE: 5.0 wt%.

[0056] Layer C, accounting for 10% of the total layer thickness. Substrate: epoxy-modified copolyester chips containing 8.5 wt% epoxidized components, benzotriazole UV absorber: 0.2 wt%, lubricant: 0.5 wt%.

[0057] Layer B, accounting for 75% of the total layer thickness. Substrate: Polyester chips for film (intrinsic viscosity 0.62 dL / g), TiO2: 22 wt%, porous SiO2 hollow microspheres: 8 wt%, KH-570: 0.5 wt%.

[0058] Overall parameters: Total thickness: 60μm, light transmittance: 16%, reflectance: 84%, tensile strength: 122MPa, thermal stability: approximately 208℃.

[0059] Example 5

[0060] Layer A, accounting for 12% of the total layer thickness. Substrate: Copolymer modified polyester chips containing 4 wt% isophthalate, barium sulfate nanoparticles: 3 wt%, boron nitride nanosheets: 0.5 wt%, TPEE: 5.5 wt%.

[0061] Layer C, accounting for 14% of the total layer thickness. Substrate: epoxy-modified copolyester chips containing 9 wt% epoxidizing components, benzotriazole UV absorber: 0.3 wt%, lubricant: 0.3 wt%.

[0062] Layer B, accounting for 74% of the total layer thickness. Substrate: Polyester chips for film (intrinsic viscosity 0.63 dL / g), TiO2: 25 wt%, porous SiO2 hollow microspheres: 8.5 wt%, KH-570: 0.3 wt%.

[0063] Overall parameters: Total thickness: 19μm, light transmittance: 18%, reflectance: 82%, tensile strength: 118 MPa, thermal stability: approximately 202℃.

[0064] Example 6

[0065] Layer A, accounting for 14% of the total layer thickness. Substrate: Copolymer modified polyester chips containing 4.5 wt% isophthalate, barium sulfate nanoparticles: 2 wt%, boron nitride nanosheets: 1.0 wt%, TPEE: 3.5 wt%.

[0066] Layer C, accounting for 12% of the total layer thickness. Substrate: epoxy-modified copolyester chips containing 10 wt% epoxidizing components, benzotriazole UV absorber: 0.7 wt%, lubricant: 0.3 wt%.

[0067] Layer B, accounting for 74% of the total layer thickness. Substrate: Polyester chips for film (intrinsic viscosity 0.63 dL / g), TiO2: 24 wt%, porous SiO2 hollow microspheres: 7 wt%, KH-570: 0.5 wt%.

[0068] Overall parameters: Total thickness: 30μm, light transmittance: 12%, reflectance: 88%, tensile strength: 135 MPa, thermal stability: approximately 215℃.

[0069] Example 7

[0070] Layer A, accounting for 13% of the total layer thickness. Substrate: Copolymer modified polyester chips containing 3 wt% isophthalate, barium sulfate nanoparticles: 1 wt%, boron nitride nanosheets: 0.5 wt%, TPEE: 3 wt%.

[0071] Layer C, accounting for 13% of the total layer thickness. Substrate: epoxy-modified copolyester chips containing 8 wt% epoxidizing component, benzotriazole UV absorber: 0.1 wt%, lubricant: 0.4 wt%.

[0072] Layer B, accounting for 74% of the total layer thickness. Substrate: Polyester chips for membranes (intrinsic viscosity 0.64 dL / g), TiO2: 23 wt%, porous SiO2 hollow microspheres: 9 wt%, KH-570: 0.3 wt%.

[0073] Overall parameters: Total thickness: 35μm, light transmittance: 20%, reflectance: 80%, tensile strength: 110 MPa, thermal stability: approximately 195℃.

[0074] Example 8

[0075] Layer A, accounting for 14% of the total layer thickness. Substrate: Copolymer modified polyester chips containing 5 wt% isophthalate, barium sulfate nanoparticles: 3 wt%, boron nitride nanosheets: 0.8 wt%, TPEE: 4 wt%.

[0076] Layer C, accounting for 11% of the total layer thickness. Substrate: epoxy-modified copolyester chips containing 9 wt% epoxidizing components, benzotriazole UV absorber: 0.5 wt%, lubricant: 0.3 wt%.

[0077] Layer B, accounting for 75% of the total layer thickness. Substrate: Polyester chips for film (intrinsic viscosity 0.65 dL / g), TiO2: 26 wt%, porous SiO2 hollow microspheres: 10 wt%, KH-570: 0.5 wt%.

[0078] Overall parameters: Total thickness: 70μm, light transmittance: 12%, reflectance: 88%, tensile strength: 140 MPa, thermal stability: approximately 220℃.

[0079] Example 9

[0080] Layer A, accounting for 11% of the total layer thickness. Substrate: Copolymer modified polyester chips containing 4 wt% isophthalate, barium sulfate nanoparticles: 2.5 wt%, boron nitride nanosheets: 0.6 wt%, TPEE: 4.9 wt%.

[0081] Layer C, accounting for 14% of the total layer thickness. Substrate: epoxy-modified copolyester chips containing 9 wt% epoxidizing components, benzotriazole UV absorber: 0.4 wt%, lubricant: 0.3 wt%.

[0082] Layer B, accounting for 75% of the total layer thickness. Substrate: Polyester chips for film (intrinsic viscosity 0.63 dL / g), TiO2: 25 wt%, porous SiO2 hollow microspheres: 8 wt%, KH-570: 0.3 wt%.

[0083] Overall parameters: Total thickness: 75 μm, light transmittance: 15%, reflectance: 85%, tensile strength: 128 MPa, thermal stability: approximately 210℃.

[0084] Summary and Explanation. The above nine embodiments fine-tuned the raw material ratios of layers A, C, and B within the ACB structure. Results show that, within the predetermined raw material ratio range, all embodiments achieved a total thickness control of 19–75 μm and a light transmittance maintained within the target range of 12%–20%. Adjusting the toughening agent and nanofiller in layer A improved surface smoothness and scratch resistance. The ratio of epoxy-modified polyester to UV absorber and lubricant in layer C effectively enhanced interlayer adhesion and UV resistance, ensuring uniform interface transition. The combination of TiO2 and porous SiO2 hollow microspheres in layer B, while ensuring optimal light scattering efficiency, replaced the traditional pore structure, thus avoiding moisture absorption and expansion issues. Key performance parameters (such as tensile strength, thermal stability, and optical properties) were relatively consistent across all embodiments, indicating that the product possesses excellent practicality and stability within the designed formulation range.

[0085] The above nine embodiments demonstrate that, within the range of raw material ratios, by rationally controlling the composition of each layer, not only can the requirements of optical thin films for low light transmittance, weather resistance, mechanical strength, and thermal management be met, but also reliable technical support can be provided for further process optimization and industrialization promotion.

[0086] In conjunction with the above embodiments, the present invention further provides comparative examples, the relevant performance parameters of which are shown in the table below. The parameters not shown in each comparative example are the same as those in the corresponding embodiments; the table only shows the comparison schemes with adjusted parameters.

[0087]

[0088] The analysis of the comparative examples above demonstrates that the content and selection of each component in the original embodiments are crucial for ensuring the overall performance of the film, including low light transmittance, mechanical strength, and weather resistance. Appropriate raw material ratios and component selection can produce low-light-transmittance white polyester films with uniform and stable performance and excellent practicality.

[0089] 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.

[0090] 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. A low-transmission white polyester film, characterized by, The film has a three-layer gradient composite structure, comprising an A-layer functional surface layer; a C-layer interface transition layer; and a B-layer main light-blocking layer; wherein the A-layer, the C-layer and the B-layer are sequentially stacked, the total thickness of the film is 19 to 75 μm, and the light transmittance is 12% to 20%.

2. The low-transmission white polyester film according to claim 1, characterized by The thickness of the A-layer accounts for 10%-15% of the total thickness of the film, and is made of a copolymer-modified polyester containing 3%-5% of isophthalate as a base material, and added with 1%-3% of nano-barium sulfate, 0.5%-1.0% of boron nitride nanosheet and 3%-5% of polyester elastomer TPEE based on the total mass of the A-layer.

3. The low-transmission white polyester film according to claim 1 or 2, characterized by, The thickness of the B-layer accounts for 70%-80% of the total thickness of the film, and is made of a film polyester with an intrinsic viscosity of 0.62 to 0.65 dL / g as a base material, and added with 20%-25% of anatase titanium dioxide, 8%-10% of porous silica hollow microspheres and 0.3%-0.5% of silane coupling agent KH-570 based on the total mass of the B-layer.

4. The low-transmission white polyester film according to any one of claims 1 to 3, characterized by The thickness of the C-layer accounts for 10%-15% of the total thickness of the film, and is made of an epoxy-modified copolyester containing 8%-10% of an epoxy component as a base material, and added with 0.1% to 0.3% of a benzotriazole ultraviolet absorber and 0.5% of a lubricant based on the total mass of the C-layer.

5. The low-transmission white polyester film according to any one of claims 1 to 4, characterized in that, The A-layer is an outer layer, the B-layer is an inner layer, and the C-layer is located between the A-layer and the B-layer.

6. A method for producing the low-transmission white polyester film according to any one of claims 1 to 5, characterized by, The method comprises the following steps: preparing raw material master batches by pre-mixing, granulating and drying the raw materials of the A-layer, the B-layer and the C-layer respectively; melt-extruding the master batches of the layers by using at least three independent extruders to form A, B and C three-layer melts; obtaining a three-layer composite structure film by die co-extrusion and stretching; stretching the film in the longitudinal and transverse directions after die co-extrusion; and performing plasma surface modification treatment on the stretched film.

7. The method of claim 6, wherein, The drying temperature of the raw materials of the A-layer, the B-layer and the C-layer before melt-extrusion is 120-150°C, and the drying time is 4-6 hours.

8. The method according to claim 6 or 7, characterized in that, The temperature range for melt-extrusion is 310-315°C for the A-layer, 305-310°C for the B-layer and 295-300°C for the C-layer.

9. The method according to any one of claims 6 to 8, characterized in that, The longitudinal stretching ratio is 3.0-3.5, and the longitudinal stretching temperature is 100-110°C; the transverse stretching ratio is 3.2-3.8, and the transverse stretching temperature is 115-120°C.

10. The method according to any one of claims 6 to 9, characterized in that, The temperature for plasma surface modification treatment is 200-230°C, and the treatment time is 3-5 seconds.

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