Double-layer-structure polyester film with high barrier property and preparation method of double-layer-structure polyester film

Abstract

The invention relates to the technical field of polymer functional film materials, in particular to a double-layer structure polyester film with high barrier property and a preparation method thereof. The invention discloses a double-layer structure polyester film with high barrier property and a preparation method thereof, and the preparation method comprises the following steps: forming a single-layer gradient melt sheet in situ in a die head through temperature difference-pressure difference cooperative driving; the upper nano barrier layer is oriented and the lower crystal network is enhanced through synchronous two-way stretching; triggering secondary cross-linking through heat setting to permanently lock the gradient structure; and finally, carrying out online surface activation. According to the invention, an integrated single-layer structure is used for replacing the traditional physical lamination, so that the interlayer stripping risk is fundamentally avoided, and the prepared film has ultrahigh barrier property, high strength, excellent interface adhesion and damage self-repairing function, and is suitable for high-end aluminum laminated films, capacitor films and carbon-nitrogen substrate films.

Description

Technical Field

[0001] This invention relates to the field of polymer functional film materials technology, specifically to a double-layer polyester film with high barrier properties and its preparation method. Background Technology

[0002] Polyester film (PET film) is widely used in packaging and electronics industries due to its excellent mechanical strength, heat resistance, and transparency. However, PET film itself has limited gas and moisture barrier properties. To meet the demand for high barrier performance, existing technologies usually require modification or lamination of PET film.

[0003] Commercially available methods to improve the barrier properties of PET include: coating the film surface with a dense barrier coating or blending or co-extruding with high-barrier materials to prepare multilayer films. For example, CN120941788A discloses a biomimetic drag-reducing protective film based on surface micro-nano structures and its preparation method. This method involves preparing a high-strength nitrogen-containing thermoplastic polyurethane film layer with flame-retardant properties, coating it with a hyperbranched polyurethane coating, and then curing it through embossing and photothermal processes to form a drag-reducing functional layer with a microstructure. The hyperbranched polyurethane coating forms a high-density cross-linked network during curing, imparting better coating properties. The hyperbranched structure of the hyperbranched polyurethane coating improves mechanical properties and weather resistance, and helps disperse shrinkage stress when heated, maintaining the smoothness of the coating surface. Furthermore, the hyperbranched polyurethane coating contains nitrogen rings, giving it a certain degree of flame retardancy. CN120307736A discloses a phase-change temperature-regulating antistatic polyester film and its preparation method. This method uses co-extrusion to prepare a double-layer polyester film with a double-layer structure. The bottom layer is a temperature-regulating functional layer doped with phase-change material, and the top layer is a functional layer doped with carbon black to prevent static electricity accumulation. This allows for temperature control while maintaining good antistatic properties. Multilayer co-extruded films typically add an adhesive layer between PET and the barrier resin to ensure interlayer adhesion. However, due to differences in thermal shrinkage and mechanical properties of the layers, there is still a risk of delamination at the interface, especially after repeated bending or heating, which reduces interlayer adhesion and affects service life. In addition, the presence of different polymer layers increases the difficulty of material recycling.

[0004] In summary, how to significantly improve the barrier properties of PET materials against water vapor and oxygen while maintaining their inherent advantages, and avoid the problem of poor interfacial adhesion in traditional multilayer structures, has become a pressing technical problem to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a double-layer polyester film with high barrier properties and its preparation method, so as to solve the technical problems of weak interlayer adhesion and difficulty in balancing barrier properties and other properties in the prior art.

[0006] The specific technical solution is as follows: A bilayer polyester film with high barrier properties and its preparation method are disclosed. The preparation method firstly involves controlling the temperature difference between multiple melt layers and the pressure difference within the die flow channel, as well as the functional component gradient enrichment effect formed under these conditions. The three factors work synergistically to directly fuse the differentiated melts into a single-layer gradient melt sheet with continuously varying composition and structure. Subsequently, synchronous biaxial stretching is used to orient the upper nano-barrier network and enhance the lower crystalline network. Then, heat setting triggers secondary crosslinking to permanently lock the gradient structure. Finally, online surface activation treatment is performed to obtain a single-layer gradient bilayer network film with high barrier properties, high strength, and damage self-healing function.

[0007] Furthermore, the three elements work synergistically by pre-setting the upper functional zone melt containing a dynamic silicon-oxygen-boron dynamic covalent network (Si-OB network) and the lower functional zone melt containing polyethylene terephthalate-co-poly(ethylene naphthalate) (PET-co-PEN) to different temperatures and introducing them into a common flow channel within the die head. Different flow pressures are imparted through the flow-blocking structure within the flow channel. Through the synergistic effect of this temperature difference, pressure difference, and gradient enrichment effect, the two melts undergo controllable lateral molecular diffusion and longitudinal interweaving in a laminar flow form, thereby fusing in situ into a single-layer gradient melt sheet with continuously varying composition and structure.

[0008] Furthermore, the synchronous biaxial stretching involves simultaneously stretching the monolayer gradient melt sheet at 120°C at a high speed of 4.0 to 4.5 times longitudinally and 4.5 to 5.0 times transversely. This process causes the hydrotalcite sheets and Si-OB network in the upper functional region to be highly oriented along the film surface plane, while inducing the PET-co-PEN molecular chains in the lower functional region to be highly crystallized and oriented along the stretching direction. This allows for the simultaneous construction of a dense barrier structure on the surface and a high-strength support skeleton in the inner layer in a single process.

[0009] Furthermore, the heat-setting triggering secondary crosslinking involves subjecting the synchronously stretched film to a heat treatment at 230-240°C for 4-10 seconds, while controlling the lateral relaxation rate to 3-8%. During this process, thermal stress serves as the final triggering condition, initiating a secondary crosslinking reaction of borate ester bonds in the dynamic Si-OB network. This causes the initially oriented network structure to undergo permanent chemical crosslinking, thereby completely locking the metastable gradient and orientation structure formed by stretching, achieving ultimate stability of the film size and maximum curing of barrier properties.

[0010] Furthermore, the online surface activation treatment involves selectively irradiating the upper surface of the Si-OB network-enriched film with atmospheric pressure plasma before heat setting and winding, thereby introducing hydroxyl, carboxyl, or carbonyl polar functional groups in situ onto the film surface and enhancing the surface energy of the film. This process allows the film to be used directly as an active substrate for aluminum or carbon nitride (C3N4) deposition without additional coating, and the interfacial bonding is significantly enhanced.

[0011] A double-layer polyester film with high barrier properties and its preparation method, comprising the following steps: S1: First, the upper layer granules, including PET-co-PEN, dynamic Si-OB prepolymer, and hydrotalcite modified with silane coupling agent, are fed into a double-cone vacuum dryer for vacuum drying. After drying, nitrogen is used for cooling. Then, the dried upper layer granules are fed into the upper extruder, while bottle-grade PET, phenyl polysiloxane nucleating agent, and carbodiimide are fed into the lower layer. The upper and lower layer temperatures are set for melting. Then, the gear pump is started, and the two melt streams enter the same coat hanger die. A flow-regulating structure is set before the die outlet to create a pressure difference. The melts converge at the lip and are extruded into melt sheets, which are immediately wound up with cooling rollers to obtain continuous single-layer gradient sheets.

[0012] S2: The cooled sheet is loaded onto the synchronous biaxial stretching line, first clamped in the chain clip, and then the preheating section is run. Then, it is stretched synchronously in the longitudinal and transverse directions. During the stretching process, the hydrotalcite and Si-OB network are oriented along the surface.

[0013] S3: After stretching, the chain clips are directly placed into a heat-setting chamber for constant temperature setting. Simultaneously, lateral relaxation occurs, and the environment triggers a secondary cross-linking reaction of dynamic borate ester bonds, thereby permanently locking the formed gradient and orientation structure. After setting, the film is immediately cooled by blowing air and then wound up to obtain a gradient double-network film.

[0014] S4: The obtained film is fed into the atmospheric pressure air plasma treatment station through the unwinding unit. Plasma scanning is performed only on the upper surface of the Si-OB network to generate -OH / -COOH polar groups in situ. After the treatment is completed, the film is immediately wound up to obtain a high-barrier polyester film product that can be directly vacuum-deposited with aluminum or deposited in aqueous phase with C3N4.

[0015] Further, in S1, the PET-co-PEN, dynamic Si-OB prepolymer, and hydrotalcite modified with silane coupling agent have mass components of 70-90 parts, 8-15 parts, and 1-5 parts, respectively; the bottle-grade PET, phenyl polysiloxane nucleating agent, and carbodiimide have mass components of 94-99 parts, 0.1-0.3 parts, and 0.03-0.15 parts, respectively; the upper and lower layer temperatures are set, wherein the temperature difference between the upper and lower layers is set to 10-25°C; and the flow-blocking adjustment structure is a protrusion with a height of 0.1-0.3 mm.

[0016] Furthermore, the longitudinal and transverse synchronous stretching described in S2 has a longitudinal stretching ratio of 4.0×~4.5× and a transverse stretching ratio of 4.3×~5.0×.

[0017] Furthermore, the isothermal shaping described in S3 is set to a constant temperature of 230~240℃ for 4~10 seconds.

[0018] Furthermore, the plasma scanning described in S4 is set to an output power of 200~400W and a film travel speed of 1~3m / min.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) Barrier performance: The excellent barrier performance of the film of the present invention is mainly due to the barrier labyrinth effect constructed by the layered filler in the upper functional area and the inhibition of polymer chain segment movement by the cross-linking network. The two work together to significantly improve the barrier effect.

[0020] (2) Mechanical strength and toughness: The lower crystalline polyester provides the film with good tensile strength and modulus, while the upper cross-linked structure further enhances the dimensional stability and temperature resistance of the film, making the overall mechanical properties more balanced.

[0021] (3) Electrical properties: Based on the highly crystalline polyester matrix and the uniform, defect-free internal structure, the film has good dielectric properties and insulation strength, which can meet the basic requirements of film capacitors for dielectric materials.

[0022] (4) Interface adhesion: After surface activation treatment, the upper layer of the film is rich in polar groups, which significantly enhances the bonding force with the vacuum aluminum plating layer and improves the barrier performance by about an order of magnitude compared with the uncoated layer. Attached Figure Description

[0023] Figure 1 This is a flowchart illustrating a double-layer polyester film with high barrier properties and its preparation method according to the present invention.

[0024] Figure 2 This is a schematic diagram of the structure of the single-layer gradient double-network high-barrier polyester film in Embodiment 1 of the present invention.

[0025] Figure 3 This is a schematic diagram of melt convergence and gradient formation in the same hanger die head exit area in Embodiment 1 of the present invention.

[0026] Figure 4 This is a comparison chart of the experimental results of water vapor transmission rate, oxygen transmission rate, aluminum layer peeling force, and 10μm scratch self-healing rate in the experimental examples of this invention. Detailed Implementation

[0027] The following embodiments further explain and illustrate the technical solutions of the present invention. It should be specifically noted that each specific embodiment is a concretization and explanation of the technical solution and should not be considered as a limitation on the scope of protection of the present invention. Those skilled in the art still have the right to modify the technical solutions of these embodiments and make equivalent substitutions for some or all of the technical features, and these modifications or substitutions do not change the essence of the corresponding technical solutions, nor do they cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions described in the present invention.

[0028] The technical solution designed by this invention to solve the existing problems includes the following key points: 1. Single-layer gradient dual-network structure thin film This invention constructs a single-layer gradient dual-network integrated structure, such as... Figure 2 As shown, unlike the physical morphology of traditional high-barrier films, this structure appears macroscopically as a homogeneous single polyester thin layer without any physical bonding interface. However, at the microscopic level, it is composed of a rigid PET-co-PEN crystalline network and a dynamic Si-OB barrier network that interpenetrate each other and exhibit a continuous gradient distribution along the thickness direction. The dynamic Si-OB network is enriched in the surface layer within 5-30% of the thickness, while the rigid PET-co-PEN crystalline network is enriched in the inner layer within 70-95% of the thickness. The two have no interface and no adhesive, forming an intrinsically integrated structure with a macroscopic single layer and a microscopic dual region.

[0029] To achieve the aforementioned gradient structure, the core strategy lies in the following: First, during the melt co-extrusion stage, zero-pressure interfacial bonds are used to achieve molecular-level pre-interweaving of the upper and lower layers of raw materials. Based on this, the precursor of the dynamic Si-OB network during processing, due to the surface energy difference between itself and the matrix polymer PET-co-PEN, exhibits a directional migration tendency towards the film surface. This spontaneously forms a continuous distribution with decreasing concentration from the surface to the interior along the film thickness direction, constituting the gradient enrichment of functional components. Based on the enriched state of the precursor on the surface, subsequent heat treatment can trigger its cross-linking and curing, thereby constructing a high-concentration three-dimensional dynamic network in situ on the surface. Simultaneously, the layered nanofillers synergistically oriented along the surface in the shear flow field. This specific configuration of gradient enrichment and surface synergistic orientation achieved by the dynamic Si-OB network not only eliminates the interface through chemical bonding but also constructs a dense and ordered reinforced region on the surface through the directionally arranged network and fillers at the microstructural level. Furthermore, it achieves strong integration with the matrix through gradient interpenetration, ultimately enabling the film to exhibit designable anisotropy in barrier and mechanical properties.

[0030] Correspondingly, the PET-co-PEN copolymer enriched in the lower layer undergoes high orientation and induces the formation of dense, fine crystalline regions under the axial stress generated by synchronous biaxial stretching, thereby constructing a rigid support framework that bears the main mechanical load. Ultimately, through the chemical cross-linking of the surface network and the physical locking of the lower crystallization, a stable interpenetrating dual-network structure is formed at the nanoscale: the Si-OB prepolymer and PET segments diffuse into each other in the 0~1.5µm transition region to form a semi-interpenetrating network, providing a chemical bridge for the reversible ester exchange between the borate ester and the PET end groups. Simultaneously, the highly oriented PET crystals and Si-O-Si segments become physically entangled, achieving true nanoscale interpenetration and ensuring a macroscopically interface-free, microscopically dual-functional integrated mechanical and barrier synergy. This intrinsically integrated design with a macroscopic single layer and a microscopic gradient fundamentally eliminates the risk of interlayer delamination inherent in traditional multilayer co-extrusion or coated films due to mechanical splicing and poor interfacial compatibility, elevating the simple physical superposition of different materials to a functional chemical integration within a single material system.

[0031] 2. In-situ composite and locking process The in-situ composite and gradient-locking preparation process system comprises two core steps. The first is the in-situ preparation of gradient melts through zero-pressure interfacial bonding, such as... Figure 3 As shown, the left channel of the co-extrusion die contains the upper layer of granular melt at 265°C, and the right channel contains the lower layer of granular melt at 280°C. A flow-regulating structure is located near the outlet of the left channel, while the right channel remains straight. This structure causes localized shrinkage of the cross-sectional area of ​​the upper melt and controlled deflection of its flow direction, creating a stable pressure difference between the two melt streams. Melt convergence and laminar diffusion are entirely driven by the pressure difference and shear traction force generated by this geometry. Gravitational potential energy is perpendicular to the flow plane and its effect is negligible, thus preventing premature mixing and not affecting gradient formation. Simultaneously, independent temperature control ensures the upper melt temperature is slightly lower than the lower layer. During this process, the bulk flow driven by the pressure difference, the molecular diffusion driven by the temperature difference, and the directional migration and gradient enrichment effect of functional components formed under these conditions work together to construct a synergistic field that continuously melts the melt interface and drives microstructural reorganization. Two melt streams undergo continuous viscous shear penetration and molecular thermal motion intertwining in a laminar flow state within the converging channel. The migration and local concentration distribution of functional components further regulate the molecular diffusion and intertwining process at the interface, thereby achieving deep composite rather than simple superposition. Ultimately, during extrusion, a single-layer gradient melt sheet with a continuous gradient distribution of components along the thickness direction and no macroscopic phase interface is directly formed. The entire process does not rely on mechanical extrusion at the die lip.

[0032] Next, the monolayer gradient melt sheet was simultaneously biaxially stretched. By cleverly utilizing the activation energy provided by the tensile stress for the dynamic borate ester bonds, a secondary cross-linking reaction was precisely triggered during the heat setting stage. This chemical reaction caused the active borate ester groups in the stretching precursor to condense with the hydroxyl groups on the siloxane chains, forming a robust and dynamic -BO-Si- three-dimensional cross-linked network. Simultaneously, the metastable physical entanglement and orientation formed during stretching were transformed into a thermodynamically stable chemical cross-linked structure. This cross-linked network, in synergy with the highly oriented sheet-like filler, blocked the permeation channels at the molecular scale, while the filler extended the permeation path at the nanoscale. Together, they achieved an exponential improvement in barrier performance. This synergistic mechanism of stress pre-activation + thermal triggering successfully anchored and locked the metastable oriented structure formed during stretching.

[0033] 3. Self-healing high-barrier synergistic effect The thin film prepared by this invention ultimately exhibits a self-healing, high-barrier synergistic intelligent effect, achieving a fundamental shift in its performance mode from passive to active. The ultra-high barrier properties of the film no longer rely solely on the static labyrinthine physical barrier formed by layered fillers, but more importantly, benefit from the reversible breaking and recombination capabilities of borate ester bonds in the dynamic Si-OB network enriched on its surface. This enables the film to autonomously repair defects such as microcracks and pinholes when microscopic damage occurs during subsequent processing or in harsh operating environments, thereby dynamically maintaining and restoring its barrier integrity.

[0034] To ensure the reliability and long-term stability of this property in applications, this invention integrates online plasma surface engineering at the end of the process. This treatment is not a simple surface cleaning or roughening, nor is it the same as traditional corona treatment, which is prone to degradation. Instead, it utilizes high-energy plasma to selectively reconstruct the surface layer of the Si-OB network in situ. Specifically, active particles in the plasma first break the surface molecular chains through physical bombardment, generating a large number of free radical active sites. When the treatment environment is oxygen-containing gas, these active sites then react chemically with the reactive oxygen components in the environment, stably constructing a chemical transition layer rich in highly polar oxygen-containing functional groups in situ on the outermost layer of the film. This chemically reconstructed layer, as a chemical anchor for bonding with subsequent functional layers, elevates the interfacial bonding force from primarily physical adsorption to a new level dominated by chemical bonding and strong polar interactions. This process transforms products from consumables with gradually declining performance in the traditional sense into durable structures with self-maintaining performance capabilities. It provides a solution to the long-standing technical pain point of premature product failure due to microscopic defects in fields such as high-end aluminized film substrates, long-lasting stable film capacitors, and C3N4 functional films with strong interface bonding.

[0035] Example 1 Table 1 Raw Material Information Table

[0036] A double-layer polyester film with high barrier properties and its preparation method, comprising the following steps: S1: 100 parts of terephthalic acid and 65 parts of 2,6-naphthalenedicarboxylic acid were added to a polymerization reactor, along with 45 parts of ethylene glycol and 0.03 parts of antimony glycolate as catalysts. The mixture was heated to 250°C under nitrogen protection for esterification. After the reaction, 0.04 parts of triphenyl phosphate were added as a heat stabilizer, followed by polycondensation under low vacuum to obtain PET-co-PEN copolymer. 100 parts of amino-terminated polydimethylsiloxane and 15 parts of 4-formylphenylboronic acid were placed in a three-necked flask, along with 300 parts of toluene as a solvent and 0.5 parts of p-toluenesulfonic acid as a catalyst. The mixture was refluxed under nitrogen protection. After the reaction, the temperature was lowered to 60°C, and 15 parts of triethyl borate and 5 parts of deionized water were added. The mixture was kept warm and stirred for 4 hours to obtain dynamic Si-OB prepolymer. The upper layer granules, including 85 parts PET-co-PEN, 12 parts dynamic Si-OB prepolymer, and 3 parts hydrotalcite modified with γ-aminopropyltriethoxysilane, were fed into a double-cone vacuum dryer and vacuum dried at 135°C for 6 hours. After drying, the mixture was cooled with nitrogen. The dried upper layer granules were then fed into the upper extruder. The lower layer consisted of 96 parts bottle-grade PET, 0.3 parts phenyl polysiloxane nucleating agent, and 0.05 parts carbodiimide. The upper layer was set to 265°C and the lower layer to 280°C. After melting for 20 minutes, the gear pump was started, and the two melt streams entered the same coat hanger die. Before the confluence area at the die lip, a flow control block with a height of 0.2 mm was installed at the end of the right channel. This caused the local cross-sectional area of ​​the upper melt to shrink and the melt flow direction to be deflected in a controlled manner, thereby creating a pressure difference between the upper melt and the lower melt, which remained straight. This structural design creates a stable pressure difference between the two melt streams. The melts converge at the lip and are extruded into 120μm melt sheets, which are then immediately wound up with a 20℃ cooling roller to obtain a continuous single-layer gradient sheet.

[0037] S2: The cooled sheet is mounted on a synchronous biaxial stretching line and first clamped in a chain clamp. The preheating section is run at 115°C for 30 seconds, followed by synchronous stretching at 120°C in the longitudinal direction (4.2×) and transverse direction (4.5×), with a stretching rate of 2000% / s. During the stretching process, the hydrotalcite and Si-OB network are oriented along the surface, and the sheet is instantly thinned to 25μm.

[0038] S3: After stretching, the chain clips are directly placed into a heat-setting chamber and kept at 235°C for 6 seconds, while simultaneously undergoing 5% lateral relaxation. This environmentally triggered secondary cross-linking of the dynamic borate ester bonds, thereby permanently locking the formed gradient and orientation structure. After setting, the film is immediately cooled to 50°C by air blowing and then wound up to obtain a 12μm gradient double-network film.

[0039] S4: The obtained film is fed into the atmospheric pressure air plasma treatment station through the unwinding unit. Plasma scanning is performed only on the upper surface of the Si-OB network enriched therein. The output power is 300W and the film running speed is 2m / min. In-situ -OH / -COOH polar groups are generated. After the treatment is completed, the film is immediately wound up to obtain a high-barrier polyester film product that can be directly vacuum-deposited with aluminum or deposited in C3N4 aqueous phase.

[0040] Example 2 The preparation method is the same as in Example 1, except that: S1: 85 parts PET-co-PEN, 12 parts dynamic Si-OB prepolymer, and 3 parts surface-modified hydrotalcite with γ-aminopropyltriethoxysilane were replaced with 70 parts PET-co-PEN, 8 parts dynamic Si-OB prepolymer, and 1 part surface-modified hydrotalcite with γ-aminopropyltriethoxysilane; 96 parts bottle-grade PET, 0.3 parts phenyl polysiloxane nucleating agent, and 0.05 parts carbodiimide were replaced with 94 parts bottle-grade PET, 0.1 parts phenyl polysiloxane nucleating agent, and 0.03 parts carbodiimide; the upper layer temperature of 265℃ and the lower layer temperature of 280℃ were replaced with the upper layer temperature of 270℃ and the lower layer temperature of 275℃; the flow control block with a height of 0.2 mm was replaced with a flow control block with a height of 0.1 mm. S2: The longitudinal 4.2× and transverse 4.5× synchronous stretching is replaced with the longitudinal 4.0× and transverse 4.3× synchronous stretching; S3: Replace 235℃ constant temperature for 6 seconds with 230℃ constant temperature for 4 seconds; S4: Output power of 300W and film running speed of 2m / min can be replaced with output power of 200W and film running speed of 1m / min; All other steps are the same.

[0041] Example 3 The preparation method is the same as in Example 1, except that: S1: 85 parts PET-co-PEN, 12 parts dynamic Si-OB prepolymer, and 3 parts surface-modified hydrotalcite with γ-aminopropyltriethoxysilane were replaced with 90 parts PET-co-PEN, 15 parts dynamic Si-OB prepolymer, and 5 parts surface-modified hydrotalcite with γ-aminopropyltriethoxysilane; 96 parts bottle-grade PET, 0.3 parts phenyl polysiloxane nucleating agent, and 0.05 parts carbodiimide were replaced with 99 parts bottle-grade PET, 0.5 parts phenyl polysiloxane nucleating agent, and 0.15 parts carbodiimide; the upper layer temperature of 265℃ and the lower layer temperature of 280℃ were replaced with the upper layer temperature of 260℃ and the lower layer temperature of 285℃; the flow control block with a height of 0.2 mm was replaced with a flow control block with a height of 0.3 mm. S2: The longitudinal 4.2× and transverse 4.5× synchronous stretching is replaced with the longitudinal 4.5× and transverse 5.0× synchronous stretching; S3: Replace 235℃ constant temperature for 6 seconds with 240℃ constant temperature for 10 seconds; S4: Output power of 300W and film running speed of 2m / min is replaced with output power of 400W and film running speed of 3m / min; All other steps are the same.

[0042] Comparative Example 1 Polyester film was prepared using a traditional process: layer A consisted of 85 parts PET-co-PEN, 12 parts dynamic Si-OB prepolymer, and 3 parts hydrotalcite composite; layer B consisted of a mixture of 96 parts bottle-grade PET, 0.3 parts phenyl polysiloxane nucleating agent, and 0.05 parts carbodiimide. A three-layer A / B / A film with a thickness of 12 μm was prepared by traditional multilayer co-extrusion process.

[0043] Comparative Example 2 The preparation method is the same as in Example 1, except that: S1: Replace 12 parts of dynamic Si-OB prepolymer with 12 parts of ordinary aminosilane coupling agent; All other steps are the same.

[0044] Comparative Example 3 The preparation method is the same as in Example 1, except that: S1: Before melt extrusion, the upper granules have undergone a 30-minute pre-curing treatment at 250°C to achieve a Si-OB crosslinking degree of over 80% before being subjected to synchronous biaxial stretching. Comparative Example 4 The preparation method is the same as in Example 1, except that: S1: The upper and lower granules are simply physically blended at a mass ratio of 50:50, and then cast into a sheet through conventional single-layer melt extrusion and casting process, omitting the temperature difference-pressure difference process; All other steps are the same.

[0045] Experimental Example 1 The high-barrier polyester film products prepared in Examples 1-3 and Comparative Examples 1-4 were measured: (1) Water vapor transmission rate: Referring to ASTM F1249, "Standard Test Method for Measuring Water Vapor Transmission Rate of Plastic Films and Sheets Using Modulated Infrared Sensors", the film sample was sealed in the test chamber to form a stable humidity gradient on both sides. The test conditions were 38°C and 90% relative humidity. Water vapor diffused from the high-humidity side through the film into the dry carrier gas, and its concentration was detected by a high-precision infrared sensor. Finally, the mass of water vapor passing through a unit area of ​​film per unit time was calculated, and the result was expressed in grams per square meter per day (g / (m²)). 2·day) indicates the day. Three samples were taken for the experiment, and the average value of the results was taken.

[0046] (2) Oxygen Transmission Rate: Referring to ASTM D3985-2017, "Standard Test Method for Measuring the Transmission Rate of Oxygen Through Plastic Films and Sheets Using a Coulomb Detector," the film sample was sealed in the test chamber to create a stable oxygen concentration gradient on both sides. The test conditions were 23°C and 0% relative humidity. Oxygen diffused through the film from the high-concentration side into the carrier gas and was transported by the carrier gas to the coulomb sensor to undergo an electrochemical reaction. By measuring the current required to maintain this reaction, the volume of oxygen passing through a unit area of ​​film per unit time was accurately calculated. The results were expressed in cubic centimeters per square meter per day (cm²). 3 / (m 2 ·day)) means that three samples were taken for the experiment and the average value of the results was taken.

[0047] (3) Peel strength of aluminum layer: Referring to ASTM D903, "Standard Test Method for Peel Strength of Adhesive-bonded Strips", the aluminum layer of a 15 mm wide aluminized film was bonded to a standard adhesive tape with known strength and adhesion. The film was then peeled at a 180° peel angle and constant rate on a universal testing machine. The force required for peeling was recorded in real time using sensors. The average force required to peel the aluminum layer per unit width from the film surface was calculated and reported, expressed in Newtons per centimeter (N / 15 mm). Three samples were tested, and the average value was taken.

[0048] (4) Self-healing rate of 10μm scratch: The functional recovery rate characterization method was used. The specific process was as follows: A standardized scratch with a depth of (10±1)μm was prepared on the surface of the film using a micro-scratcher, and its oxygen permeability was immediately measured as the initial value; then the sample was placed in a standard environment of (23±2)℃ and (50±5)% relative humidity and left to stand for (24±0.5) hours, and the oxygen permeability at the same location was measured again. The self-healing rate of 10μm scratch was finally obtained by calculating the oxygen permeability recovery ratio between the scratched state and the intact state. Three samples were taken for the experiment, and the average value of the results was taken.

[0049] Table 2 Comparison of experimental results of Examples 1-3 and Comparative Examples 1-4

[0050] The experimental results of Examples 1-3 and Comparative Examples 1-4 are shown in Table 2 and Figure 4As shown, the high-barrier polyester film produced by this invention has the lowest water vapor permeability and oxygen permeability. The aluminum layer peel strength of 6.8N and the 10μm scratch self-healing rate of 92.3% all meet the standards, which are significantly better than the traditional multilayer co-extrusion or uniform blending control. It achieves the synergy of macroscopic single layer, micro-gradient, self-healing and high barrier properties, and meets the application requirements of high-end substrates such as aluminized and C3N4.

[0051] Although Example 2 still had better water vapor transmission and oxygen transmission rates than the comparative example, its Si-OB prepolymer and hydrotalcite content were reduced to the lower limit, the gradient slope was too gentle, and the surface crosslinking density was insufficient, resulting in a decrease of 0.9N and 9 percentage points in aluminum peel strength and self-healing rate, respectively. The overall performance had reached the edge of the specification, so it was not in the optimal window. In Example 3, the filler and stretch ratio were both taken to the upper limit, the surface was over-crosslinked and the hydrotalcite concentration was too high. Although the barrier performance and peel strength were the best, the extended heat setting time led to increased energy consumption, and the improvement in water vapor transmission and oxygen transmission was limited, resulting in a decrease in cost-effectiveness. Therefore, it deviated from the economic-reliability balance zone and was not selected as the optimal point.

[0052] Comparative Examples 1-4, lacking key technologies, showed varying degrees of reduced overall performance compared to the examples. Comparative Example 1 employed a traditional A / B / A three-layer co-extrusion method, relying on the mechanical bonding of ethylene-vinyl alcohol copolymer and tackifying resin, resulting in no chemical bonding at the interface. This led to a sharp 75% drop in aluminum peel strength, with water vapor transmission and oxygen transmission rates 2.5 and 2.3 times higher than Example 1, respectively. The self-healing rate for 10µm scratches was only 45%, indicating a comprehensive deterioration in overall performance. This validates the irreplaceable advantages of the gradient dual-network single-layer structure in terms of reliability and barrier performance. Comparative Example 2, using a common aminosilane coupling agent, lost its dynamic Si-OB three-dimensional cross-linked network, resulting in a self-healing rate of only 46%. While water vapor and oxygen transmission rates increased to some extent, the aluminum peel strength decreased by 2.6N. Because the coupling agent only provides single-point bonding, it cannot form a three-dimensional barrier wall or undergo secondary cross-linking during the heat setting stage. Consequently, both interface reliability and barrier performance deteriorated simultaneously, validating the dynamic borate ester network. It is the core chemical basis for achieving high barrier properties, high interfacial reliability, and self-healing function; Comparative Example 3 pre-cured the upper layer granules at 250°C for 30 min before extrusion, resulting in a Si-OB crosslinking degree >80%, a sharp increase in melt viscosity, and a significant increase in tensile stress, leading to frequent film breakage during the synchronous biaxial stretching stage. The remaining film surface was rough and could not be crosslinked again. The water vapor permeability and oxygen permeability were more than 4 times higher than those of Example 1; the pre-crosslinking locked the chain segment orientation ability and also lost the stress-crosslinking synergy during the heat setting stage, proving that the process sequence of stretching first and then crosslinking is irreplaceable; Comparative Example 4 eliminated the temperature difference-pressure difference design, and simply blended the upper and lower layer granules at a mass ratio of 50:50, then formed a homogeneous casting through conventional single-layer melt extrusion, and then performed synchronous biaxial stretching and heat setting; the surface Si-OB and hydrotalcite concentrations were only 42% of those of the gradient film, and a surface enriched barrier layer could not be formed, and the water vapor permeability increased to 0.088 g / (m 2 •day), oxygen permeability increased to 0.058cm 3 / (m 2 (day), the aluminum layer peel force decreased by 2.8N, and the self-healing rate of 10µm scratches dropped to 44%; the lack of composition gradient caused the film to degenerate from a bilayer functional partition to a uniform dilution, and the barrier, interface and self-healing properties were comprehensively degraded, verifying that the temperature difference-pressure difference gradient formation mechanism is the key prerequisite for the high barrier and reliable interface synergy of the present invention.

[0053] In summary, this invention, through its innovative single-layer melt gradient dual-network structure, achieves a compositional gradient distribution between the dynamic Si-OB barrier network and the PET-co-PEN rigid chain within the same extrusion cycle, and completes secondary cross-linking locking during the synchronous stretching-heat setting stage, thereby simultaneously obtaining ≤0.036 g / (m 2 ·day) and ≤0.019cm 3 / (m 2This invention boasts ultra-high water and oxygen barrier properties (·day), aluminum layer adhesion of ≥5.9N / 15mm, and a self-healing rate of ≥83.1% for 10μm scratches. Compared with traditional multilayer co-extrusion, gradient-free uniform blending, pre-curing, or coupling agent substitution, this invention significantly outperforms the comparative examples in terms of barrier properties, interface reliability, and damage self-healing. Furthermore, it maintains a single PET material, simplifying the recycling process and providing a one-stop solution for high-end applications such as aluminized films and C3N4 base films, offering high barrier properties, long lifespan, and easy recycling.

Claims

1. A method for preparing a bilayer polyester film with high barrier properties, comprising co-melting, biaxial stretching, heat setting, and surface activation, characterized in that, The co-melting process involves laminarly combining multiple layers of melt with differentiated components within a shared die flow channel to form a single-layer melt sheet. The temperature difference between the multiple melt layers before lamination, the pressure difference within the die flow channel, and the functional component gradient enrichment effect formed under these conditions work synergistically to drive the diffusion of at least one functional component into other melt layers, thereby forming a three-dimensional network at the interlayer interface region. This achieves a structure with continuous gradients of different components and no macroscopic interface. The biaxial stretching process involves simultaneously biaxially stretching the single-layer melt sheet, causing the different components in the gradient regions of the single-layer melt sheet to align in a functionalized microstructure orientation, achieving different preset functions. The heat setting process involves heat-setting the single-layer melt sheet after simultaneous biaxial stretching, triggering a secondary cross-linking reaction of its internal dynamic covalent bonds, locking the formed metastable gradient distribution and orientation structure through chemical cross-linking. The surface activation process involves online energy beam treatment after heat setting, acting on the surface layer enriched with the dynamic covalent bonds to generate polar functional groups in situ, achieving high surface reactivity.

2. The method for preparing a double-layer polyester film with high barrier properties as described in claim 1, characterized in that, The monolayer melt sheet is achieved through the synergistic driving force of temperature difference, pressure difference, and gradient enrichment effect, combined with the functional design of the upper and lower melt raw materials. Before entering the composite die, the upper and lower melt layers are respectively controlled to a state with a preset temperature difference, and a preset pressure difference is formed within the die flow channel through a flow-blocking adjustment structure. Based on this, the driving force field established by the temperature and pressure differences, and the gradient enrichment effect of the functional components formed in this field, jointly promote the controlled diffusion and interweaving of the two melt streams in a laminar flow manner. Simultaneously, the precursors of the crosslinkable network in the functional layer melt undergo in-situ reactions under thermal action, forming a three-dimensional interpenetrating network. During the diffusion process, this network chemically bonds and physically entangles with the adjacent matrix resin layer melt, jointly constituting a monolayer gradient melt sheet with barrier properties.

3. The method for preparing a double-layer polyester film with high barrier properties as described in claim 2, characterized in that, The upper and lower melt raw materials comprise upper granules and lower granules. The upper granules are polyethylene terephthalate-co-polynaphthalene glycol ester copolymer, dynamic silicon-oxygen-boron prepolymer, and hydrotalcite surface modified with silane coupling agent. The lower granules are bottle-grade polyethylene terephthalate, phenyl polysiloxane nucleating agent, and carbodiimide.

4. The method for preparing a double-layer polyester film with high barrier properties as described in claim 3, characterized in that, Includes the following steps: S1: First, the upper layer of granules is vacuum dried. After drying, nitrogen is used for cooling. Then, the dried upper layer of granules and the untreated lower layer of granules are simultaneously fed into the machine. The upper and lower temperatures are set for melting. Then, the two melts enter the same instrument. By setting a flow control structure in the instrument, a pressure difference is formed to extrude the melt sheet. It is then immediately cooled and wound up to obtain a single-layer gradient melt sheet. S2: The single-layer gradient melt sheet is stretched longitudinally and laterally simultaneously. During the stretching process, the hydrotalcite and the dynamic silicon-oxygen-boron covalent network are oriented along the surface. S3: After stretching, constant temperature setting is performed, and lateral relaxation is carried out simultaneously. After setting, air is blown to cool and then wound up to obtain a gradient double network film. S4: The gradient dual-network film is subjected to plasma treatment, and only the upper surface of the dynamic silicon-oxygen-boron network is plasma scanned. After the treatment is completed, it is immediately wound up to obtain a high-barrier polyester film product that can be directly vacuum-deposited with aluminum or carbon-nitrogen aqueous phase deposition.

5. The method for preparing a double-layer polyester film with high barrier properties as described in claim 4, characterized in that, The mass ratio of the dried upper layer granules and the untreated lower layer granules in S1 is 15:85~40:60; the temperature difference between the upper and lower layers is set to 10~25℃; the flow-blocking adjustment structure is a protrusion with a height of 0.1~0.3mm.

6. The method for preparing a double-layer polyester film with high barrier properties as described in claim 4, characterized in that, The longitudinal and transverse synchronous stretching described in S2 has a longitudinal stretching ratio of 4.0×~4.5× and a transverse stretching ratio of 4.3×~5.0×.

7. The method for preparing a double-layer polyester film with high barrier properties as described in claim 4, characterized in that, The constant temperature setting described in S3 is set at 230~240℃ for 4~10 seconds.

8. The method for preparing a double-layer polyester film with high barrier properties as described in claim 4, characterized in that, The plasma scanning described in S4 is set to output power of 200~400W and film travel speed of 1~3m / min.

9. A double-layer polyester film with high barrier properties as described in any one of claims 1 to 8, characterized in that, The double-layer polyester film is divided into an upper functional area composed of the upper granules and a lower functional area composed of the lower granules. The thickness of the upper functional area accounts for 5-30% of the thickness of the polyester film, and the thickness of the lower functional area accounts for 70-95% of the thickness of the polyester film.

10. A double-layer polyester film with high barrier properties as described in claim 9, characterized in that, The polyester film has a water vapor permeability ≤0.036 g / (m²). 2 • day), oxygen permeability ≤ 0.019 cm 3 / (m 2 • Day), aluminum layer peel strength ≥5.9N / 15mm and 10μm scratch self-healing rate ≥83.1%.

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