High-transmittance non-coated release bopet base film and preparation method thereof

Abstract

The application discloses a high-transmittance non-coated release BOPET base film and a preparation method thereof. The base film is a co-extrusion bidirectional stretching composite film comprising at least an upper surface layer and a PET base layer. The upper surface layer takes a PET resin as a matrix, contains a layered material nanosheet modified by a surface charge, and the layered material nanosheet is horizontally oriented and arranged in the upper surface layer. The upper surface layer also contains a polymer electrolyte which forms a charge induction layer in the upper surface layer to induce the layered material nanosheet to be horizontally oriented. The application has intrinsic release performance without an external coating release agent, and has high transmittance and low haze, thereby breaking the technical bottleneck that release function and optical performance are difficult to be combined.

Description

Technical Field

[0001] This invention relates to the field of polymer functional thin film materials technology, specifically to a high-transmittance uncoated release BOPET base film and its preparation method. Background Technology

[0002] BOPET (biaxially oriented polyethylene terephthalate) film has become the mainstream substrate in the release film field due to its excellent mechanical properties, thermal stability, dimensional stability, and optical transparency. Release films are widely used in pressure-sensitive adhesive labels, MLCC (multilayer ceramic chip capacitor) casting, electronic product die-cutting, optical adhesive protection, and other applications.

[0003] Based on the formation method of the release layer, existing release films are divided into two main categories: offline coating type and online coating type. Offline coating involves coating a release agent onto a pre-made BOPET film and then curing it. This method involves multiple steps, low efficiency, and the organic solvents in the coating solution pose hazards to the environment and human health. Online coating involves coating the release agent online between the longitudinal and transverse stretching stages of the BOPET production line. Due to its integrated process and low cost, it has gradually become the mainstream. Regardless of whether it is offline or online coating, most current release films rely on an external release agent layer to achieve the release function. Release agents are mainly divided into silicone systems and non-silicone systems (such as modified polyolefins and fluoropolymers). Some solutions also incorporate particles into the surface layer to form micro-protrusions, using physical anti-adhesion to reduce the contact area and achieve the release effect. This method usually still requires the use of an external release agent.

[0004] In the prior art, CN111634088A discloses a highly transparent antistatic BOPET release film, whose surface contains organic spherical anti-adhesion particles. The anti-adhesion is achieved by reducing the contact area through micro-protrusions, but this solution still requires an external release agent to achieve the release function. CN101623947A discloses an ultrathin BOPET film, including an upper layer, a middle layer, and a lower layer. The anti-adhesion agent used in the upper and lower anti-adhesion layers is nano-silica with a particle size of less than 1.5 μm. CN104592848A discloses a BOPET release film, using a BOPET film as the base film, on which a release agent layer is coated, which is a typical chemical coating release solution. CN103182822A discloses a low-release-force antistatic BOPET release film, employing a three-layer structure: a BOPET film layer, a silicone oil layer, and a release layer. The silicone oil layer and the release layer are sequentially coated and cured using a linear coating method, essentially still relying on an externally coated release layer. CN119662121A discloses a release agent formulation that enhances the adhesion between the release agent and the base film surface by adding an anchoring agent, but it is essentially still an external coating solution. CN119371700A discloses a BOPET release film with high residual release force and its preparation process, which still uses an online coating method for silicone release agents. Non-silicone release coating solutions (such as modified polyolefin or fluoropolymer coatings) avoid silicone contamination, but still rely on external coatings and are more expensive.

[0005] The aforementioned existing technologies have the following problems: First, the release function depends on the outer coating. Existing commercial release films all coat the base film surface with a release agent. The interfacial adhesion between the coating and the base film is limited. With long-term use or under high temperature and humidity conditions, the release agent is prone to migrate to the surface of the material being released. The silicon migration problem is particularly prominent in silicone systems, and it is difficult to eliminate even with the addition of anchoring agents or the use of non-silicone release agents. Second, it is difficult to achieve both optical performance and release function. When physical anti-adhesion is achieved by incorporating inorganic particles (such as silica) to form surface micro-protrusions, the large difference in refractive index between inorganic particles and the PET matrix and the uneven particle size distribution lead to light scattering, resulting in increased haze and decreased light transmittance (e.g., CN101623947A uses nano silica as an anti-adhesion agent). Although the external release agent solution has better release performance, optical performance will still be lost due to the coating itself or the coating process. The haze of existing external release agent solutions is generally not less than 2.5%, which is difficult to meet the requirements of high-end optical applications for haze below 2% and light transmittance above 92%. In addition, the external coating process requires steps such as preparing release liquid, controlling coating amount, and curing. The process window is narrow and the defect rate is high. The organosilicon system involves the emission of organic solvents, which poses environmental risks.

[0006] It is evident that the fundamental drawback of existing technologies lies in the fact that obtaining release functionality requires the addition of an external functional layer, which introduces interface problems and a decline in optical performance. Therefore, how to impart intrinsic release properties to BOPET base films without the need for external release agents, while simultaneously maintaining or even improving their optical performance, is a pressing technical challenge that needs to be addressed in this field. Summary of the Invention

[0007] To address the aforementioned technical problems, the present invention aims to provide a high-transmittance uncoated release BOPET base film and its preparation method. This invention possesses intrinsic release properties without the need for external release agent coating, while simultaneously exhibiting high light transmittance and low haze, overcoming the technical bottleneck of the difficulty in simultaneously achieving release function and optical performance.

[0008] To achieve the above-mentioned technical objectives and effects, the present invention is implemented through the following technical solution: A high-transmittance uncoated release BOPET base film, wherein the base film is a co-extruded biaxially oriented composite film comprising at least an upper surface layer and a PET base layer; the upper surface layer is based on PET resin and contains layered material nanosheets modified with surface charge, wherein the layered material nanosheets are horizontally oriented in the upper surface layer; the upper surface layer also contains a polymer electrolyte, wherein the polymer electrolyte forms a charge-inducing layer in the upper surface layer, inducing the layered material nanosheets to be horizontally oriented.

[0009] Furthermore, the layered material nanosheets are hexagonal boron nitride nanosheets with positive charges on their surfaces; the polymer electrolyte is an anionic polyelectrolyte, forming a negative charge-inducing layer on the upper surface; the positively charged hexagonal boron nitride nanosheets are horizontally oriented under the electrostatic effect of the negative charge-inducing layer. Hexagonal boron nitride has a layered crystal structure, with the layers bound together only by weak van der Waals forces, resulting in an extremely low coefficient of slip friction. This endows the film with intrinsic release properties. After being modified with positive surface charges, it generates electrostatic coupling with the negative charge-inducing layer on the film surface, forcing the nanosheets to be horizontally oriented in a large-area, flat arrangement. This maximizes the parallelism of the interlayer slip surface to the film surface, thus fulfilling the release function.

[0010] Furthermore, the median particle size D50 of the layered material nanosheets is 0.2–0.6 μm, and the aspect ratio is greater than 100. Within this particle size and aspect ratio range, the layered material nanosheets possess sufficient specific surface area to achieve adequate electrostatic coupling with the charge-induced layer on the film surface, while also being less prone to aggregation and able to be uniformly dispersed in the PET matrix. Simultaneously, the larger aspect ratio facilitates horizontal orientation of the nanosheets under electrostatic influence and reduces light scattering.

[0011] Furthermore, the polymer electrolyte is a polymer electrolyte containing sulfonic acid groups or a polymer electrolyte containing carboxylic acid groups.

[0012] Furthermore, the sulfonic acid-containing polymer electrolyte is a perfluorosulfonic acid resin. Perfluorosulfonic acid resin has excellent thermal stability and can withstand the high temperatures during the lateral stretching and heat setting stages of BOPET, ensuring that the charge-induced layer does not decompose or fail during high-temperature processing and maintaining the electrostatic induction effect on the layered nanosheets.

[0013] Further, the upper surface layer, by weight, comprises: 70-85 parts PET resin, 10-25 parts layered nanosheets, 0.5-3 parts polymer electrolyte, and 2-10 parts toughening compatibilizer. PET resin serves as the matrix resin, providing mechanical support and film-forming properties; the layered nanosheets serve as a release filler, imparting intrinsic release properties to the film; the polymer electrolyte serves as a charge inducer, forming a charge-inducing layer on the film surface; and the toughening compatibilizer improves the interfacial compatibility between the layered nanosheets and the PET matrix, preventing interfacial debonding due to differences in thermal expansion coefficients.

[0014] Furthermore, the base film has a three-layer co-extruded structure and also includes a lower surface layer; the lower surface layer contains an antistatic agent and / or an anti-sticking agent.

[0015] Furthermore, the base film has a light transmittance of not less than 92%, a haze of not more than 2.0%, a release force of 15–50 g / inch, and a residual adhesion rate of ≥90%. These optical and release performance indicators enable the base film to meet the usage requirements of high-end application scenarios such as MLCC release films, polarizer release films, and optical adhesive release films.

[0016] Another aspect of the present invention provides a method for preparing a high-transmittance uncoated release BOPET base film, comprising the following steps: (1) Surface charge modification treatment is applied to layered nanosheets to make their surface carry charge; (2) The surface charge-modified layered nanosheets were mixed with PET resin to prepare the upper surface masterbatch; (3) Preparation of PET-containing base layer masterbatch; (4) The upper surface masterbatch and the base layer masterbatch are co-extruded through a multi-layer co-extrusion die to form a thick sheet containing at least an upper surface layer and a PET base layer; (5) The thick sheet is stretched longitudinally; (6) After longitudinal stretching, a polymer electrolyte is applied to the outer surface of the upper layer to form a charge-induced layer; (7) Perform transverse stretching and heat setting. During the transverse stretching process, the layered material nanosheets are arranged in a horizontal orientation under the electrostatic effect of the charge-induced layer to obtain the high-transmittance uncoated release BOPET base film.

[0017] Further, in the above preparation method, the layered material nanosheets in step (1) are hexagonal boron nitride nanosheets, and the surface charge modification treatment includes: ultrasonically dispersing hexagonal boron nitride powder and polycyclic aromatic hydrocarbon compounds containing benzene rings in an alcohol / water mixed solvent to perform non-covalent lipophilic modification; then adding oligomeric polyethyleneimine to perform protonation or amination positive charge modification to obtain hexagonal boron nitride nanosheets with positive charges on the surface; the polymer electrolyte in step (6) is an aqueous solution of perfluorosulfonic acid resin, which is applied to the outer surface of the upper layer by ultrasonic atomization spraying; the heat setting temperature in step (7) is 210-230℃, and the setting time is 20-30s.

[0018] The beneficial effects of this invention are as follows: In this invention, layered nanosheets are dispersed in the upper surface layer after being modified with surface charge. Under the influence of charge, they are arranged in a horizontal orientation. The weak interlayer forces give the film surface intrinsic release properties, eliminating the need for external release agents and removing the risk of release agent migration from the source.

[0019] The horizontally oriented layered nanosheets are parallel to the film surface, resulting in extremely low light scattering. This allows the film to achieve both release properties and high light transmittance and low haze, breaking through the technical bottleneck of the difficulty in achieving both release function and optical performance.

[0020] This preparation method eliminates the release agent coating and curing steps, simplifies the process, reduces environmental impact, and can be implemented based on existing BOPET production lines, making it easy to scale up industrially.

[0021] In addition, horizontally oriented layered nanosheets can form conductive channels on the surface, giving the film antistatic properties. Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] This invention provides a high-transmittance uncoated release BOPET base film, wherein the base film is a co-extruded biaxially oriented composite film comprising at least an upper surface layer and a PET base layer. The upper surface layer uses PET resin as a matrix and contains layered material nanosheets modified with surface charge, wherein the layered material nanosheets are horizontally oriented in the upper surface layer; the upper surface layer also contains a polymer electrolyte, which forms a charge-inducing layer on the film surface during film formation, inducing the layered material nanosheets to be horizontally oriented.

[0024] For example, the upper surface layer comprises, by weight: 70-85 parts PET resin, 10-25 parts layered nanosheets, 0.5-3 parts polymer electrolyte (based on solids), and 2-10 parts toughening compatibilizer. PET resin serves as the matrix resin, providing mechanical support and film-forming properties; the layered nanosheets serve as a release filler, imparting intrinsic release properties to the film; the polymer electrolyte serves as a charge inducer to form a charge-induced layer in the upper surface layer; and the toughening compatibilizer improves the interfacial compatibility between the layered nanosheets and the PET matrix, preventing interfacial debonding or porosity defects caused by differences in thermal expansion coefficients, while simultaneously improving the stability of nanosheet orientation during biaxial stretching.

[0025] The layered material nanosheets of this invention are exemplarily hexagonal boron nitride nanosheets. Hexagonal boron nitride has a layered crystal structure, with the layers bound together only by weak van der Waals forces, resulting in an extremely low coefficient of slip friction. These weak van der Waals forces impart intrinsic release properties to the film. However, the nonpolar surface of hexagonal boron nitride has poor compatibility with PET, and direct addition easily leads to interface defects and uneven dispersion. Therefore, it is necessary to first perform non-covalent lipophilic modification to improve its interfacial compatibility with the PET matrix. Simultaneously, to enable the hexagonal boron nitride nanosheets to achieve horizontal orientation through electrostatic induction in subsequent processes, their surface also needs to be positively charged. Meanwhile, the polymer electrolyte is preferably anionic polyelectrolyte, which is used to form a negative charge-inducing layer. When the positively charged hexagonal boron nitride nanosheets migrate to the upper surface layer, the positively charged nanosheets align horizontally under the electrostatic effect of this negative charge-inducing layer.

[0026] The median particle size D50 of the layered material nanosheets is preferably 0.2–0.6 μm, and the aspect ratio is greater than 100. Within this particle size and aspect ratio range, the layered material nanosheets have sufficient specific surface area to achieve full electrostatic coupling with the charge-induced layer, and are not prone to aggregation, thus enabling uniform dispersion in the PET resin matrix. At the same time, the larger aspect ratio is beneficial for the nanosheets to achieve horizontal orientation under electrostatic coupling and reduce light scattering.

[0027] The polymer electrolyte is preferably a sulfonic acid-containing polymer electrolyte or a carboxylic acid-containing polymer electrolyte. Polymer electrolytes containing sulfonic acid or carboxylic acid groups carry a large amount of negative charge after ionization in water. Exemplarily, in this invention, the polymer electrolyte can be sprayed onto the upper surface (film surface) in solution form after longitudinal stretching to form a high-density negative charge-inducing layer. To further enhance the overall charge environment of the upper surface layer, the polymer electrolyte can also be added during the preparation of the masterbatch, allowing it to be uniformly distributed within the upper surface layer, providing a charge anchoring substrate for the film surface induction layer. The two work synergistically to ensure that the layered nanosheets are subjected to continuous and sufficient electrostatic coupling during transverse stretching, resulting in a horizontally oriented arrangement. Exemplarily, the sulfonic acid-containing polymer electrolyte is a perfluorosulfonic acid resin. Perfluorosulfonic acid resin has excellent thermal stability (decomposition temperature > 280℃), capable of withstanding the high temperatures of 210–230℃ during the transverse stretching and heat setting stages of BOPET, ensuring that the charge-inducing layer does not decompose or fail during the high-temperature process, maintaining the electrostatic induction effect on the layered nanosheets.

[0028] During the preparation of this base film, the terminal carboxyl groups of the PET resin undergo deprotonation during melt extrusion to form -COO. - This process imparts a weak negative charge to the surface of the extruded material. Based on this inherent negative charge, the present invention enhances and regulates the charge density of the film surface by applying an anionic polymer electrolyte, forming a high-density negative charge-induced layer. This negative charge-induced layer generates electrostatic coupling with the positively charged layered nanosheets. Due to the extremely high aspect ratio of the layered nanosheets, when they approach the film surface in a horizontal orientation (large area parallel to the film surface), the coupling area between positive and negative charges is maximized, the interaction is strongest, and the system energy is lowest; conversely, if they approach in a vertical orientation (edges towards the film surface), the interaction is extremely weak, the energy is higher, and the system is unstable. Therefore, driven by the synergistic effect of electrostatic coupling and the two-dimensional planar spatial confinement, the layered nanosheets are forced to achieve a horizontal orientation by spreading across the film surface over a large area, thereby maximizing the parallelism of the interlayer slip surface to the film surface and fully utilizing their intrinsic release function.

[0029] In the upper surface layer, the toughening compatibilizer is preferably an olefin copolymer containing epoxy functional groups. For example, an ethylene-acrylate-glycidyl methacrylate terpolymer (E-MA-GMA type) can be used. The epoxy groups on its molecular chain can react with the carboxyl groups at the PET ends. The acrylate segments have good compatibility with the adsorption layer on the surface of the layered nanosheets, thereby forming a flexible transition layer between the layered nanosheets and the PET matrix, improving the interfacial bonding force, and simultaneously enhancing the stability of the nanosheet orientation during biaxial stretching.

[0030] The base film is preferably a three-layer co-extruded structure, further comprising a lower surface layer. The lower surface layer contains an antistatic agent and / or an anti-sticking agent. The antistatic agent in the lower surface layer imparts antistatic properties to the film, preventing the film from adsorbing impurities due to static electricity during winding, unwinding, and downstream processing; the anti-sticking agent prevents interlayer adhesion of the film, ensuring smooth winding and high yield in subsequent processing. For example, the antistatic agent may be a polymeric permanent antistatic agent, and the anti-sticking agent may be nano-silica.

[0031] This invention also provides a method for preparing a high-transmittance uncoated release BOPET base film, comprising the following steps: (1) Surface charge modification treatment is performed on layered nanosheets to make their surface carry charge.

[0032] For example, when the layered material is hexagonal boron nitride, its surface charge modification treatment includes two steps: The first step is non-covalent lipophilic surface modification. Hexagonal boron nitride powder and a benzene-containing polycyclic aromatic hydrocarbon (PAH) compound are ultrasonically dispersed in an alcohol / water mixed solvent. The PAH compound is adsorbed onto the hexagonal boron nitride surface using π-π stacking interactions, resulting in a dispersed suspension of hexagonal boron nitride, thereby improving the interfacial compatibility between hexagonal boron nitride and PET melt. The benzene-containing PAH compound can be, for example, ruthenium red or o-phenanthroline.

[0033] The second step is protonation or amination for positive charge modification. Oligomeric polyethyleneimine is added to the above suspension, and the mixture is stirred at 50–70°C. The amino groups of the oligomeric polyethyleneimine undergo a protonation reaction with the B-OH groups at the edge of the hexagonal boron nitride, resulting in a positive charge on the surface of the hexagonal boron nitride nanosheets. After centrifugation, washing, and drying, hexagonal boron nitride nanosheets with a positively charged surface are obtained, with a preferred Zeta potential of +30–+45 mV. The median particle size D50 of the treated nanosheets is preferably 0.2–0.6 μm, and the aspect ratio is greater than 100.

[0034] (2) The surface charge modified layered material nanosheets are mixed with PET resin and polymer electrolyte to prepare the upper surface masterbatch.

[0035] For example, surface-charge-modified layered nanosheets, PET chips, polymer electrolytes, and toughening compatibilizers can be mixed in a high-speed mixer according to a certain ratio, and then melt-mixed and granulated using a twin-screw extruder at 265–275°C to obtain the upper surface masterbatch. When adding the polymer electrolyte, the polymer electrolyte is first vacuum-dried with a small amount of PET at 60°C to obtain a powdered premix, which is then mixed with other components. This facilitates the uniform dispersion of the polymer electrolyte in the masterbatch.

[0036] (3) Prepare a base layer masterbatch containing PET.

[0037] For example, PET chips and toughening compatibilizer can be mixed in a certain proportion and then granulated by twin-screw extrusion to obtain a base layer masterbatch. The base layer preferably contains 90-95 parts of PET resin and 5-10 parts of toughening compatibilizer by weight.

[0038] When the base film has a three-layer co-extrusion structure, the process further includes the step of preparing a lower surface layer masterbatch. For example, PET chips, a permanent antistatic agent, and a nano-silica anti-sticking agent can be mixed in a specific ratio and then granulated via twin-screw extrusion to obtain the lower surface layer masterbatch. The lower surface layer masterbatch preferably contains 88-97 parts by weight of PET resin, 2-6 parts by weight of the permanent antistatic agent, and 1-6 parts by weight of the nano-silica anti-sticking agent.

[0039] (4) The upper surface masterbatch, the base layer masterbatch, and the optional lower surface masterbatch are co-extruded through a multi-layer co-extrusion die to form a thick sheet containing at least the upper surface layer and the PET base layer.

[0040] The top and base layer masterbatches are fed separately into an extruder for melt extrusion. The melt passes through a filter and a metering pump into a multi-layer co-extrusion distributor, where it is extruded into thick sheets through a T-die. The sheets are cooled and shaped by cooling rollers, with the preferred roller temperature being 25–30°C and the preferred air knife pressure being 0.2–0.4 MPa. In three-layer co-extrusion, the extrusion temperature of the top and bottom layers can be 265–275°C, and the extrusion temperature of the base layer can be 270–280°C.

[0041] (5) The thick sheet is stretched longitudinally.

[0042] The thick sheet is longitudinally stretched at 75–95°C, with a stretching ratio preferably of 3.0–3.5 times. Within this temperature range, PET is in a highly elastic state, and the molecular chains can be oriented longitudinally; a stretching ratio of 3.0–3.5 times ensures that the film has sufficient longitudinal mechanical strength.

[0043] (6) Apply the polymer electrolyte to the longitudinally stretched membrane surface to form a charge-induced layer.

[0044] For example, the polymer electrolyte is prepared as an ultra-diluted aqueous solution, preferably with a solid content of 0.02% to 0.1%, and uniformly applied to the longitudinally stretched membrane surface (upper surface) by ultrasonic atomization spraying. Ultrasonic atomization spraying ensures that the droplets adhere evenly to the membrane surface, avoiding localized over-coating or uneven coating. The wet film thickness is preferably 2 to 5 μm. After rapid evaporation of water, the polyelectrolyte chains are anchored to the very shallow layer of the membrane surface through electrostatic interaction, forming a charge-induced layer.

[0045] Specifically, during melt extrusion, the terminal carboxyl groups of PET resin undergo deprotonation to form -COO. -This process imparts a weak negative charge to the surface of the thick sheet. Applying an anionic polymer electrolyte at this specific point, after longitudinal stretching and before transverse stretching, utilizes the electrostatic interaction between the pre-existing negative charge on the membrane surface and the polymer electrolyte to uniformly anchor the polyelectrolyte chains in the very shallow layer of the membrane, forming a high-density negative charge-induced layer. This provides a charge environment for the horizontal orientation of the layered nanosheets during transverse stretching. Furthermore, the polymer electrolyte, pre-added during the masterbatch preparation stage, is uniformly distributed within the upper surface layer, creating a uniform charge background across the entire upper surface. This provides a charge anchoring substrate for the very shallow negative charge-induced layer on the membrane surface, ensuring that the layered nanosheets undergo continuous and sufficient electrostatic coupling during subsequent transverse stretching.

[0046] For example, the polymeric electrolyte is an aqueous solution of perfluorosulfonic acid resin.

[0047] (7) Perform transverse stretching and heat setting.

[0048] The film with the charge-induced layer applied is fed into the transverse stretching section. The preheating temperature for transverse stretching can be 90–105°C, the stretching temperature is preferably 100–120°C, and the stretching ratio is preferably 3.2–4.0 times. Subsequently, heat setting is performed, with the heat setting temperature preferably 210–230°C and the setting time preferably 20–30 seconds. After cooling to room temperature, it is wound up to obtain a high-transmittance uncoated release BOPET base film.

[0049] During the transverse stretching process, the layered nanosheets align horizontally under the electrostatic influence of the charge-induced layer. Specifically, the positively charged layered nanosheets are forced to achieve a horizontal alignment by being electrostatically coupled to the negatively charged layer on the film surface during transverse stretching, thus maximizing the parallelism of the interlayer slip planes to the film surface and enabling intrinsic release function. Simultaneously, heat setting fixes the molecular chain orientation structure, eliminates internal stress, and ensures the dimensional stability and heat resistance of the film.

[0050] The total thickness of the base film obtained by the present invention can be adjusted according to the application requirements, and can be 32 μm for example. The thickness of the upper and lower surface layers can account for 13% to 15% of the total thickness of the film, respectively.

[0051] The high-transmittance uncoated release BOPET base film prepared using the above technical solution has a transmittance of no less than 92%, a haze of no more than 2.0%, a release force of 15–50 g / inch (TESA7475 tape test), and a residual adhesion rate of ≥90%, which can meet the usage requirements of high-end application scenarios such as MLCC release films, polarizer release films, and optical adhesive release films. Furthermore, since no external release agent is required, this base film eliminates the risk of release agent migration at the source, avoiding silicon contamination problems.

[0052] The core of this invention lies in transferring the release function from the "outer coating" to the "film body". Specifically, the inherent release properties of the BOPET base film are given by the weak van der Waals forces between the layers of the nanosheet material (such as hexagonal boron nitride nanosheets), eliminating the need for any external release agent and thus eliminating the risk of release agent migration from the source.

[0053] To achieve the above objectives, this invention modifies the surface of layered nanosheets to carry a charge, and utilizes the inherent weak negative charge of the PET matrix and a polymer electrolyte to construct a charge-induced layer. Through electrostatic coupling, the layered nanosheets are horizontally oriented on the film surface. This orientation maximizes the parallelism of the interlayer slip planes of the nanosheets to the film surface, fully leveraging their intrinsic release function. Simultaneously, the horizontally oriented nanosheets exhibit extremely low light scattering. Combined with the refractive index matching effect between them and PET (e.g., hexagonal boron nitride has a refractive index of approximately 1.80–2.10, while PET has a refractive index of approximately 1.57–1.64), the film achieves higher transmittance and lower haze than pure PET while simultaneously gaining release function. Furthermore, the conductive channels formed by the horizontally oriented nanosheets on the film surface endow the film with antistatic properties, eliminating the need for additional antistatic agents. Thus, the layered nanosheets simultaneously serve as a release layer, a light-gaining layer, and an antistatic layer in the film.

[0054] The present invention will be further described below through specific embodiments.

[0055] The raw materials used in the following embodiments are shown in Table 1.

[0056] Table 1

[0057] Example 1

[0058] (1) Preparation of positively charged hexagonal boron nitride nanosheets 5 g of hexagonal boron nitride powder was placed in 500 mL of ethanol / water (volume ratio 1:1) mixed solvent, and 0.15 g of ruthenium red was added. The mixture was ultrasonically dispersed for 1.5 h (power 300 W, frequency 40 kHz). Subsequently, 0.3 g of oligomeric polyethyleneimine (M_w≈800) was added, and the mixture was stirred in a 60 °C water bath for 2 h. After the reaction was completed, the mixture was cooled to room temperature, centrifuged, and washed three times alternately with deionized water and anhydrous ethanol. The nanosheets were then vacuum dried at 50 °C for 12 h to obtain positively charged hexagonal boron nitride nanosheets with a D50≈0.35 μm, an aspect ratio of approximately 120, and a Zeta potential of +38 mV.

[0059] (2) Preparation of the upper surface masterbatch By weight, weigh 18 parts of positively charged hexagonal boron nitride nanosheets, 72 parts of PET chips, 1.5 parts of Nafion™ D520 (on a solid basis), and E-MA-GMA terpolymer (DuPont Elvaloy). ® 8.5 parts of PTW. First, the Nafion™ D520 dispersion and a small amount of PET chips were vacuum dried at 60°C to obtain a powdered premix. Then, it was mixed with the remaining components in a high-speed mixer for 8 minutes (500 r / min). The mixture was then melt-kneaded and granulated at 265-275°C using a twin-screw extruder to obtain the upper surface masterbatch.

[0060] (3) Preparation of base layer masterbatch By weight, 95 parts of PET chips and 5 parts of E-MA-GMA terpolymer are mixed, melt-blended and granulated by a twin-screw extruder to obtain the base layer masterbatch.

[0061] (4) Preparation of the lower surface masterbatch By weight, 94 parts of PET chips, 4 parts of permanent antistatic agent, and 2 parts of nano silica are mixed, melt-blended and granulated using a twin-screw extruder to obtain the lower surface masterbatch.

[0062] (5) Three-layer co-extrusion biaxial stretching film formation The top, base, and bottom masterbatches are fed into three separate extruders for melt extrusion (top / bottom layers 265–275°C, base layer 270–280°C). After passing through filters and metering pumps, the masterbatches enter a three-layer co-extrusion distributor and are extruded into thick sheets via a T-die. The cooling roller temperature is 25–30°C, and the air knife pressure is 0.2–0.4 MPa. The sheets are longitudinally stretched 3.0–3.5 times at 75–95°C. Then, a 0.05 wt% Nafion™ D520 aqueous solution is uniformly sprayed onto the top surface using an ultrasonic atomizing nozzle (wet film thickness 2–5 μm). Following this, transverse stretching is performed (preheating 90–105°C, stretching temperature 100–120°C, stretch ratio 3.2–4.0 times), followed by heat setting at 210–230°C for 20–30 seconds. After cooling to room temperature, the sheets are wound up to obtain a high-transmittance uncoated release BOPET base film with a total thickness of 32 μm.

[0063] Example 2

[0064] (1) Preparation of positively charged hexagonal boron nitride nanosheets Same as step (1) in Example 1.

[0065] (2) Preparation of the upper surface masterbatch Weigh out 22 parts by weight of positively charged hexagonal boron nitride nanosheets, 70 parts of PET chips, 2.0 parts of Nafion™ D520 (based on solids), and 6 parts of E-MA-GMA terpolymer. The preparation method is the same as step (2) in Example 1.

[0066] (3) Preparation of base layer masterbatch Same as step (3) in Example 1.

[0067] (4) Preparation of the lower surface masterbatch Same as step (4) in Example 1.

[0068] (5) Three-layer co-extrusion biaxial stretching film formation Following the same steps (5) as in Example 1, a high-transmittance uncoated release BOPET base film with a total thickness of 32 μm was obtained.

[0069] Example 3

[0070] (1) Preparation of positively charged hexagonal boron nitride nanosheets Same as step (1) in Example 1.

[0071] (2) Preparation of the upper surface masterbatch By weight, weigh 16 parts of positively charged hexagonal boron nitride nanosheets, 76 parts of PET chips, 1.2 parts of Nafion™ D520 (on a solid basis), and E-MA-GMA terpolymer (Arkema Lotader). ® AX8900) 6.8 parts. The preparation method is the same as step (2) in Example 1.

[0072] (3) Preparation of base layer masterbatch Same as step (3) in Example 1.

[0073] (4) Preparation of the lower surface masterbatch Same as step (4) in Example 1.

[0074] (5) Three-layer co-extrusion biaxial stretching film formation Following the same steps (5) as in Example 1, a high-transmittance uncoated release BOPET base film with a total thickness of 32 μm was obtained.

[0075] Comparative Example 1 Comparative Example 1 is a conventional online silicone release film. A three-layer co-extruded BOPET base film (without hexagonal boron nitride nanosheets and Nafion™ D520 in the upper layer, and without ultrasonic atomization spraying) was prepared according to the steps of Example 1. After longitudinal stretching and before transverse stretching, an organosilicon release agent was applied online. After thermosetting, transverse stretching and thermosetting were continued to obtain an online silicone release film with a total thickness of 32 μm.

[0076] Comparative Example 2 Comparative Example 2 is a pure PET film. A three-layer co-extruded BOPET film (each layer is pure PET, without hexagonal boron nitride nanosheets, Nafion™ D520, antistatic agent and anti-sticking agent, and without ultrasonic atomization spraying) was prepared according to the steps of Example 1. After longitudinal stretching, transverse stretching and heat setting, a pure PET film with a total thickness of 32 μm was obtained.

[0077] Performance testing Haze and transmittance were determined according to GB / T 2410; release force and residual adhesion were determined according to TESA 7475 tape and JIS Z 0237; and surface resistivity was determined according to GB / T 1410. The performance test results of each embodiment and comparative example are shown in Table 2.

[0078] Table 2

[0079] As shown in Table 2, the embodiments of the present invention achieved release force comparable to conventional online silicone-coated release films without any release agent coating. Regarding optical performance, the transmittance of the embodiments of the present invention was higher than that of pure PET film, while the haze was essentially the same. In contrast, conventional online silicone-coated release films exhibited significantly increased haze and decreased transmittance due to light scattering from the coating itself, indicating that the horizontally oriented hexagonal boron nitride nanosheets achieved a synergistic gain in release function and optical performance. Residual adhesion rate is a key indicator for measuring the degree of release agent migration. The residual adhesion rates of the embodiments of the present invention were significantly better than those of Comparative Example 1, indicating that no release agent transferred to the surface of the material being released, eliminating the risk of release agent migration at the source.

[0080] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0081] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A high-transmittance uncoated release BOPET base film, characterized in that, The base film is a co-extruded biaxially oriented composite film comprising at least an upper surface layer and a PET base layer; the upper surface layer is based on PET resin and contains layered material nanosheets modified with surface charge, wherein the layered material nanosheets are arranged in a horizontal orientation in the upper surface layer; the upper surface layer also contains a polymer electrolyte, wherein the polymer electrolyte forms a charge-inducing layer in the upper surface layer, inducing the layered material nanosheets to be horizontally oriented.

2. The high-transmittance uncoated release BOPET base film according to claim 1, characterized in that, The layered material nanosheets are hexagonal boron nitride nanosheets with positive charges on their surface; the polymer electrolyte is an anionic polyelectrolyte that forms a negative charge induction layer on the upper surface; the positively charged hexagonal boron nitride nanosheets are horizontally oriented under the electrostatic effect of the negative charge induction layer.

3. The high-transmittance uncoated release BOPET base film according to claim 1, characterized in that, The median particle size D50 of the layered material nanosheets is 0.2–0.6 μm, and the aspect ratio is greater than 100.

4. The high-transmittance uncoated release BOPET base film according to claim 1, characterized in that, The polymer electrolyte is a polymer electrolyte containing sulfonic acid groups or a polymer electrolyte containing carboxylic acid groups.

5. The high-transmittance uncoated release BOPET base film according to claim 4, characterized in that, The sulfonic acid-containing polymer electrolyte is a perfluorosulfonic acid resin.

6. The high-transmittance uncoated release BOPET base film according to claim 1, characterized in that, The upper surface layer, by weight, comprises: 70-85 parts of PET resin, 10-25 parts of layered material nanosheets, 0.5-3 parts of polymer electrolyte, and 2-10 parts of toughening compatibilizer.

7. The high-transmittance uncoated release BOPET base film according to claim 1, characterized in that, The base film has a three-layer co-extruded structure and also includes a lower surface layer; the lower surface layer contains an antistatic agent and / or an anti-sticking agent.

8. The high-transmittance uncoated release BOPET base film according to claim 1, characterized in that, The base film has a light transmittance of not less than 92%, a haze of not more than 2.0%, a release force of 15-50 g / inch, and a residual adhesion rate of ≥90%.

9. A method for preparing a high-transmittance uncoated release BOPET base film as described in any one of claims 1 to 8, characterized in that, Includes the following steps: (1) Surface charge modification treatment is applied to layered nanosheets to make their surface carry charge; (2) The surface charge-modified layered nanosheets were mixed with PET resin to prepare the upper surface masterbatch; (3) Preparation of PET-containing base layer masterbatch; (4) The upper surface masterbatch and the base layer masterbatch are co-extruded through a multi-layer co-extrusion die to form a thick sheet containing at least an upper surface layer and a PET base layer; (5) The thick sheet is stretched longitudinally; (6) After longitudinal stretching, a polymer electrolyte is applied to the outer surface of the upper layer to form a charge-induced layer; (7) Perform transverse stretching and heat setting. During the transverse stretching process, the layered material nanosheets are arranged in a horizontal orientation under the electrostatic effect of the charge-induced layer to obtain the high-transmittance uncoated release BOPET base film.

10. The preparation method according to claim 9, characterized in that, The layered material nanosheets mentioned in step (1) are hexagonal boron nitride nanosheets. The surface charge modification treatment includes: ultrasonically dispersing hexagonal boron nitride powder and polycyclic aromatic hydrocarbon compounds containing benzene rings in an alcohol / water mixed solvent to perform non-covalent lipophilic modification; then adding oligomeric polyethyleneimine to perform protonation or amination positive charge modification to obtain hexagonal boron nitride nanosheets with positive charges on the surface; the polymer electrolyte mentioned in step (6) is an aqueous solution of perfluorosulfonic acid resin, which is applied to the outer surface of the upper layer by ultrasonic atomization spraying; the heat setting temperature mentioned in step (7) is 210-230℃, and the setting time is 20-30s.

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