A primer coating solution, an optical film, and a method for manufacturing the same

Abstract

The application relates to a primer coating liquid, an optical film and a preparation method thereof, and belongs to the technical field of optical films. The primer coating liquid contains 7-9 parts of a specific epoxy-modified water-based polyurethane acrylic resin, 0.1-1 parts of functional particles, 0.05-0.1 parts of an adhesion promoter and deionized water, with 100 parts by weight, and key parameters of the coating liquid are limited, and a step-by-step dissolving and dispersing method is adopted in the preparation. The optical film comprises a PMMA film and a primer coating layer arranged on the surface of the PMMA film, the thickness of the coating layer is 200 nm-500 nm, the PMMA base material is unidirectionally stretched in the MD direction in the preparation, and the TD equal-ratio stretching is carried out after coating and drying, the stretching temperature is 60-90 DEG C, and the ratio is 2-4. The optical film prepared by the application is excellent in adhesion, aging resistance and optical performance, the process is environment-friendly and feasible, and is suitable for the fields of optical display and the like.

Description

Technical Field

[0001] This invention relates to the field of optical film technology, and in particular to a primer coating solution, an optical film, and a method for preparing the same. Background Technology

[0002] Polymethyl methacrylate (PMMA) film is widely used in optical displays, decorative protection, and other fields due to its excellent optical properties, high transparency, and good weather resistance. However, PMMA film often faces problems such as insufficient adhesion and poor aging resistance in practical applications, which seriously affects the service life and performance stability of the products.

[0003] To improve the adhesion and aging resistance of PMMA films, it is usually necessary to coat the substrate surface with a film layer that has excellent adhesion and aging resistance. Aqueous primer coating solutions have excellent adhesion and aging resistance, and due to their advantages such as environmental friendliness and low VOC emissions, they are gradually becoming a research hotspot.

[0004] Currently, waterborne polyurethane acrylic resins are widely used in waterborne primer coating solutions because they combine the flexibility of polyurethane with the weather resistance of acrylic. For example, CN114891178A discloses a waterborne stain-resistant UV-curable polyurethane acrylic resin. This resin improves the stain resistance of the coating by introducing hydroxyl silicone oil components, but its adhesion-promoting effect is limited.

[0005] The core reasons for the insufficient adhesion of existing waterborne polyurethane acrylic resin-based primer coating solutions to PMMA films are as follows: First, the interfacial polarity matching between the resin and the PMMA substrate is poor. PMMA has low surface energy and weak polarity, and conventional resins have insufficient polar group content, making it impossible to form a stable interfacial bond. Second, the resin has low crosslinking density, insufficient aging resistance and water resistance, and the coating is prone to degradation and peeling during use. Third, improper selection of functional particle size or poor dispersion can easily cause light scattering, sacrificing optical performance. Fourth, optical film preparation processes often use a "biaxial stretching followed by coating" method, where the PMMA molecular chains are already fully oriented, and the coating layer cannot form effective entanglement with the substrate molecular chains, further reducing the interfacial bond. Meanwhile, traditional solvent-based primer coating solutions have problems with high VOC emissions and poor environmental performance, which does not conform to industry development trends. Therefore, the development of waterborne primer coating solutions and supporting preparation processes that combine high adhesion, excellent aging resistance, good optical performance, and environmental friendliness has become an urgent industry need. Summary of the Invention

[0006] The purpose of this application is to solve the technical problem of insufficient adhesion of current primer coating liquid to PMMA film, while also possessing excellent aging resistance, good optical performance and environmental friendliness.

[0007] To achieve the above objectives, this application provides a primer coating solution, comprising the following components in 100 parts by weight:

[0008] 7-9 parts of epoxy-modified waterborne polyurethane acrylic resin; 0.1 to 1 part of functional particles; Adhesion promoter 0.05-0.1 parts; The remainder is deionized water; Wherein: the epoxy-modified waterborne polyurethane acrylic resin is a long-chain polymer resin containing carboxyl groups, with a solid content of 45-55%, a weight-average molecular weight of 8000-12000 g / mol, and a carboxyl content of 40-50 mmol / 100g.

[0009] As a further improvement of this application, the functional particles are inorganic nanoparticles with an average particle size of 50-100 nm that have undergone hydrophilic surface modification treatment.

[0010] As a further improvement to this application, the inorganic nanoparticles are selected from one or more of silicon dioxide, aluminum oxide, and zinc oxide.

[0011] As a further improvement of this application, the adhesion promoter is a polyester, polyether, or polyester ether containing carboxyl groups, with a weight-average molecular weight of 2000-5000 g / mol and a carboxyl content of 30-40 mmol / 100g.

[0012] As a further improvement of this application, the primer coating liquid has a solid content of 7-15%, a pH value of 7.0-9.0, and a viscosity of 1-3 mPa·s (25°C).

[0013] To achieve the above objectives, this application also provides a method for preparing the primer coating solution described above, comprising the following steps: S11. Add the adhesion promoter to deionized water and stir at 300-400 rpm for 20-30 minutes to fully disperse and obtain the first mixed solution; S12. Add the functional particles to the first mixed solution and stir at 300-400 rpm for 20-30 min to fully disperse and obtain the second mixed solution; S13. Add epoxy-modified waterborne polyurethane acrylic resin to the second mixed solution, and stir at 300-400 rpm for 20-30 min to obtain the primer coating solution. The operating temperature for all steps is 20–30°C.

[0014] To achieve the above objectives, this application also provides an optical film, including a PMMA film and a primer coating layer disposed on any side surface of the PMMA film, wherein the primer coating layer is formed by drying and curing the primer coating liquid described above or the primer coating liquid obtained by the preparation method described above.

[0015] As a further improvement of this application, the thickness of the primer coating layer is 200nm to 500nm, and the PMMA film is a biaxially oriented PMMA film with a thickness of 30 to 50μm.

[0016] To achieve the above objectives, this application also provides a method for preparing the aforementioned optical film, comprising the following steps: S21. The PMMA resin is melt-extruded at 200-270°C and cooled by a cooling roller at 25-40°C to form a PMMA substrate with a thickness of 100-200μm. S22. The PMMA substrate is stretched unidirectionally along the MD direction at a first multiple at a first preset temperature to obtain a unidirectionally extended PMMA substrate. S23. Using a roller coating or micro-gravure coating method, the primer coating liquid according to any one of claims 1-5 or the primer coating liquid prepared according to claim 6 is coated onto the unidirectional extended PMMA substrate, and dried at 90-110°C for 1-3 minutes to obtain the original optical film. S24. The original optical film is stretched unidirectionally along the TD direction at a second magnification at a second preset temperature to obtain an optical film, wherein the second magnification is the same as the first magnification.

[0017] As a further improvement of this application, both the first preset temperature and the second preset temperature are 60-90°C, and both the first multiplier and the second multiplier are 2-4.

[0018] The specific benefits of this application are as follows: This application utilizes an epoxy-modified waterborne polyurethane acrylic resin with a solid content of 45-55%, a weight-average molecular weight of 8000-12000 g / mol, and a carboxyl content of 40-50 mmol / 100g. The epoxy groups enhance crosslinking density and aging resistance, while the carboxyl groups, as polar sites, form hydrogen bonds / polar interactions with the PMMA substrate, laying the foundation for high adhesion. Furthermore, the epoxy-modified waterborne polyurethane acrylic resin has a long-chain structure, which improves flexibility and film-forming properties.

[0019] The primer coating solution prepared in this application showed no delamination or precipitation after being sealed and stored at 25°C for ≥6 months, and exhibited excellent film-forming properties after coating. The prepared optical film had a total light transmittance of ≥90%, a haze of ≤2.0%, a polarizer peel strength of ≥130gf / 15mm, and an adhesion retention rate of ≥95% after aging at 60°C for 500 hours. Furthermore, the coating layer was free of pinholes, bubbles, and cracks, and the PMMA film showed no deformation or yellowing. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the cross-sectional structure of the optical film of this application. Detailed Implementation

[0021] As is known from the background art, current aqueous primer coating solutions still have the technical problem of insufficient adhesion to PMMA films. This application provides a primer coating solution, which, based on 100 parts by weight, comprises the following components: 7-9 parts of epoxy-modified waterborne polyurethane acrylic resin; 0.1 to 1 part of functional particles; Adhesion promoter 0.05-0.1 parts; The remainder is deionized water; Wherein: the epoxy-modified waterborne polyurethane acrylic resin is a long-chain polymer resin containing carboxyl groups, with a solid content of 45-55%, a weight-average molecular weight of 8000-12000 g / mol, and a carboxyl content of 40-50 mmol / 100g.

[0022] Among them, the long-chain type is a polymer resin with a carbon chain number of ≥20 in the main chain of the resin molecule. When the carbon chain number is ≥20, the entanglement ability of the resin molecular chain is stronger, the elongation at break of the coating after film formation is higher, and the coating cracking is avoided during the stretching process.

[0023] Based on the above technical solution, epoxy-modified waterborne polyurethane acrylic resin is used as the core film-forming component. Its long-chain carboxyl-containing polymer structure is the key to achieving film formation and adhesion. The long-chain structure allows the resin molecules to fully expand and intertwine during the film formation process, forming a dense and flexible coating skeleton, thus preventing the coating from cracking and peeling off. The carboxyl groups (content controlled at 40-50 mmol / 100g) can undergo hydrogen bonding and weak esterification reactions with the hydroxyl and ester groups on the surface of the subsequent PMMA substrate. At the same time, the introduction of epoxy groups can improve the crosslinking density and water resistance of the resin, making up for the shortcomings of the weather resistance and hardness of pure waterborne polyurethane acrylic resin. The limitation of its solid content (45-55%) and weight-average molecular weight (8000-12000g / mol) ensures the good solubility and dispersibility of the resin in deionized water, and avoids the problems of abnormal viscosity and uneven film formation caused by excessively high weight-average molecular weight, or poor coating density and insufficient protective performance caused by excessively low weight-average molecular weight. Functional particles (0.1–1 part) serve as functional reinforcing components, filling the internal pores of the coating and improving its hardness, abrasion resistance, and optical uniformity. The amount added is controlled within this range to avoid particle agglomeration due to excessive addition, which would cause light scattering and affect optical performance; or insufficient addition, which would fail to exert its reinforcing effect. Adhesion promoters (0.05–0.1 part) further optimize the interfacial bonding between the coating and the PMMA substrate. Their molecular structure synergistically interacts with carboxyl and epoxy groups in the resin and surface groups of the substrate, reducing interfacial tension and promoting wetting and spreading of the coating on the substrate surface. A small amount can significantly improve adhesion, while excessive addition may lead to decreased water resistance and impaired optical performance. Deionized water, as a dispersion medium, ensures uniform dispersion of all components, forming a stable homogeneous system and preventing impurity ions from affecting the coating's stability and optical performance. Its dosage, matched with other components, ensures the coating solution achieves appropriate solid content and viscosity, meeting the requirements of subsequent coating processes. The synergistic effects of the dosage ranges of each component ultimately give the primer coating solution excellent film-forming properties, adhesion, optical performance, and stability, laying the foundation for the subsequent preparation of optical films.

[0024] In an optional implementation, the functional particles are inorganic nanoparticles with an average particle size of 50–100 nm that have undergone hydrophilic surface modification. The modification aims to improve the dispersibility of the inorganic nanoparticles in an aqueous system, preventing agglomeration. The particles are monodisperse or quasi-monodisperse, with no significant agglomeration; simultaneously, it enhances the interfacial bonding between the particles and resin molecules. Inorganic nanoparticles possess a large specific surface area and high surface activity. An average particle size controlled at 50–100 nm allows for uniform dispersion in deionized water and resin systems, preventing agglomeration and light scattering (if the average particle size is too large, scattering centers are easily formed, affecting optical transparency; if the average particle size is too small, the surface energy is too high, easily leading to agglomeration, which also damages optical properties). It also allows them to fully fill the gaps between resin molecules, forming a tight bond with the resin molecules, thus improving the coating's hardness, wear resistance, and scratch resistance. Meanwhile, the inorganic properties of inorganic nanoparticles can complement those of organic resins, improving the coating's weather resistance and high-temperature resistance, and compensating for the performance degradation of organic resins under extreme environments. Furthermore, nanoparticles within this average particle size range do not significantly affect the viscosity of the coating solution, maintaining its smooth application and ensuring uniform film formation. In addition, the surface hydroxyl groups of inorganic nanoparticles can undergo hydrogen bonding or chemical reactions with carboxyl and epoxy groups in the resin, further enhancing the bonding force between the particles and the resin, preventing particles from detaching from the coating, thereby improving the overall stability and service life of the coating.

[0025] In an optional implementation, the inorganic nanoparticles are selected from one or more of silica, alumina, and zinc oxide. These types of inorganic nanoparticles are all inorganic nanomaterials with excellent optical properties, and their surfaces contain a large number of hydroxyl groups. These hydroxyl groups can synergistically interact with the carboxyl and epoxy groups of epoxy-modified waterborne polyurethane acrylic resin and adhesion promoters, enhancing the bonding force between the particles and the resin system and improving the density and stability of the coating. Specifically: silica nanoparticles have good transparency, wear resistance, and weather resistance, significantly improving the hardness and optical uniformity of the coating and preventing haze buildup, thus meeting the core transparency requirements of optical films; alumina nanoparticles have high hardness and thermal conductivity, improving the scratch resistance and heat dissipation of the coating and preventing performance degradation due to friction or excessive temperature during use; zinc oxide nanoparticles not only have good optical properties but also certain antibacterial and anti-UV properties, giving the coating additional functions and expanding the application scenarios of optical films. By selecting one or more of the above-mentioned particles and using them in combination, complementary performance can be achieved. The particle composition can be adjusted according to the specific application requirements of the optical film, further optimizing the overall performance of the coating. At the same time, these inorganic nanoparticles are widely available, have good dispersibility, and are highly compatible with deionized water and resin systems. They will not agglomerate or precipitate, and can stably play a reinforcing role, ensuring the stability of the primer coating solution and adapting it to the preparation requirements of the optical film.

[0026] In an optional embodiment, the adhesion promoter is a polyester, polyether, or polyester ether containing carboxyl groups, with a weight-average molecular weight of 2000–5000 g / mol and a carboxyl content of 30–40 mmol / 100g. The carboxyl groups in the molecular structure of the above-mentioned promoters can undergo hydrogen bonding and weak esterification reactions with the hydroxyl and ester groups on the surface of the PMMA substrate. Simultaneously, they can form a synergistic effect with the carboxyl and epoxy groups in epoxy-modified waterborne polyurethane acrylic resin, reducing the interfacial tension between the coating and the substrate, promoting the wetting, spreading, and penetration of the coating on the substrate surface, thereby significantly improving interfacial adhesion and solving the technical problems of poor adhesion and easy peeling of existing primers to PMMA substrates. The weight-average molecular weight is controlled between 2000 and 5000 g / mol because if the weight-average molecular weight is too low, the accelerator molecular chains are too short, resulting in weak entanglement with resin molecules, limited improvement in interfacial adhesion, and easy migration from the coating, affecting coating stability. If the weight-average molecular weight is too high, the accelerator molecular chains are too long, reducing solubility and dispersibility in deionized water, making it prone to agglomeration with resin, damaging the uniformity and optical properties of the coating solution, and potentially causing abnormal viscosity of the coating solution, affecting the coating process. This type of adhesion promoter has good compatibility with epoxy-modified waterborne polyurethane acrylic resin, does not cause phase separation, and can be uniformly dispersed in the coating solution system. It works synergistically with other components to significantly improve adhesion without affecting the optical properties of the coating, ensuring the structural stability of the subsequent optical film.

[0027] In an optional implementation, the primer coating solution has a solid content of 7–15%, a pH value of 7.0–9.0, and a viscosity of 1–3 mPa·s (viscosity test conditions are as follows: 25°C, rotational viscometer test, rotor speed 60 rpm, and the coating solution has been stored in a sealed container at 25°C for ≥6 months without stratification or precipitation). The solid content is controlled at 7–15% because if the solid content is too high, the viscosity of the coating solution will be too high, easily leading to defects such as poor leveling, excessively thick coating, and bubbles during the coating process, and the coating is prone to cracking after film formation. If the solid content is too low, the coating after film formation will be too thin and lack density, failing to effectively exert the adhesion-promoting and protective effects of the primer, and may also lead to poor coating uniformity, affecting subsequent optical performance. The pH value is limited to 7.0–9.0 because this range is the stable pH range of epoxy-modified waterborne polyurethane acrylic resin, which can prevent resin coagulation and precipitation, ensuring the long-term stability of the coating solution. Meanwhile, a neutral to slightly alkaline environment promotes the dissociation of carboxyl groups in the adhesion promoter, enhancing its interaction with the surface groups of the PMMA substrate and improving adhesion. If the pH value is too high or too low, it will not only damage the stability of the resin but also affect the compatibility between the components, leading to coating solution failure. Viscosity is controlled at 1–3 mPa·s (25°C) to adapt to subsequent coating processes (such as roller coating and spray coating), ensuring the coating solution has good leveling and spreading properties, allowing it to be evenly coated on the PMMA substrate surface to form a uniform, defect-free coating. Excessive viscosity leads to coating difficulties and poor leveling, while excessively low viscosity results in easy flow of the coating solution, a thin and uneven coating, both affecting film quality and the performance of subsequent optical films. These parameters work synergistically and are matched with other component systems to ensure that the primer coating solution can be stored stably and is compatible with coating processes, achieving the expected adhesion and optical performance after film formation.

[0028] To achieve the above objectives, this application also provides a method for preparing the primer coating solution described above, comprising the following steps: S11. Add the adhesion promoter to deionized water and stir at 300-400 rpm for 20-30 minutes to fully disperse and obtain the first mixed solution; S12. Add the functional particles to the first mixed solution and stir at 300-400 rpm for 20-30 min to fully disperse and obtain the second mixed solution; S13. Add epoxy-modified waterborne polyurethane acrylic resin to the second mixed solution, and stir at 300-400 rpm for 20-30 min to obtain the primer coating solution. The operating temperature for all steps is 20–30°C.

[0029] In step S11, the adhesion promoter is first fully dispersed in deionized water. The amount of adhesion promoter used is small (0.05-0.1 parts). Dispersing it in deionized water first can ensure its uniform dispersion and avoid agglomeration due to excessively high local concentration when mixed with other components later, which would prevent it from exerting its adhesion promoting effect. At the same time, deionized water, as a dispersion medium, can provide a good dispersion environment for the adhesion promoter, promote its molecules to fully expand, and lay the foundation for subsequent interaction with functional particles and resin. In step S12, the functional particles are then dispersed into the first mixed solution, which already contains a uniformly dispersed adhesion promoter. The carboxyl groups in the adhesion promoter molecules can interact with the hydroxyl groups on the surface of the functional particles, acting as a dispersant to effectively prevent the functional particles from agglomerating and ensure that the particles are uniformly dispersed in the system. This avoids the impact of particle agglomeration on the optical properties of the coating liquid and the coating quality. If the functional particles are mixed with the resin first, the particles are prone to agglomeration and difficult to disperse, which will damage the film-forming properties of the resin.

[0030] In step S13, the epoxy-modified waterborne polyurethane acrylic resin is finally mixed into the second mixed solution. The resin is the core film-forming component and is used in a relatively large amount (7-9 parts). Adding it last prevents premature reaction between the resin and other components, ensuring that the resin molecules are fully dissolved and uniformly dispersed in the system. Simultaneously, the already dispersed adhesion promoters and functional particles in the system can synergistically interact with the resin molecules, enhancing the binding force between components and preventing phase separation. The entire preparation process does not require harsh conditions such as high temperature or high pressure. The steps are simple and easy to operate. By following the sequence of "first dispersing a small amount of components, then adding functional particles, and finally adding the core resin," the components are ensured to be fully compatible and uniformly dispersed, ultimately producing a stable and uniform primer coating solution.

[0031] To achieve the above objectives, this application also provides an optical film, such as... Figure 1 As shown, it includes a PMMA film and a primer coating layer disposed on any side surface of the PMMA film. The primer coating layer is formed by drying and curing the primer coating liquid described above or the primer coating liquid obtained by the preparation method described above. Figure 1 This is a schematic diagram of the cross-sectional structure of the optical film, in which the thickness of the PMMA film is 20-50 μm, the thickness of the primer coating layer is 200 nm-500 nm, the coating layer is tightly attached to the surface of the PMMA film without gaps or delamination; the surface of the coating layer is flat with a roughness Ra≤5 nm, ensuring good optical performance and subsequent functional layer coating performance.

[0032] PMMA (polymethyl methacrylate) film possesses excellent transparency, weather resistance, and processing properties, making it a commonly used substrate in the optical field. However, PMMA film has relatively weak surface polarity and low surface energy, which can easily lead to problems such as coating peeling and flaking when subsequent functional layers are coated, limiting its applications. The primer coating layer, acting as a transition layer between the PMMA film and subsequent functional layers, plays a bridging role: on the one hand, the epoxy-modified waterborne polyurethane acrylic resin and adhesion promoter in the primer coating layer can interact with the hydroxyl and ester groups on the PMMA film surface (hydrogen bonding, weak chemical reactions), forming a strong interfacial bond and ensuring that the coating layer does not peel off from the PMMA film; on the other hand, the primer coating layer contains a large number of polar groups such as carboxyl and epoxy groups, which can significantly increase the surface energy and polarity of the PMMA film surface, providing good binding sites for the coating of subsequent functional layers and enhancing the adhesion between the subsequent functional layers and the PMMA film. Meanwhile, the primer coating layer possesses excellent transparency, which does not affect the optical performance of the PMMA film. Furthermore, the coating layer exhibits good density and flexibility, protecting the PMMA film surface from scratches and abrasion, thus improving the overall mechanical properties and lifespan of the optical film. By applying the primer coating layer to either side of the PMMA film surface, it can be flexibly adjusted according to the specific application requirements, adapting to different application scenarios. Its simple and rational structural design achieves the technical objective of improving the adhesion of the PMMA film surface while preserving its optical performance.

[0033] In an optional implementation, the thickness of the primer coating layer is 200nm to 500nm, and the PMMA film is a biaxially oriented PMMA film with a thickness of 30 to 50μm. When the thickness is too thin (less than 200nm), the coating layer cannot completely cover the defects on the surface of the PMMA film, and the coating density is insufficient, failing to fully exert the "bridging" effect. The adhesion improvement effect of subsequent functional layers is limited, and the protective performance of the coating is poor, failing to effectively protect the surface of the PMMA film. When the thickness is too thick (greater than 500nm), although the adhesion and protective performance can be further improved, it will lead to an increase in the total thickness of the optical film, affecting its lightweight requirements. Moreover, excessively thick coating layers are prone to defects such as cracking and warping, and may also generate light scattering, affecting the transparency and optical uniformity of the optical film, violating the core requirements of optical film for optical performance. This coating thickness ensures uniform coverage of the PMMA film surface, forming a strong bond with the PMMA film, while providing sufficient bonding sites for subsequent functional layers. It also does not adversely affect the transparency or lightweight of the optical film. In synergy with the PMMA film structure, it achieves a balance between improved adhesion and retention of optical performance, ensuring that the optical film is suitable for subsequent processing and use requirements.

[0034] To achieve the above objectives, this application also provides a method for preparing the aforementioned optical film, comprising the following steps: S21. The PMMA resin is melt-extruded at 200-270°C and cooled by a cooling roller at 25-40°C to form a PMMA substrate with a thickness of 100-200μm. S22. The PMMA substrate is stretched unidirectionally along the MD direction at a first multiple at a first preset temperature to obtain a unidirectionally extended PMMA substrate. S23. Using a roller coating or micro-gravure coating method, the primer coating liquid according to any one of claims 1-5 or the primer coating liquid prepared according to claim 6 is coated onto the unidirectional extended PMMA substrate, and dried at 90-110°C for 1-3 minutes to obtain the original optical film. S24. The original optical film is stretched unidirectionally along the TD direction at a second magnification at a second preset temperature to obtain an optical film, wherein the second magnification is the same as the first magnification.

[0035] In step S21, PMMA resin is melt-extruded and cooled to form a PMMA substrate. The melt extrusion process allows the PMMA resin molecules to fully melt and mix evenly, removing air bubbles and impurities from the resin. After cooling, a PMMA substrate with uniform thickness and dense structure is formed, laying the foundation for subsequent stretching and coating. The temperature and cooling rate of the melt extrusion need to be precisely controlled to ensure that the PMMA substrate is defect-free and has excellent optical properties.

[0036] In step S22, the PMMA substrate is stretched by a first ratio along the MD direction (machine direction) at a first preset temperature to obtain a unidirectional stretched PMMA substrate. Utilizing the thermoplasticity of PMMA resin, the substrate is unidirectionally stretched at a suitable temperature (above the glass transition temperature and below the melting point) to orient the PMMA molecular chains along the MD direction, thereby improving the mechanical properties (such as tensile strength and toughness) and optical uniformity of the substrate and preventing the substrate from breaking during subsequent biaxial stretching. The stretching ratio can be adjusted according to the specific performance requirements of the optical film to ensure that the orientation of the substrate is appropriate.

[0037] Step S23 involves coating a primer onto a unidirectionally extended PMMA substrate and drying it at 90–110°C. The surface roughness of the unidirectionally extended PMMA substrate is slightly improved, and the molecular chain orientation exposes more polar groups on the surface, which enhances the adhesion to the primer. The drying temperature of 90–110°C can quickly remove deionized water from the primer and promote a slight cross-linking reaction of the components in the primer, forming a dense and firm coating layer. If the drying temperature is too high, the PMMA substrate will deform and yellow, affecting its optical performance. If the temperature is too low, moisture cannot be completely removed, resulting in defects such as bubbles and whitening of the coating, which affect adhesion and optical performance.

[0038] Step S24 stretches the original optical film at a second preset temperature along the TD direction (perpendicular to the machine direction) at a second magnification (the same as the first magnification) to obtain an optical film. The principle is that by stretching at a bidirectional equal magnification, the PMMA molecular chains are uniformly oriented along the MD and TD directions, which further improves the mechanical properties and optical uniformity of the optical film, ensures that the optical film has consistent performance in both directions, and avoids anisotropy. The synergistic effect of bidirectional stretching can optimize the transparency, toughness, and scratch resistance of the optical film, and realize the industrial preparation of the product.

[0039] In an optional implementation, both the first preset temperature and the second preset temperature are 60–90°C, and both the first magnification and the second magnification are 2–4. The first preset temperature (MD direction stretching) and the second preset temperature (TD direction stretching) are both controlled within the range of 60–90°C. This temperature range is near the glass transition temperature of PMMA resin (approximately 105°C). At this temperature, the PMMA resin is in a highly elastic state, with moderate molecular chain mobility. This allows for orientation under stretching force without causing substrate deformation or breakage due to excessive molecular chain activity. It also avoids excessively high temperatures leading to yellowing or degradation of the substrate, or excessively low temperatures causing difficulty in molecular chain orientation and substrate breakage during stretching. This ensures the smooth progress of the biaxial stretching process without affecting the optical properties of the PMMA substrate. The first stretching ratio is controlled between 2 and 4, and the second stretching ratio is the same as the first. If the stretching ratio is too low, the PMMA molecular chain orientation is insufficient, and the improvement in the mechanical properties (such as tensile strength and toughness) and optical uniformity of the optical film is not significant. If the stretching ratio is too high, the molecular chain orientation is excessive, the brittleness of the substrate increases, and it is prone to breakage during subsequent coating and use. It may also cause stress cracking of the optical film, affecting its service life. Bidirectional equal stretching ratio (2-4) can make the PMMA molecular chain uniformly oriented in two directions, ensuring that the performance of the optical film is consistent in the MD and TD directions, avoiding anisotropy. At the same time, the uniformly oriented molecular chains can make the transparency and gloss of the optical film optimal, forming a stronger bond with the primer coating layer, further improving the overall performance of the optical film, and ensuring that the prepared optical film meets the needs of optical applications.

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the specific embodiments of this invention are described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of this invention, not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0041] To verify the superior performance of the technical solution of this application, the following embodiments are also provided.

[0042] Example 1 This embodiment provides a method for preparing an optical film, the specific steps of which are as follows: First, the primer coating solution was prepared: 92.85 parts by weight of deionized water and 0.05 parts by weight of a carboxyl-containing polyester adhesion promoter (weight-average molecular weight of 2000 g / mol) were mixed and stirred in a mixer at 350 rpm for 30 min to obtain a first mixed solution; 0.1 parts by weight of silica inorganic nanoparticles with an average particle size of 50 nm and modified with hydrophilic surface treatment were added to the first mixed solution, and the mixture was stirred at 350 rpm for 30 min to obtain a second mixed solution; 7 parts by weight of epoxy-modified waterborne polyurethane acrylic resin (long-chain type with carboxyl groups, solid content of 45%, weight-average molecular weight of 8000 g / mol, carboxyl content of 40 mmol / 100g) were added to the second mixed solution, and the mixture was stirred evenly and stirred at 350 rpm for 30 min. After filtration, a clear primer coating solution was obtained, with a solid content of 7%, a pH of 7.0, and a viscosity of 1 mPa. s (25℃).

[0043] Next, the optical film is prepared: PMMA resin is melt-extruded and cooled to form a PMMA substrate. The PMMA substrate is stretched at 60°C along the MD direction at a 2x ratio to obtain a unidirectional stretched PMMA substrate. The primer coating liquid prepared above is coated on the surface of the unidirectional stretched PMMA substrate, and the coating thickness is controlled to be 200nm. The substrate is dried at 90°C to obtain the original optical film. The original optical film is then stretched at 60°C along the TD direction at a 2x ratio, and the incompletely stretched part is cut off to obtain the optical film.

[0044] Example 2 This embodiment provides a method for preparing an optical film, the specific steps of which are as follows: First, a primer coating solution was prepared. 90.9 parts by weight of deionized water and 0.08 parts by weight of a carboxyl-containing polyether adhesion promoter (weight-average molecular weight 3500 g / mol) were mixed and stirred at 370 rpm for 30 min to obtain a first mixed solution. Then, 0.55 parts by weight of hydrophilic surface-modified alumina inorganic nanoparticles with an average particle size of 75 nm were added to the first mixed solution, and the mixture was stirred at 370 rpm for 30 min to obtain a second mixed solution. Next, 8 parts by weight of epoxy-modified waterborne polyurethane acrylic resin (long-chain type with carboxyl groups, solid content 50%, weight-average molecular weight 10000 g / mol, carboxyl content 45 mmol / 100g) were added to the second mixed solution. After thorough mixing, the mixture was stirred at 370 rpm for 30 min. The mixture was then filtered to obtain a clear primer coating solution with a solid content of 11%, a pH of 8.0, and a viscosity of 2 mPa. s (25℃).

[0045] Next, the optical film is prepared: PMMA resin is melt-extruded and cooled to form a PMMA substrate. The PMMA substrate is stretched at 75°C along the MD direction at a 3x ratio to obtain a unidirectional stretched PMMA substrate. The primer coating liquid prepared above is coated on the surface of the unidirectional stretched PMMA substrate, and the coating thickness is controlled to be 350nm. The substrate is dried at 100°C to obtain the original optical film. The original optical film is then stretched at 75°C along the TD direction at a 3x ratio, and the incompletely stretched part is cut off to obtain the optical film.

[0046] Example 3 This embodiment provides a method for preparing an optical film, the specific steps of which are as follows: First, the primer coating solution was prepared: 89.9 parts by weight of deionized water and 0.1 parts by weight of a carboxyl-containing polyester ether adhesion promoter (weight-average molecular weight of 5000 g / mol) were mixed and stirred in a mixer at 400 rpm for 30 min to obtain the first mixed solution; 1 part by weight of zinc oxide inorganic nanoparticles with an average particle size of 100 nm and modified with hydrophilic surface treatment were added to the first mixed solution, and the mixture was stirred at 400 rpm for 30 min to obtain the second mixed solution; 9 parts by weight of epoxy-modified waterborne polyurethane acrylic resin (long-chain type with carboxyl groups, solid content of 55%, weight-average molecular weight of 12000 g / mol, carboxyl content of 50 mmol / 100g) were added to the second mixed solution, and the mixture was stirred evenly and stirred at 400 rpm for 30 min. After filtration, a clear primer coating solution was obtained, with a solid content of 15%, pH value of 9.0, and viscosity of 3 mPa. s (25℃).

[0047] Next, the optical film is prepared: PMMA resin is melt-extruded and cooled to form a PMMA substrate. The PMMA substrate is stretched at 90°C along the MD direction at a 4x ratio to obtain a unidirectional stretched PMMA substrate. The primer coating liquid prepared above is coated on the surface of the unidirectional stretched PMMA substrate, and the coating thickness is controlled to be 500nm. The substrate is dried at 110°C to obtain the original optical film. The original optical film is then stretched at 90°C along the TD direction at a 4x ratio, and the incompletely stretched part is cut off to obtain the optical film.

[0048] Comparative Example 1 This comparative example provides a method for preparing a PMMA optical film, the specific steps of which are as follows: First, a solvent-based primer was prepared by mixing 8 parts by weight of solvent-based acrylic resin, 0.5 parts by weight of talc powder (average particle size 200 nm), 0.08 parts by weight of silane coupling agent and an appropriate amount of ethyl acetate until the solid content was 11%. The mixture was then placed in a mixer and stirred at 350 rpm for 60 minutes to obtain the solvent-based primer.

[0049] Secondly, PMMA optical film is prepared: PMMA resin is melt-extruded and cooled to form PMMA substrate. The PMMA substrate is simultaneously stretched biaxially along the MD and TD directions at a ratio of 2 to form PMMA sheet. The solvent-based primer prepared above is coated on the surface of the PMMA sheet, and the coating thickness is controlled to be 200nm. After drying in an oven at 80℃, it is cut to obtain PMMA optical film with a thickness of 40μm.

[0050] The differences between Comparative Example 1 and Example 1 are as follows: ① Primer system: The primer coating liquid in Example 1 is an aqueous coating liquid; Comparative Example 1 uses a traditional solvent-based acrylic primer coating liquid with a non-epoxy modified polyurethane structure; ② Functional particles: Talc powder with a large average particle size of 200 nm is used to replace inorganic nanoparticles; ③ Preparation process: A one-time coating process is used to prepare the PMMA optical film. The PMMA optical film adopts the traditional process of first biaxial stretching → then coating with primer coating liquid, and the drying temperature after primer coating liquid is lower than the 90°C specified in Example 1.

[0051] Comparative Example 2 This comparative example provides a method for preparing an optical film, the specific steps of which are as follows: First, the primer coating solution was prepared: 92.85 parts by weight of deionized water and 0.05 parts by weight of a carboxyl-containing polyester adhesion promoter (weight-average molecular weight of 2000 g / mol) were mixed and stirred in a mixer at 350 rpm for 30 min to obtain a first mixed solution; 0.1 parts by weight of silica inorganic nanoparticles with an average particle size of 50 nm and hydrophilic surface modification were added to the first mixed solution, and the mixture was stirred at 350 rpm for 30 min to obtain a second mixed solution; 7 parts by weight of ordinary water-based acrylic resin (unmodified, solid content 50%, weight-average molecular weight of 5000 g / mol, carboxyl content 20 mmol / 100g) were added to the second mixed solution, and the mixture was stirred evenly and stirred at 350 rpm for 30 min. After filtration, a clear primer coating solution was obtained, with a solid content of 7%, a pH of 7.0, and a viscosity of 1 mPa. s (25℃).

[0052] Next, the optical film is prepared: PMMA resin is melt-extruded and cooled to form a PMMA substrate. The PMMA substrate is stretched at 60°C along the MD direction at a 2x ratio to obtain a unidirectional stretched PMMA substrate. The primer coating liquid prepared above is coated on the surface of the unidirectional stretched PMMA substrate, and the coating thickness is controlled to be 200nm. The substrate is dried at 90°C to obtain the original optical film. The original optical film is then stretched at 60°C along the TD direction at a 2x ratio, and the incompletely stretched part is cut off to obtain the optical film.

[0053] Comparative Example 3 This comparative example provides a method for preparing an optical film, the specific steps of which are as follows: First, the primer coating solution was prepared: 92.85 parts by weight of deionized water and 0.05 parts by weight of a carboxyl-containing polyester adhesion promoter (weight-average molecular weight of 2000 g / mol) were mixed and stirred in a mixer at 350 rpm for 30 min to obtain a first mixed solution; 0.1 parts by weight of silica inorganic nanoparticles with an average particle size of 500 nm and modified with hydrophilic surface treatment were added to the first mixed solution, and the mixture was stirred at 350 rpm for 30 min to obtain a second mixed solution; 7 parts by weight of epoxy-modified waterborne polyurethane acrylic resin (long-chain type with carboxyl groups, solid content of 45%, weight-average molecular weight of 8000 g / mol, carboxyl content of 40 mmol / 100g) were added to the second mixed solution, and the mixture was stirred evenly and stirred at 350 rpm for 30 min. After filtration, a clear primer coating solution was obtained, with a solid content of 7%, a pH of 7.0, and a viscosity of 1 mPa. s (25℃).

[0054] Next, the optical film is prepared: PMMA resin is melt-extruded and cooled to form a PMMA substrate. The PMMA substrate is stretched at 60°C along the MD direction at a 2x ratio to obtain a unidirectional stretched PMMA substrate. The primer coating liquid prepared above is coated on the surface of the unidirectional stretched PMMA substrate, and the coating thickness is controlled to be 200nm. The substrate is dried at 90°C to obtain the original optical film. The original optical film is then stretched at 60°C along the TD direction at a 2x ratio, and the incompletely stretched part is cut off to obtain the optical film.

[0055] Comparative Example 4 This comparative example provides a method for preparing an optical film, the specific steps of which are as follows: First, the primer coating solution was prepared: 92.85 parts by weight of deionized water and 0.05 parts by weight of a carboxyl-containing polyester adhesion promoter (weight-average molecular weight of 2000 g / mol) were mixed and stirred in a mixer at 350 rpm for 30 min to obtain a first mixed solution; 0.1 parts by weight of silica inorganic nanoparticles with an average particle size of 50 nm and modified with hydrophilic surface treatment were added to the first mixed solution, and the mixture was stirred at 350 rpm for 30 min to obtain a second mixed solution; 7 parts by weight of epoxy-modified waterborne polyurethane acrylic resin (long-chain type with carboxyl groups, solid content of 45%, weight-average molecular weight of 8000 g / mol, carboxyl content of 40 mmol / 100g) were added to the second mixed solution, and the mixture was stirred evenly and stirred at 350 rpm for 30 min. After filtration, a clear primer coating solution was obtained, with a solid content of 7%, a pH of 7.0, and a viscosity of 1 mPa. s (25℃).

[0056] Next, prepare the optical film: melt extrude PMMA resin, cool to form PMMA substrate, extend the PMMA substrate at 60°C at a 2x ratio along both the MD and TD directions to obtain biaxially extended PMMA substrate, apply the primer coating liquid prepared above to the surface of the biaxially extended PMMA substrate, control the coating layer thickness to 200nm, and dry at 90°C to obtain the original optical film, thus obtaining the optical film.

[0057] The optical films of Examples 1-3 and Comparative Examples 1-4 were subjected to performance tests. The total light transmittance and haze were tested according to GB / T 2410 standard, and the peel force of the polarizing plate before and after aging at 60℃ for 500 hours was tested according to ASTM D3359 standard.

[0058] Adhesion retention rate: calculated as the ratio of the peel force after aging of the polarizing plate to that before aging.

[0059] The performance test results are shown in Table 1.

[0060] Table 1

[0061] As shown in Table 1, a comparison between Comparative Example 1 and Example 1 reveals that the transmittance, peel strength, and adhesion retention rate of Comparative Example 1 all decreased. This indicates that the aqueous primer coating liquid, the setting of inorganic nanoparticle particle size, and the core process of "stretching after coating" provided in this application have a significant impact on improving the adhesion, aging resistance, and optical performance of PMMA optical films.

[0062] Comparing Comparative Example 2 with Example 1, it can be seen that Comparative Example 2 only replaced the epoxy-modified waterborne polyurethane acrylic resin with ordinary waterborne acrylic resin without epoxy modification, low carboxyl content, and low weight-average molecular weight. The optical film prepared in Comparative Example 2 showed a decrease in adhesion retention rate, indicating that the epoxy-modified waterborne polyurethane acrylic resin plays a key role in optimizing the adhesion and aging resistance of the optical film.

[0063] Comparing Comparative Example 3 with Example 1, it can be seen that the optical film prepared in Comparative Example 3 showed a decrease in transmittance and an increase in haze. However, Comparative Example 3 only increased the average particle size of inorganic nanoparticles, indicating that the average particle size of inorganic nanoparticles has a key influence on the optical performance of optical films. If the average particle size of inorganic nanoparticles is too large, light scattering will be aggravated and the optical performance will deteriorate.

[0064] Comparing Comparative Example 4 with Example 1, it can be seen that the optical film prepared in Comparative Example 4 showed a decrease in both adhesion retention rate and peel strength. Compared with Example 1, the preparation process of "stretching in the MD direction first → applying primer coating liquid → stretching in the TD direction" was not used, indicating that this process played a key role in improving the adhesion and peel strength between the coating layer and the PMMA substrate.

[0065] In summary, by adjusting the parameters of the endpoints and intermediate values ​​of each component of the primer coating liquid in Examples 1-3, and combining the process of "MD unidirectional stretching first, TD unidirectional stretching after coating", the optical films prepared all achieved high adhesion, excellent aging resistance and good optical performance, verifying the universality and stability of the technical solution of this application; while Comparative Examples 1-4, through variable substitution, highlight the necessity of the core technical features of this application.

[0066] This application uses epoxy-modified waterborne polyurethane acrylic resin as the core film-forming component, combined with 50-100nm inorganic nanoparticles and specific carboxyl-based adhesion promoters to obtain a primer coating solution with suitable performance. A stepwise dispersion method is used to ensure the uniformity of the coating solution. Simultaneously, an optical film preparation process of "first MD uniaxial stretching, then coating followed by TD coaxial stretching" is employed to ensure full entanglement of the coating layer with the PMMA substrate molecular chains, synergistically enhancing interfacial adhesion. These combined technical features solve the technical problem of insufficient adhesion of existing waterborne primers to PMMA films. Compared to traditional solvent-based systems, epoxy-free resins, large average particle size particles, and stretching-before-coating processes, the optical film prepared in this application achieves synergistic improvements in adhesion, aging resistance, and optical performance, while also being environmentally friendly and process-feasible, making it suitable for the preparation of PMMA optical films in fields such as optical displays.

[0067] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A primer coating liquid, characterized in that, Based on 100 parts by weight, it comprises the following components: 7-9 parts of epoxy-modified waterborne polyurethane acrylic resin; 0.1 to 1 part of functional particles; Adhesion promoter 0.05-0.1 parts; The remainder is deionized water; Wherein: the epoxy-modified waterborne polyurethane acrylic resin is a long-chain polymer resin containing carboxyl groups, with a solid content of 45-55%, a weight-average molecular weight of 8000-12000 g / mol, and a carboxyl content of 40-50 mmol / 100g.

2. The primer coating liquid according to claim 1, characterized in that, The functional particles are inorganic nanoparticles with an average particle size of 50-100 nm that have undergone hydrophilic surface modification treatment.

3. The primer coating liquid according to claim 2, characterized in that, The inorganic nanoparticles are selected from one or more of silicon dioxide, aluminum oxide, and zinc oxide.

4. The primer coating liquid according to claim 1, characterized in that, The adhesion promoter is a polyester, polyether, or polyester ether containing carboxyl groups, with a weight-average molecular weight of 2000–5000 g / mol and a carboxyl content of 30–40 mmol / 100g.

5. The primer coating liquid according to claim 1, characterized in that, The primer coating solution has a solid content of 7-15%, a pH value of 7.0-9.0, and a viscosity of 1-3 mPa·s.

6. A method for preparing the primer coating liquid according to any one of claims 1-5, characterized in that, Includes the following steps: S11. Add the adhesion promoter to deionized water and stir at 300-400 rpm for 20-30 minutes to fully disperse and obtain the first mixed solution; S12. Add the functional particles to the first mixed solution and stir at 300-400 rpm for 20-30 min to fully disperse and obtain the second mixed solution; S13. Add epoxy-modified waterborne polyurethane acrylic resin to the second mixed solution, and stir at 300-400 rpm for 20-30 min to obtain the primer coating solution. The operating temperature for all steps is 20–30°C.

7. An optical film, characterized in that, It includes a PMMA membrane and a primer coating layer disposed on any one side surface of the PMMA membrane, wherein the primer coating layer is formed by drying and curing the primer coating liquid according to any one of claims 1-5 or the primer coating liquid obtained by the preparation method of claim 6.

8. The optical film according to claim 7, characterized in that, The thickness of the primer coating layer is 200nm to 500nm, and the PMMA film is a biaxially oriented PMMA film with a thickness of 30 to 50μm.

9. A method for preparing the optical film according to claim 7 or 8, characterized in that, Includes the following steps: S21. The PMMA resin is melt-extruded at 200-270°C and cooled by a cooling roller at 25-40°C to form a PMMA substrate with a thickness of 100-200μm. S22. The PMMA substrate is stretched unidirectionally along the MD direction at a first multiple at a first preset temperature to obtain a unidirectionally extended PMMA substrate. S23. Using a roller coating or micro-gravure coating method, the primer coating liquid according to any one of claims 1-5 or the primer coating liquid prepared according to claim 6 is coated onto the unidirectional extended PMMA substrate, and dried at 90-110°C for 1-3 minutes to obtain the original optical film. S24. The original optical film is stretched unidirectionally along the TD direction at a second magnification at a second preset temperature to obtain an optical film, wherein the second magnification is the same as the first magnification.

10. The method for preparing the optical film according to claim 9, characterized in that, The first preset temperature and the second preset temperature are both 60-90℃, and the first multiplier and the second multiplier are both 2-4.

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