English
Abstract
This paper focuses on the key technology research and development of CPI (colorless polyimide) optical film for foldable OLED display screens, and explores the latest research hotspots in AR anti reflective film, hydrophobic film, and reflective glass. By analyzing the stringent performance requirements of foldable OLED displays for CPI optical films, this study explores the progress in core technologies such as molecular structure design, preparation process optimization, and surface modification. At the same time, analyzing the innovative applications and technological breakthroughs of AR anti reflective films, hydrophobic films, and reflective glass in related fields, revealing their reference significance for the research and development of CPI optical films, and providing comprehensive technical support and new ideas for promoting the development of the foldable OLED display industry.
keywords
Foldable OLED display screen; Colorless polyimide; CPI optical film; AR anti reflective film; Hydrophobic membrane; Reflective glass
1. Introduction
In the field of consumer electronics, foldable OLED displays have become a new direction and focus of display technology development due to their unique flexibility and excellent display performance, such as high contrast and wide viewing angle. As the core component of foldable OLED displays, the performance of CPI optical films directly affects the display effect, durability, and reliability of the display screen. At present, it is not only necessary to meet the conventional requirements of high optical transparency, low haze, excellent flexibility, and thermal stability, but also to draw on the cutting-edge technologies of other optical films and related glass materials to achieve further performance improvement and innovation. At present, the key technology of CPI optical film is still controlled by a few foreign companies, and there is a gap between China and the international advanced level in material properties and preparation processes. It is urgent to carry out in-depth research and development. At the same time, new research achievements and application cases continue to emerge in the fields of AR anti reflective films, hydrophobic films, and reflective glass, bringing new insights and opportunities for the development of CPI optical films.
2、 Performance requirements of CPI optical film for foldable OLED display screens
2.1 Optical Performance
CPI optical films need to have a high transmittance of ≥ 90% and a low haze of ≤ 1% in the wavelength range of 400-700nm, in order to ensure accurate color reproduction and clear and sharp display images for OLED displays. Moreover, during long-term use and frequent folding, its optical performance must remain stable to prevent situations such as decreased transmittance and increased haze caused by material aging or structural deformation.
2.2 Mechanical Properties
To meet the repeated bending scenarios of foldable OLED displays, CPI optical films must have excellent flexibility and fatigue resistance. Generally speaking, under the condition of a curvature radius of 5mm, the film layer should be able to withstand more than 100000 bending cycles, and after bending, there should be no cracking or significant attenuation of optical performance. In addition, the film material also needs to have a certain strength and modulus to resist the damage of external mechanical stress to the display screen.
2.3 Thermal performance
In the preparation process of OLED display screens, high-temperature evaporation, packaging and other processes are involved. CPI optical films need to withstand a high temperature environment of 200-300 ℃ to ensure that deformation, yellowing, decomposition and other problems do not occur; In practical use, it is necessary to maintain stable dimensional stability and mechanical performance in the face of temperature fluctuations in different environments.
2.4 Chemical Properties
CPI optical films must have excellent chemical stability, which can effectively resist the erosion of water, oxygen, acid, alkali, organic solvents, etc., prevent the deterioration of film performance due to chemical corrosion, and thus affect the service life of OLED displays. At the same time, when composite with other materials, good chemical compatibility should be demonstrated to avoid chemical reactions.
3、 Research and development of key technologies for CPI optical films
3.1 Molecular Structure Design and Modification Technology
1. Introduction of Fluorine Containing Groups: Cleverly introducing fluorine containing groups (such as trifluoromethyl CF ∝) into polyimide molecular chains, leveraging the strong electronegativity and small atomic radius of fluorine atoms, disrupts the regular arrangement of molecular chains, weakens intermolecular forces, and reduces the formation of charge transfer complexes (CTCs) between molecular chains, thereby significantly improving optical transparency. Experiments have shown that introducing 15% -20% fluorinated monomers can increase the transmittance of CPI films from 85% to 92%, and reduce the dielectric constant of the material, meeting the requirements of high-frequency signal transmission.
2. Adipose ring structure construction: Adipose ring dianhydride (such as hexafluoroisopropyldianhydride 6FDA) or diamine monomer is integrated into the polyimide molecular structure, utilizing the rigidity and non planarity of the lipid ring to suppress the tight stacking of molecular chains, further improving optical transparency and thermal stability. Research has found that CPI membranes with an aliphatic ring structure have a glass transition temperature (Tg) of over 350 ℃ and a thermal decomposition temperature (Td) exceeding 500 ℃.
3. Co polymerization modification: Using binary or multicomponent copolymerization methods to polymerize monomers with different properties, achieving complementary performance advantages. For example, copolymerizing ether bonded monomers with rigid aromatic monomers can improve the flexibility and processing performance of film materials while ensuring optical properties.
3.2 Optimization of Preparation Process
1. Solution casting process: The synthesized polyamide acid (PAA) solution is uniformly coated on a smooth substrate through a casting machine, and then dried, imidized, and other processes to prepare CPI film. During the casting process, it is necessary to accurately control parameters such as solution concentration, coating speed, drying temperature and time to ensure film thickness uniformity (thickness deviation ≤± 2%) and surface flatness. For example, high solution concentration can cause difficulties in casting and wrinkles on the film surface; If the concentration is too low, the film layer will be too thin, which is prone to defects such as pinholes.
2. Bi directional stretching process: To improve the mechanical properties and optical uniformity of CPI film, bi directional stretching process can be used. Under specific temperature and stretching rate, the preliminarily formed CPI film is stretched horizontally and vertically to promote the alignment of molecular chains along the stretching direction. Research has shown that appropriate biaxial stretching (with a stretching ratio of 2-3 times) can increase the tensile strength of film materials by 30% -50% and reduce haze by 20% -30%. However, during the stretching process, attention should be paid to avoiding excessive stretching that may cause the film material to break or the optical properties to decrease.
3. Coating composite process: Coating functional coatings (such as scratch resistant and wear-resistant coatings, anti-static coatings, water vapor barrier coatings, etc.) on the surface of CPI film can further optimize the performance of the film material. By using precision coating techniques such as slit coating and gravure coating, the coating thickness is controlled to nanometer level accuracy, ensuring coating uniformity and stability. For example, a scratch resistant and wear-resistant coating composed of silicon dioxide (SiO ₂) nanoparticles and organic resin can increase the surface hardness of the film material to over 3H, effectively resisting external scratches.
3.3 Surface modification technology
1. Plasma treatment: Using high-energy particles in plasma to bombard the surface of CPI film, introducing polar groups (such as hydroxyl OH, carboxyl COOH) to increase the surface energy of the film material and enhance its adhesion performance with other materials (such as adhesives and barrier layers). At the same time, plasma treatment can also improve the roughness and chemical activity of the membrane surface, promoting the uniform coating of functional coatings.
2. UV curing modification: A UV curing coating containing a photoinitiator and an active monomer is applied to the surface of CPI film. After UV irradiation, the active monomer undergoes polymerization reaction, forming a dense cross-linked network structure on the film surface. This method can improve the surface hardness, wear resistance, and chemical corrosion resistance of membrane materials, and has a fast curing speed, making it suitable for industrial production.
4、 The latest research hotspots of AR anti reflective film, hydrophobic film and reflective glass, and their implications for the development of CPI optical film
4.1 AR anti reflective film
Recently, the AR film used in the Huawei Mate 70 Pro lens has become a focus of attention. This AR film utilizes advanced nanotechnology to construct a special microstructure on the surface of the lens, with a thickness that precisely matches the wavelength of light. When light is irradiated, the reflected light interferes and cancels each other out, greatly reducing the intensity of the reflected light and significantly increasing the light transmittance. The reflectivity can be reduced to below 1%, which is 10% -15% higher than that of ordinary mobile phone lenses. This technical approach can be borrowed from the development of CPI optical films. By designing nanoscale structures on the surface of CPI films, the interference effect on different wavelengths of light can be optimized, further improving the transmittance of CPI films in the visible light range, reducing display interference caused by reflection, and enhancing the display clarity and color vividness of foldable OLED displays.
4.2 Hydrophobic membrane
In the research of superhydrophobic photothermal film, the intelligent polymer material team of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, has prepared Janus film (SPCM) with superhydrophobic and photothermal properties. This film is formed by partially embedding candle ash particles into a transparent elastic thin film, forming a layered nanostructure with superhydrophobic properties (contact angle~159.7 °) and good solar thermal conversion ability (up to~68 ° C under one sunlight). For CPI optical films, water vapor erosion is often encountered in foldable OLED display applications. The preparation method of this superhydrophobic film can be referred to, and a similar microstructure can be constructed on the surface of CPI film or hydrophobic groups can be introduced to enhance its waterproof and vapor permeation ability, prevent water vapor from damaging the internal organic light-emitting layer and other components, and improve the reliability and service life of the display screen in humid environments.
4.3 Reflective Glass
Silver plated polymer reflector, as an innovative form of reflective glass, forms a high reflectivity reflective surface by silver plating and multi-layer protective film on one side of a polymer film with high transmittance and strong weather resistance. It can be attached to a curved substrate to make a curved reflector. The technical means of improving reflectivity and substrate bonding stability in its preparation process provide reference for how CPI optical films can enhance interface stability and improve optical performance stability when combined with other components. For example, when combining CPI optical film with packaging glass and other components of OLED display screens, reference can be made to the film layer design and composite process of silver plated polymer mirrors to optimize the interface adhesion between CPI film and other materials, ensuring that there is no separation between layers during the folding process and maintaining good optical performance.
5、 Challenges and Solutions Faced by Research and Development
5.1 Cost control challenges
The raw materials for CPI optical films, especially high-performance modified monomers, are expensive and the preparation process is complex, resulting in high product costs. The solution includes vigorously developing low-cost modified monomers and raw materials; Continuously optimizing the preparation process, improving production efficiency, reducing energy consumption and raw material loss; Realize cost sharing through large-scale production. In addition, when drawing on AR anti reflective film, hydrophobic film, and reflective glass technology, it is preferred to choose cost-effective technology routes, such as the PVG technology developed by the Southeast University team to prepare AR glasses optical films, which uses wet coating and holographic exposure processes to significantly reduce costs. This process idea can be attempted to be applied to the preparation of CPI optical films.
5.2 Stability of Mass Production Process
In the large-scale production process, it is difficult to ensure the consistency of performance and process stability of CPI optical films. Further optimization of production equipment and process parameter control is needed, and an online monitoring system should be introduced to monitor the optical properties, thickness, surface quality, and other indicators of film materials in real time; Establish a comprehensive quality control system to ensure the stability and reliability of product quality. At the same time, referring to the experience of other membrane materials in mass production, such as precise control of temperature, pressure and other parameters in the industrial production of hydrophobic membranes, to achieve stable reproduction of superhydrophobic performance, relevant experience will be applied to optimize the mass production process of CPI optical membranes.
5.3 Compatibility with other materials
During the assembly process of foldable OLED displays, CPI optical films need to work in conjunction with various materials such as organic light-emitting layers, electrode materials, and packaging materials. Compatibility issues between materials may affect the overall performance of the display screen. By conducting research on interfacial compatibility between materials, optimizing material formulations and surface treatment processes, enhancing the bonding strength and chemical stability between materials. For example, the technology of interface treatment when composite reflective glass with other components can be applied to the composite of CPI optical film with other components of OLED display screen to enhance overall compatibility.
6、 Conclusion and Prospect
The key technology research and development of CPI optical film for foldable OLED display screens, combined with the latest hotspots of AR anti reflective film, hydrophobic film, and reflective glass, is the core task to promote the upgrading of the display industry. Through in-depth research on the molecular structure design, preparation process optimization, surface modification and other related technologies of CPI optical films, as well as the reference and integration of technologies from other related fields, the comprehensive performance of CPI optical films can be effectively improved to meet the application needs of foldable OLED displays. Although there are still challenges in cost control, mass production process stability, and material compatibility, with the increase of scientific research investment and continuous technological innovation, CPI optical films will develop towards higher performance, lower cost, and more environmentally friendly directions. In the future, it is expected to further explore the synergistic potential of AR anti reflective film, hydrophobic film, and reflective glass technology with CPI optical film research and development through cross disciplinary cooperation, laying a solid foundation for the widespread application of foldable OLED displays and the vigorous development of the display industry.
