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Preliminary study on the core material of liquid crystal pressure sensitive blackboard

Browse:160         Release time:2020/4/15

1. Overview

In the course of human development, writing and recording are the hallmarks of human civilization, and paper has continued for nearly 2000 years, and is still an indispensable writing material in life and work until now. With the intensification of a series of problems such as the greenhouse effect and paperless dependence on electrical energy, mankind urgently needs alternative new environmentally friendly materials for continuous communication, writing and recording. Research on reusable materials has always been at the forefront of global R & D innovation, product innovation, and market innovation. The introduction of new technologies, new processes, and new processes has become the focus of research investment.

Under this opportunity, the research and innovation of flexible liquid crystal writing film came into being. It can solve the problems of complicated combination, repeated procurement, pollution consumption and other problems during the use of traditional writing materials such as pen, ink, paper; Combined with the circuit for control refresh, it can repeat display and writing without backlight, and deeply simulate the display and writing effect of copy paper, which truly preserves the traditional writing experience while being greener and more environmentally friendly.

2. The origin and status of the technology
The flexible liquid crystal writing film is made by a special process using the characteristics of bistable cholesteric liquid crystal. Bistable cholesteric liquid crystal is similar to electrophoresis technology and has the characteristics of bistable display, that is, it can display without electricity when the picture is stationary, so it has the same power saving characteristics as electrophoresis technology; bistable cholesteric The liquid crystal is a reflective display, which can be displayed with the help of daily external lighting environment. It is still visible under strong light and there is no problem of reverse white; bistable cholesteric liquid crystal does not have liquid crystallinity, but when its hydroxyl group is replaced by halogen Halogen-forming compounds and cholesterol derived from esterification reaction with carbonic acid (or fatty acid); bistable cholesteric liquid crystal material has a special helical structure, which induces selective light scattering, optical rotation and circular polarized dichroism, can use bistable The applied voltage, gas adsorption, and temperature of the cholesteric liquid crystal material cause color changes. Therefore, the bi-stable cholesteric liquid crystal display allows the viewer to see the comfort of the liquid crystal display as if it were paper.

As early as the 1960s, when liquid crystal displays were first invented, there were many scholars and laboratories in the United States and Europe engaged in cholesteric liquid crystal related research; in the early 1980s, phase-change mode cholesteric liquid crystal displays began to appear; American research institutes began to use the bistable characteristics of cholesteric phase to conduct research on reflective cholesteric liquid crystal display; at the beginning of the 21st century, domestic research and development of bistable cholesteric liquid crystal technology began in China. It was launched on the market in 2015. At present, the scale of the domestic industry has surpassed that of foreign countries, the overall technical level has been flat, and some application areas are still in a leading position.

The flexible LCD writing film displays and writes without power consumption, refreshes short-term power consumption, and can be partially modified. The matching circuit design involves inverter boost and waveform control, and partial erasure involves continuous waveform control and output. The current research and development focus includes:
• Improve the writing display effect and service life of materials in extreme environments such as high and low temperature, strong irradiation, etc ;
• Improve the display and writing contrast, solve the key function of partial erasure, and meet the needs of more usage scenarios;
• Simplify processes, reduce costs, and continue to explore, summarize, and transform large-scale production processes and processes;
• Develop wider and deeper application fields and products, and target a wider consumer group.

3. Technical principles
3.1 Utilize the characteristics of bistable cholesteric liquid crystal, mix liquid crystal with polymer to form liquid crystal polymer, then fill the liquid crystal polymer into ITO film, and make a flexible liquid crystal writing film by UV curing.

3.2 Material structure:

① Flexible substrate: The flexible substrate is made of high-transmittance PET material with a thickness of only 0.125mm, which has good bending resistance. Due to the need for writing, the front of the substrate is specially processed by atomization, anti-glare, hardening and scratch resistance, which has less reflection and scratch resistance. The reflective layer of the flexible substrate is a black PET substrate, which is used to improve the display contrast and needs to have excellent bending resistance.
② Transparent conductive layer: the transparent conductive layer is the ITO coating, the metal oxide is coated on the flexible substrate by the magnetron sputtering method, so that the flexible substrate has conductivity, the coating thickness is only micron level, and the conductivity is good And transparent, the sunlight transmittance is more than 80%.
③ Bistable cholesteric liquid crystal: Cholesteric liquid crystal has unique optical properties. The liquid crystal molecules are arranged in parallel in the plane, but the molecular direction in the adjacent plane changes slightly, and the spiral changes along the normal direction of the plane. About 300NM, it has extremely strong optical rotation, obvious circular dichroism and selective reflection on wavelength, which is the theoretical basis of liquid crystal writing film.
④ Cured polymer: The polymer is made up of UV glue and liquid crystal. During the UV curing process, the UV glue is cured to form a network structure to divide the liquid crystal into small droplets uniformly and bond the substrate up and down. Encapsulate small liquid crystal droplets, the cured polymer plays the role of wrapping liquid crystal and bonding substrate.

3.3 Writing and display principles:
When writing on a flexible liquid crystal writing film, the flexible substrate is subjected to the pressure of the pen tip, and the pressure is transmitted to the liquid crystal polymer to change the arrangement structure of the liquid crystal. The changed part reflects light of a specific wavelength and appears as a handwriting of a specific color; Under the circumstances, the liquid crystal structure will not rebound and the handwriting displayed will not disappear; when an electric field is applied, the liquid crystal structure will be neatly arranged under the action of the electric field, showing that the handwriting disappears.

3.4 Process flow:

3.5 Principle of partial erasure:
After the writing changes the arrangement of the liquid crystal, a weak electric field is applied to the liquid crystal structure to reduce the anchoring force of the liquid crystal structure at the writing position to maintain the current status. At this time, the external force is applied to restore the liquid crystal structure to an approximately neatly arranged state. Partially erased.

4. Material characteristics and innovation
No power consumption, no blue light, no flicker, no consumables for writing and display, no ink and dust pollution during the writing process, and no harm to human health caused by ink, dust and radiation in various writing scenarios.
Repeated use and even inkless printing, the number of repeated use of a piece of flexible liquid crystal writing material is about 100,000 times of sheets of the same area, which can greatly reduce paper consumption in writing scenarios, energy saving and environmental protection
Material application areas include: educational equipment, commercial display displays, personal consumption, public services, etc.
Existing application products: LCD pressure-sensitive blackboard, smart blackboard, LCD handwriting board, drawing board, notice board, label board, etc.

5. Key technologies
•Polymer liquid crystal ratio: The compatibility of polymer and liquid crystal is the core technology for making flexible liquid crystal writing films. Different compatibility will affect the contrast and display color of the writing film. The ratio of liquid crystal to polymer, the choice and ratio of polymer monomers affect the scratch resistance of the diaphragm, the stability of the writing handwriting, the removal effect, and the weather resistance of the writing film.
•Partial erasure: It is the key technology for the application of flexible liquid crystal writing materials in the field of educational equipment. The feasible technical solutions include heating and circuit control. Heating and erasing have high energy consumption, high temperature, and irreversible damage to the diaphragm. It is limited to the principle of demonstration and does not have the prospect of mass production. Circuit control requires liquid crystal deployment and drive circuit design, which is constrained by many materials.
•UV phase separation and curing: It is the key to the liquid crystal writing film manufacturing process, which directly affects the feasibility of writing film production, and is related to the film's adhesive strength, bending resistance, and writing film's aging resistance. It is better to solve the adhesion strength and aging resistance of the film, but it needs to be improved for the ultimate bending.
•Preparation process consistency control: It involves material compatibility, preparation temperature control, filling pressure consistency, liquid crystal material viscosity control, material transmission speed control, etc. The existing technology can achieve mass production, but the consistency of the prepared product is slightly There are differences, which have an impact on yield.
•Cutting: The short-circuit solution for cutting small gap conductive film materials is the key to solving the production and application of liquid crystal writing films. Although the writing film has the advantages of arbitrary cutting, the characteristics and insulation interval of the ITO film are in the micron level How to solve the short circuit caused by the contact between the two layers of ITO film during cutting is the key to the film cutting.

In recent years, the flexible liquid crystal writing film as a new type of low-power writing panel material has been widely used in personal consumption fields (writing tablets, calculators, digital accessories, etc.) At present, with the continuous updating of technology, related application products based on large-size liquid crystal pressure-sensitive blackboards are entering the field of education, which will definitely bring healthy and intelligent changes to educational equipment.


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