Liquid crystal printing principle, process and use
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Liquid crystal printing is a printing method in which a liquid crystal-incorporated ink is used to make a printed product have a reversible reaction or an irreversible reaction.
Principle of liquid crystal printing
Liquid crystal is an intermediate state exhibited by certain organic substances within a certain temperature range. In this state, due to the special orientation of the molecular arrangement, the molecular motion also has a specific law, so that the liquid crystal has both fluidity and surface tension of the liquid, and exhibits optical properties of some crystals, such as a temperature higher than the upper limit of the temperature of the liquid crystal phase. The liquid crystal becomes an ordinary transparent liquid, and loses optical properties. If the temperature is lower than the lower limit of the temperature of the liquid crystal phase, the liquid crystal becomes an ordinary crystal and loses fluidity. Nematic liquid crystals in liquid crystals are used as display materials in the electronics industry, and cholesteric liquid crystals can be used for temperature indication.
The liquid crystal is irradiated by natural light and artificial white light, and the color light of a certain wavelength enhances the reflection due to the refraction phenomenon, and as the temperature rises, the color from the long wavelength changes to the color of the short wavelength (ie, changes from red to green to cyan), and the liquid crystal thereof The color forming mechanism is formed by the liquid crystal selectively reflecting light of a specific wavelength. The liquid crystal must be printed on a black or dark background. Now the liquid crystal can react between -10 °C and +70 °C with an accuracy of +/- 0.5 °C.
When printing, the liquid crystal is sealed in the microcapsules, and the ink is printed, which can prevent the pollution of the liquid crystal itself, can protect the skin, prolong the protection life, facilitate long-term preservation, and improve the printability. The amount of liquid crystal added to the ink is 40% of the total ink amount, and the diameter of the capsule is between 10 and 30 microns.
Composition of liquid crystal ink
A compound having crystallinity is added to the liquid crystal ink. Under the influence of weak current and temperature, the liquid crystal displays light and dark patterns and colors due to changes in the crystal lattice. Packages printed with this ink can react with changes in temperature, and if used for printing calendars, will vary in color as the temperature of the seasons changes.
The liquid crystal ink is mainly composed of a water-soluble resin, a liquid crystal capsule, an auxiliary agent, a binder, and an antifoaming agent.
Printing art
Both screen printing and gravure printing can be used. When liquid crystal ink is used, the color temperature range should be determined according to the purpose of use and the durability of the liquid crystal should be considered according to the use.
The key to liquid crystal printing technology is to be able to display bright liquid crystals at different temperatures, and to form a series in the color gamut value; the fabrication technology of liquid crystal microcapsules is also the key. The microcapsules should be small and uniform, the capsules are transparent and thin, and the inks required to be solvent-resistant, stable, reliable, and long-lived; in the printing process, the liquid crystal microcapsules are not crushed; the surface is covered with a protective film. Most of the liquid crystal ink printing is screen printing, and the screen printing process is as follows:
A. Screen printing plate
Generally, the mesh can be made of nylon mesh or nylon mesh. If the amount of ink required on the plate is large, a stainless steel mesh plate can be used. The mesh size is generally 100-175 mesh. The diameter of the microcapsules is generally between 10 and 30 μm. The plate is made of a water-resistant emulsion.
B. Screen printing liquid crystal ink requirements
In terms of its function, liquid crystal printing is mainly used for two aspects of temperature and humidity and color change.
1. To have enough ink on the plate, it is best not to refill the ink in the middle to prevent foaming.
2. Since the microcapsule ink is easy to block the plate, when the net is found in the printing and foaming, it should be stopped immediately and washed thoroughly with alcohol water.
3. When printing, it should be noted that the ink is even and sufficient, and the amount of ink on the printed substrate is insufficient, which will affect the color development effect.
4. The printing pressure should not be too large to prevent the liquid crystal capsule from breaking, which will also reduce the coloring effect.
5. The background color should be ink or dark tone. The base color may be a water-soluble ink or an organic solvent ink. It must be sufficiently dried after printing, and the organic film must not remain in the ink film.
6. The ink layer should be smooth and smooth, and the thickness of the ink layer should be controlled to 15-35μm. If the ink layer is not flat or too thin, the color effect will be reduced.
7. The printing sequence can first print the ground color with black (or dark color) ink, and then print the desired pattern with liquid crystal ink. It can also print the thin base with liquid crystal ink first, and then print with water-based ink (or deep color) ink. The negative pattern is covered to improve the color development.
8. The surface of the printed matter is preferably coated with a varnish or a protective film to protect the gloss of the ink layer and improve wear resistance and temperature resistance.
C. Post-press drying
1. The drying method is preferably natural drying, and it can also be dried by warm air at around 40 °C.
2. Do not press heavily when the printed materials are stacked, and it is not advisable to add pressure when cutting.
The characteristics and main uses of liquid crystal printing
The liquid crystal ink does not use pigments, and its viscosity is between 4000 and 6000 centipoise. It has the characteristics of small thixotropy and good leveling. It is mainly used for the printing of substrates with temperature display requirements such as thermometers, automatic temperature recorders, and detectors. It is also widely used in advertisements, trademark labels, arts and crafts and other decorative items that require color change. It is printed as a calendar picture, and the picture will change into different colors at different temperatures throughout the year; On hot products, you can understand the quality changes of the contents according to the change of the pattern color. If the bright picture on the candy wrapper becomes dark, you should eat the candy as soon as possible; LCD printing can also be used as a reminder card to save the temperature. LCD printing paper tape is also used abroad as a one-time body temperature tester. Over time, LCD printing will have a wider range of applications.