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1. The composition and characteristics of liquid crystal
Generally, a solid substance is heated to a melting point to become a liquid, but some substances having a special molecular structure do not directly change from a solid to a liquid, but first pass through an intermediate state. This intermediate state, which is not an ordinary solid, liquid or gas, is called the fourth state of matter, and its appearance is a fluid turbid liquid with liquid fluidity and surface tension, and at the same time is unique to crystals. Birefringence of optical anisotropy. Such a substance capable of combining both liquid and crystal properties in a certain temperature range is called a liquid crystal, and is generally classified into a nematic type, a smectic type, a cholesteric type, and a heterotypic type.
Nematic type. When observed with a polarizing microscope, many filament-like optical patterns can be seen. Such a liquid crystal still exhibits positive refractive properties, and its viscosity is small and fluid.
Near crystal form. The rod-like molecules form a layered structure, and the molecules are arranged in parallel with each other. The lining liquid crystal exhibits two-dimensional fluid properties and high viscosity characteristics.
Cholesteric type. Most of them are compounds derived from cholesterol, and the liquid crystal forms a spiral structure as a whole, and has birefringence properties, and is a main type of liquid crystal ink.
Shaped liquid crystal. It is divided into re-enter liquid crystal and disc type liquid crystal. Re-entering the liquid crystal is a liquid crystal in which the same phase appears again during the phase change. The disc-shaped liquid crystal is a liquid crystal whose molecular structure is a disk shape.
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. For example, the humidity is higher than the upper limit of the temperature of the liquid crystal phase, and the liquid crystal becomes a common transparent liquid, losing optical properties, such as a temperature lower than the lower limit of the liquid crystal phase temperature. It will become a normal crystal and lose its mobility.
2. Composition and process of liquid crystal ink
Liquid crystal inks originated in the United States in the 1970s. Later, with the gradual maturity of its technology and the continuous expansion of the printing market, liquid crystal ink printing has developed rapidly in the United States, Japan and other countries, and its application range has also expanded. From the point of view of the production method, the liquid crystal ink belongs to the microcapsule structure ink type. From the application point of view, it belongs to the temperature-sensitive variable ink category of the anti-counterfeiting printing ink. However, from the characteristics of the liquid crystal ink, the liquid crystal temperature is mainly used for discoloration. .
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, and is prepared by dispersing a liquid crystal and an auxiliary agent enclosed in a microcapsule in a binder. It is caused by the change of the direction of the ordered molecules in the temperature of the liquid crystal in the ink layer, so as to selectively reflect the visible light of a specific wavelength, and absorb the optical characteristics of the light of other wavelengths to exhibit a color change instead of the color of the ink layer. Graphic.
When the liquid crystal is irradiated by natural light and artificial white light, and a certain wavelength of color light, the reflection is enhanced by the refraction phenomenon, and as the temperature rises, the color of the long wavelength changes to the color of the short wavelength (ie, changes according to red-green-cyan). The color forming mechanism of the liquid crystal is due to the fact that the liquid crystal selectively reflects light of a specific wavelength.
The LCD must be printed on a black or dark background, and now the LCD can reflect -100 to +700 degrees Celsius. The accuracy is 0.50 degrees Celsius. Since the arrangement of the liquid crystal molecules is not as strong as the crystal structure, it is easily affected by external stimuli such as electric field, magnetic field, temperature, stress, and adsorption impurities, and various optical properties are changed. This characteristic of liquid crystal is applied under liquid crystal ink.
Since liquid crystal is an organic compound between liquid and crystal, it cannot be directly added to the binder like ordinary ink. In order to prevent the liquid crystal from being contaminated by the chemical substance to ensure the coloring effect, the liquid crystal is wrapped in the microsphere capsule, and then mixed with the connecting material to prepare a so-called microcapsule type ink. The microcapsules in the liquid crystal ink are different from the microcapsules in the foaming and flavoring inks.
The microcapsules of the foaming ink are foaming agents (or low-boiling liquefied gases). After heating, the foaming agent in the microcapsules releases a large amount of gas (or low-boiling liquefied gas swells) to increase the volume of the microcapsules to foam. effect. In addition, pigments are added to the foaming ink, which are printed into foamed graphics of various colors, and have the dual functions of foaming and coloring.
The characteristics of the microcapsules in the scented ink are not crushed and damaged during printing. After printing into a product, it is required to be pressed and torn with nails, pencils, etc. to make it easy to break, thereby achieving the purpose of emitting fragrance. The microcapsules in the liquid crystal ink are used to adhere the liquid crystal in the capsule to the ink layer through the connecting material.
3. Printing technology requirements for liquid crystal inks
The liquid crystal ink can be printed by screen or gravure, but it is better to use silk screen printing. The key to liquid crystal printing technology is the ability to display bright colors of liquid crystal at different temperatures, and to form a series in the color temperature domain. The liquid crystal microcapsules should be small and uniform, the capsule wall is transparent and thin, and the ink is mixed, and the solvent resistance is required to be stable, reliable, and long-lived; the liquid crystal microcapsules are not crushed during the printing process; the surface is covered with a protective film.
Plate making
The mesh size is preferably 100-175 mesh, and is selected according to the requirements of the ink microcapsule diameter and the thickness of the ink layer. Since the screen liquid crystal microcapsule ink is water-soluble, a photosensitive resin having high water resistance should be used for plate making.
2. Printing
Pay attention to the following points when printing LCD inks:
(1) The printing plate should be filled with enough ink at a time, and the ink should be evenly distributed. It is best not to refill the ink in the middle to prevent foaming;
(2) When it is found that the net is blocked or foamed during printing, it should be immediately stopped and removed, and thoroughly cleaned with alcohol water;
(3) The printing pressure should be moderate. The pressure is small, the amount of ink on the printed substrate is insufficient; if the pressure is too large, the liquid crystal capsule will be broken, which will affect the color development effect;
(4) In order to make the printed part bright, the printed background should be black or dark. Water-soluble inks can be used for the bottom layer, and organic solvent inks can also be used. Printing methods can be screen printing, offset printing, gravure printing. When using an organic solvent ink, the ink must be sufficiently dried after printing, and the ink film must not remain organic solvent, otherwise it will react with the liquid crystal to affect the color development effect;
(5) It should be noted that the ink layer should be smooth and smooth, and the thickness of the ink layer should be controlled to 15~35μm to improve the color development effect;
(6) Printing color sequence arrangement. The background color can be printed first with black (or dark color) ink, and then the desired pattern can be printed with liquid crystal ink. It can also be printed with liquid crystal ink and then printed with the water-soluble black (or deep color) ink. cover.
3. Postpress processing
(1) The drying method is preferably natural drying, or it can be dried by hot air at about 40 degrees Celsius, and it is not allowed to accelerate and heat at a high temperature;
(2) Do not stress when the printed materials are stacked. After drying, try not to stack too much overlap, and it is not advisable to add excessive partial pressure when cutting, so as to prevent the liquid crystal microcapsules from being crushed;
(3) When using a liquid crystal ink, the color temperature range should be determined according to the purpose of use and the durability of the liquid crystal should be considered depending on the application. In order to improve the surface wear resistance, temperature resistance, gas barrier properties and maintain the gloss of the ink layer, the surface may be coated with a varnish or a protective film.
Fourth, the market prospects of liquid crystal inks
Liquid crystal ink is a new type of ink material that is very vital and needs to be developed. Its printing application range is very wide, in terms of its function, it is mainly used for two directions of temperature display and color change.
Temperature display: mainly the thermometer, body temperature, temperature automatic recorder, testing machinery and testing equipment and other substrates with temperature display requirements. At present, LCD printing paper tape is also used abroad as a one-time body temperature tester.
Hair color changes. Advertising, trademark labels, arts and crafts, medals, jewelry and other decorative and anti-counterfeiting printing products. For printing calendar drawings, different colors will appear as the temperature of the four seasons changes;
If printed in a children's album, you can change the color of the picture with the child's touch; print the trademark, wrapping paper, etc., and stick it on some products that are afraid of heat. You can understand the quality changes of the contents according to the change of the color of the pattern, such as candy packaging. If the bright picture on the paper becomes gray, you should eat the candy as soon as possible, otherwise it will melt. Printing a liquid crystal pattern on the container can display the humidity of the contents of the container. On a large item that is expensive, hot, or cold, liquid crystal printing can be used as a reminder card for storing temperature.
As for the use of liquid crystal ink as anti-counterfeiting ink, it mainly uses the characteristics of liquid crystal temperature and discoloration, and now it has played a great role in the field of anti-counterfeiting packaging and decoration, and its anti-counterfeiting effect is also ideal.
In the ink field, liquid crystal temperature change materials are currently of most interest. It mainly causes changes in optical properties through changes in the crystal lattice.
March 09, 2021