In the field of inks, liquid crystal temperature change materials are currently the most interesting. It mainly causes changes in optical properties through changes in the crystal lattice. 1. Composition and characteristics of LCD Generally, a solid substance becomes a liquid when heated to a melting point. However, some substances with a special molecular structure do not directly change from a solid to a liquid, but first go through an intermediate state. This intermediate state, which is not an ordinary solid, liquid and gas, is called the fourth state of matter. Its appearance is a fluid turbid liquid, with the fluidity and surface tension of the liquid, and at the same time unique to crystals. Optically anisotropic birefringence. This kind of substance that can combine the characteristics of both liquid and crystal in a certain temperature range is called liquid crystal, which is generally divided into nematic, smectic, cholesteric and atypical. Nematic. Observation with a polarizing microscope reveals many filament-like optical patterns. This type of liquid crystal still shows positive refraction, its viscosity is small, and it is rich in fluidity. Smectic. The rod-shaped molecules form a layered structure, and the molecules are arranged parallel to each other. The force between the arranged molecular layers is relatively weak, and it is easy to slide between each other, so smectic liquid crystals exhibit two-dimensional fluid properties and have high viscosity characteristics. Cholesteric. Most of the compounds are derived from cholesterol, and the liquid crystal forms a spiral structure as a whole, with birefringence, and is the main type of liquid crystal ink. Shaped LCD. It is divided into reentrant liquid crystal and disc type liquid crystal. The reentrant liquid crystal is the liquid crystal with the same phase again during the phase transition. Disc-type liquid crystals are liquid crystals whose molecular structure is disc-shaped. Liquid crystal is an intermediate state that certain organic substances exhibit in a certain temperature range. In this state, due to the special orientation of the molecules and the specific movement of the molecules, if the humidity is higher than the upper limit of the temperature of the liquid crystal phase, the liquid crystal becomes an ordinary transparent liquid and loses its optical properties, such as the temperature is lower than the lower limit of the temperature of the liquid crystal phase , It will become an ordinary crystal and lose its fluidity. 2. Composition and process of liquid crystal ink Liquid crystal ink originated in the United States in the 1970s. Later, as its technology gradually matured and the printing market continued to expand, liquid crystal ink printing developed rapidly in the United States, Japan, and other countries, and its application scope was also expanding. From the perspective of production method, liquid crystal ink belongs to the type of microcapsule structure ink. From the perspective of application, it belongs to the category of thermosensitive thermochromic ink on anti-counterfeiting printing ink. However, from the perspective of the characteristics of liquid crystal ink, it mainly uses the characteristics of liquid crystal to change temperature. . The liquid crystal ink is mainly composed of water-soluble resin, liquid crystal capsules, additives, linking materials, defoamers, and is prepared by dispersing liquid crystals and additives enclosed in microcapsules in the linking materials. It is based on the temperature of the liquid crystal in the ink layer to cause the change of the direction of the ordered molecules, so as to selectively reflect the visible light of a specific wavelength, and absorb the light of other wavelengths. Graphic. When the liquid crystal is irradiated by natural light, artificial white light, and colored light of a certain wavelength, the reflection is strengthened due to the refraction phenomenon. As the temperature rises, the color of the long wavelength changes from the color of the long wavelength to the color of the short wavelength (that is, changes according to red-green-cyan) The color-forming mechanism of the liquid crystal is due to the liquid crystal selectively reflecting light of a specific wavelength. The liquid crystal must be printed on a black or dark background, and now the liquid crystal can reflect -100 to +700 degrees Celsius. The accuracy is 0.50 degrees Celsius. Because the arrangement of liquid crystal molecules is not as strong as the crystal structure, it is easily affected by external stimuli such as electric fields, magnetic fields, temperatures, stresses, and adsorbed impurities, which changes its various optical properties. Under the liquid crystal ink, this characteristic of liquid crystal is applied. 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 polluted by its chemical substances and to ensure the color rendering effect, it is necessary to wrap the liquid crystal in microsphere-shaped capsules, and then mix with the connecting material to make the so-called 'microcapsule' type ink. The microcapsules in liquid crystal ink are different from the microcapsules in foaming and fragrance ink. The microcapsules of the foaming ink are foaming agents (or low boiling point liquefied gas). After heating, the foaming agent in the microcapsules emits a large amount of gas (or low boiling point liquefied gas swelling) to increase the volume of the microcapsules and foam. effect. In addition, the foaming ink is added with pigments, printed into various colors of foaming graphics and text, and has the dual function of foaming and coloring. The characteristics of microcapsules in scented inks are that they are not crushed and damaged during printing. After printing into products, they are required to be pressed and torn with nails, pencils, etc. to make them easily broken, so as to achieve the purpose of emitting fragrance. The microcapsules in the liquid crystal ink fix the liquid crystal in the capsule in the ink layer through the connecting material. [next] 3. The printing technical requirements of liquid crystal ink The liquid crystal ink can be screen or gravure printing, but screen printing is better. The key to the liquid crystal printing technology is to be able to display liquid crystals with bright colors at different temperatures, and to form a series in the color temperature domain value. The liquid crystal microcapsules should be small and uniform. The wall of the capsule is transparent and thin. Incorporating ink requires stable, reliable and long life of solvent resistance. During the printing process, it is necessary to ensure that the liquid crystal microcapsules are not crushed. 1. Plate making The screen mesh number is preferably 100-175 mesh, which is selected according to the requirements of the ink microcapsule diameter and ink layer thickness. Since the silk-screen liquid crystal microcapsule ink is water-soluble, a photosensitive resin with strong water resistance should be used when making plates. 2. Printing Pay attention to the following points when printing liquid crystal ink: (1) The printing plate should be filled with sufficient ink at one time, and the ink supply should be uniform. It is best not to refill the ink halfway to prevent blistering; (2) When the network is blocked or blistered during printing, it should be immediately shut down to remove and thoroughly cleaned with alcoholic water; (3) The printing pressure should be moderate. Low pressure, insufficient ink on the printed substrate; excessive pressure will cause the liquid crystal capsules to rupture, affecting the color development effect; (4) In order to make the printed part have obvious color, the background color of the printing should be black or dark. The bottom layer can be used with water-soluble ink or organic solvent ink. The printing method can be screen printing, offset printing, gravure printing. When using organic solvent ink, it must be fully dried after printing, and the ink film must not leave organic solvent, otherwise it will react with the liquid crystal and 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. You can print the background color with black (or dark color) ink first, and then print the desired pattern with liquid crystal ink; you can also print the full background with liquid crystal ink, and then print the necessary negative pattern with water-soluble black (or dark color) ink. cover. 3. Post-press processing (1) The drying method is preferably natural drying, and it can also be dried by hot air at about 40 degrees Celsius, and must not be accelerated and heated at a high temperature for rapid drying; (2) Do not press heavily when stacking printed materials. After drying, try not to stack too much overlap, too much local pressure should not be added when cutting, so as not to crush the liquid crystal microcapsules; (3) When using liquid crystal ink, the color change temperature range should be determined according to the purpose of use and the durability of the liquid crystal should be considered according to the application. In order to improve the abrasion resistance, temperature resistance, gas barrier and maintain the gloss of the ink layer surface, the surface can be coated with varnish or with a protective film. 4. The market prospect of liquid crystal ink Liquid crystal ink is a new type of ink material with great vitality and yet to be developed. Its printing application range is very wide, in terms of its function, it is mainly used for temperature display and color development. Temperature display: mainly printed materials with temperature display requirements such as thermometer, body temperature, automatic temperature recorder, testing machinery and testing instruments. At present, foreign countries also use liquid crystal printing paper tape as a disposable temperature tester. Hair color changes. Decorations such as advertisements, trademark labels, arts and crafts, commemorative badges, jewelry, and anti-counterfeiting printed products, etc. If it is used for printing calendar pictures, it will show different colors with the temperature changes of the four seasons; if it is printed as a children's picture book, it can change the color of the picture with the children's pointing and touching; printed as a trademark, wrapping paper, etc., and affixed to some products that are afraid of heat According to the change of the color of the pattern, you can understand the quality change of the contents. For example, if the bright picture on the candy wrapping paper becomes gray, you should eat the candy as soon as possible, otherwise it will melt. The liquid crystal pattern is printed on the container, which can display the humidity of the contents in the container. On the large items that are expensive, afraid of heat and cold, liquid crystal printing can be used as a reminder card to save the temperature. As for the use of liquid crystal ink as anti-counterfeiting ink, it mainly uses the characteristics of liquid crystal temperature-sensing discoloration. Now it has played a great role in the field of anti-counterfeiting packaging and decoration, and its anti-counterfeiting effect is also ideal.
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