In the field of inks, the temperature change materials for liquid crystals are currently of greatest interest to people. It mainly causes changes in optical properties through changes in the crystal lattice.
1. The composition and characteristics of liquid crystals Generally, solids are heated to a melting point and become liquid. However, some substances with a special molecular structure do not directly change from solids to liquids, but first pass through an intermediate state. This kind of intermediate state, which is not an ordinary solid, a liquid, or a gas, is called the fourth state of the substance. Its appearance is a fluid turbid liquid with the fluidity and surface tension of the liquid, and is also unique to crystals. Optically anisotropic birefringence. This substance that can both characterize liquids and crystals within a certain temperature range is called liquid crystal and is generally classified into nematic, smectic, cholesteric, and heteromorphic types.
Nematic. Observed with a polarizing microscope, many filament-like optical patterns can be seen. This liquid crystal still shows positive refraction, its viscosity is small, and it is rich in fluidity.
Smectic type. The rod-like molecules form a layered structure, and the molecules are arranged in parallel to each other. The molecular forces between the arranged molecular layers are relatively weak and easily slide between each other. Therefore, the smectic liquid crystal exhibits two-dimensional fluid properties and has high viscosity characteristics.
Cholesterol type. Most of them are compounds derived from cholesteryl alcohol. The liquid crystal forms a spiral structure as a whole and has a birefringent property. It is the main type of liquid crystal ink.
Shaped liquid crystal. Also divided into re-entry liquid crystal and disc type liquid crystal. Reentry liquid crystals are liquid crystals that again exhibit the same phase during the phase transition. The discotic liquid crystal is a liquid crystal having a discotic molecular structure.
Liquid crystal is an intermediate state that certain organic substances are present in a certain temperature range. In this state, due to the special orientation of molecular alignment, there are also specific laws of molecular motion. 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, losing optical properties such as the temperature lower than the lower limit of the liquid crystal phase temperature It will become ordinary crystals and lose fluidity.
2. Composition and Process of Liquid Crystal Inks Liquid crystal inks originated in the United States in the 1970s. Later, as its technology matured and the printing market continued to expand, liquid crystal ink printing was rapidly developed in the United States, Japan, and other countries, and its application range was also expanding. The liquid crystal ink is a microcapsule structure ink type in terms of production methods. From the application point of view, it belongs to the category of temperature-sensitive thermochromic inks for anti-counterfeit printing inks, but in terms of the characteristics of liquid crystal inks, it mainly uses the characteristics of temperature-sensitive discoloration of liquid crystals. .
The liquid crystal ink is mainly composed of a water-soluble resin, a liquid crystal capsule, an auxiliary agent, a binder, and a defoamer. The liquid crystal ink is prepared by dispersing liquid crystals, additives and the like enclosed in the microcapsules in a binder. It changes the orientation of the aligned molecules by the temperature of the liquid crystal in the ink layer, thereby selectively reflecting the visible light of a specific wavelength, absorbing the optical characteristics of other wavelengths and showing a color change, instead of forming a color with the ink layer pigment. Graphically. When the liquid crystal is irradiated with natural light, artificial white light, and color light of a certain wavelength, reflection is strengthened due to the refraction phenomenon, and as the temperature rises, the color changes from a long wavelength to a short wavelength (i.e., changes in 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 LCD must be printed on a black or dark background, and the LCD can now reflect -100 to +700 degrees Celsius. The accuracy is 0.50 degrees Celsius. Since the alignment of the liquid crystal molecules is not as strong as the crystal structure, it is easily affected by external stimuli such as electric fields, magnetic fields, temperature, stress, and adsorption impurities, causing various optical properties to change. Liquid crystal inks use this characteristic of liquid crystals.
Since the liquid crystal is an organic compound between the liquid and the crystal, it cannot be directly added to the binder like ordinary ink. In order to prevent the liquid crystal from being contaminated by its chemical substances, in order to ensure the color rendering effect, it is necessary to enclose the liquid crystal in microsphere capsules and then mix with the connecting material to make a so-called `microcapsule` type ink. The microcapsules in the liquid crystal ink are different from the microcapsules in the foaming and aroma inks. The microcapsules of the foaming ink are foaming agents (or low-boiling liquefied gases). Upon heating, the foaming agent in the microcapsules emits a large amount of gas (or a low-boiling liquefied gas expander) to increase the volume of the microcapsules to foaming. effect. In addition, pigments are added to the foaming ink, printed into various colored foaming graphics, and have dual functions of foaming coloration. The characteristics of microcapsules in scented inks are that they are not crushed and damaged during printing, but they are required to be easily broken by pressing and tearing with nails, pencils, etc. after printing into products, thereby achieving the purpose of emitting fragrance. The microcapsules in the liquid crystal ink are used to fix the liquid crystal in the capsule in the ink layer through the connecting material.
3. Printing Technology Requirements for Liquid Crystal Inks Liquid crystal inks can be screen or gravure, but silk screens are preferred. The key to liquid crystal printing technology is the ability to display brightly colored liquid crystals at different temperatures, and to make a series of color temperature values. The liquid crystal microcapsules have to be small and uniform, the capsule wall is transparent and thin, and the ink is incorporated, and the solvent resistance is required to be stable, reliable, and long-lived; the liquid crystal microcapsules are also not to be crushed during the printing process, and the surface is covered with a protective film.
1. The number of screen meshes is 100-175 mesh, which is selected according to the requirements of ink microcapsule diameter and ink layer thickness. Since the screen liquid crystal microcapsule ink is water-soluble, the photosensitive resist which has strong water resistance should be used when making the plate.
2. The following points should be noted when printing printing liquid crystal 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 fill the ink in the middle to prevent foaming.
(2) When plugging or blistering is found during printing, it is necessary to stop the machine immediately and thoroughly clean it with alcohol.
(3) The printing pressure should be moderate. The pressure is small and the amount of ink on the printing substrate is insufficient. If the pressure is too high, the liquid crystal capsule will be broken and affect the color effect.
(4) In order to make the color of the printing part obvious, the black color or dark tone should be used for the printing background. The bottom layer can be water-soluble inks, organic solvent inks can also be used. Printing methods available screen printing, offset printing, gravure. When organic solvent ink is used, it must be fully dried after printing. The organic solvent should not remain in the ink film, otherwise it will react with the liquid crystal and affect the color effect.
(5) Pay attention to the smoothness and smoothness of the ink layer. The thickness of the ink layer should be controlled to 15~35μm to improve the color effect.
(6) Printing color sequence arrangement. Can use black (or dark color) ink to print the background color first, then use liquid crystal ink to print the desired pattern; also can use liquid crystal ink to fill the bottom first, and then use water-soluble black (or dark color) ink to print the desired negative pattern. cover.
3. Post-printing treatment (1) The drying method is preferably natural drying. It can also be dried by hot air at about 40 degrees Celsius, and it must not be accelerated and the high temperature of heating must be drastically dried.
(2) Do not press the printed matter when it is stacked. After drying, try not to pile too much overlap, when breaking, etc. should not be too large partial pressure, so as not to be liquid crystal microcapsules are crushed;
(3) When using liquid crystal inks, the temperature range of the discoloration should be determined according to the purpose of use and the durability of the liquid crystal should be considered according to the use. In order to improve the surface wear resistance, temperature resistance, gas barrier and maintain its gloss, the surface can be coated with varnish or protective film.
IV. Market Prospects of LCD Inks LCD ink is a kind of new ink material that is very vital and yet to be developed. The range of its printing applications is very wide, and in terms of its function, it is mainly used in the two major directions of temperature display and hair color change.
Temperature display: Mainly thermometers, body temperature, automatic temperature recorders, testing machines and testing instruments, etc. have temperature display requirements of the substrate. At present, foreign countries also use liquid crystal printing tape as a one-time body temperature tester.
Hair color changes. Advertising, trademark labels, arts and crafts, medals, jewelry and other accessories and security printing products. If used for printing calendar pictures, it will show different colors as the temperature of the four seasons changes; if it is printed into a children's album, it can change the color of the picture with the touch of the child's fingertips; printed trademarks, wrapping paper, etc., are affixed to some products that are afraid of heat , According to the change of pattern color, to understand the quality of the contents of the contents, such as the candy on the bright paper screen becomes gray, you should eat candy as soon as possible, or to dissolve. A liquid crystal pattern is printed on the container to display the humidity of the contents of the container. On large items such as precious, heat-resisting and cold, the liquid crystal printing can be used as a reminder card for preserving the temperature. As for the use of liquid crystal ink as a forgery-proof ink, the main use is the use of the characteristic of temperature-sensitive discoloration of liquid crystals, and now it has also played a significant role in the field of anti-counterfeiting packaging and decoration, and its anti-counterfeiting effect is also ideal.