3.3 Thermal Printing
Here only take the liquid crystal microcapsule ink as an example. A substance that combines the optical characteristics of both liquid and solid crystals within a certain temperature range is called a liquid crystal. The liquid crystal is a turbid liquid that can flow, and at the same time has the unique optical anisotropy of the crystal, such as birefringence and the like. According to the characteristics of liquid crystal molecular structure, they are divided into nematic, smectic and cholesteric three categories. For example, the unique optical property of cholesteric liquid crystals is that when the light is irradiated, it does not regularly absorb light of a certain wavelength like the usual liquid, but instead selectively reflects, transmits, and scatters the light like a crystal. When the ambient temperature changes due to changes in spatial structure and minor changes, the composition of the refraction and reflected light also changes accordingly. That is, there is a sensitive and strictly fixed correspondence between the temperature and the color of the light reflected by the cholesteric liquid crystal. Features can record temperature. Different types of cholesteric liquid crystals have different ranges of temperature-reflectance color correspondence. Can be changed in the range of -20 ~ 250 °C, and some color change sensitivity can reach 0.1 °C, so you can choose different cholesteric liquid crystal or use a combination of several kinds of cholesteric liquid crystal according to the needs of measuring temperature range .
The cholesteric liquid crystal microcapsules are generally prepared by the complex coacervation method using gelatin and gum arabic as raw materials, and are actually made into thermochromic liquid crystal microcapsule printing inks. When this ink is printed on a black back panel, a liquid crystal temperature display is available, indicating the temperature with its color. If the ink is printed on a certain part of the hat, the body temperature of the person wearing the hat can be displayed. This kind of hat is particularly suitable for children in sports or games, so that children can learn about the color change of the “thermometer†on the hat. Whether the exercise is excessive; if the liquid crystal microcapsule ink is printed on a certain part of the frozen food packaging bag to indicate the temperature, the freezing temperature can be directly judged from the appearance, and the quality of the frozen food can be guaranteed, and the food can be prolonged. Storage shelf life; can also be applied to the long-term operation of the machine shell, to play a role in the prevention of mechanical overheating; such as adding to the anti-counterfeiting as a sign, when touched by the hand will change due to temperature changes will play anti-counterfeit effect If printed on the surface of the baby bottle, it can be used to indicate the temperature of the milk in the bottle and ensure that the milk is sucked by the baby at a suitable temperature.
Thermochromic liquid crystal microcapsules have many applications in addition to the above applications, such as non-destructive inspection of thermal conductive systems. It is difficult to inspect the flaws existing in the material in the heat transmission pipeline after painting. The ultrasonic detection, infrared photography and other methods are usually used to detect the heat transmission pipeline. However, it is also possible to use the heat conduction in the defective part to cause the disorder to form different temperatures. The characteristics of the area are checked with thermal microcapsules. There is also a thermosensitive liquid crystal microcapsule "thermometer". When the liquid crystal microcapsules are mixed in a binder to form a liquid crystal printing ink, it is printed on a black backed board to obtain a liquid crystal temperature display. The color indicates the temperature.
In short, the principle of liquid crystal thermochromism will be widely used in the printing field.
3.4 Pressure-sensitive printing
In packaging and printing, if the prints are subject to different forces resulting in different appearance colors, it will be very intuitive to show the force of the product or the strength of the same product in different parts.
The microcapsule print consists of two layers. Several different color pigment microcapsules are bonded on the back of the first layer. The same color pigment microcapsules have the same wall thickness and strength, and are subject to the same When pressure is applied, the paint will burst and color develop. The wall thickness and strength of the microcapsules of different pigments are different, and some will break at higher pressures. The second layer is the base layer, made of paper or other material, and when the pigment microcapsules on the back of the first layer are crushed, they will be blotted in the same place on the second layer. When some articles with different weights are placed on such microcapsules, the different types of color microcapsules on the microcapsules on the surface are broken due to the different pressures generated in different parts, leaving different colors on the second layer of backing paper. The blot. Microcapsules that are generally broken under greater pressure are loaded with darker pigments. For example, a packaged print consisting of three colors of microcapsules. The broken microcapsules contain a white pigment at the lowest pressure. The microcapsules broken at higher pressures are loaded with color pigments, and the microcapsules broken at higher pressures are filled with microcapsules. With black pigments, the weight of an item or the application of force at different locations can be judged based on the color of the print surface when it is white, orange, and black.
3.5 Printing Resisting Environmental Impact
The pigments used in the printing not only require a rich color, but also need to be able to resist the influence of the external environment and keep the pigments from fading for a long time.
Although some pigments are rich in color, their lightfastness and lightfastness are poor. If they are microencapsulated, the ability to protect pigments can greatly improve their ability to resist environmental influences, thereby improving the quality of pigments, due to the use of coated pigments. The wall materials are colorless and transparent, so there is no adverse effect on the gloss of the pigments, and the pigments can maintain good color after long-term exposure to sunlight (ultraviolet rays) after use. Practice has proved that after a variety of color material microcapsules, its dispersion performance, heat resistance, light resistance and chemical stability, anti-diffusion performance than the micro-encapsulated pigment has significantly improved. For example, phthalocyanine pigment is a brightly colored pigment, but its light resistance and corrosion resistance are poor. After the microcapsule is formed, its performance is greatly improved. According to U.S. Patent No. 3,806,364, it is reported that a phthalocyanine pigment containing a halogen and a copper ion or a quinoline (C20H12N2) pigment is formed into a microcapsule with an acrylic copolymer as a wall material, and the microcapsules of the pigment and the aluminum powder are 9:1 Proportioned pigments made of ink printed on metal surfaces exhibit a wonderful two-tone effect, which is clearly better than before unencapsulation.
3.6 Pearlescent, shading effect printing
Many inorganic powders such as titanium dioxide, calcium carbonate, barium sulfate, bauxite (Al2O3·3H2O), zinc oxide, clay, and the like are inorganic pigments or sunscreens with good hiding power. If these inorganic pigments are made into air-containing sunscreen microcapsules having a particle size of less than 1 μm, a better light-shielding effect can be obtained. This microcapsule opacifier can be printed on the surface of paper, plastic, metal, or wood to give pearlescent or shading effects. Because the microcapsules contain air to reduce their density, when printed on the surface of the paper, it can penetrate into the gaps of the fibers, which not only increases the shading effect but also does not increase the thickness of the paper.
(to be continued)