Section 3 Thermal Ink - 2

(C) temperature-change color ink printing process temperature-induced color ink printing before the use of screen printing, and later multi-gravure printing, in addition, with the development of flexible printing technology, flexo printing process will also become a major printing tool. The following outlines the process requirements for using common printing types.
1. The screen printing process is suitable for Longlong (nylon) screen, polyester (polyester) screen or stainless steel wire, etc. The mesh number is ≤ 250 mesh. Due to the wide availability of substrates for screen printing, screen printing is available for substrates that require the use of thermochromic inks. At present, screen printing is conducted at room temperature, and the printing speed is not fast, and the problem of drying is better solved. For small batches, but with some special shapes of substrates, especially for screen printing.
In addition, when performing screen printing, attention should be paid to the relationship between the thickness of the ink layer and the effect of thermochromic discoloration. Therefore, it is recommended that before a large amount of screen printing, the number of corresponding mesh lines, the height distance between the screen plate and the substrate, the thickness of the photosensitive resin coating, and the surface characteristics of the substrate should be determined. At the same time, the viscosity, volatility, etc. of the ink must also be considered because these characteristics affect the printing process as well as the color rendering effect. Dry after printing, be careful not to overheat, it is best to use no heating method.
2. Gravure printing process should use 175 lines / cm, version of the same depth of gravure printing plate, should not use copper plate printing. At all times, it should be chrome-plated to prevent darkening of the ink, which can be lined with polyterpenes or stainless steel ink tanks. Long-term use of the squeegee blade should be nickel, chromium. The guide roller touched by the printing surface should also be chrome plated. It is not appropriate to print directly on aluminum foil, but on paper and plastic. The solvents and diluents for ink transfer should be used exclusively. After the ink is fully stirred, it should be placed in the ink tank. After the light printing, the hue can be printed. Need cold air drying, heating should be moderate so as not to destroy the discoloration properties can not be restored.
3. Flexo Printing Since flexo printing inks are required to have good fluidity, fast drying, good fastness, bright color, and the like, in addition, flexographic printing is not like using gravure printing inks because of the use of rubber reliefs or resin reliefs. Solvents are free to choose.
In the flexographic printing, it is necessary to solve the contradiction between the high solvent content and the influence of the drying speed and printability, and the flexibility of the flexographic printing to dry and fast. When you want to consider the temperature requirements, such as due to high temperature easily lead to substrate deformation or temperature change color ink effect.
For substrates (such as paper, plastic, etc.) that are bulky, do not deform due to high temperature, etc., as well as substrates that do not require very sensitive thermal discoloration, flexographic printing should be used.

Second, the LCD ink temperature change material is currently the most interesting people, mainly through the lattice changes caused by changes in optical properties. To date, thermal inks still account for a relatively large proportion in security printing. For example, security labels on certain cigarettes are made of this material. This ink's printing performance and temperature change color effect are ideal, can be used for one-time use of product security, this mark will be destroyed after a test, the color can not be restored to the original color, so the product packaging can not be repeated use.
LCD inks originated in the United States in the 1970s. Subsequently, liquid crystal ink printing has rapidly developed in the United States, Japan, and other countries, and its application range has been expanding. In terms of manufacturing methods, liquid crystal inks also belong to the type of microcapsule structure inks, but in terms of the characteristics of liquid crystal inks, they are mainly characterized by the use of thermochromic liquid crystals.
(a) Classification of Liquid Crystals In general, solids become liquid when heated to the melting point. However, some substances with a special molecular structure do not change from solids directly to liquids, but go through an intermediate state before they are converted into liquids. This intermediate state, which is not any of ordinary solids, liquids, and gases, is called the fourth state of matter. Its appearance is a fluid, turbid liquid with the birefringence characteristic of optically anisotropic crystals. This kind of material that can both characterize liquids and crystals within a certain temperature range is called liquid crystal.
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 from red to green to cyan) . The color forming mechanism of the liquid crystal is due to the selective reflection of light of a specific wavelength by the liquid crystal, and the liquid crystal must be printed on a black or dark background. The liquid crystal can reflect the temperature of -100+700°C and the accuracy is 0.50°C.
Liquid crystals generally fall into four categories, namely nematic, smectic, cholesteric, and heterotypic. In 1922, French Friedel proposed the classification of nematic, smectic, and cholesteric classifications based on the use of special optical microscopes to observe the optical states of liquid crystals.
a. Nematic type. Observed with a polarizing microscope, many filament-like optical patterns can be seen. The term nematic is derived from (μη), which means filiform.
b. Smectic type. The term smectite originates from Greek (oμeληa), meaning grease or clay. The unique polarizing microscope pattern is shown as thick as a grease.
c. Cholesterol type. Most of them are compounds derived from cholesteryl, so they are named after the cholesterin.
After entering the 1970s, people have successively discovered reentry liquid crystals and discotic liquid crystals, which are collectively referred to as abnormal liquid crystals, namely the fourth type:
a. Re-enter the LCD. The liquid crystals that again appear the same phase during the phase transition are called reentrant liquid crystals.
b. Disc type liquid crystal. The molecular structure is a disc-shaped liquid crystal. For example, the ether and ester molecules with triphenylene core and benzene ring as the core are discotic liquid crystals, as shown in Figure 2-48.
(B) The Molecular Arrangement of Liquid Crystals Here are three highlights. In smectic liquid crystals, the rod-like molecules form a layered structure, each molecule is arranged perpendicular to the plane or at an angle to the plane, as shown in Figure 2-49(a). And no matter what kind of arrangement, the molecules are arranged in parallel with each other. The molecular forces between the arranged molecular layers are relatively weak and easily slide between each other, and thus the smectic liquid crystal exhibits two-dimensional fluid properties. In this kind of liquid crystal, the speed of light passing in a direction perpendicular to the layer is slower than the speed of light passing in a direction parallel to the layer. Here, the slow light transmittance in the direction of the molecular axis means that it exhibits optically positive birefringence. In addition, smectic liquid crystals have high viscosity characteristics as compared with ordinary liquids.
The rod-like molecules of the nematic liquid crystal also remain in a square-arrangement with the molecular axis, as shown in Fig. 2-49(b). However, there is no layered structure in smectic liquid crystals. This liquid crystal still shows positive refraction. In addition, compared with smectic liquid crystals, nematic liquid crystals have a small viscosity and are rich in fluidity. The reason for this fluidity is mainly due to the fact that each molecule of the nematic liquid crystal easily moves itself along the major axis.
The cholesteric liquid crystal [Fig. 2-49(c)] has the same layered structure as the smectic liquid crystal, but the molecular arrangement within the layer is similar to that of the nematic liquid crystal, and the molecular axis of each layer and the molecular axis of the adjacent layer. The directions are slightly offset, and the liquid crystal as a whole forms a spiral structure. The length of the pitch is of the order of the wavelength of visible light. The optical properties of cholesteric liquid crystals such as optical rotation, selective light scattering, and monochromaticity of circularly polarized light are caused by this special spiral structure. Also, its optical properties are different from those of smectic and nematic liquid crystals, and have birefringent properties.
From the above, it can be seen that the molecular arrangement of the liquid crystal is not as strong as the crystal structure, so it is easily affected by external stimuli such as electric field, magnetic field, temperature, stress, and adsorption impurities, causing various optical properties to change. Liquid crystal inks use this characteristic of liquid crystals.
(C) The Optical Properties of Cholesteric Liquid Crystals Because cyano-type liquid crystals are mainly used for making liquid crystal inks, this section focuses on the light properties of cholesteric liquid crystals.
1. Selective light scattering and optically active cholesteric liquid crystals exhibit various specific optical properties due to their helical molecular arrangement. One of the specific properties is the selective light scattering phenomenon that causes rainbow glow, as shown in Figure 2-50. When the light is incident parallel to the planar arrangement of the helical axis, it is divided into two types of circularly polarized light: right-handed light and left-handed light, in which one component of light is transmitted and the other component of light is totally reflected. This phenomenon is called circular polarization dichroism. Here, it is assumed that the direction of circularly polarized light is the incident direction, and circularly polarized light having the same direction of rotation as the cholesteric liquid crystal helical axis is selectively scattered and reflected. The wavelength of the largest selective light scattering is λ0=n·p where p is the pitch and n is the average refractive index (nn+n1)/2 perpendicular to the plane of the helical axis.
At this time, the band width Δλ of the scattered light can be expressed by the following equation:
Δλ = Δn·p where Δn = nn - n1, on the other hand, the wavelength λφ of the selective scattered light obliquely incident with respect to the spiral direction of the planar array is:
Λφ=np cos 1/2 [sin-1(1/n sin φi)+sin-1(1/n sina φs)] where φi and φs are the angles of incidence and scattering of light with respect to the helical axis, respectively. From the equation, it can be seen that λφ moves to a side shorter than the wavelength of λ0 and thus contains high-frequency scattered light.
As shown by the dotted line in FIG. 2-50 , in the wavelength region on both sides of the selective light scattering band of the cholesteric liquid crystal, there is strong rotation and the directions of the optical rotations in the light scattering band are different from each other. Because the pitch p of most cholesteric liquid crystals has a strong dependence on temperature, the wavelength (color) of the selected scattered light changes greatly as long as the temperature is slightly changed. According to this feature, cholesteric liquid crystal films can be used to determine the temperature and distribution of the temperature.
2. A variety of substances that can cause a color change due to a temperature change can present a cholesteric phase. Only when the temperature is relatively high, the liquid crystal phase can be displayed. However, there are few substances that can display a liquid crystal phase at room temperature. Therefore, it is usually necessary to mix several kinds of cholesteric liquid crystals to prepare a mixed liquid crystal which is suitable for use at normal temperature and has a color temperature range of 1-10C.
Fig. 2-51 shows the hue-temperature characteristics of various cholesteric liquid crystals for measuring temperature. The temperature range from red to purple is 0-15°C and 40-55°C, with a range of 30°C. By observing the hue, a temperature change of approximately 0.5°C can be roughly determined. However, it should be noted in Figure 2-51 that any cholesteric liquid crystal can change from red to green due to a small temperature change, but the temperature change from green to purple is relatively large, and the hue and temperature changes. Not a linear relationship. In order to improve these imperfections, it has recently been attempted to add a small amount of nematic liquid crystals to a cholesteric mixed liquid crystal to broaden the temperature range from red to green.
In addition, because the scattering spectrum intensity of red, orange, yellow and other colors is relatively weak, some people are trying to add dyes or pigments that have an external color effect on weakly scattered light in the cholesteric mixed liquid crystal to improve the defect.
In general, a cholesteric mixed liquid crystal changes its hue in the order of red, orange, yellow, green, blue, and violet when the temperature rises from a low temperature to a high temperature. For a composition, there is only one color developing temperature region, and the color temperature of the cholesteric mixed liquid crystal formed after the lipid-based nematic liquid crystal is incorporated in the cholesteric ester may be two. One or three, and the order of the hue changes in each area is not the same (Figure 2-51). liquid crystal