Section III heat sensitive ink

Section III heat sensitive ink

(III) printing process of thermochromic ink

before thermochromic ink printing, silk printing was mostly used, and then gravure printing was adopted. In addition, with the development of flexible printing technology, flexographic printing process will also become a major means of printing. The following briefly introduces the process requirements of common printing types

1. Silk printing process

Dragon (nylon) wire, polyester (polyester) wire or stainless steel wire should be used, and the number of wire mesh ≤ 250. Due to the wide range of experimental data and curves of substrate suitable for silk printing, they are automatically saved; The experimental process, measurement, display and analysis are all completed by microcomputer; For samples with the same parameters, silk printing can be carried out on substrates that need thermochromic ink. At present, silk screen printing is carried out at room temperature, and the printing speed is not fast, so the drying problem is relatively easy to solve. It is especially suitable for silk printing for substrates with small batch but some special shapes

in addition, during silk printing, attention should be paid to the relationship between the thickness of the ink layer and the effect of temperature sensitive discoloration. Therefore, it is recommended to determine the corresponding number of EYELINES, the height distance between the plate and the substrate, the thickness of the photoresist coating and the surface characteristics of the substrate before a large number of screen printing. At the same time, we should also consider the viscosity and volatility of the ink, because these characteristics not only affect the printing process, but also affect the color rendering effect. Be careful when drying after printing. Do not heat too much. It is best to use the method of not heating

2. Gravure printing process

175 lines/cm should be used, and the plate depth is the same as the gravure plate. Copper plate printing is not suitable. At all times, chrome should be plated to prevent the ink color from darkening. Polyurethane can be used as the lining or stainless steel ink tank. The blade of the doctor blade used for a long time should be nickel plated and chromium plated. The guide roller touched by the printing surface should also be chrome plated. It is not suitable to print directly on aluminum foil, but on paper and plastic. The solvent and diluent used for ink mixing should be special. The printing ink should be fully stirred and put into the ink tank. The formal printing can be carried out only after the hue is observed by light printing. It needs to be dried by cold air, and the heating should be moderate to prevent the discoloration performance from being damaged and cannot be recovered

3. Flexographic printing

because flexographic printing ink requires good fluidity, fast drying, good fastness, bright color and other properties, in addition, flexographic printing uses rubber relief or resin relief, so it is not free to choose solvents like gravure printing ink

when carrying out flexographic printing, we should solve the contradiction between the high solvent content which affects the drying speed and printing suitability and the fast and high drying speed of flexographic printing. When considering the temperature requirements, such as high temperature is easy to cause substrate deformation or the color rendering effect of thermochromic ink

flexographic printing should be used for substrates that are large in batch and do not cause their own deformation and other reactions due to high temperature (such as paper, plastic, etc.), as well as substrates that do not have strict requirements for temperature sensitive discoloration effect

II. Liquid crystal inks

liquid crystal thermotropic materials are the most interesting at present, which mainly cause changes in optical properties through lattice changes. So far, thermal ink still accounts for a relatively large proportion in anti-counterfeiting printing. For example, the anti-counterfeiting marks on some cigarettes are made of this material. The printing performance and temperature sensitive discoloration effect of this ink are relatively ideal, and it can be used for the anti-counterfeiting of disposable goods. This mark is damaged after one inspection, and the color cannot be restored to the original color, so the product packaging cannot be reused

liquid crystal ink originated in the United States in the 1970s. Then, liquid crystal ink printing has developed rapidly in the United States, Japan and other countries, and its application scope is expanding day by day. Liquid crystal ink also belongs to the type of microcapsule structure ink in terms of manufacturing method, but in terms of the characteristics of liquid crystal ink, it mainly uses the characteristics of liquid crystal temperature sensitive discoloration

(I) classification of liquid crystals

generally, solids become liquids when heated to the melting point. However, some substances with special molecular structure do not directly change from solid to liquid, but first go through an intermediate state, and then change into liquid. This intermediate state, which does not belong to any ordinary solid, liquid or gas, is called the fourth state of matter. Its appearance is a turbid liquid with fluidity, and at the same time, it has the unique birefringence of optically anisotropic crystals. This kind of substance that can combine the characteristics of liquid and crystal in a certain temperature range is called liquid crystal

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 refraction. With the rise of temperature, the color of the long wavelength changes to the color of the short wavelength (that is, it changes according to red → green → cyan). The color forming mechanism of its liquid crystal is formed by the selective reflection of liquid crystal to light of specific wavelength. The liquid crystal must be printed on the black or dark background color. Now the liquid crystal can reflect the temperature of -100+700 ℃, and the accuracy is 0.50 ℃

liquid crystals are generally divided into four types, namely nematic, smectic, cholesteric and heteromorphic. In 1922, namely HRA, HRB and HRC, Friede of France proposed the nomenclature of nematic, smectic and cholesteric types on the basis of observing the optical pattern of liquid crystal state with a special polarizing microscope

a. nematic. Many filamentous optical patterns can be seen by polarizing microscope. The word nematic comes from（ μη）， It means filiform

b. smectic form. The word smectic comes from Greek (o μ e λη a) , which means grease or clay. It shows a unique polarizing microscope pattern, which is as viscous as grease

c. cholesteric type. Most of them are compounds derived from cholesterol, so they are named after cholesterol

after entering the 1970s, people have found reentrant liquid crystals and disk liquid crystals, which are collectively referred to as heteromorphic liquid crystals, that is, the fourth type:

a. reentrant liquid crystals. The liquid crystal that appears the same phase again in the process of phase transition is called reentrant liquid crystal

b. disc liquid crystal. Liquid crystal with disc-shaped molecular structure. For example, ether and ester molecules with triphenylene core and benzene ring as the core are disc liquid crystals, as shown in the figure

(II) molecular arrangement of liquid crystal

three kinds are mainly introduced here. In smectic liquid crystals, rod-shaped molecules form a layered structure, and each molecule is arranged vertically or at an angle to the layer, as shown in figure (a). And no matter what kind of arrangement, the molecules are arranged in parallel with each other. The force between the molecular layers of this arrangement is relatively weak, and it is easy to slide between each other, so the smectic liquid crystal presents two-dimensional fluid properties. In this liquid crystal, the speed of light passing in the direction perpendicular to the layer is slower than that in the direction parallel to the layer, such as the sample knocked down by hammering. Here, the so-called slow light transmission along the molecular axis refers to the optical display of positive birefringence. In addition, smectic liquid crystals have high viscosity properties compared with ordinary liquids

the rod-shaped molecules of nematic liquid crystals also remain aligned with the square of the molecular axis, as shown in figure (b). But there is no lamellar structure in smectic liquid crystal. This liquid crystal still shows positive refraction. In addition, compared with smectic liquid crystals, nematic liquid crystals have low viscosity and rich fluidity. The main reason for this fluidity is that each molecule of nematic liquid crystal is easy to move along the long axis

cholesteric liquid crystal [figure (c)] has a layered structure like smectic liquid crystal, but the molecular arrangement in the layer is similar to that of nematic liquid crystal. The molecular axis direction of each layer is slightly offset from that of the adjacent layer, while the liquid crystal as a whole forms a spiral structure. The length of the pitch is the order of magnitude of the wavelength of visible light. The optical properties of cholesteric liquid crystals, such as optical rotation, selective light scattering and circularly polarized light monochromaticity, are caused by this special spiral structure. Moreover, its optical properties are also different from those of smectic and nematic liquid crystals, and it has birefringence properties

from the above, it can be seen that the molecular arrangement of liquid crystal is not as firm as the crystal structure, so it is easy to be affected by external stimuli such as electric field, magnetic field, temperature, stress and adsorbed impurities, which will change its various optical properties. Liquid crystal ink is the application of this characteristic of liquid crystal

(III) optical properties of cholesteric liquid crystals

because cholesteric liquid crystals are mainly suitable for making liquid crystal inks, this section focuses on the optical properties of cholesteric liquid crystals

1. Selective light scattering and optical rotation

cholesteric liquid crystals exhibit various specific optical properties due to their helical molecular arrangement. One of its unique properties is the selective light scattering phenomenon that leads to the generation of rainbow glow, as shown in the figure. When the light is incident parallel to the spiral axis of the plane arrangement, it is divided into right-handed and left-handed circularly polarized light, in which the light of one component is transmitted and the light of the other component is completely reflected. This phenomenon is called circularly polarized dichroism. Here, if the direction of circularly polarized light is the incident direction, circularly polarized light with the same rotation direction as the spiral axis of cholesteric liquid crystal is selectively scattered and reflected. The maximum selected wavelength of light scattering is

λ 0 = n · P, where p is the pitch and N is the average refractive index (nn+n1)/2 in the plane perpendicular to the spiral axis

the frequency band width of scattered light at this time △ λ It can be expressed by the following formula:

△ λ= In △ n · P formula, △ n=nn-n1, on the other hand, the wavelength of scattered light is selected when the incident light is inclined relative to the spiral direction of the plane arrangement λφ Is:

λφ= np cos1/2[sin-1(1/n sin φ i)+sin-1(1/n sina φ s) ] where, φ I and φ S is the incident angle and scattering angle of light relative to the helical axis, respectively. According to the formula, λφ Move to ratio λ 0 wave length, so it contains high-frequency scattered light

as shown by the dotted line in the figure, the cholesteric liquid crystal has strong rotation in the wavelength region on both sides of the selected light scattering band, and the direction of left and right optical rotation of the light dispersive RF is mainly used to process artificial rubber, thermosetting resin and thermoplastic tape with a temperature below 200 ℃. Because the pitch P of most cholesteric liquid crystals is strongly dependent on temperature, as long as the temperature changes slightly, the wavelength (color) of the scattered light will change greatly. According to this feature, cholesteric liquid crystal films can be used to measure the temperature and its distribution

2. Temperature changes cause color changes

many substances that can display cholesteric phase can display liquid crystal phase only in relatively high temperature regions, while few substances can display liquid crystal phase at room temperature. Therefore, it is usually necessary to mix several kinds of cholesteric liquid crystals and modulate them into a mixed liquid crystal suitable for use at room temperature and with a color development temperature range of ℃

figure shows the hue temperature characteristics of various cholesteric mixed liquid crystals used for temperature measurement. The temperature range from red to purple is ℃ and ℃, with a total of 30 ℃. Through the observation of the hue, we can roughly judge the temperature change of 0.5 ℃. However, it should be noted that any cholesteric liquid crystal can change from red to green due to small temperature changes, but the temperature change from green to purple is relatively large, so the hue change is not linear with the temperature change. In order to improve these shortcomings, recently someone tried to add a small amount of nematic liquid crystals to cholesteric mixed liquid crystals to broaden the temperature region from red to green