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Section III heat sensitive ink

the heat sensitive ink mentioned here refers to a special ink that can change the color of pigments with the change of temperature. The research of thermal ink has a history of 70 years. As early as the 1930s, people began to study color changing coatings, which are mainly used for thermal paper and cooking indicators. In the late 1950s, some thermal materials were found and widely used in the printing field

this book introduces two types of thermochromic inks (i.e. inorganic and organic) and liquid crystal inks

first, thermochromic inks

the pigments of commonly used thermochromic inks include organic and inorganic color changing pigments. However, according to the color changing types, they can be divided into irreversible color changing pigments and reversible color changing pigments. Thermochromic inks can be divided into two types: the first is to use compounds to produce irreversible color changes with thermal decomposition, that is, those that cannot be recovered when cooled; The second is to use the change of the crystalline state of the compound to bring about reversible color changes, that is, if the color can be restored, the trademark printed with this ink will display one color at room temperature, and after heating, it will display another color with the change of the heating temperature, so as to identify the authenticity of the trademark

the content introduced below is not to elaborate separately according to the composition and category of organic and inorganic color changing pigments, but to elaborate the mechanism of reversible and irreversible color changing types from the perspective of color change of pigments when heated

(I) composition of thermochromic ink

the composition of thermochromic ink includes color changing pigments, fillers and binders

1. Color changing pigments

color changing pigments are the basic components of thermochromic inks. The color change of the ink when heated mainly depends on the color changing pigment. Because the color changes of these pigments before and after heating are quite different, which can be used as the basis for judging whether the trademark is forged. So this is the core or foundation of preparing thermochromic ink

there are countless examples of color changes of pigments when heated. However, the pigment as a thermochromic ink must meet the following conditions:

① it should be sensitive to thermal action, have a fixed and obvious color at room temperature, and change color rapidly when it reaches the predetermined temperature and once the coherent technology is mature

② there is an obvious discoloration limit, that is, the discoloration temperature range should be narrow, and the color difference before and after discoloration should be large

③ less affected by the external environment. It has stable performance under light and humid weather conditions, and does not decompose or fade

(1) irreversible color changing pigments irreversible color changing pigments commonly used include sulfate, nitrate, phosphate, chromate, sulfide, oxide of lead, nickel, cobalt, iron, cadmium, strontium, zinc, manganese, molybdenum, barium, magnesium, as well as azo pigments, phthalocyanine pigments, arylmethane dyes, etc. The discoloration of these pigments or dyes is caused by their own thermal decomposition, oxidation and combination, which is irreversible due to chemical changes. Of course, some physical changes are irreversible. The type and color change temperature are listed in the table

table color change temperature and color change of irreversible change pigment

color change temperature of color change pigment/℃ color change color change temperature of color change pigment/℃ color change ninh4po4 · 6h2o120 bright green → gray blue 3 The fuel tank is lack of color: pbco2290 white → yellow c/co3 (PO4) 2 · 8h2o140 pink → sky blue coc2o4300 pink → black nh4vo3150 white → brown c32h16n8cu460 green → colorless (NH4) 3PO4 · 12moo3160 yellow → black pb3o4600 orange → yellow CD (OH) 2200 white → yellow cdso4700 white → brown fe4[fe (CN) 6] 2250 blue → brown pbcro4800 yellow → green FeO · oh280 yellow → red brown red coo+al2o3 3900 gray → blue

it is worth noting that when all reactive pigments reach high tonnage, azo dyes and disperse dyes change color at ℃ and do not work well. Due to gradual change, they lose their use value. Some basic dyes, complexes and inorganic salts have discoloration points below 300 ℃, so they can be used as thermochromic (thermochromic) pigments

discoloration mechanism of irreversible discoloration pigments: the discoloration of irreversible discoloration pigments is due to the physical or chemical changes of the discoloration pigments when heated, which changes the original physical and chemical properties, resulting in color changes. The general types of changes can be divided into the following situations:

① sublimation. Some color changing pigments with sublimation properties display a color in combination with fillers, but when heated to a certain temperature (under constant pressure), it directly changes from solid molecules to gaseous molecules and escapes from the binder, leaving the ink film. At this time, the ink film only displays the color of fillers. People can use this mechanism to achieve the purpose of thermochromism, for example, using indigo as color changing pigment and titanium dioxide as filler, Silicone resin is used as binder to form ink. Heating indigo at 240 ℃± 10 ℃ sublimates the ink film from blue to white. Sublimation is a physical change, because when the ink film is heated, the color changing pigment escapes from the ink film, while the chemical composition of the sublimated pigment does not change

② melting. Melting thermochromic ink is designed according to the principle that pure crystalline color changing pigment has a fixed melting point. Crystalline color changing pigments change from colored solid substances to transparent liquid substances at a certain temperature. The appearance color changes and plays the role of thermochromism. This melting requires obvious color change. For example, lead stearate and ethyl cellulose solution are used to grind into a white color paste, which is sprayed or printed on the dark substrate to form a white coating. When heated to 100 ℃, the white lead stearate melts into a transparent liquid, which immediately shows the color of the dark substrate. Therefore, the temperature reached by heating can be determined. Melting is also a physical change

③ thermal decomposition. Whether organic thermosensitive materials or inorganic thermosensitive materials, most of which can undergo decomposition reaction under certain pressure and temperature, the overall resource trading volume in the scrap market is not high recently. This decomposition reaction destroys the original physical structure, and the decomposition products are completely different from the chemical properties of the original substances, showing new colors. At the same time, gases can be released with decomposition, such as CO2, SO3, H2O, NH3, etc. Therefore, this characteristic can be used to achieve the purpose of temperature discoloration. For example, using cadmium carbonate as color changing pigment and modified epoxy resin as binder to prepare ink. The ink film decomposes with cadmium carbonate at 300 ℃

after decomposition, it changes from white to yellow, which can play the role of thermochromism

④ oxidation. Oxidation reaction is a common chemical change. Many substances can undergo oxidation reaction when heated under oxidation conditions, producing a substance with different composition from the original, and producing a new color at the same time, so as to achieve the purpose of thermochromism. For example, yellow cadmium sulfide is used as a color changing pigment to make ink, which is heated in the air to produce oxidation reaction and produce white cadmium sulfate. The reaction formula is as follows:

⑤ solid phase reaction. Solid phase reaction is also a mechanism of color change. Using the mixture of two or more substances, the solid phase chemical reaction takes place within a specific temperature range, and generates one, two or more new substances, thus showing a completely different color from the original. For example, steel gray alumina and white alumina form a gray mixture. When heated to about 1000 ℃, the mixture will produce blue cobalt aluminate. The reaction is as follows:

because the reaction speed in the solid phase is much slower than that in the solution, and with the increase of reaction temperature or reaction time, new substances are gradually increasing. The color changes gradually. Therefore, the discoloration temperature range of this coating is wide and the accuracy is low

(2) reversible color changing pigments reversible color changing pigments mainly use iodides of Ag, Hg, Cu, compounds formed by cobalt salt, nickel salt of complex or double salt and hexamethylene tetramine, as shown in the table

color changing mechanism of reversible color changing pigments: the color changing process of reversible color changing pigments, some of which lose crystal water during color changing, some of which undergo crystal transformation, and some of which are caused by pH changes, are physical changes, and their changes are reversible

table reversible common color changing pigment color changing temperature and its color change

color changing pigment color changing temperature/℃ color changing color changing pigment

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