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UV drying ink
Although the method of using ultraviolet drying ink was patented in the 1940s, it was not successfully trial-produced until 1969. It began to be used in production in 1971. Currently, the countries that use it most are the United States, Canada and Japan. The advantages of ultraviolet drying ink are that it has stable performance on the printing press, which is conducive to long-term preservation; it dries quickly and is not affected by the printing material during the drying process, and it will not damage the printing material; it has little pollution; the ink film after drying and hardening has excellent characteristics and wear resistance.
In addition to gravure printing, flexographic printing and silk screen printing, ultraviolet drying ink can be used in all printing methods using single-sheet paper and web rotary presses. For example, the United States has used it on a web offset press for printing, and 56 ultraviolet lamps are installed on one production line. From a technical point of view, the advantages of ultraviolet drying ink can be confirmed. However, ultraviolet ink is expensive and it is still difficult to popularize it.
1. The mechanism of ultraviolet drying ink
The components of ultraviolet ink are ultraviolet-sensitive binders, pigments, etc. In order to reduce pollution when the ink is drying and not be affected by the printing material or surface structure. The binder of the ink must have self-polymerization reactivity. In addition, to achieve fast drying speed, the self-polymerization reaction speed of the binder must be very fast. The self-polymerization reaction of the binder in UV ink belongs to the photopolymerization reaction in photochemistry. And the photopolymerization reaction is usually a free radical polymerization reaction.
Free radical polymerization can be divided into two types: polymerization without initiator and polymerization with initiator. Within the scope of photopolymerization, polymerization without initiator is generally less than the free radical polymerization reaction caused by light absorption in the energy range of more than 140kcal/mol with ultraviolet light in the short wavelength region of 200nm. The drying process of UV ink usually uses polymerization with initiator, and its ultraviolet band is 200~400nm. Therefore, the photosensitive binder must be composed of photopolymerization compounds and photopolymerization initiators.
2. Types and characteristics of lamps
The most ideal light source for drying UV ink is a tubular high-pressure mercury lamp, which has a high power density and an effective ultraviolet band (200~400nm). The luminous length of the lamp is determined by the drying area. The power density of this type of lamp is generally 80W/cm. If the light length is 50cm, then 4kW is selected. If the light length is 1 meter, then 8kW is selected. Of course, the power density can be increased to 160W/cm. However, if the temperature of the lamp tube exceeds 800°C, quartz is prone to crystal folding and devitrification. Therefore, it is necessary to force air cooling to keep the tube wall temperature within 500~800°C, otherwise the life of the lamp tube will be greatly affected.
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