Composition and application of offset ultraviolet (UV) ink
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With the advancement of China's printing industry, China's ink manufacturing industry is developing rapidly. The technical indicators of major domestic ink manufacturing enterprises have reached or are close to the advanced level of similar foreign products. In addition to meeting the needs of the domestic printing industry and the packaging and decoration industry, they are also exported to many countries and regions in the world, realizing export earnings. However, we must realize that compared with the international advanced level, there is still a certain gap in China's ink manufacturing industry, mainly due to the lag in production technology and packaging level, the poor consistency of domestic raw materials, research and development capabilities and analytical methods need to be further improved. , environmental awareness is poor. The key targets for the future development of the ink industry are commercial rotary offset inks, high-grade offset inks, environmentally friendly inks and special varieties of inks. Offset UV ink is one of the special varieties of ink.
1. Characteristics of offset UV (UV) ink
Ultraviolet inks have good printability, color stability, overprinting, and brightness compared to conventional inks.
1.1 Its curing speed is fast, it can be cured in a few seconds under ultraviolet light irradiation, especially in packaging printing, it is often necessary to print a large area of the field, and a higher density is required, so it is necessary to increase the amount of ink. The problem of drying becomes very important. The ink is not easy to dry and often affects the printing production efficiency. With ultraviolet light-solid ink, this problem is solved.
1.2 UV ink can also prevent ink crust, which solves the worries of offset presses.
1.3 Its conjunctival property is good, and it has good adhesion to all kinds of substrate materials. Since the ultraviolet ink does not contain solvent, the thickness of the ink layer after curing is basically the same as the thickness of the printing ink layer, and the ultraviolet ink of unit weight is compared with ordinary ink. The printing area is 30%-50%. It has no penetration during curing and can be printed on non-absorbent substrates, such as card printing, which is an area where ordinary offset inks cannot be used.
1.4 Because the UV ink does not need to be sprayed during printing, the printing environment is well improved, and the troubles caused by the powder coating to the post-press processing, such as the effect of glazing and film coating, are avoided. Wire processing is possible.
1.5 UV inks are brightly colored, and their scratch resistance, abrasion resistance, acid and alkali resistance are better than ordinary inks. Therefore, although their price is much more expensive than ordinary inks, they are widely used.
2. The composition of offset UV ink (UV)
The ultraviolet ink is composed of a binder, a pigment, a photosensitizer, an adjuvant, and the like.
2.1 The binder is a kind of adhesive fluid that acts as a bonding agent. It is composed of a photo-curable resin and a prepolymer, a crosslinking agent (a monomer crosslinking agent and a prepolymer crosslinking agent), and the like. The binder increases the fluidity of the ink, allowing the ink to pass from the ink fountain to the printing plate to the substrate, completes good ink transfer, and produces chain polymerization under the action of a certain wavelength of light, cross-linking and solidifying into a film, and the pigment is firmly adhered. On the printing surface. The binder is the main flow part of the ink and is also the heart of the ink.
2.2 Crosslinking agent as a diluent can reduce the viscosity of the ink, improve the printability, and play the role of cross-linking and solidification in the photopolymerization reaction.
2.3 Because some substances in the binder cannot directly absorb the energy of photons, it is necessary to add a photosensitizer, also called a photoinitiator, to the ink. Under the action of a certain wavelength of light, the photosensitizer molecules are decomposed into two radicals. The electrons in the molecule leap from a low energy level to a high energy level, and generate vibration, which causes the unsaturated double bonds in the photo-solid resin and the photosensitive thinner to split, produce a chain polymerization, and further crosslink and solidify.
2.4 adjuvant is an additional part of the ink except for the main part, mainly some wax substances, such as microcrystalline wax polyethylene wax. The auxiliary agent can reduce the viscosity of the ink, enhance the abrasion resistance of the ink film, and the like.
3. Precautions when using offset UV (UV) inks
3.1 When UV inks are formulated, the light-solid inks can be blended with each other, but they cannot be mixed with ordinary inks. When printing, when the ordinary ink is changed into ultraviolet ink, the machine printing part must be thoroughly cleaned with a special cleaning agent, otherwise the phenomenon of poor ink drying and poor inking may occur.
3.2 UV ink auxiliary materials mainly include debonding agent, drier, hardener, thinner and so on. The main function of the debonding agent is to reduce the viscosity of the ink. The amount of the detacking agent is generally 5% of the ink, too much will cause the adhesion of the ink to decrease; the role of the drier is to accelerate the curing speed of the ink, and the amount is generally ink. 2%, the role of the hardener is to enhance the wear resistance of the ink, the amount is generally 5% to 10% of the ink; the role of the diluent is to increase the fluidity of the ink, the amount is generally ink
3% to 5%, too much usage will cause poor drying of the ink film.
3.3 Insufficient ink curing method: The back of the printed product should be clean, non-sticky, and the ink film is not sticky by hand; the thumb is pressed under the pressure of 50N, the ink film is not indented or cracked; Ink film, no scratches.
4. Offset UV (UV) ink printing process requirements
4.1 color sequence arrangement: The problem of UV offset color sequence arrangement is more complicated. Because different colors absorb different ultraviolet photons, the drying rate of different inks is also different. Magenta inks have the best UV transmission, followed by yellow ink, green ink, and black ink. Since black ink is physics in total absorption spectrum, white ink is a total reflection spectrum, which affects the absorption of ultraviolet light by photosensitizer, thus affecting the drying speed, so its color sequence should be white, black, blue, yellow, and Red ink makes the ink with poor light transmittance absorb photons as much as possible, enhancing its curing effect. Such a color sequence arrangement is particularly effective in monochrome or two-color printing. Because this is equivalent to adding a light-solid opportunity to white ink and black ink, the drying effect is naturally good; in a multi-color printing machine with multiple sets of UV lamp drying devices, this color sequence arrangement is also reasonable; When printing in color, the line or text version should be printed first. The solid version with dark ink and small area should also be printed first. The large-area version is printed on the ground. This arrangement can make the ink volume reach the highest and the color is fuller. The ink film of the area is smeared and scratched.
4.2 dampening solution: multi-color offset printing machine with alcohol dampening device, the water bucket original liquid should be replaced with special UV water bucket original liquid, the alcohol does not change. In monochrome offset printing, alcohol can be added to the water to reduce the amount of water, reduce the emulsification of the ink, reduce the surface tension of the ink, and increase the drying speed of the ink. Cleaning should be carried out with special UV car wash water. If there is no special UV car wash water, the effect of using absolute ethanol or xylene is not bad.
4.3 printing plate: Due to the corrosiveness of UV ink and UV car wash water, the printing resistance of ordinary PS plate is very low, generally around 1000~2000 printing, so it is necessary to use special UVPS plate or baked ordinary PS plate. In the case of a temperature of 250 ° C, the ordinary PS plate prepared according to the conventional method requires a baking plate for 10 to 15 minutes, and the CTP plate also requires a baking plate for 10 to 15 minutes.
4.4 UV lamp selection: Experiments show that when the printing speed is 300 rev / hr, use 3 8KW UV lamps; when the printing rate is 6000 ~ 8000 rev / h, with 6 8KW UV lamps can achieve complete print dry. When the drying effect is not ideal, the photosensitizer may be appropriately added depending on the degree of drying. The effective wavelength range of UV curing is 200-400 nm. The wavelength of some light sources in the printing workshop is also within this range, which will cause the surface layer of the ink to be slightly cured. Therefore, attention should be paid to the effect of ambient light on the drying of UV ink.
Due to the special drying method of UV inks, it is suitable for printing absorbent and non-absorbent materials as well as other special The material has excellent printing effect and stable printing performance. With the solution of UV printing technology, the future UV offset printing will have more room for development.