First, what is printing UV coating

The so-called printing glazing is the surface of a replica that has been completely printed in the graphic and text printing process. The printing plate is printed once or twice with a solid printing plate or a graphic plate, and the surface of the printing surface obtains a bright UV film layer. Methods. Printing glazing can enhance the lightfastness of the ink, increase the heat resistance and moisture resistance of the ink layer, and protect the print, beautify the product, and replace the film.

Printing glazing and other methods to increase the gloss coating on the surface of the printed matter are compared with the coating, gluing, and brushing. Not only is it inexpensive, it is simple and easy to operate, and its biggest advantage is that it can be used according to actual needs. Selective glazing allows for full glazing (no special plate, no glossy plate for letterpress printing, no special new plate for offset printing, use of the original plate on the machine) or partial glazing. Printing glazing is the same as the transfer principle of printing and graphic printing. The printing plate is used to transfer the adsorption layer on the substrate, and the principle of non-adsorption of the blank is used. According to the customer's requirements and the special requirements of the actual printing process, the printing is applied.

Second, the characteristics of UV printing oil

UV glazing is a method of curing a glazing coating using UV (abbreviation of Ultra Viole, ie, ultraviolet ray) irradiation. The characteristics of rapid solidification and low-temperature curing of the coating help to solve many problems in the process of glossy processing of paper prints. Therefore, it has been widely applied in gloss processing of printing paper devices, trademarks, seals, pictures, and tape sleeves. In foreign countries, the glossy finish of books, magazines, and covers is UV glazing, which is also more common.

UV glazing has rapidly emerged and has greatly replaced plastic film and solvent-based glazing in many products, depending on its own characteristics:

1. The air pollution is small. Because the UV varnish contains almost no solvent and the organic volatile matter emission is very small, the air pollution is reduced, the working environment is improved, and the risk of fire is also reduced.

2. Curing speed is fast, UV coating solvent does not contain solvent, curing does not require heat, the energy required for curing only infrared curing ink and infrared curing type coating oil is about 20%. In addition, this coating oil has strong affinity for ink and is firmly attached. The curing speed can reach 100-300 m/min under the irradiation of 80-120 W/cm UV lamp.

3. The quality of the coating is good. After printing by the UV coating process, the color is obviously brighter and brighter than other processing methods, the gloss is full and moist, and the cured coating is slippery and wear-resistant. It is also more resistant to chemicals and chemicals. Good stability, able to scrub with water and ethanol.
4. The cost is low, UV coating oil has high effective components and less volatilization, so the dosage is low. The coated amount of coated paper is usually about 4g/m2, and the cost is about 60% of the cost of coating.

5. Can avoid the plastic film coating process often appear defects, such as Alice, blistering, wrinkling, delamination and other phenomena, UV coating products do not stick, after stacking can be stacked up, is conducive to binding and other post-processing operations.

6. Can be recycled, such as recycled paper, to solve the problem of environmental pollution caused by the plastic-covered paper base can not be recycled.

At present, in addition to a small amount of products imported from Europe and the United States, most of the domestic UV coating oil products are produced in Taiwan, and the quality of our UV coating products has reached the level of similar foreign products.

III. Composition and Types of UV Varnish

UV varnish is made of ultraviolet light sensitive resin, photoinitiator, active monomer and auxiliary agent, and processed by scientific compounding. Compared with traditional solvent-based varnish and coating, UV varnish has strong gloss, excellent adhesion, fast curing, good toughness, scratch resistance, no fading, no discoloration, and strong dimensional stability of paper products. Etc. Compared with the coated film, it is almost non-toxic after drying, which is beneficial to environmental protection and can be relatively safely applied to the packaging and printing of paper products such as foods and pharmaceuticals. UV varnish types can be divided into UV varnish UV varnish, oil varnish UV varnish, and both uv varnish, but also according to the coating process is divided into offline UV varnish , On-line UV varnish or both UV varnish.

Fourth, the application of paper printing UV Varnish

UV Varnish can be widely applied in roll-type (roll coating) coater, gripper coater, gravure coater and flexo coater.
The high-efficiency roller coating machine for thick paper is composed of rollers rotating in the same direction, and can be divided into many types according to different combinations of rollers and different center line angles between the rollers. The gripper coater is a paper suitable for sizes 80 to 120 g/m2. It is equipped with a gripper for transporting paper, so as to hold the paper on the impression cylinder, the intermediate cylinder, and the paper discharge section. The other two are gravure gravure coating machines and letterpress flexo coating machines. These two kinds of coating machines are based on partial coating. Gravure coater coating can make the local image clear and uniform coating. However, the coating amount can only be adjusted by diluting the varnish. As long as the number, depth, and shape of the gravure lines are not changed, the coating amount cannot be increased or decreased. In the case of the flexographic coating, the edge of the image is prone to edge contours and lacks sharpness, but it is relatively easy to adjust the coating amount and modify the printing plate.

After coating with any of the above coating machines, the solvent is generally evaporated in a hot air oven or an infrared oven, and then cured by ultraviolet rays or directly by ultraviolet rays. When the paper is coated with UV varnish, it can be printed first, and the varnish can be coated offline at the time of post-processing. This processing speed is generally 30 to 60 m/min.

UV Varnish on-line glazing mainly has the following three methods: offset printing blanket coating, processing speed 7000 ~ 90000 / h; gravure rotary press final coating machine, processing speed 100 ~ 200m/min; flexible Final printing press and dedicated coating machine, processing speed 100 ~ 200m/min.

V. Several issues that should be paid attention to when using UV lacquer in paper printing

1. light source. UV irradiation light source - generally uses high-pressure mercury lamp or metal halide lamp (ideal lamp is preferred), the power of the light source is also required to be stable, the output power of the high-pressure mercury lamp is generally 80 ~ 120W/cm, in order to ensure the UV coating curing speed <0.5s.

2. Paper. UV coatings are not suitable for the processing of easily permeable paper, because the low molecular weight materials in UV coatings can easily penetrate into the paper, causing the paper to become dark and even saturated. In this case, only the printing speed can be increased and the lamps can be increased. In order to prevent the infiltration phenomenon, the primer layer may be coated first with an infrared primer to prevent the penetration of the UV coating material.

3. Ink. Need to pay attention to the combination of UV coating and ink, if necessary, first coat a primer.

4. coating. After the gloss coating is irradiated with ultraviolet light, the photoinitiator is initiated to generate free radicals or ions, and these radicals or ions react with the double bonds in the prepolymer or unsaturated monomer to form a monomer gene. These monomeric genes start a chain reaction to form a polymer solid polymer and complete the drying process of the coating coating. If the coating layer is too thick, the drying time is relatively long under the irradiation of the same power source. On the one hand, it affects the drying of the UV oil, on the other hand, the water content of the paper is relatively reduced, and the paper becomes brittle and breaks, affecting the UV oil layer. The wear resistance.

5. Thinner. UV varnish generally requires the use of special thinner, dilution control in the 1:0.5 to 1 or so, too much will affect the glazing effect (depending on the quality of the coating may be).

6. Gloss difference. 1 poor paper quality, different surface roughness, large oil absorption, should use a high-grade paper; 2 uneven coating or coating oil dilution ratio is too high, the viscosity is too low, should adjust the polishing machine and improve the coating oil viscosity .

7. Poor adhesion. 1 Improper use of lacquer ink layer crystallization or ink additives and anti-slip agent, etc., should improve the printing process, with or without ink additives; 2 coating oil viscosity is too low, should adjust the glazing machine and improve the varnish viscosity; 3 use The UV varnish type is not suitable.

8. The glazing layer is white and the cross-linking density is low. (1) Improper use of thinners. Due to the high volatilization rate of alcohol solvents in summer, thinners should be changed or ester diluents and special thinners should be used.

9. The coating layer is yellowish. 1 The coating of glazing oil is too thick and the viscosity is too large, and the viscosity should be reduced. 2The matte oil expires (usually 6 months) and should be stopped.

10. Adhesion (not dried). 1 The coating is too thick and the viscosity should be reduced. 2 UV lamp power shortage or aging (UV lamp life is generally about 800h, more than this time, curing efficiency will decrease), should replace the UV lamp to achieve rated power. 3 The coating oil has expired and the use of the extended coating oil should be stopped.

11. Poor smoothness. 1 The coating roller of the coating machine is damaged or the viscosity of coating oil is high. The roller should be replaced or reground to reduce the viscosity of the coating varnish.

12. Safe Production. Some imported UV varnish is irritating to the skin and does not volatilize. If it adheres to skin or clothing, it may be in the hot summer weather. It is possible to enter the skin with sweat and cause inflammation. Therefore, if the skin is covered with UV varnish during the production process, it should be washed with soapy water and replaced as soon as possible.

In addition, the temperature and humidity control of the environment and the hot stamping properties of paper prints after coating with UV varnish must also be noted.

Sixth, concluding remarks

UV UV curing technology has been widely used since the invention in the 1960s until the end of the 20th century. UV printing and UV polishing have been widely applied in aluminum foil and aluminum foil paperboard printing, magnetic card printing, commercial printing, metal printing, food and drug, cigarette package, stickers and other packaging printing and polishing and other fields in China. UV paper coatings are growing at a rate of more than a dozen percent each year. Therefore, the application of UV coatings is very promising. (Wuhan Institute of Modern Industrial Technology Jin Zhenhua Qin Mingnan)