The Definition, Characteristics and Application of UV Varnish for Paper Prints

1 What is printing glazing

The so-called printing glazing, is to complete the surface of the printed copy of the graphic, printed with a solid plate or graphic printing plate once or twice a coating of oil, so that the surface of the printed matter to obtain a bright UV film layer. Printing glazing can enhance the light resistance of the ink, increase the ink layer's ability to prevent heat and moisture, and protect the print, beautify the product, and replace the film.

Compared with lamination, printing glazing is not only cheap and easy to process, but it can also be coated in full or partially.

2 Paper Print UV Polishing Features

    UV glazing is a method of curing a glazing coating by irradiating with UV (abbreviation of Ultra viole, ie, ultraviolet ray). With the characteristics of rapid curing and low temperature curing, it can help solve many problems in gloss processing of paper prints. Therefore, it has been widely used in glossy processing of paper containers, trademarks, seals, pictures, tape covers and other printed materials. In foreign countries, the glossy processing of books, magazines, and covers is commonly used UV coating.

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

1) UV varnish contains almost no solvent and very little organic volatiles emissions, thus reducing air pollution, improving the working environment, and reducing the risk of fire;

2) UV varnish contains no solvent and does not require thermal energy for curing. The energy required for curing is only about 20% of the infrared curable ink and the infrared curable varnish. In addition, this varnish has strong affinity for ink and adheres firmly, and is in the range of 80 to 120 W/cm. Curing speed of up to 100 ~ 300m/min under lamp irradiation;

3) After the UV coating process, the color is obviously brighter than other processing methods, and the cured coating wears, and it is more resistant to chemicals and chemicals, has good stability, and can be scrubbed with water and ethanol. ;

4) UV Varnish is high in active ingredient and less volatile, so the dosage is less. The coated amount of coated paper is usually only about 4g/m2, and the cost is about 60% of the cost of coating film.

5) It can avoid the defects that often occur in the plastic film coating process, such as the phenomenon of curling, blistering, wrinkling, delamination, etc. The UV glazing products do not stick together and can be stacked after curing, which is beneficial to the post-processing such as binding. operation;

6) Can be recycled. Solved the problem of environmental pollution caused by the recycling of plastic composite paper base.

At present, except for a small amount of UV coatings imported from Europe and the United States, most of the UV coatings on the domestic market are products produced in Taiwan.

3 UV Varnish Composition and Type UV Varnish is made of ultraviolet light sensitive resin, photoinitiator, active monomer and additive, and processed by scientific compounding. UV varnish types can be divided into UV varnish UV varnish, oil varnish UV varnish and both UV varnish, can also be divided into off-line UV varnish, according to the coating process, In-line UV Varnish or both UV Varnish.

4 Application of UV Varnish for Paper Prints

UV varnish can be widely applied to off-line coating such as roll (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. According to different combinations of rollers and different center line angles between rollers, it can be divided into many types. Paper grippers are suitable for papers with a specification of 80 to 120 g/m2; gravure coaters and flexo coaters are predominantly for topical application. Gravure coater coating can make the local image clear and uniform coating. However, the amount of coating can only be adjusted by the amount of dilution. As long as the number, depth, and shape of gravure lines are not changed, the amount of coating cannot be increased or decreased. However, when the flexographic printing is applied, the edge of the image is prone to edge contours and lacks sharpness. However, it is relatively easy to adjust the amount of coating and make revisions.

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. When the paper is coated with UV varnish, it can be printed first, and the varnish can be coated in an off-line manner after finishing. The processing speed is generally 30-60 m/min.

UV Varnish coating is mainly in the following three ways:

1) offset blanket blanket coating, processing speed 7000 ~ 9000 / h;

2) The last unit of gravure rotary press printing, processing speed 100 ~ 200m/min;

3) The last unit of the flexographic printing press and the special coating machine are coated with a processing speed of 100-200 m/min.

5 Several issues that should be noticed when using UV Varnish

1) Light source. UV light sources generally use high-pressure mercury lamps or metal halide lamps (ideal lamps are preferred), and the power of the light source is also required to be stable. High-pressure mercury lamp output power 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 materials in the UV coating easily penetrate into the paper, causing the paper to become dark or even saturated. In this case, only speed up printing and increase the number of tubes. In order to prevent the infiltration phenomenon, the underlayer may be coated with an infrared primer to prevent the infiltration of UV coating.

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

4) Coating. After the glazing 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 monomeric groups. These monomer radicals form a polymer solid polymer through a chain reaction and complete the drying process of the gloss coating. The coating layer is too thick, drying time under the same power light source is relatively long time, on the one hand affect the UV light oil drying, on the other hand, the water content of the paper is relatively reduced, the paper will become brittle and easy to break, affect the UV The wear resistance of the oil layer;

5) Thinner. UV varnish generally requires the use of special thinner, the dilution amount is controlled at about 1:0.5 to 1:1, too much will affect the glazing effect (depending on the amount of coating film);

6) Difference in gloss. 1 Poor paper quality or surface roughness, large oil absorption, should be replaced by a high-grade paper; 2 uneven coating or dilution of coating oil is too thin, the viscosity is too low, should adjust the polishing machine and improve the viscosity of the coating oil;

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 Incorrect type of UV varnish;

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

9) The coating layer is yellowish. 1 The coating of glazing oil is too thick, the viscosity is too large, and the viscosity should be reduced; 2 The glazing oil is expired (the service life is generally 6 months) and should be stopped;

10) Adhesions (not dried). 1Coating is too thick, should reduce the viscosity; 2UV lamp power shortage or aging (UV lamp life is generally about 800 hours, more than this time curing efficiency will decrease), should replace the UV lamp to achieve rated power; 3 coating Oil has expired and should be stopped;

11) Poor smoothness. The coating roller of coating machine is damaged or the viscosity of coating oil is high. The roller should be replaced or re-polished to reduce the viscosity of varnish.

12) Safety in production. Some imported UV varnish is irritating to the skin, plus it does not volatilize on the skin. When the clothes are attached, if it is in the hot weather in summer, it may enter the skin with sweat and cause inflammation. Therefore, if the skin is stained with UV varnish during the production process, wash it with soapy water and replace the clothes as soon as possible.

In addition, attention must be paid to the environmental temperature and humidity control and the hot stamping properties of paper prints after being coated with UV varnish.

6 Conclusion

UV UV curing technology has been widely used since the invention in the 1960s until the end of the 20th century. In China, UV printing and UV coating have been widely used in aluminum foil and aluminum foil jam printing, magnetic card printing, commercial printing, metal printing, food, medicine, cigarette packs, stickers and other packaging and printing. 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.