Application of UV Varnish for Paper Prints and Problems to Be Noted

Editor's note: Since the UV curing technology was invented in the 1960s, it has been widely used by the end of the 20th century. In China, UV printing and UV coating have been widely used in aluminum foil paper and aluminum foil card printing, magnetic card printing, commercial printing, metal printing, food and drug, cigarette packs, stickers and other packaging printing and polishing and other fields. UV coating oil has a very broad application prospect.

First, the significance of printing glazing

The so-called printing glazing is a method of obtaining a glossy film layer on the surface of a replica of a printed matter that has been completely printed by a solid printing plate or a graphic printing plate by means of a printing machine for once or twice. Printing glazing can enhance the lightfastness of the ink, increase the heat resistance and moisture resistance of the ink layer, and play a role in protecting the print, beautifying the product, and replacing the prose film.

Printing glazing is not only cheaper, simpler, and easier to handle than other treatments that add a glossy layer to the surface of a printed product (eg, laminating, laminating, brushing, etc.). It can be either full version or partial. This is because the transfer principles of printing gloss and printing graphics are exactly the same.

Second, the characteristics of paper print UV coating oil

UV glazing is a method of curing a glazing coating using UV (Ultra Violet) irradiation.

The characteristics of UV curing and low-temperature curing are helpful to solve many problems in the processing of paper prints. Therefore, it has been widely used in the processing of glossy paperware, trademarks, seals, pictures, and tape sleeves. In foreign countries, the gloss processing of books, magazines and magazine covers using UV coating is also more common.

UV glazing quickly rises and greatly replaces laminating and solvent-based glazing, depending on the following characteristics:

(1) The air pollution is small. Because UV Varnish is almost solvent-free, organic volatiles emissions are minimal, thus reducing air pollution, improving the working environment, and reducing the risk of fire.
(2) Fast curing. UV varnish does not require thermal energy when curing, and the energy required for curing is only about 20% of infrared curable ink and infrared curable varnish. In addition, this glazing oil has strong affinity for ink and adheres firmly. The curing speed can reach 100-300m/min under the irradiation of 80-120W/cm ultraviolet light.
(3) The coating quality is good. After the UV coating process, the color is obviously brighter than other processing methods, the luster is plump and nourished, and the cured coating is slippery and wear resistant. It is also more resistant to chemicals and chemicals, and has good stability. Scrub with water and ethanol.
(4) Low cost. UV Varnish is high in active ingredient and less volatile, so the dosage is less. The coated amount of coated paper is usually about 4g/m2, and the cost is about 60% of the cost of coating.
(5) It can avoid the defects that often appear in the plastic film coating process, such as edge-up, blistering, wrinkling, and delamination. UV glazing products do not stick, they can be stacked after curing, which is beneficial for binding and other post-processing operations.
(6) Can be recycled again. It can be reused for papermaking, solving the problem of environmental pollution caused by the non-recyclable plastic composite paper base.
At present, except for a small amount of products imported from Europe and the United States, most of the domestic UV coating oil market is produced in Taiwan.

III. Composition and Types of UV Varnish

UV varnish is made from diesel exterior photosensitive resin, photoinitiator, reactive monomer and additives. UV varnish is almost non-toxic after being dried, which is good for environmental protection and can be applied safely to the surface decoration of food products, pharmaceuticals and other packaging products. 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.

Fourth, the application of paper printing UV Varnish

UV varnish can be applied with a roll (roll coat) coater, a gripper coater, a gravure coater, and a 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 suitable for 80-120g/m2 paper. The gravure gravure coater and the letterpress flexo coater are based on topical coating. Gravure coater coating can make the local image clear, uniform coating, but the coating amount can only be adjusted by dilution amount, does not change the number of gravure lines, depth and shape, can not increase or decrease the coating amount; flexo version When coating, the edges of the image are prone to edge contours, and there is a lack of clarity. However, it is relatively easy to adjust and modify the coating amount.
After coating with any of the above coating machines, the solvent is generally volatilized in a hot air oven or an infrared oven, and then cured using ultraviolet rays. When the paper is coated with UV varnish, it can be printed first, and then it is not lighted off-line. This processing speed is generally 30-60m/min.

UV glazing can also be done by on-line glazing, with the same processing speed as the press.

Fourth, several issues that should be paid attention to when using UV varnishes for paper prints

(1) Light source. The UV irradiation light source generally uses a high-pressure mercury lamp or a metal halide lamp (ideal lamp is preferred), and the power requirement of the light source is stable. High-pressure mercury lamp output power is generally 80-120W/cm, in order to ensure that UV curing oil curing speed <0.5s.
(2) UV coatings are not suitable for operation on 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 order to prevent the infiltration phenomenon, the primer layer may be coated with an infrared primer first to prevent the infiltration of UV coating oil.
(3) Ink. Need to pay attention to the combination of UV coating and ink, if necessary, first apply a primer.
(4) Coating. The photo-initiator is initiated after the coating layer has been exposed to ultraviolet light to generate free radicals or ions. These radicals or ions react cross-linked with the double bonds of the prepolymer or unsaturated monomer to form monomeric groups. These monomer groups start a chain reaction to form a polymer solid polymer and complete the drying process of the coating coating. If the coating is too thick, the drying time is relatively long under the irradiation of the same power source, on the one hand affecting the drying of the UV oil, on the other hand, the water content of the paper is reduced, the paper becomes brittle and easy to break, and the resistance of the UV oil layer is affected. Grinding.
(5) Thinner. UV varnish requires special thinner. The dilution amount is controlled from 1:0.5 to 1. Too much will affect the glazing effect (depending on the amount of coating film).
(6) The main reason for the difference in gloss is: bad paper quality, different surface roughness, large oil absorption, high-grade paper; uneven coating or too thin coating oil, low viscosity, adjust the coating machine And improve the viscosity of the varnish.
(7) The main reason for poor adhesion is: improper use of printing ink layer crystallization or ink additives and anti-slip agent, etc., should improve the printing process, without or with less ink additives; coating oil viscosity is too low, should adjust the polishing machine And improve the viscosity of the varnish; the type of UV varnish is not suitable.
(8) The reason why the glazing layer is white and the density of cross-linking is low is that: if the thinner is used improperly, the thinner should be changed or ester diluent or special diluent should be used instead.
(9) The yellowing of the coating layer is due to the fact that the coating of the coating is too thick and the viscosity is too high, and the viscosity should be reduced. When the coating oil expires (the service life is generally 6 months), it should be stopped.
(10) The reason for adhesion (not dried out) is that: if the coating is too thick, the viscosity should be reduced; the power of UV lamp is insufficient or aging (UV lamp life is generally 800 hours, the curing efficiency will be reduced beyond this time), and it will be replaced. The UV lamp makes it reach the rated power; when the varnish has expired, the extended gloss coating should be stopped.
(11) Poor smoothness may be caused by damage to the coating roller of the coating machine or high viscosity of the coating varnish. The rollers should be replaced or reground to reduce the viscosity of the coating varnish.
(12) Some imported UV varnish is irritating to the skin, and if it does not volatilize, it adheres to the skin or clothes. If it is in the hot weather in summer, it may enter the skin with sweat to cause inflammation. If the skin is covered with UV varnish during the production process, wash it with soapy water as soon as possible and replace it in time.

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.
Source: HC Business

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