Application of environmentally friendly waterborne varnish and several problems that should be paid attention to
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The so-called printing glazing is to use the printing plate or the graphic printing plate to print one or two varnishes on the surface of the replica of the graphic printing process, so that the surface of the printed matter obtains a bright film layer. method. Printing glazing can enhance the light resistance of the ink, increase the heat resistance and moisture resistance of the ink layer, and protect the imprint and beautify the product.
Compared with other methods, such as laminating, plastic coating, and spray coating, the printing glazing is not only cheap, simple, and easy to operate. The biggest advantage is that it can be based on actual needs. Choose the ground light, you can not only full version of the glazing (no special printing plate, letterpress printing with lithographic printing, offset printing, no special new printing plate, using the original printing plate on the machine), you can also partially glaze, because The printing glazing is the same as the transfer principle of the printed graphic. The printing plate is used to transfer the film on the substrate, and the blank portion is not adsorbed. According to the customer's needs and the actual printing process, the printing glazing is carried out.
1 characteristics of water-based varnish
Waterborne varnish (or dispersion varnish) has colorless, odorless, organic volatiles, low cost, wide material source, good gloss, folding resistance, abrasion resistance and chemical resistance. Economic and health characteristics.
Water-based glazing oil can be divided into three types according to product use: high gloss, ordinary gloss and matt gloss. From the processing technology can be divided into: ink glazing process, water bucket glazing process (using ordinary offset printing machine water, ink fountain) special glazing glazing process, online glazing process. Water-based glazing oil can eliminate harm to the human body and pollution to the environment during operation, and has been increasingly valued by food and tobacco printing companies.
What are the advantages of water-based varnish compared with solvent-based varnish in addition to environmental protection? It can be summarized as follows:
a. Transparency is strong, the printed matter is bright, and it can be preserved for a long time on the printed matter it is coated; under the long-term strong sunlight, the aqueous varnish is not easy to yellow and does not change color;
b. Strong abrasion resistance, packaging materials need strong abrasion resistance, measured with 50 pressures per square centimeter, generally can achieve 150 rubs per minute, general brightness, can reach 50 to 70 brightness units, high-grade light The type can exceed 80, the non-absorbent substrate can reach 90, especially the cigarette case. In the high speed operation of speed up to 300-500 packs/min, the cigarette label is subject to a large mechanical load. For this reason, the printed matter not only needs to be glazed, but also wearable, and the aqueous varnish can fully satisfy this requirement;
c. The conjunctival speed is fast and the drying is rapid. The drying process of the aqueous glazing oil is 30% by evaporation, 70% is absorbed by the material, and the drying is rapid, and it can be completed in about several seconds. Conjunctiva is formed when the glazing oil still contains 20% to 30% moisture. Therefore, it does not use any drying method. However, if infrared or hot air drying method is used, the drying can be improved quickly;
d. It is convenient and safe to use and store, which is conducive to safe production and improvement of working conditions of employees. Since solvent-based varnish is classified as dangerous goods, transportation, storage, and use are quite inconvenient. The water-based glazing oil uses water as a solvent without any danger. It is safe and reliable to use and can be washed with water.
e. Non-toxic and tasteless, because water-based varnish uses water as solvent and a small amount of volatile edible ethanol as auxiliary solvent, it is especially suitable for use in food and tobacco packaging. Tobacco is a very odor-absorbing plant, and the use of water-based varnish fundamentally eliminates the possibility of odor;
f. High temperature resistance and good heat sealing performance. At present, the PP film is widely used as an outer package, and the water-based varnish has good heat-sealing performance, and even if the celluloid is used, a good heat-sealing effect can be obtained;
g. The finished product has good flatness and strong anti-rolling force.
One headache in gravure printing is the continuous drying of the printed matter through several color groups. The paper has a very low moisture content and is particularly susceptible to curling; so that the tobacco factory cannot use it. The application of water-based varnish to the soft-packed work light, the product is cut into large pieces, and then bronzing and cutting, still maintaining good flatness and anti-rolling property. This will provide a new way for the future offset printing to change the gravure, especially the need for bronzing.
h. The price is reasonable. Currently, the marketed water-based varnish is 26 yuan per kilogram (including tax), and the solvent-type varnish is 10 yuan per kilogram (tax), but the coating amount of water-based varnish is only solvent-type varnish. 30% to 40%, so the actual cost is about 80% of solvent-based varnish.
In addition to the above advantages, the use of aqueous varnish on a gravure press is more convenient than that of a general solvent type varnish. Mainly to control the viscosity of the varnish, the depth of the anilox roll and the temperature of the oven. According to experience, the viscosity of water-based glazing oil is generally controlled at about 15 seconds (measured at 20 ° C); the anilox roller depth is 20 um, 200 L / in; the oven temperature is 120 ° C (100 Kw × 2), at 150 m / min Light, coating amount of 5g / m2, glazing oil is instantly dry, can be directly bronzing, die cutting. When waterborne varnish is used on an offset press, it is different in manner from oil glazing. Oil-based glazing can use the original printing unit and ink fountain to directly print the glazing with the PS plate. The disadvantage of this glazing method is that the brightness is not enough, the fastness is not strong, the color is easy to fade, and the wet glazing cannot be wet, especially the full-page printing. Offset line light is applied by coating method for water-based glazing, which is different from oil-based embossing of the original offset printing unit. It uses the gravure ink transport principle to transfer the varnish to the blanket or resin plate through a metal anilox roller and a doctor blade, and then apply it to the substrate. In general, the viscosity of the glazing oil is about 20 seconds (measured at 20 ° C), the electro-engraving depth of the metal anilox roller is 23 μm, 150 l / in, the coating amount is 5 g / m 2 , the temperature is 65 ° C, infrared power It is 5.4Kw×5××2, the total power is about 54Kw, the machine running speed is 13000 sheets/h, it is instantly dry, no powder is sprayed, it can be bronzing and die-cutting immediately, but it is especially important to wet on the wet machine. Under the conditions, the ink may not be fully dried, and it needs to be dried naturally for about 4 hours to bronze.
In summary, water-based varnish has the advantages of advanced technology, convenient use, economical and practical, non-toxic and non-polluting, and has broad development prospects. It will gradually replace solvent-based varnish and oil-based varnish, occupying in the printing industry. leading position.
2 composition of waterborne varnish
The aqueous glazing oil is mainly composed of three main categories: main agent, solvent and auxiliary agent.
a. The main agent. The main agent of water-based varnish is a film-forming resin, which is a film-forming substance of a glazing agent, usually a synthetic resin, which affects and supports various physical properties of the deep layer and the glazing quality of the film layer, such as gloss and adhesion. Sex, dryness, etc.;
b. Auxiliary. Additives are intended to improve the physical and chemical properties and processing characteristics of aqueous polishes. The types of additives are:
Hardener. Improve the film forming property of the aqueous main agent, increase the cohesive strength of the film layer; surfactant. Reduce the surface tension of aqueous solvents, improve leveling; defoamer. It can control the foaming of the polishing agent for a long time, eliminate the quality defects such as fish eyes and pinholes; desiccant. Increase the drying speed of water-based glazing agent, improve the printability of paper printing; adhesion promoter. Improve the adhesion of the film-forming substance to the substrate; wetting the dispersant. Improve the dispersibility of the main agent, prevent dirt and improve wear resistance; other additives are plasticizers that can improve the folding resistance;
c. Solvent. The main function of the solvent is to disperse or dissolve the synthetic resin and various auxiliary agents.
The solvent for aqueous glazing oil is mainly water. The volatility of water is almost zero and its leveling performance is very good. However, water as a solvent for aqueous glazing oil also has disadvantages such as slow drying speed and easy process size instability.
Therefore, ethanol is added during use to improve the drying performance of the aqueous solvent and to improve the processing suitability of the aqueous varnish.
3 water glazing application
With the rapid and steady development of the market economy and the commodity economy, people pay more and more attention to the printing and decoration of books, magazine covers, calendars, pictures, kits, cigarette packets, not only the printing requirements in the beautiful color picture, but also the magnificent The surface gloss, which is the special effect produced by applying varnish to the printed matter. Coating the varnish on the printed matter can not only increase the value of the printed product, but also enhance the light resistance of the ink, increase the heat-proof and moisture-proof ability of the ink, and protect the print to improve the performance of the printed product, bringing considerable benefits to the printing factory. Economic benefits.
Coated glazing market is a diversified market. Various book printing houses, commercial printing houses, trademark printing factories and folding carton printing factories have different levels of glazing demand for various packaging products. The application of water-based glazing oil must be comprehensively considered according to different glazing methods, paper types, product quality and other factors to achieve good results.
4 affecting the quality of glazing and several issues that should be noted
The main problems affecting the quality of glazing are paper properties, paper type, viscosity, coating amount, drying conditions, and the like.
4.1 Paper performance
The effect of paper on the quality of glazing is mainly reflected in the influence of paper smoothness on the quality of glazing. High-smooth paper has a remarkable effect after glazing, while paper or paperboard with low smoothness has a poor effect after glazing because the glazing oil is almost completely absorbed by the paper or cardboard on the rough surface. In order to solve this problem, you can use the primer or the last two lights before glazing.
4.2 Paper Type
Thick paper has good dimensional stability and poor dimensional stability of tissue. Dimensional stability is one of the key indicators of print quality, and has a close relationship with the amount of water used in printing, the water content of the paper, the drying time and mode, and the viscosity control. If the printed product is on paper below 90g/m2, please use water-based varnish carefully.
4.3 Varnish viscosity and solid content
The viscosity and solids content of the varnish must be properly controlled during the coating process. Therefore, the dilution can only be carried out within a certain viscosity and solid content range. The diluent of the aqueous glazing oil generally adopts a mixture of ethanol and water in a ratio of 1:1, and the dilution amount is 10% to 30%, which should not be too large, and should be strictly controlled.
4.4 coating amount
The coating amount of the aqueous glazing oil should be controlled at 4 to 6 g/m2. Since the evaporation rate of ethanol is very fast in the coating varnish, the control of the viscosity is crucial. The content of ethanol should also be increased in time to ensure the stability of the product.
4.5 Dry conditions
Solvent-based glazing oils are almost all volatile and dry, while water-based varnishes; osmotic drying has great value. However, in multi-color printing, if the last color is coated with varnish, the drying time will be slower because the inside of the paper has absorbed more ink and moist water. An infrared drying device must be used. During the drying process, the glazing oil is heated, and the bottom layer is rapidly infiltrated and dried, and the moisture in the glazing oil is heated and evaporated by the high temperature, and is discharged through the exhaust pipe.
Another way to increase the drying speed is to increase the content of ethanol, but in combination with the actual situation, the viscosity of the glazing oil, can not be adjusted at will.
5 Conclusion
With the development of society, human beings' awareness of the protection of the living environment has also been continuously improved. For the printing industry, many requirements and constraints on health, environmental protection, labor protection, etc. have been gradually increased, especially for food packaging and printing. More rigorous, only in this way can meet the requirements of international health standards. Therefore, it is imperative to develop and apply non-polluting and pollution-free “green” materials.
The water-based varnish has the characteristics of non-toxicity, odorlessness, high gloss and convenient use, and has been widely used in the field of packaging and printing in recent years, and has achieved an increasing trend.