Preparation and use of water-soluble glazing oil
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Water Base Varnish / Water Base Coating is referred to as aqueous varnish or water oil for general coating or spot coating. In print, it protects prints and enhances shine. The advantages are non-toxic, no environmental pollution, no solvent volatilization (low residual odor), can improve the production environment, is conducive to the health of operators; and can meet the environmental import standards of Europe and the United States. The water-based varnish is fast, has good abrasion resistance, is not easy to turn yellow, and has high gloss. It is widely used in various types of paper. Specially formulated water-based varnishes can be used in non-absorbent materials such as gold, silver, PET, PP, PVC, etc.
Waterborne varnishes generally consist of 80-90% aqueous resin and 10-20% other additives. After coating with glazing, a thin layer of polymer is left on the surface of the substrate, giving the coating different properties due to different formulation combinations. Such as high light, matte, abrasion and so on.
Waterborne resin is the most important component in waterborne varnish. It determines and affects the characteristics of water-based varnish, such as gloss, adhesion, abrasion resistance, dryness, etc., so the proper selection of water-based resin is the key to the success of water-based varnish.
There are many kinds of resins, such as water-based rosin-modified maleic acid resin, water-based urethane resin, water-based acrylic resin, water-based alkyd resin, and water-based amino resin. For water-based varnish, the selected resin should have the characteristics of easy to dissolve salt, good water release, good gloss after film formation, heat resistance, good chemical resistance, fast drying speed, etc., and water-based acrylic copolymer resin. It can meet the requirements of printing glazing to the utmost extent. Therefore, when formulating water-based varnish, the copolymer resin of water-based acrylic system is our best choice.
The aqueous acrylic copolymer resin can be classified into an aqueous acrylic resin solution, an aqueous acrylic dispersion, and an acrylic emulsion. Acrylic emulsions can be divided into film-forming acrylic emulsions and non-film-forming acrylic emulsions. The properties of the aqueous acrylic copolymer resin are composed of monomers. Performance and the impact of synthetic processes vary. Some monomers can improve gloss and hardness, while others provide chemical resistance and adhesion. A large number of experiments have proved that the selection of aqueous acrylic acid solution and acrylic emulsion is ideal for the performance of water-based varnish which is scientifically compounded.
Other additives also play an important role in improving and improving the performance of waterborne varnishes. Such as leveling agents, wetting agents, defoamers, anti-wear agents. The leveling agent improves the fluidity, prolongs the leveling time, and helps the coating form a flat coating film after drying to avoid water ripple and streaking. The wetting agent mainly reduces the surface tension of the aqueous varnish system and improves the wetting ability of the coating on the surface of the substrate. The defoamer just inhibits and eliminates the foaming of the aqueous varnish in the circulation system, avoiding excessive foaming affecting the operation. Anti-wear agents can improve the anti-friction, scratch resistance and smoothness of water-based varnish. In addition, a small amount of auxiliary solvent is appropriately added to the formulation, so that the components of the aqueous varnish can be more miscible with each other, the cohesive force of the system is lowered, and the glazing adaptability is improved.
Water-based varnish is suitable for special connection line light unit, off-line roll coating glazing machine, flexographic glazing, gravure glazing, etc., and the main application is more connected light (such as KBA KBA104, KBA105), offline The roller is coated with light. In particular, the connection of light, in addition to less and no dusting, can also greatly improve the printing efficiency (reduced spacer), reducing the waiting time for post processing. After the light is printed on the line, the die-cutting and other post-processing can be performed after 2 to 6 hours.
However, no matter which way you choose to glaze, if you want to achieve a satisfactory glazing effect. Must master the use of water-based varnish. Therefore, in the use of water-based varnish, we recommend to carefully grasp the following three factors:
1) Coating amount: After a uniform amount of the coating film is formed, the gloss is good and the abrasion resistance is high. If the coating amount is small, the aqueous varnish cannot form a complete continuous coating film, resulting in poor gloss and failing to provide the protective and decorative effects. However, if the coating amount is large, the coating film is thick, which increases the cost of glazing, and also affects the drying speed, and a serious backing may occur. We generally recommend that the wet film be 4 to 10g / m2, the specific coating amount needs to be different according to the paper and equipment characteristics.
2) Viscosity: The viscosity of the upper roller is generally recommended to be 18 to 25 seconds (DIN 4, 25 ° C). The viscosity of the wire (such as Roland 705) is generally recommended to be 40 to 70 seconds (DIN4, 25 °C). The viscosity is too low, and the function of protection and decoration is not obtained; if the viscosity is high, there may be a phenomenon that the leveling is not good and the drying is slow. The aqueous varnish and diluent are generally water, and a mixture of isopropyl alcohol: water 1:1 may be used as a diluent according to specific conditions. The specific viscosity should be adjusted according to the paper and equipment characteristics.
3) Drying temperature and time: The aqueous varnish mainly relies on permeation drying and cross-linking curing. Common drying devices are hot air drying and infrared drying. Generally, infrared drying is the best. Since the main solvent of water-based varnish is water, and the heat of fluidization of water is relatively high, the drying speed is slower than that of oil. By the same token, it is higher in drying temperature than oily varnish. Generally recommended at 80 ~ 100 ° C, the drying tunnel is about 2m. If the temperature is not enough, the drying tunnel is too short, the coating film will not dry, and the sticky back and the wiping will occur seriously, but if the temperature is too high, the drying time is long, the paper fiber loses too much water, the paper becomes brittle, and it is easy to be Broken and bursting occurred during processing.
The above is only a brief description of the three key issues when using water-based varnish, but the use of water-based varnish is also based on different printing methods, printing equipment, paper type, ink characteristics and glazing equipment to make water-based varnish The excellence of the performance.
Water-based varnish has gradually replaced solvent-based varnish and oil-based varnish due to its advantages of environmental protection and ease of use. In the future, it will be adopted by more and more printing manufacturers.