UV varnish
The printing and varnishing process involves applying a varnish (or varnishing paint) to the surface of the printed matter to form a dry film. Its function is similar to that of laminating, mainly to increase the smoothness and gloss of the printed surface. Printed matter that has been varnished is more vibrant and has a thicker texture, which beautifies it and enhances the viewing effect. At the same time, the printed matter after glazing also has post-press functions such as waterproof, moisture-proof, friction-resistant, and chemical-resistant, which can extend the service life of the printed matter. Paper glazing has gone through the processes of water-based paint glazing, solvent-based glazing oil glazing, plastic lamination, and UV glazing oil glazing. Although plastic lamination has better performance, the paper after lamination cannot be recycled and reused, and during post-press processing, processes such as bonding and bronzing cannot be carried out.
Therefore, when UV varnish appeared in the 1980s, it was gradually replaced by the UV varnishing process, which has even better performance. Because the UV varnishing process has the advantages of being simple, convenient and inexpensive, and because UV-varnished paper does not affect recycling and can conserve resources, it meets environmental requirements and is the mainstay of green packaging. It is widely used in various books, catalogues, packaging and decorative printed products, and is more competitive than the traditional lamination process in the surface gloss treatment technology of printed products.
Varnishing with varnish has better surface properties than lamination or calendering, and can meet the high requirements of packaging cartons in terms of abrasion resistance, gloss, and stain resistance. It can achieve results comparable to laminating BOPP film to high-grade printing paper. Therefore, varnishing paper products with UV varnish is indeed the best choice.
According to the film-forming mechanism, varnishes can be divided into three main categories: solvent-evaporable, emulsion-aggregating and cross-linked curing, corresponding to solvent-based varnishes, water-based emulsion varnishes and UV varnishes respectively. According to the substrate, varnishes can be divided into paper varnishes, plastic film varnishes and wood varnishes. According to the drying method, varnishes can be divided into natural drying, infrared drying and UV curing. Among them, the classification according to the film-forming mechanism is more scientific, as it can reflect the main characteristics of various types of varnishes and also coincides with the technological development direction of varnishes.
(1) Solvent-based varnishes will be replaced
Early glazing oils were volatile solvents and mainly consisted of film-forming resins, solvents and additives. Film-forming resins were often natural resins such as Cuba resins and rosin resins. Natural resins can cause poor transparency of the film, yellowing and, in high temperature and high humidity environments, re-adhesion. With the development of polymer synthesis technology, film-forming resins were replaced with synthetic nitrocellulose resins, amino resins and acrylic resins. The use of these synthetic resins has effectively improved the film-forming properties of glazing varnishes. Compared with natural resins, synthetic resins have the significant characteristics of good film-forming properties, high gloss and high transparency. However, due to their high viscosity, film-forming resins cannot be applied directly to paper. Organic solvents are used to dissolve and dilute the synthetic resins in the organic solvents, reducing the viscosity of the resins to meet the requirements for glazing varnish application.
After the solvent-based varnish is applied to the surface of the printed matter and dried by infrared or hot air, the solvent in the varnish is volatilized and the film-forming resin is left on the surface of the printed matter to form a shiny film. The volatilized organic solvents pollute the environment and harm the operator’s health. Moreover, if the organic solvents do not volatilize completely, some of them will remain or penetrate into the paper, causing secondary pollution. Commonly used organic solvents include benzene, ketones, alcohols and esters. These solvents are used in large quantities and are expensive. They are ultimately evaporated, resulting in a waste of resources. In the end, only a resin remains on the surface of the printed product, and it seems that the organic solvent does not contribute much to the final film. However, organic solvents play a very important role in the film formation process. A series of processes such as dissolving, diluting, dispersing, wetting, leveling and drying are directly related to its type and amount. The organic solvents in the varnish are not without harm, but they are also very useful. The best way to resolve the conflict between the two is to find a substitute product. Naturally, people think of water, the world’s most abundant and economical resource. The advantages of water – abundant, cheap and easy to obtain, non-flammable and non-explosive – have become the driving force behind the race to develop water-based varnishes.
(2) Water-based varnishes also have their drawbacks
In today’s world, where building a harmonious society and calling for green materials are the order of the day, people have begun to pay attention to the VOC (volatile organic compounds) around them. The VOC content in solvent-based varnishes is generally high, generally 40% to 60%, and most of it volatilizes during the film-forming process, polluting the environment. Water-based varnishes have a very low VOC content and are generally favored by colleagues in the printing industry. The film-forming resin in water-based varnishes is a high-molecular compound. Because oil and water repel each other, the high-molecular resin cannot be dissolved directly in water. It can only be dispersed in water in the form of particles to obtain a uniform and stable emulsion-like varnish. The polymerization process, dispersion process and particle size of the high-molecular resin determine the stability of the emulsion-like varnish and the comprehensive performance of the film.
Generally speaking, there are two methods for preparing water dispersions. The first is the direct dispersion method, in which the main resin (such as styrene-butadiene block copolymers, ethylene-vinyl acetate copolymers, etc.) is dispersed in water under high-speed shearing force by mechanical stirring in the presence of a surfactant. However, if the resin particles are not crushed into a sufficiently small and uneven size, the type and quantity of the surfactant is not selected appropriately, or the emulsification process is not carried out properly, the resulting dispersion system will be thermodynamically unstable and will also undergo particle sedimentation and flocculation over time.
Therefore, the stability of the dispersion system obtained by the direct dispersion method will deteriorate over time, and the quality of the water-based glazing oil obtained by this method is limited by a certain period of time. The second method is the emulsion polymerization method. Waterborne dispersions prepared using the emulsion polymerization method are thermodynamically stable systems with small particle sizes and narrow particle size distributions. Their stability does not deteriorate over time.
Compared with the direct dispersion method, the emulsion produced using the emulsion polymerization method and formulated into a waterborne varnish film has good compactness and high gloss. When producing waterborne varnishes using the emulsion polymerization method, acrylate monomers are generally used. Acrylate monomers can be polymerized alone or copolymerized with other monomers such as ethylene and vinyl acetate. Acrylate polymers are water-resistant, colorless, glossy and have good adhesion to paper. Copolymerization with different monomers can produce copolymer resins with different hardness and film properties. The emulsion polymerization process is the key to the performance of the varnish. The emulsion polymerization process for varnish generally uses acrylates or unsaturated olefins as monomers, anionic or nonionic surfactants as emulsifiers, and persulfates as initiators. The process is carried out at a certain temperature to produce free radical emulsion copolymers, and then a small amount of additives are added, neutralized with ammonia and filtered.
The water-based glazing oil prepared by emulsion polymerization belongs to the emulsion coalescence drying type of coating. It can dry quickly under the action of infrared rays or hot air. After the water evaporates and penetrates into the paper, the isolated latex particles spread and accumulate with each other, leaving a shiny polymer film on the surface of the paper. Water-based varnishes are easy to use, inexpensive and environmentally friendly, but they also have some significant disadvantages, such as relatively poor water resistance, relatively low gloss, and high energy consumption during drying and water removal.