Surface engineering of UV varnish carton packaging
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In recent years, the packaging and printing industry, which is known as the “urban industry”, has achieved continuous growth at a rate of 10% to 12%, making it one of the industries with the most growth space. With the continuous development of printing technology, all walks of life have higher and higher requirements for the appearance of packaging. However, the carton packaging often suffers from scratches and smudging on the production line of the folder gluer or during the handling process, which seriously affects the value of the goods inside. Therefore, the surface of the carton packaging should have good wear resistance, scratch resistance and water resistance. Resistant to dirt and other properties. At the same time, in order to improve the quality of the products, it is often necessary to carry out surface finishing processing on the carton packaging. The surface finish of carton packaging is processed in a variety of ways, including UV glazing, calendering, and lamination. Among them, the UV glazing process is more common and the cost performance is higher. UV varnish not only meets environmental protection requirements, but also has the advantages of fast curing speed, stable machine, easy cleaning, short processing waiting time, etc., and the carton packaging has good wear resistance and scratch resistance after UV glazing. And high gloss. In this article, the author will focus on the application of UV varnish coating on carton packaging for industry reference.
UV varnish selection
There are many kinds of UV varnish, which are divided into flexible UV varnish, wear-resistant UV varnish, recoatable UV varnish, yellowing resistant UV varnish, high adhesion UV varnish and low volatility according to product quality and functional requirements. UV varnish. Each UV varnish has a suitable product, but it also has certain application limitations. In general, printing companies have the misunderstanding that UV varnishes are required to have all the properties, both flexibility and wear resistance; both surface recoating and scratch resistance. Of course, all ink suppliers want to find a product that has all the properties, but because the structure of the raw materials determines the flexibility and wear resistance of UV varnish is contradictory, recoating and scratch resistance are in conflict. Therefore, it is impossible to have a versatile UV varnish, only the most suitable UV varnish.
Therefore, it is necessary for the printing company to rationally select UV varnish. Some products have complicated post-press processing. After UV coating, it is necessary to carry out multiple processing steps such as die-cutting and indentation. At this time, it is necessary to select a flexible UV varnish. However, it should be noted that the flexible UV varnish uses a monomer with a flexible segment length and a prepolymer. Because of its small degree of cross-linking of the polymer, the cured film layer has good flexibility, but the reaction speed of the cured film formation is relatively Slower, as the degree of cross-linking of the polymer increases, the mechanical strength and hardness of the UV varnish will increase, eventually losing flexibility and becoming brittle. Therefore, the better the flexibility of the UV varnish, the slower the curing speed and the lower the surface hardness, which may result in poor curing after printing, adhesion on the surface of the printed product, and easy scratching in the paste box.
In addition, some printed products need to be subjected to finer finishing processes such as hot stamping and paste box after UV glazing. At this time, recoatable UV varnish can be selected. The reason why the hot stamping can be achieved on the surface of the recoatable UV varnish is because the resin used is matched with the hot melt adhesive in the electrochemical aluminum, and the reaction speed of the resin is slow, the shrinkage rate after curing into a film is small, and the surface tension is low. High, the surface is soft. Therefore, the better the hot stamping effect, the better the flexibility, but the worse the surface wear resistance and scratch resistance, the slower the curing speed, the poor curing after printing, the adhesion of the printed surface, The risk of scratching when pasting the box.
The wear-resistant UV varnish uses a resin having a high crosslink density and a high reaction speed, and therefore has the advantages of high surface hardness and good wear resistance, and such resins have a fast curing speed and a large shrinkage rate after film formation. The surface tension is low and the surface hardness is high. Therefore, the better the abrasion resistance, the higher the surface hardness of the UV varnish, but the worse the surface recoatability and flexibility, and the risk of poor color and adhesion may occur in the paste box.
Since UV varnish has different surface hardness and flexibility under different light-solid conditions and temperatures, it is necessary to select a suitable UV varnish in combination with the printing process.
UV varnish use precautions
(1) Viscosity. Generally controlled in about 20s or less (3# Zahn Cup), the printing effect is better. However, UV varnish does not have a low viscosity diluent like solvent or water. It is difficult to adjust the viscosity during operation. Generally, the viscosity of the UV varnish is adjusted by controlling the temperature of the varnish container.
(2) Save. Avoid direct sunlight during use of UV varnish and store in a cool place below 30 °C. When heating and adjusting the viscosity of UV varnish, take care to avoid high temperature or long-time heating, so as not to cause gelation or deterioration of UV varnish.
(3) Labor protection. UV varnish is irritating to the skin, and it is non-volatile. In the high temperature environment in summer, if UV varnish gets on the skin or clothing, it may enter the skin with sweat and cause inflammation. Therefore, if the skin is stained with UV varnish, it should be cleaned as soon as possible and the work clothes should be replaced in time.
UV varnish problem to be solved
(1) Odor. Although UV varnish is gradually moving toward low odor, the odor problem still exists. In the future, it is necessary to increase the molecular weight and reactivity of prepolymers and monomers, and to develop odorless materials including photoinitiators.
(2) Security. UV varnish is inferior to aqueous varnish in terms of odor and food hygiene, and UV coatings are not approved by the FDA (US Food and Drug Administration) and cannot be in direct contact with food.
(3) Reduce costs. In recent years, although the cost of UV varnish has dropped a lot, it is still too high compared with general inks, and further reduction is needed, mainly the operating costs of equipment such as UV glazing and UV curing devices.
In general, although UV varnish has the above problems, it has great advantages in protecting the environment, saving energy, simplifying operation and increasing the added value of products. It is believed that with the continuous advancement of UV curing technology, UV varnish will There is more room for development.