Part A The characteristics and application of shrink label printing ink
At present, there are four types of inks that can be used as heat-shrinkable materials: cationic UV inks, radical UV inks, solvent-based inks, and water-based inks.
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1. Cationic UV ink
Cationic UV inks are used less often in the field of heat shrink film printing, mainly due to the fact that their price is too high and a specific device needs to be used to catalyze the polymerization reaction by using photoinitiators under UV light to generate acidic substances. The cationic agent induces a chain reaction of epoxy groups until the oxygen bond breaks or a glassy polymer is formed, and the chain reaction stops. The infrared light emitted by the UV lamp is the key to controlling the reaction rate and complete reaction. Cationic polycondensation reaction must fully understand the nature of the film to avoid ink cracking, to ensure complete ink curing.
Cationic ink advantages
The reaction type is a gradual ring-opening polymerization reaction of a typical epoxy group, which increases the content of the active component of the ink and is almost completely converted into an ink film.
Cationic ink disadvantages
(1) Longer reaction time. Since the cationic polycondensation reaction is slower than the radical polycondensation, it takes a long time and the ink film is cross-linked and adhered to the substrate to reduce the pressure on the substrate. The shrinkage of the ink film of the cationic cured product is generally less than 5%.
(2) Since the reaction rate is slower than radical polymerization, high temperature curing is often required. High-temperature curing, combined with low ink film shrinkage, can often cause the film edges to be uneven and not smooth. In general, the selection of 80W UV light source can ensure the cationic ink is cured quickly and completely. If it is lower than this temperature, the durability and adhesion of the ink film will be significantly reduced, and the odor will be large.
(3) The cation polycondensation reaction is terminated by the influence of atmospheric humidity. High humidity weather can cause constant changes in the ink properties. Therefore, the temperature and humidity control system is an essential device for cationic UV printing.
(4) The choice of ink composition is difficult. First, the price of the basic ingredients of cationic inks is higher than that of free radicals. Second, since the alkaline substances delay or even inhibit the progress of the reaction, the content of such substances in the ink cannot be too high. Acidic substances can also not be used in excess, otherwise it will increase the viscosity of the ink into a gel. For this reason, ink engineers must strictly control the type of pigment in the ink composition, and perform surface treatment on the pigment to make its pH value neutral. Therefore, many pigments cannot be used because they cannot meet the UV ink fluidity and viscosity requirements.
(5) There are high requirements for dispersants. Cationic inks have strict regulations on the type of dispersant, which in turn limits the amount of pigment added in the ink, resulting in poor color performance of the printing ink layer, and graphic colors are not bright.
(6) Cationic initiators contain heavy metal elements, but also generate a small amount of benzene, which pollutes the environment and endangers the health of workers.
2. Free radical UV ink
The free radical UV ink device also utilizes a UV lamp to irradiate the photoinitiator to induce an acrylate-linked polycondensation reaction until the free radicals react with pigments, oxygen, additives or other free radicals, or vitrification to terminate the chain reaction.
Under normal circumstances, there will be a small number of secondary splits after the ink is cured under UV light, but there are few real post-curing processes. Therefore, when the ink cools and adheres to the substrate, sticking, scratches, etc. may occur. The infrared light emitted by the UV lamp can accelerate the curing speed. This is slightly worse than the cation reaction in the free radical reaction, but there are other ways to obtain the desired curing effect.