Currently, there are four types of inks that can be used as heat-shrinking materials: cationic UV inks, free-radical UV inks, solvent-based inks, and water-based inks.
The following uses one-to-one analysis of the inks used on shrink labels.
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 is broken or a glassy polymer is formed and the chain reaction is terminated. 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.
â— 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.
â— Disadvantages of cationic ink
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 Because 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 rapid and complete curing cationic ink. If the temperature 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 will be terminated by the influence of atmospheric humidity. Hot and humid 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 ink composition is difficult to choose. First, the price of the basic ingredients of cationic inks is higher than that of free radical inks. 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 and release 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 rate. This is slightly worse than the cation reaction in the free radical reaction, but there are other ways to obtain the desired curing effect.
â— free radical ink advantages
1 The reaction speed is fast. The curing of radical ink does not require too much temperature, and it can be quickly cured under low-energy light sources, which greatly reduces the heat of the substrate and avoids the shrinkage of the heat-shrinking film during the printing process. Its ink requirements are also less demanding than cationic inks. In this way, a wide selection of pigments is sufficient to ensure that the ink has a good rheology on the YAG anilox roll. In addition, high pigment filling reduces the volatile components of the ink, reducing shrinkage and odor.
2 wide printability. Because free radical ink does not have too many restrictions on ingredient selection, it can print substrates with strong alkalinity, or alkali-soluble, water-soluble flexographic printing inks.
3 Drying speed. High adhesion enables radical inks to adhere instantly to substrates and rapidly cure under low energy light sources. In this process, substrates are not exposed to heat, and cationic inks cannot be printed at high speed because of the high energy requirement for curing. This precisely solves the problem of lower productivity of UV flexo printing than gravure printing. With proper curing equipment and pressure settings, free radical inks will overcome this bottleneck.
4 The new free-radical UV technology can produce heavy metal-free and benzene-free materials, but the printing speed is slightly reduced.
â— The disadvantages of free radical ink
1 Free radical ink reacts quickly and quickly cures all ingredients together, which increases the pressure on the substrate. When printing a heat-shrink film, excessive pressure and a small amount of thermal energy of the UV lamp can cause shrinkage and deformation of the film. The contraction will also make the free radical UV ink as soft as other UV inks, and the affinity and adhesion will be worse.
2 Free radical ink is easily oxidized. This can be done with high-energy light sources, high-efficiency ingredients, or oxygen-absorbing initiators (such as diamines and triamines). To illustrate, this is an experimental result obtained under the condition that the air condition is relatively stable and there is no great fluctuation. If it is used in actual production, further demonstrations are needed.
Now some companies, such as XSYS's Flexocure XS UV radical polymerization flexographic inks, have been tested to show that they can achieve 70% shrinkage, and the product has good adhesion, with a wide range of applicable materials, such as PVC, OPS, OPP, PETG and so on.
Even at a shrinkage rate of 70%, these new-technology free-radical-polymerized flexographic printing inks can exhibit good abrasion resistance without the use of primers and varnish. These products have the advantages of high color strength, fast curing speed, excellent printability and print performance. It can realize both surface and in-print processes, while its excellent curing performance and low viscosity are effective guarantees for productivity.
Reprinted from: Shenzhen Printing Network
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