Performance of UV Cationic Inks for Heat Shrink Film Printing (I)

The turnover of the sleeve-type heat-shrinkable film market is expected to reach 52.5 billion U.S. dollars by 2007, which is regarded as a credit for the low-cost flexo market. Since engraving gravure cylinders and solvent recovery devices are expensive, gravure printing costs are much higher than flexo printing, and this lack of gravure printing also limits the development speed of the sleeve type heat shrinkable film market.

Cationic UV inks When printing heat-shrinkable films, UV drying systems must be used to cure the ink. The print quality, printing speed, and customer requirements cannot be achieved in the past with cationic UV inks. Therefore, people usually choose gravure to print heat-shrinkable films.

This new type of UV radical flexo technology for sleeve heat shrink film has made the graphic quality of printed products exceed gravure printing. This kind of white opaque ink has low environmental requirements, not only low cost, but also meets the requirements of plate deformation, ink flow control, and printing speed.

Selection of UV Ink Chemical Composition

Today, acrylates (radicals) and cations are the two most important chemical methods in UV printing. Cation UV ink printing used for many years in the short sleeve type heat shrinkable film printing; free radical ink is due to the ink film shrinkage is small and mostly used in paper substrates. Although these two methods are many forms of application, their basic chemical composition is discussed here.

Cationic UV device

The cationic UV device utilizes a photoinitiator under irradiation of a UV lamp to produce an acidic material that catalyzes the polymerization reaction. The cationic agent induces a chain reaction of epoxy groups until the oxygen bond is broken or a glassy polymer is formed and the bond reaction is terminated. This reaction is slower than the free radical reaction. However, its composition has a high degree of post-curing. The infrared (IR) emitted by the UV lamp is the key to controlling the reaction rate and complete reaction. In the cationic polycondensation reaction, it is necessary to fully understand the properties of the film, to avoid ink cracking, and to ensure that the ink is completely cured.

The advantage of cationic inks is that the type of reaction is a gradual ring-opening polymerization of typical epoxy groups, which increases the active ingredient content of the ink and is almost completely converted into an ink film.

Because the cationic polycondensation reaction is slower than the free radical polycondensation, it takes a long time and the ink film cross-links and adheres to the substrate in order to reduce the pressure on the substrate. The ink film shrinkage of the cationic cured product is less than 5%.

The disadvantages of cationic ink

The reaction rate is slower than radical polymerization and requires high temperature curing. However, the low ink film shrinkage and high temperature curing often cause uneven and smooth edge of the ink film. In general, a UV light source of 800 W or more is selected to ensure rapid and complete curing of the 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 increased.

The cation polycondensation reaction is terminated by the influence of atmospheric humidity is another weakness of it. 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.

For cationic inks, the choice of ingredients is also a challenge. First of all, the basic cost of cationic inks is higher than that of free radicals. In addition, the amount of such substances in the ink cannot be too high because the substances delay or even inhibit the progress of the reaction. The same can not be used excessively, otherwise it will make the viscosity of ink become gelation.

For this reason, ink engineers must strictly control the types of pigments in ink components. They often have to surface-treat the pigments to make their pH neutral. Therefore, many pigments cannot be used because they cannot meet the requirements of UV ink fluidity and viscosity.

Also, there are high requirements for dispersants. In particular, the type of dispersant has been more stringent after being reported to hinder the reaction of the cationic ink. This in turn limits the amount of pigment added in the ink, resulting in a thick printing ink but poor color performance, and graphic colors are not bright.

In addition, cationic initiators also contain heavy metals, which release a small amount of benzene, which pollutes the environment and harms the health of workers.

However, this new free-radical UV technology can produce heavy metal-free and benzene-free materials, but the printing speed is only slightly reduced.

Radical UV ink device

The free radical UV ink device also uses a UV lamp to irradiate the photoinitiator to induce the chain polycondensation reaction of acrylic acid until the radicals react with the pigment, oxygen, additives or other free radicals, or vitrification to terminate the chain forging reaction. Under normal circumstances, there will be a small number of secondary splits after the ink is cured under UV light irradiation, but in fact there are few real post-curing processes. Therefore, when the ink is cooled and adhered on the substrate, sticking, scratches, etc. may occur. The infrared (IR) emitted by the UV lamp can speed up the curing process. The free radical reaction is indeed slightly inferior to the cation reaction, but there are other ways to obtain the desired curing effect.

(to be continued)

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