Cationic UV inks and free radical ink properties

Selection of UV Ink Chemical Composition

Today, acrylates (radicals) and cations are the two most important chemical methods in the UV process. Cation UV ink printing has been used for many years in short-sleeve sleeve-type heat-shrinkable films; free-radical inks are used in paper-based substrates because of the small shrinkage of ink films. 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 breaks or a glassy polymer is formed, and the chain reaction stops. The infrared (1R) 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.

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.

Since the cationic polycondensation reaction is slower than the radical polycondensation, it takes a long time and the ink film cross-links and adheres to the substrate to reduce the pressure on the substrate, and 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 cure often cause the edge of the ink film to be uneven and smooth. In general, the selection of 80w or more UV light source can ensure the 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 large.

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, the price of the basic ingredients of cationic inks is higher than that of free radicals. In addition, the content of such substances in the ink cannot be too high because the alkaline substances delay or even inhibit the progress of the reaction. Acidity 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 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 UV ink fluidity and viscosity requirements.

Also, there are high requirements for dispersants. In particular, after some reactions have been reported to hinder the cationic ink, the types of dispersants have been more stringent, which in turn has limited the amount of pigments added to the ink, resulting in poor color performance of the printing ink layer and graphic colors.

In addition, cationic initiators also contain heavy metals, which generate 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 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 forging 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. Infrared rays (IR) emitted by UV lamps 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.

The advantages of free radical ink

The speed of this device is fast. The curing of free radical ink is not too high temperature. Under low energy light source, it can be quickly cured. This greatly reduces the heat of the substrate and avoids shrinkage of the heat shrinkable 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 ink volatility, reducing shrinkage and odor.

Because free radical ink does not have too many restrictions on ingredient selection, it can print substrates with strong alkalinity and co-printing with alkali-soluble, water-soluble flexo inks.

The disadvantages of free radical ink

The free radical ink reacts quickly and quickly cures all components together, which increases the pressure on the substrate. When printing a heat-shrinkable film, excessive pressure and a small amount of thermal energy of a curing UV lamp can cause shrinkage and deformation of the film. The shrinkage will also make the free radical UV ink as soft as other UV inks to reduce, affinity, adhesion worse.

Radical inks are also very susceptible to oxidation. However, this can be done with high-energy light sources, high-efficiency ingredients, or oxygen-absorbing initiators (such as diamines and triamines). It should be noted that this is an experimental result obtained under the conditions of relatively stable air conditions and no significant fluctuations. If it is used in actual production, further demonstrations are needed.

speed

Fast drying speed and high adhesion make the radical ink adhere to the substrate instantaneously and cure rapidly under the irradiation of low-energy light source: In this process, the substrate is not exposed to heat, while the cationic ink can not be printed at high speed due to high curing energy requirements. . This precisely solves the problem of UV flexo productivity lower than gravure. With proper curing equipment and pressure settings, free radical ink will break through this bottleneck.

Source: 21st Century Fine Chemicals Network