The advantages and disadvantages of cationic UV ink

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.

Choice of Chemical Composition of UV Inks Nowadays, 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%.

Disadvantages of cationic inks The reaction rate is slower than radical polymerization and requires high temperature cure. 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.

Advantages of self-based ink This device is fast. Radical ink curing is not too high temperature requirements, low-energy light source can be quickly cured, which greatly reduces the heat of the goods, avoid shrinkage of the heat shrink film in the printing process, and its requirements for ink composition Nor is it as harsh as 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. Since the free radical ink has no restrictions on the choice of ingredients, it can print substrates that are more salty and can be printed in combination 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 (eg, 2.4, 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, it needs further demonstration.

new technology

The low ink film shrinkage and good flexibility make the free radical ink suitable for printing high shrinkage films.

speed

Drying fast. High adhesion allows radical inks to instantly adhere to the substrate and rapidly cure under low-energy light; in the process, the substrate is not exposed to heat. Cationic inks, however, are unable to print at high speed due to high energy requirements for curing. 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.

shrink

This new ink quickly combines with the substrate. Because the speed is too fast, the ink film itself generates internal stress, but it is not enough to make it break. This internal stress is the same as the formation of the shrink film. The film is cooled after preheating and stretched, and the film retains the tension and memory tensile stress inside the film. When reheated, the disorientation occurs and the curl returns to its original state. The free radical ink ink film also has such helium memory tensile stress." When the shrink film print is heated, this feature is incomparable with gravure and aqueous flexo inks. Therefore, the use of this new technology and high shrinkage PETG materials in the shrink sleeve label market has reached 75%.

Graphic quality

The print quality of this new type of ink is better than the quality of the flexo printing with a 150 - 175LPI photosensitive resin plate. The amount of ink used in this ink is half that of the gravure ink. Different from gravure printing with low viscosity ink and double ink layer thickness to obtain high quality dot reproduction rate, this new type of flexo ink is rapidly cured by thin ink layers and brightly colored small dots to obtain the same quality as gravure printing. Text. Opaque white can also do a very good opacity, can be used in conjunction with water-based ink to make up for the lack of water-based ink at this point.

cost

The cost of flexo printing is lower than that of gravure printing. In terms of printing, the photosensitive resin plate - engraved gravure cylinder, no solvent recovery device, low environmental requirements for UV lamp drying - traditional sleeve dryer. In addition, the flexographic version has a relatively low labor intensity, and long and short prints can be converted quickly. But if you think carefully, you will find that the price of the selected pigment for UV ink is twice that of gravure ink. In this way, the two printing costs are not much different.

Curing concept

If you want to fundamentally solve the problem of shrinking <except ink properties> during printing, it is necessary to avoid shrinkage of the printing material by the UV lamp during curing. There are many methods, but none are suitable for thin film printing and solving the heat generated by the printing pressure.

Cooling roller

Cooling rollers are the best choice for cooling and high-speed printing shrinkage. Although there are some other solutions, they have problems such as low speed and high waste.

Low-energy light source

The next step is to ensure that the ink film is firmly bonded to the substrate and that it has good expressiveness. Of course, the ink should not be overcured. Therefore, it is not possible to use the same light source for all color inks. The light drying time for light-colored inks is a bit longer, and some inks themselves can fully absorb the heat of UV lamps, such as carbon black ink, so no cooling is required. Adding a thin black ink layer before the four-color printing can effectively absorb heat and get a bright, dense black color. This is a very effective method.

Doped light source

The doped light source is a low-energy light source that provides sufficient curing energy. This is a good choice for high-speed operation of UV devices.

The use of a thermometer The thermometer can accurately give you the energy speed ratio of your current working condition. If not, you can control the temperature by comparing the UV thermochromic ink with the color ladder.

Cold UV ink

The light source system is the driving force of this new technology. Dichroic mirrors, cold mirrors, and infrared filters reflect and filter light sources, leaving usable light. This new professional light source has been applied in the sleeve type heat shrinkable film printing industry. This low-energy light source can provide the complete curing energy of the ink, realizing low-energy, high-speed printing.

in conclusion

The application of sleeve type heat shrink film is both a challenge and an opportunity for UV printing. With the continuous development of various technologies, special free radical UV devices have been able to get the ideal heat shrinkable film printing effect, accelerating the pace of the industrial development of the shrink sleeve. Low-cost, good expressiveness has also brought free radical UV technology to the forefront of trends.

Reprinted from: Long Chen Printing Technology Network