Industry Focus: Overview of Specialty Inks (3)

2, UV curing ink.

Due to the relatively high cost of the baking process in the printing iron process, and time-consuming, fashion can cause ink discoloration. Therefore, the ultraviolet curing process has been widely used in the iron printing process.

The use of UV-dry ink, as long as the printer is equipped with a few UV quartz lamps on it, but the lamp power and installation methods and other factors will affect its curing rate.

Curing of UV inks requires proper energy to activate the components (photoinitiators) in the ink. Ultraviolet lamps provide such energy. Such lamps are generally high-pressure quartz mercury lamps (ie, high-purity mercury vapor and inert gases such as argon are enclosed in an arc tube made of quartz glass), and the emission wavelength is about 200 nanometers (up and down). The wavelength of the light used for curing is generally between 200 and 450 nm. Although the energy below 200 nm is relatively large, it is easily absorbed and the penetrating power is weak. The power is generally 200 watts/hour. The wall temperature of this lamp when working can reach 400-800 °C. The service life is generally 1000-1500 hours.

In fact, only about 25% of the rays of this lamp are in the ultraviolet region, and only 8% of the wavelength can be used to activate the photoinitiator, and another 17% of the ultraviolet light is not effective for the initiator. Other rays are visible light and infrared rays and are also useless for the curing of the ink.

Since ultraviolet rays can damage human eyes and skin, and the ozone released from such lamps is also toxic, the use of such lamps should be equipped with an exhaust device, a device for cooling the lamp tube, a protective cover, and the like. Currently widely used protective cover is elliptic and parabolic two, the internal surface of the cover is generally coated with a layer of aluminum film, its function is two: the reflected light and play a protective role.

As for the installation location of the lamp, it depends on the structure of the printing machine, and it is also various. In multi-color printing presses, there is a color to install a lamp, there are also practices for installing multiple lamps behind four colors, and so on. Only when the light emitted by the lamp matches the absorption spectrum of the photosensitizer, will there be an optimum effect.

We know that some drying oils and resins can be partially polymerized (cured) after adding some appropriate photoinitiators and irradiated with ultraviolet rays, and they are slowly oxidatively dried at a later stage. Admittedly, this type of print cannot be applied immediately (or re-applied) because of its long drying time. Therefore, people have further developed and developed inks that can be fully cured immediately after UV irradiation.

Modern UV drying inks generally have three major components, namely, monomers (as binders), pigments, and photoinitiators (photosensitizers). Monomers can combine with free radicals to polymerize, resulting in the continued production of high molecular weight polymers as the activity is transferred to new units. Since this situation occurs almost instantaneously, ink drying takes only about 1/150 seconds.

In general, substances containing polymerizable unsaturated double bonds can be used almost as photocurable materials. The photo-curing rate of the double bonds is in the following order: Acrylic base> Methacrylic acid> Vinyl. Unsaturated polyesters, acrylic polyethers, polyesters and alkyds, acrylic epoxies, acrylic urethanes, polybutadienes, and the like can be used as light-curable materials.

Ideally, pigments do not absorb any UV light. It is virtually impossible, of course, that a variety of color bodies can absorb different levels of UV light, and black absorbs more than other colors. The UV light is well known. It can be understood that the transmittance of the pigment should be as high as possible because the photocurable material must absorb ultraviolet rays to cure. If the transmittance of the pigment is low, ultraviolet rays cannot be irradiated to the inside of the film, which may delay the curing or make the curing incomplete. .

From the ultraviolet transmittance of several colors of the four-color printing ink, it can be clearly seen that their curing rates are magenta, yellow, cyan, and black. This is because the transmittance of black pigment is the worst, and the transmittance of carbon black is zero. Later developed a carbon black grafted with ethylene monomer, when using this carbon black to make light-curable inks, the curing speed is much faster.

The principle of photo-curing is that the free radicals of the photoinitiator initiate the double bonds in the resin (linking material), so that a chain polymerization reaction causes the ink to cross-link and solidify, and the reaction mechanism is the same as the general free-radical type reaction. Chain initiation, chain growth, chain transfer, and chain termination (endylation).

Photoinitiators are stable under normal conditions but form radicals upon proper light (energy) irradiation. This more active unit will be transferred and combined with the monomer.

The principle of curing of UV-induced inks initiated by light can be roughly expressed as follows:

(1) Initiation. High-energy short-wavelength light is capable of directly initiating polymerization. For example, the photochemical initiator benzophenone can be decomposed to produce free radicals upon irradiation with ultraviolet light.

(2) Growth. The chain groups formed in the initiation phase form chain growth due to the addition of monomers.

(3) Termination (end-based). The growth response can continue until the monomer is used up. The end-group reaction can have two forms of coupling and disproportionation.

Photosensitizers can generally be classified into six types of (a) carbonyl compounds, (b) peroxo compounds, (c) polysulfides, (d) azo, diazo, azide, (e) elemental organic compounds, (f ) Halogen compounds. Among them, (e) mostly has a polymerization inhibitory effect on the photopolymerization reaction, (f) radicals formed as photosensitizers are extremely unstable, (c) is as difficult to control as the peroxide in the photolysis process, and tends to yellowish, ( d) When the light is decomposed, nitrogen gas escapes and bubbles are formed in the film. Relatively speaking, (a) is more ideal than the hinge.

Photoinitiators for UV inks are generally based on benzophenones, and some other initiators are properly incorporated. The specific need depends on the type of pigment and the requirements of the printed product.

The use of a chain transfer agent (such as thioglycol dodecyl ester) in such inks can increase the cure rate.

This type of ink is characterized by fast drying (curing) and can achieve so-called instant drying. However, the drying conditions may vary depending on the function of the ink, the light conditions, the thickness of the print, and the characteristics of the substrate (eg, paper, sheet metal, aluminum foil, plastic film). Usually about 0.2-1 seconds.