Ink Curing and Drying Technology (I)

At last drupa show, the maximum speed of sheetfed offset press has reached 20,000 sheets/hour, and the maximum speed of rotary newspaper press has reached 100,000 copies/hour. The customers who favor these high speed machines are China. Printers, especially cigarette packs and newspaper printing plants. It can be said that increasing the printing speed has great potential for the printing press itself, but the drying speed of the ink is a bottleneck for development. Therefore, research on UV curing, thermal drying, and quick-drying inks of inks has been active in recent years. This article mainly starts from the principle of ink drying, and discusses the latest development of UV curing and thermal drying light source and device design of ink.

Ink drying and curing principle

Drying of printing inks mainly includes oxidative conjunctiva drying, osmotic drying, volatile drying and radiation drying.

1. Oxidative conjunctiva drying

The ink contains a dry oil. The unsaturated fatty acids in the molecular structure of the oil chemically react with oxygen in the air, and are polymerized into a macromolecular network structure from small molecule linear structures to form a solid substance. This chemical reaction process is called The oxidized conjunctiva is dry.

2. Infiltration drying

Ink linking materials contain more mineral oils, such as gasoline, high boiling point kerosene, etc. When the mineral oil in the ink penetrates into the porous paper fibers, the pigments are fixed on the surface of the paper to form a solid film layer, which is called osmotic drying.

3. Volatile drying

The ink contains a lot of volatile substances, such as aliphatic hydrocarbon rosin esters, aromatic hydrocarbon solvents, alcohols, ketones, ester solvents. After the ink is transferred to the surface of the printing material, the solvent is volatilized and the pigment is fixed to form a solid film layer, which is called volatile drying.

4. Radiation curing

Radiation curing is a chemical reaction in which a chemical monomer is cross-linked into a macromolecular network structure to form a solid, and the curing speed is fast, and the formed film layer is hard and bright, but the adhesion is not good. At present, there are three kinds of radiation curing methods commonly used.

(1) Infrared radiation drying. The ink contains a chelating agent which can be irradiated with medium-wave infrared light with a wavelength of 3-30 μm to accelerate the polymerization reaction. At the same time, the generated heat promotes the volatilization of the solvent and accelerates the ink drying. Short wave, long wave infrared is not conducive to ink drying.
(2) Ultraviolet (UV) radiation curing. The ink contains photoinitiators such as benzophenone (absorption wavelength 240-340 nm), 2-chlorothianthone (absorption wavelength)
(200-400nm) derivatives such as photoinitiators can rapidly open unsaturated double bond initiating resins, such as polyurethane, epoxy acrylic, in less than 0.1 second under the irradiation of UV light in the wavelength range of 250-420nm. Cross-linking of esters and the like are solidified and dry. UV curable inks are essentially solvent-free.
(3) Electron beam (EB) radiation curing. The thermionic cathode tube emits a high-energy radiation electron beam, and the radiation wavelength is selected by adjusting the cathode tube emitter so that there is no visible light and infrared ray accompanying radiation, and it is completely a cold light source. The electron beam has a high effective radiation intensity and can directly initiate the polymerization of the resin into a solid. Therefore, photoinitiators may not be used in the EB ink. The photoinitiator in the UV ink is removed or added in small amounts as electron beam curing ink.

In general, printing ink drying is not a single drying method, but several kinds of drying occur simultaneously. For example, the drying of offset printing ink is oxidative conjunctiva, infiltration, and volatilization at the same time. The drying of oxidized conjunctiva is the main method; the newspaper printing ink is mainly osmotic drying; the gravure printing ink is mainly volatile and dry.

Common ink drying and curing equipment

Ink drying is often compound drying, so what drying equipment, how to set the parameters, so that the best drying efficiency while saving energy is the printer's most concern.

Infrared drying equipment

(1) Light source. For ink drying, the effect of medium-wave infrared is best. There are three kinds of infrared light sources.

The first is the heating wire radiation source, and the common is the non-enclosed resistance wire jacket quartz lamp. The heat radiation intensity of the lamp tube cannot be controlled with the distribution of light wavelength, the efficiency is low, the price is low, and the service life is about 2000-3000 hours.

The second type of light source is to place the resistance wire inside the closed quartz tube, and fill different gases in the tube to change the wavelength distribution characteristics of the radiation light, and at the same time, the filament is isolated from the oxygen in the air, and the radiation efficiency is high. Most of the imported printing presses nowadays are equipped with such light sources and have a lifetime of 5,000 to 10,000 hours. However, a large part of their radiation energy is still distributed in the visible light and near-infrared rays. Medium-wave infrared light is not very abundant.

The third type is electrothermal semiconductor light tubes. The outer shape of the ceramic tube is coated with a red or gray matt semiconductor material, and the inner wall of the tube is wound with a resistance wire. Electric heating induces radiative emission of the semiconductor material. When the lamp ignites, the light is dark red or black. By selecting the semiconductor material, the wavelength distribution of the radiant energy can be changed, and the radiation efficiency is extremely high.

(2) Infrared lamp reflector. At present, there are two kinds, one is a gold-plated reflector, coated with a metal film 180 ° around the lamp tube, the metal film reflects infrared efficiency up to 90%; the other is the aluminum plate or aluminum plate pressed oval or figure eight Reflector, as shown in Figure 2, the closer the lamp is to the lampshade, the higher the reflection efficiency and the highest reflectivity of 85%.

(3) Heat dissipation of infrared lamps. The heat dissipation of the surface of the infrared lamp can be blown, and the air volume does not affect the infrared power output of the lamp. Since only about 1% of the energy radiated by the infrared lamp is absorbed by paper and ink, and the rest of the energy is completely converted into heat, causing the temperature around the machine to rise, it needs to be exhausted to the outdoors or recycled.

(4) Scope of application. Infrared lamps are used for drying infrared inks. The drying efficiency of non-dedicated infrared inks is very low and does not even work for some inks. Infrared penetrating power can dry the upper and lower layers of the ink film at the same time, accelerating the ink penetration, and promoting the oxidized conjunctiva, especially for the application of thick ink layer printing and drying of coating oil. For the lower solvent content offset, letterpress, screen printing, ink printing, flexographic printing inks and water-based gloss oil drying effect is very good.

(to be continued)