Sources of exposure

ELECTRIC SOURCES

Ultraviolet radiators

Ultraviolet radiators are an ideal source of energy used in many physical, chemical and biochemical processes. Currently, gas-dicharge lamps are used as electric sources of ultraviolet radiation. Among the ultraviolet radiators, the following groups can be distinguished:

    • low pressure mercury radiators with or without phosphors (UV-A, UV-B, bactericide UV-C, actinic and superactic fluorescent lamps, Wood lamps, etc.),
    • medium-pressure and high pressure mercury radiators with short or long arc,
    • metal halide radiators,
    • xenon and halogen lamps.

High-pressure mercury vapor lamps with special design are used, for example, for laboratory analyzes, induction of various types of photochemical reactions, identification of materials, diagnostics of skin diseases. They mainly emit a UV-A band, with a maximum of 366 nm, to a lesser extent visible radiation and shortwave ultraviolet.

Low-pressure mercury lamps (special-purpose fluorescent lamps) emit a few narrow lines, of which the most important, containing over 80% of the total radiation energy, is the 253,7 nm line. These lamps are mainly used for decontamination of air, clothing, water and other substances, because their radiation is strongly bactericidal. Examples of other devices in which UV lamps are installed are insecticide lamps and banknote testers. The radiation emission level for these devices is from several to several hundred mW / m2.

The use of ultraviolet radiators includes many industries, including mainly: food, electronic, chemical, furniture, printing, pharmaceutical, cosmetics and medicine (treatment of physiological jaundice, psoriasis, therapeutic devices), cosmetics (sunbeds, phototherapy), water supply companies, dialysis stations and swimming pools (disinfection).

Examples of electric ultraviolet radiation sources and their selected applications are shown in Figures 1 to 6.

Fig. 1. Metal-halide UV lamps.

 

 

RFig. 2. UV-C radiator in the form of a 11 W fluorescent lamp.

Fig. 3. An example flow bactericidal lamp.

 

 

Fig. 4. Application of UV lamps in an insecticide lamp

 

Fig. 5. UV-A radiators (Wood lamp) in the form of a fluorescent lamp and a mercury lamp.

Fig. 6. Application of the UV-A radiator in the banknote tester.

 

Disinfection with ultraviolet radiation

This is one of the most popular applications of ultraviolet radiation. There are three basic uses:

    • air disinfection (Figure 3): hospitals, food production, ventilation systems, air purifiers,
    • surface disinfection: packaging, vegetables, spices,
    • disinfection of liquids: drinking water, technological water, fish ponds, aquariums, sewage (Figure 7).

In the case of disinfection systems, there are two versions: open, in which the radiators emit ultraviolet directly into space and closed - in the form of flow type devices. Flow devices are more universal, because there is no risk of this radiation.

In the water treatment process, ozone is used to oxidize pollutants and disinfect water. After this process, free ozone remains in the water, which UV rays catalyze to a harmless form of oxygen. In the process of obtaining ultra pure water, UV rays are used to remove organic carbon compounds from the water. UV radiation causes the formation of radicals that oxidize organic carbon compounds to carbon dioxide and water. UV radiation can also be used for the destruction of free chlorine and chloramines - as an alternative to traditional methods, ie adsorption on activated carbon or dosing of chemicals.

 

Fig. 7. Application of UV lamps for water disinfection

 

Application of UV technology in printing

In recent years, the use of ultraviolet radiation in printing has significantly increased, as evidenced by the growing annual consumption of UV inks and varnishes. Most of them are used in screen printing and flexo printing, in subsequent places there are varnishing and offset. Also on the Polish market, the use of paints, and above all lacquers hardened with UV radiation, is becoming more and more common.

Printing with UV inks is currently used to the greatest extent when printing non-absorbent materials, mainly due to their ability to cure momentarily. On the other hand, UV varnishes are used for finishing finished prints. It has an aesthetic and protective function. Its gloss is unrivaled, and you can also use matt lacquer, of course. Can be painted full-surface and fragments. Painting enhances the gloss and contrast of colors, enhances the impression of depth of the image. Clearly improves the overall impression of finished print. The varnish layer is also a very excellent protection for printing and printouts against scratches, chemicals, oils, fats, moisture, frost, high temperatures. Such printouts can be bent, stamped, and hot foil stamping is also possible.

In Fig. 8 An example of a universal, mobile drying tunnel behind offset printing machines is shown, which allows both curing of conventional dispersion paints and UV inks and varnishes.

 


Fig. 8. Universal mobile drying tunnel behind offset printing machines.

 

Often, there are printing machines which, after the last printing tower, must be equipped with at least UV curing, so that the UV varnish is applied to at least partially hardened UV inks. In the unloading device, an efficient UV curing system is then installed so that it can harden all the layers applied at once.

In the case of UV off-line painting, a drying machine and a hardening tunnel must be located behind the printing machine. The tunnel ends with an automatic stacker and can now be equipped with infrared radiators, hot and cold air drying and UV curing. The parameters of UV lamps in the curing tunnel must be selected for the thickness of the lacquer layer and the power of the printing machine. In this way, you can dry or harden classic, dispersion and UV paints and varnishes (fig. 9). In this tunnel, two 7 kW UV lamps were used.

Fig. 9. Curing UV tunnel for screen printing.

Other devices using UV radiation are semi-automatic and automatic screen printing machines. These are devices for one- or multi-colored prints on flat, round or oval objects by screen printing using UV inks. The radiant-reflector system contains one or more high-pressure UV mercury lamps, whose maximum radiation is at 366 nm and a semiparabolic mirror made of electrolytically polished aluminum. Drying can take place immediately in the machine (fig. 10) or in a separate dryer cooperating in an automatic cycle with a printing device.

Fig. 10. Automatic device type ASP 3000 designed for printing multi-color printing directly on cylindrical and oval plastic and metal packaging.

The lamp shown in Fig. 11 is designed for exposure  of offset plates, silk-screen matrices,  etc. The fitting features a ozone-free metal halide UV radiator with a capacity of 2 500 kW. This device is designed for hanging, as well as for horizontal installation, so that it can be used as a standalone lamp, or for incorporation into an existing imaging device.

Fig. 11. Example UV lamp for copying.

Another group of machines in which UV radiation is used are UV array plotters. The UV board plotter is designed for direct printing on flat surfaces (PVC, wood, sheet, cardboard, textiles, glass, stone, tiles and others) and from the role (fig. 12).

 

Fig. 12. View of an exemplary UV table plotter with an open and closed cover.

Fig. 13 shows a UV contact printing machinel , which is used to expose films using ten UV-A lamps from PHILIPS with a total power of 200 W.

Fig. 13. View of the B2 UV contact printing machine.

In the above mentioned printing machines, mainly high-power, linear, mercury-based sources are used. These lamps are made of quartz glass, which in some types of lamps is doped with titanium oxide to eliminate short waves of ultraviolet radiation responsible for ozone formation. They are radiators with very high powers, from 1 kW to 20 kW and the voltage of several kilovolts. They are used in the manufacture of screen printing matrices, drying of acrylic varnishes, paints and in photochemical processes. In contrast, radiators with a higher gas pressure in the discharge bulb are used for drying polyester coatings and also in photochemical processes. In the production of offset plates and printed circuit boards as well as in the drying of varnishes, metal halide radiators are used. They emit radiation in the 300 - 440 nm range and are produced with power from 400 to 1,750 W.