In simplest form, drying of an ink film occurs with conventional inks when the ink vehicle (solvent or water) evaporates or is absorbed, leaving being the solids (pigments, resins, waxes, etc.) to form a film on the substrate surface. Radiation cured inks are cured, not dried. The components of the ink or coating remain on the surface of the substrate, but are chemically transformed in the case of ultraviolet (UV) inks into a hard film through exposure to UV light or for electron beam (EB) inks, a concentrated beam of highly energized electrons. The difference lies in the chemistry of the materials in the inks and coatings and in the pressroom equipment needed to energize the curing process.
Most inks contain pigments and special additives, but what separates UV inks from conventional solvent or water-based inks is the presence of oligomers, monomers and photoinitiators, that replaces the typical resins and solvents.
UV inks consist of one or more monomers and a photosynthesizer that selectively absorbs UV energy to start the rapid curing reaction. Benefits of using UV inks are that the inks contain no solvent. The substrate is not heated above 50oC and a minimum amount of moisture is lost in the process. Because the inks do not cure until exposed to UV light, and may therefore be allowed to remain in the ink fountains (and plates) for long periods of time, the need for clean-up is reduced. UV inks do not lose ingredients to evaporation and therefore, do not require additives during a press run.
After the UV ink is printed on the substrate, it is passed under a source of UV energy, typically a UV curing system. The UV curing system is composed of a UV lamp, a cooling unit, a reflector, a power supply and a control unit.
The lamp is a tube of transparent quartz filled with an inert gas (typically argon) and a small amount of mercury. Unlike a normal glass tube, the quartz tube is transparent to UV. Mercury is used because of its strong emissions in the ultraviolet range. The systems are rated in watts per linear inch and newer systems are normally 400 to 600 watts per inch. The reflectors for UV curing units typically employ elliptical or focused geometry to optimize the energy delivered to the chemistry.
Best Management Practices & Pollution Prevention
The use of UV inks can offer several advantages, such as instant drying with virtually no air emissions, and a generally higher-quality product. The major drawback of UV ink is that some unresolved technical problems still exist. UV inks are more costly, and the technology only has been perfected in the narrow-web segment of the industry.
For access to vendors who may supply alternative materials and equipment, see the PNEAC Vendor Directory.
At this time there are no regulated air emissions associated with the inks themselves, however there may be VOC or HAP emissions from the materials used to clean up the press equipment used to print with the UV or EB inks.
The waste UV and EB inks must be disposed according to state and federal regulations. If the waste is not reblended or recycled, it may be classified as a hazardous waste. UV and EB ink must never be commingled with water based or solvent based ink wastes as they are not compatible and may make it difficult for the waste disposal company to treat the waste.
Health & Safety
UV inks can be a skin irritant. Appropriate personal protective equipment including gloves and aprons should be worn when handling the ink.
The UV and EB curing equipment can cause skin and eye damage due to the high intensity UV light and the EB. Proper shielding should be in place at all times. When the equipment must be maintained, such as replacing bulbs, the printing press and curing unit should be locked out using the OSHA Lock Out/Tag Out procedures to assure personnel safety.
UV Curing: Health and Safety, FLEXO Magazine