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For many years, offset, letterpress and flexographic printing have been important techniques used for packaging printing, label printing and printing on a variety of other substrates.
The appropriate printing technique is selected according to the specific area of use and the quality requirements related to it. In this regard, flexographic printing is particularly charactensed by its cost efficient plate-manufacturing process and the ever-improving print quality.
Flexographic printing requires low viscosity inks which usually contain organic solvents. For environmental reasons, emission of volatile organic compounds (VOC) should be further reduced. Therefore, organic solvents are often being replaced by water. Such aqueous-based printing inks are especially suited to highly absorbing, fast drying substrates for which, in most cases, only moderate printing quality is required. However, aqueous inks suffer from poor adhesion on plastic substrates and do not provide the degree of chemical resistance and gloss often required for label printing. Because UVprinting inks can meet these requirements they are particularly well suited for the growing needs in the packaging area. UV-inks do not contain solvents, but require in-line UV-irradiation for hardening.
UV-irradiation activates the photoinitiator contained in the UV-ink, which subsequently causes the fast cure reaction of the UV-ink. Thus, UV-printing allows printing at very high line speed and simultaneously maintenance of a superior quality standard, such as high gloss and hardness, which is usually not readily achievable with aqueous inks.
The cure speed of an UV-ink is dependant on several factors, for example the choice of binder resins in the ink formulation, the power of the UV-lamps, and the type of substrate. In addition, reactivity and concentration of the photoinitiator system in the UV-ink are essential in this regard. This paper reports a comparison of different ommercial photoinitiator system with regard to cure speed for a four colour set of UV-flexo inks.
