Solvent/Oil-based Inks
Solvent-based inks are commonly used in industrial marking or coating applications where the printing is done on a nonporous substrate such as plastic, metal, or glass. Because no absorption or penetration occurs, the printed image relies on quick evaporation of the ink solvent to be fixed onto the substrate.
However, there are several disadvantages to these types of inks, solvent inks tend to emit a strong smell and over time the printed area can smudge. These inks also have environmental and health and safety issues, for example, inks that contain petroleum solvents emit volatile organic compounds (VOCs) into the air. VOCs can be an irritant for printshop workers when present in the form of vapours, and also contribute to the formation of smog. Inhalation of the solvent can cause drowsiness and hallucinations and, amongst other things, can be carcinogenic. Of course, these solvent vapours cause environmental problems when released in to the atmosphere and can be flammable and there is substantial pressure on ink manufacturers to develop solvent recovery and incineration systems but the development of solvent free inks remains the popular option.
Another more recent, non-aqueous ink option is oil- based ink. More often found in large-format inkjet printers (several of which utilize Nu-Kote piezo shear- mode print heads) the use of non-polar oil-based ink minimizes the effect of electrical fields on the ink and print head materials. There are also legitimate claims that with some coated media, oil-based inks enjoy faster drying time and the absence of cockle on paper substrates, when compared with water-based inks.
Wax/Polymer-based Inks
Solid (hot-melt) ink, as referred to in the Inkjet Technology Overview, is usually solvent-based and is effectively solid at room temperature. This ink is jetted out from the print head as a molten liquid and, upon hitting a recording surface, the molten ink drop solidifies immediately, thus preventing the ink from spreading or penetrating the printed media. The quick solidification feature ensures that image quality is good on a wide variety of recording media.
Inkjet image quality and durability for water-based, solvent and oil-based inks are generally acceptable when they are printed on inkjet papers or coated substrates. But when printing on non-absorbent substrates such as metal, glass and plastic, these types of inks are generally unable to produce durable and sharp images. To solve this, the idea of using UV- curable, wax or polymer-based inks was discussed for a long time. But factors relating to inkjet print head capability, photo-initiator and low-toxicity monomer availability, hindered the progress of UV-curable inkjet ink development. Today, with recognised increases both in the capability of and availability of inkjet print heads, and with UV photoinitiators, monomers and even oligomers readily available at economic scale, successful development of UV-curable inkjet inks is on the horizon.
