Commercial printing processes are dominated by lithography, flexography, letterpress, screen printing and electrophotographic printing. Rapidly evolving technologies for sublimation/melt-type printing and ink-jet are becoming more commercially attractive processes. Increasing in influence of these printing technologies is curable ink systems.
A distinguishing feature of printing ink is its visual appearance. The color, transparency, intensity or density, and gloss often determine the suitability of the ink for a particular application. Another distinguishing feature of printing ink is its adhesion to surfaces, resistance to scratching and defacement, impact resistance, resistance to heat, resistance to solvents or other media, lightfastness, UV stability, and flexibility.
In many printing processes, once these challenges are met, the ink is then evaluated for suitability for color matching. Color matching often requires the use of one colored ink in concert with other different colored inks. In one example, International Commission on Illumination (CIE) color matching, provides for an increase in the color spectrum though a process of mixing primary colors (red, green, and blue) to produce secondary colors (cyan, magenta, and yellow) and myriads of possibilities between them. For such a system to function properly, the ink must be truly compatible, not only in physical/chemical properties, but in color properties too.
There are many raw materials employed in the manufacturing of ink products. The four basic components of a printing ink are pigments and dyes, resins, solvents, and additives. These components can be broken down into further details covering potential ingredients such as pigments and dyes, oils, resins, solvents, plasticizers, waxes, driers, chelating agents, anti-oxidants, surfactants, deodorants and fragrances, defoaming agents, adhesion promoters, photo-initiators, reactive diluents, oligomers, inhibitors, and laking agents. Not all of these ingredients will be used for all inks and some ingredients are capable of serving more than one purpose.
Viscosity is a key element to the physical properties and commercial performance capabilities of an ink system. As indicated in Kipphan's Handbook of Print Media: Technologies and Production Methods (Springer Verlag, New York, 2001) and Leach and Pierce's Printing Ink Manual (Kluwer, Boston, 1999), typical viscosity ranges for inks and coatings can vary depending on the particular printing process as provided below in Table 1.
TABLE 1Typical Viscosity Ranges for Various Printing ProcessesPrinting ProcessTypical Viscosity Range (Pa * s)Lithography2 to 30Offset40 to 100Letterpress50 to 150Sublimation and Melt-Type printingsolid at room temperature andmelts at elevated temperatureElectrophotographic~0.1 to 10, for liquid tonerSolid, for dry tonerFlexography0.05 to 0.5 Gravure0.01 to 0.2 Screen1.5 to 2.0 or higherInk-jet~0.001 to 0.1    Intaglio9 to 25
Typical devices for measuring the viscosity of an ink system include capillary viscometers, falling sphere viscometers, flow cups (i.e., Zahn, Shell and Ford), rotational viscometers, cone and plate viscometers (i.e., Haake, TA Instruments), controlled stress rheometers, falling bar viscometers and the like.
As printing speeds become faster and materials more specialized, certain aspects of the printing process have evolved. For example, in some printing press applications, it is not uncommon to employ substrates that are pre-treated, by providing a primer coating to enable adhesion to the surface or surface treating with corona or flame, thereby enabling good ink performance on the substrate despite the added cost in materials and/or production time.
Printing technologies are applied to many different surfaces. For example, polyester film, polyolefin film (PE and PP), polycarbonate, polyimide film, metals (e.g., aluminum, steel, copper), glass, vinyl film, Tyvec® materials, canvas, polyvinylidene chloride films, paper, polyurethane, ceramics, wood and the like are examples of typically used substrates.
In curable ink systems, the polymerization process can be initiated by thermal effects or irradiation (α, γ□, and x-rays, UV, E-beam, and the like).
Among the properties that can be beneficially impacted by monomers are solution viscosity, cure speed, adhesion, impact resistance, toughness, coating hardness, surface tension, wetting, foaming, tensile strength, solvency, dispersive properties, flexibility, chemical resistance, abrasion resistance, and penetration.
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