Printing inks are liquids or pastes with different viscosities. After they are transferred to a substrate, the printed film must become a durable solid layer suited to that surface and resist being removed easily. This change is called drying.
The ink manufacturer must balance drying and adhesion for the intended substrate and printing process, so the finished film meets the customer’s requirements. The main methods are described below.
Physical drying by absorption
On absorbent paper, ink can set as its liquid components penetrate the paper. Newspaper printing is a familiar example of this absorption-based process.
Drying by oxidation
After printing, reactive components of the ink combine with oxygen in the air and form a stable, solid film. Resins and oils with reactive double bonds are involved in this process. Absorption and oxidation can work together on paper: some liquid components penetrate the substrate, while the remaining film hardens at the surface. The drying rate depends on the pigment, varnish, solvents and oils used in the formulation.
Drying by solvent evaporation
Many inks dry physically as solvent leaves the printed film, separating from the resins, pigments and other components. The rate depends on the volatility of the chosen solvents. Their affinity for the resin also matters: a strong affinity may mean that additional energy is needed to release the solvent. It is important that the solvent has left the film before subsequent handling. Otherwise, the surface remains tacky; in gravure or flexographic printing, the rewound roll may stick together, causing blocking.
Chemical drying
Oxidation is itself a chemical drying mechanism, but the term also covers systems in which two components, A and B, are mixed before printing and react to form a hard film. Two-component screen-printing inks are an example. Catalytic systems use an added catalyst to speed reactions in the ink’s polymers; the source mentions phosphoric acid and organometallic acid catalysts as examples. Heat can accelerate the chemical reactions involved in drying.
Radiation-assisted drying and curing
The article discusses ultraviolet (UV), infrared (IR), microwave, radio-frequency and electron-beam (EB) systems. In UV curing, reactive formulation components are activated by UV light, producing radicals that react to form a complex, cross-linked, three-dimensional polymer network and harden the film.
Short- and long-wave infrared systems supply heat that helps remove solvents from the printed film, particularly in offset lithography.
Electron-beam curing also generates reactive species that form a cross-linked network and solidify the ink film. Microwave and radio-frequency energy can help remove polar solvents, including water, alcohols and ketones, from the ink.