Drying of printing inks is divided into physical and chemical processes.
Physical processes do not imply substantial chemical reactions. The solvent is evaporated or absorbed into the printing stock. Residual solvents may cause problems.
- highly volatile solvents are evaporated at moderatly elevated temperatures below their boiling point:
- in gravure inks, toluene is virtually the only solvent used. Semi-synthetic resins (e.g. modified rosin, nitrocellulose) are the most important binders.
- in flexo inks, more polar solvents are common (e.g. ethanol, other alcohols, esters). According to the broad range of stocks, almost every resin is employed as binder.
- moderately volatile solvents are evaporated at elevated temperatures near to or even above their boiling temperature:
- in heat-set web-offset inks, mineral oils with boiling points around 250 °C are evaporated in hot-air dryers. These dryers are exceedingly energy-consuming. Only a small part of the thermal energy input is used to evaporate the oil.
- High-boiling solvents (usually mineral oil) are absorbed into the stock:
- in printing newspapers, be it letterpress or cold-set web-offset, this drying process dominates. In part, it occurs in parallel with every other drying process, provided the stock is able to absorb the solvent at all.
Chemical processes rely on chemical reactions which transform the ink (more or less a liquid) into a solid film. It is the vehicle or at least parts of it, which undergo these reactions.
UV- and electron beam-curing systems, where energy is used to cleave chemical bonds and thus to initiate a chemical chain reaction. The vehicle, usually some acrylate ester, is converted into a plastic film.
It is a drawback of these systems, that ozone is formed from atmospheric oxygen. It must be removed, or its formation prevented.
IR-drying is not comparable to the processes above. Applying infra-red radiation just means transfering heat, thus accelerating chemical reactions and absorption.
Two-pack inks, where a second chemical to form a polymer (e.g. an epoxy resin) or a catalyst are added to start the reaction. These inks are employed where absorption of solvent is impossible.
Oxidation is the classical drying mechanism for lithographic inks, involving the oxygen-induced free radical polymerisation of unsaturated (drying) vegetable oils such as linseed and tung. The oxidant is atmospheric oxygen. It is a chemical process which can be catalysed (accelerated) by small amounts of appropriate metal, usually transition metal, driers.
Physical and chemical drying combined
Physical and chemical drying combined are employed with quicksetting inks, which are common in sheet-fed offset litho.
The varnish is made up of (solid) modified natural resins, (soft) alkyd resins, "drying" vegetable oils (e.g. linseed oil), mineral oil, and others. It has been "cooked" in order to increase its viscosity; chemically, this was the same reaction as the drying reaction to come.
In a first step, the mineral oil is absorbed into the paper or some other absorptive stock quite rapidly. Thus, the ink has "set", which means, that the pile can be handled. In a second step, which may take hours or even days, the drying oils react with atmospheric oxygen to form a solid polymer. This reaction can be accelerated by cobalt and/or manganese catalysts.