Drying of printing inks is divided into physical and chemical processes.

Physical processes

Physical processes do not imply substantial chemical reactions. The solvent is evaporated or absorbed into the printing stock. Residual solvents may cause problems.

Examples are:


Chemical processes

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.

Examples are:

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

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.




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