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The drying of the ink is a combination of a physical phenomenon “penetration into the substrate” and a chemical phenomenon “air oxidation”.
Penetration into the substrate
During printing, as soon as the ink is deposited on the substrate, certain ingredients of the ink penetrate into the substrate. These are liquid ink components, i.e. mineral or vegetable diluents. Mineral diluents, which are more fluid, have a tendency to penetrate more quickly than vegetable diluents.
This penetration is very dependent on the type of substrate printed.
see article “Printability of substrates”
On paper and cardboard, the rapidity and ease with which they penetrate depends on the quality, surface condition and porosity of the substrate. The more porous the substrate, the faster the penetration. 
Conventional offset ink drying
In the case of non-absorbent subs- trates such as tracing paper or synthetic substrates, no penetration occurs. Drying is entirely through air oxidation.
Air oxidation
This chemical phenomenon begins as soon as the print leaves the press. Air oxidation corresponds to the hardening of the varnishes in the ink through a chemical process that occurs on contact with oxygen in the air. In the first stage, the reaction begins on the surface of the ink film and then spreads throughout the body of the film.
This air oxidation mechanism can be accelerated on the press itself by adding drying agents, such as Triple Drying Agent, to the ink.
The balance between penetration/ air oxidation
The drying of the ink, whatever its for- mulation, involves the two above men- tioned phenomena. By adjusting the siccativity of the ink, the balance can be switched towards penetration or air oxidation.
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