Desensitizing nonimage areas

Following exposure, the plate must be treated to increase the ink receptivity of image areas and the water receptivity of nonimage areas.

This keeps the ink and water separated so that printing only occurs in the intended area.

Generally hydrophobic materials are oleophilic and hydrophilic materials are oleophobic. However, an important class of materials known as soaps, detergents, or surfactants have both hydrophilic and oleophilic groups in the same molecule.

Surfactants are usually organic acids or salts of organic acids. The most effective surfactants are associated with relatively long chains (8-20 Carbon atoms).

Generally the acid form is more oleophilic while the salt is more hydrophilic. Because the hydrophilic part is at one end of the molecule and the oleophilic part is at the other, these molecules tend to migrate to surfaces and interfaces. In particular they populate the oil-ink interface.

These molecules tend to stabilize emulsions and in some cases can solubilize oil (or ink) in water and water in oil (or ink).

The most widely used desensitizing agent for lithographic fountain solutions is Gum Arabic, which is a polysaccharide (sugar units) based carboxylic acid which is obtained from the acacia tree. Other materials may be substituted for Gum Arabic, but their functions are similar.

Since the acid form of these materials is more oleophilic and the salt is more hydrophilic, the actual performance of the fountain solution is very dependent on pH.

If the pH is too high, too much ink is solubilized in the fountain and plate tinting or scumming results. If the pH is too low, too much water is solubilized in the ink and low ink densities result.

When the pH is just right, the fountain solution wets only the nonimage area of the plate, while carrying very little ink, and the ink wets the image area while carrying little water.