The liquid component of ink that holds the pigment and binds the pigment to the printed surface (or substrate) after drying. The speed at which the vehicle is absorbed into the paper or other surface affects the ultimate quality of the ink; in inks that dry by polymerization and oxidation, rapid absorption of the vehicle impedes proper ink drying and can lead to ink chalking. In some printing processes (such as high-speed web offset lithography), slow absorption of the vehicle can lead to smudging during such operations as folding. (See Ink Absorbency.)
The composition of ink vehicles varies greatly, depending on the printing process used and the substrate to be printed.
Drying oil vehicle inks dry primarily by oxidation of the vehicle—such as linseed oil, a common drying oil vehicle, as well as fish oil, cottonseed oil, castor oil, tung oil, petroleum oils, synthetic oils, etc.—in which the oil absorbs oxygen and then undergoes polymerization, or the hardening and solidification of the oil. Drying oil vehicle inks are the most commonly used inks in letterpress and offset printing.
Solvent-resin vehicle inks are composed of pigments dissolved in a resin-based solvent which dries primarily by evaporation of the solvent, leaving behind a dry pigment on the substrate. Solvent-resin vehicle inks are used in gravure, flexographic, and offset printing, and are the vehicles that form the basis of heatset inks.
Glycol vehicle inks form a category of inks known as moisture-set inks, which dry by precipitation. Pigments and resins are dissolved in a glycol solvent, which is solubilized in the presence of moisture. When water contacts the glycol solution, the resin and pigments precipitate out and are deposited on the substrate. The advantages of moisture-set inks include a lack of odor, and are consequently used in food-packaging printing.
Resin-oil vehicle inks dry by a combination of absorption and oxidation (called quick-setting). The solvent is absorbed by the substrate quickly, and the remaining film of resin and oil finishes hardening by means of oxidation. These types of inks are known as quick-set ink.
Resin-wax vehicle inks dry by a process called cold-setting. These inks are solid at room temperature, and are applied to the substrate by melting them, utilizing heated rollers, plates, and dampening systems, which essentially melt the ink onto the paper, which then hardens as it cools. Since special heated presses are required for cold-set inks, they are not widely used.