Any substance capable of dissolving another to form a solution. A common, everyday solvent is water, in which substances like sugar, salt, or instant coffee can be dissolved. In many printing inks, a solvent is used to dissolve the vehicle and the pigment and aid in transport and drying of the ink. Low-boiling-point solvents evaporate quickly, leaving the pigment on the surface of the substrate. Other solvents are absorbed rapidly by the paper, again leaving the pigment on the surface of the substrate. Common solvents used in inks are a variety of organic compounds, such as hydrocarbons (such as aliphatic solvents like naphtha; paraffin hydrocarbons like pentane, hexane, heptane, isooctane, and mineral oil; and aromatic solvents like benzene, toluene, and xylene), alcohols (monohydric alcohols like methyl, ethyl, propyl, and isopropyl alcohols, and polyhydric alcohols like glycol and glycerol), ketones (such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone), esters (such as ethyl acetate, propyl acetate, butyl acetate, isobutyl acetate, and amyl acetate), and other organic substances such as ethers (like diethyl ether, isopropyl ether, and tetrahydrofuran). Hydrocarbons, composed solely of hydrogen and carbon atoms, are the simplest and least expensive of the solvents, save for the aromatic hydrocarbons such as benzene and other chemicals having cyclic (or ring) molecular structures, which tend to be more expensive. Hydrocarbons are also characterized by high flammability and toxicity, as well as low solvent power for many types of materials. (The solvent power of hydrocarbons is measured using a Kauri-Butanol Value.) The other varieties of organic solvents contain other types of atoms, in particular oxygen (called polar solvents), and are more expensive, but less flammable, less toxic and more powerful as solvents. Another important characteristic of a solvent is its boiling point. High-volatility organic solvents have very low boiling points (room temperature or slightly above) and evaporate rapidly. Other solvents are much less volatile. The desired degree of volatility of a solvent is a function of the drying mechanism of the ink in which it is to be used. Inks that need to dry quickly by evaporation require highly volatile solvents, while those that dry by other means, such as oxidation or by the application of heat, may require less volatile solvents. (See Vehicle.)
See also Diluent.