In a cosolvent system, two miscible fluids with different boiling points are mixed to create a zeotropic mixture. The first fluid is a solvating agent that dissolves soil in the cleaning process. This fluid is an organic solvent with a low-boiling point and a flash point greater than the system's operating temperature. After the solvent mixes with the oil, the second fluid, a hydrofluoroether rinsing agent (HFE), rinses off the solvating agent. The solvating agent can be flammable because its mixture with the HFE is nonflammable. In bisolvent cleaning processes, the rinsing agent is separated from the solvating agent. This makes the solvating and rinsing agents more effective because they are not diluted.
Cosolvent systems are used for heavy oils, waxes, greases and fingerprints, and can remove heavier soils than processes that use pure or azeotropic solvents. Cosolvent systems are flexible in that different proportions of substances in the zeotropic mixture can be used to satisfy different cleaning purposes. For example, increasing the proportion of solvating agent to rinsing agent in the mixture increases the solvency, and thus is used for removing heavier soils.
The operating temperature of the system depends on the boiling point of the mixture, which in turn depends on the compositions of these agents in zeotropic mixture. Since zeotropic mixtures have different boiling points, the cleaning and rinse sump have different ratios of cleaning and solvating agents. The lower-boiling point solvating agent is not found in the rinse sump due to the large difference in boiling points between the agents.