Stable, unstable, diphase, and other emulsion cleaners cover a wide range of solvent and emulsifier compositions. The solvent is generally of petroleum origin and may be heterocyclic (Mpyrol), naphthenic, aromatic, or of hydrocarbon nature (kerosine).

Solvents are available with boiling points of 60 to 260 °C (140 to 500 °F) and flash points ranging from room temperature to above 95 °C (200 °F). Because the solubility factor increases as the molecular weight of the solvent approaches that of water, low-to-medium molecular weight solvents are usually more effective in removing soils. However, fire hazards and evaporation losses increase as boiling and flash points decrease.

Emulsifiers include:

• Nonionic polyethers and high-molecular-weight sodium or amine soaps of fatty acids

• Amine salts of alkyl aryl sulfonates (anionic)

• Fatty acid esters of polyglycerides

• Polyalcohols

Cationic ethoxylated long-chain amines and their salts are also used in emulsions.

Emulsifiers must have some solubility in the solvent phase. When solubility is low, it can be increased by adding a coupling agent (hydrotrope), such as a higher-molecular-weight alcohol, ester, or ether. These additives are soluble in oil and water.

Emulsion Types and Stability. The stability of emulsion cleaners depends on the properties of emulsifying agents that are capable of causing oil and water to mix uniformly. Because oil and water do not mix naturally, an oil-in-water mixture that does not contain an emulsifying agent or dispersant requires constant mechanical agitation to prevent the oil and water from separating into two layers. Emulsifying agents can be placed in two categories:

• Those that promote the formation of solvent-in-water emulsions, in which water constitutes the continuous phase and solvent constitutes the discontinuous phase

• Those that promote the formation of water-in-solvent emulsions, in which water is the dispersed discontinuous phase

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