Materials employed in the cooling systems and radiators of internal-combustion engines to ensure a liquid circulating medium at low temperatures to prevent damage from the formation of ice. 

 The requirements are that the compound must give a freezing point below that likely to be encountered without lowering the boiling point much below that of water, that it must not corrode the metals or deteriorate rubber connections, that it must be stable up to the boiling point, and that it must be readily obtainable commercially. Calcium chloride was early used for automobile radiators but corroded the metals. It is still used in fire tanks, sodium chromate being added to retard corrosion. Oils were also used, but the high boiling points permitted overheating of the engine, and the oils softened the rubber. Denatured ethyl alcohol may be used, but methanol is less corrosive and less expensive. A 30% solution of ethyl alcohol in water has a freezing point of about 5^oF (-15^oC), and a 50% solution freezes at —24^oF (-31^oC). Alcohol, however, must be renewed frequently because of loss by evaporation.

 Glycerol is also used as an antifreeze, a 40% solution in water lowering the freezing point to about 0^oF (-18^oC), and a 50% solution to - 15^oF (-2 5^oC). It has the disadvantage of high viscosity, requiring forced circulation at low temperatures, but it does not evaporate easily. Ethylene glycol lowers the freezing point to a greater extent than alcohol and has a high boiling point so that it is not lost by evaporation, but it has a higher first cost and will soften ordinary natural rubber connections. Acetamide in water solution may also be used as an antifreeze. Antifreezes are sold under various trade names. Zerone, of Du Pont, has a methanol base, while Zerex has a base of ethylene glycol. Prestone, marketed by the Union Carbide Corp., is ethylene glycol antifreeze. Pyro is an antifreeze of the U. S. Industrial Chemicals, Inc. with a low freezing point. Ramp, of the Antara Chemical Co., is ethylene glycol with anticorrosion and antifoam agents added. Antifreeze PFA55MB, of the Phillips Petroleum Co., used in jet-engine fuels, is ethylene glycol monoethyl ether with 10% glycerin. Dowtherm 209, of Dow Chemical Co., is an antifreeze material of inhibited methoxypropanol which boils off without forming gum.