Antifreezes and Deicing Fluids
Authors: Stefl Barbara A. , George Kathleen F. 
Published: July 2004

An antifreeze is defined as a chemical which, when added to a water‐based fluid, reduces the freezing point of the mixture. Antifreezes are used in a wide variety of mechanical equipment during the winter months to prevent freezing of aqueous heat‐transfer fluids. Most commonly, antifreeze refers to the freeze‐protected fluid which cools automotive engines, although antifreeze liquids are also used in ice skating rinks, refrigeration systems (as a secondary coolant), heating and air conditioning systems, solar energy units, and many other applications. Chemical antifreezes include brines, such as calcium chloride; alcohols, such as methanol, ethanol, and 2‐propanol; and glycerol and glycols. Since 1960 ethylene glycol has held the majority of the antifreeze market share because of its availability and superior performance characteristics. Many chemicals when added to water cause a freezing point depression, and thus are termed antifreezes. The reduction in freeze point depends both on the chemical itself and the concentration of the chemical in water. The freeze point depression increases as the antifreeze chemical is added to the water, until a characteristic concentration is achieved. Further addition of the antifreeze chemical to water will either result in insolubility or serve to increase the freezing point of the mixture. The colligative properties of antifreeze chemicals may also result in boiling point elevation. As the chemical is added to water, the boiling point of the mixture increases. Unlike the freeze depression, the boiling elevation does not experience a maximum; the boiling point versus concentration curve is a smooth curve that achieves its maximum at the 100% antifreeze level. The boiling point elevation can be another important characteristic for antifreeze fluids in certain heat‐transfer applications. Because the internal combustion engine is by far the largest antifreeze application, and ethylene glycol is the predominant antifreeze in use, the performance properties of an ethylene glycol‐based antifreeze and their relationship to engine cooling are the focus herein. The primary role of the antifreeze liquid in an internal combustion engine is to remove heat and thus cool the engine. If heat is not removed, the engine will be destroyed. The antifreeze fluid removes heat by passing through the engine where it picks up heat, then through the radiator where it is cooled by air. The fluid operates in a closed loop system and thus is continuously reused. To provide efficient cooling, the antifreeze must have a high specific heat and thermal conductivity and low viscosity at operating temperatures. The slight heat‐transfer penalty incurred when an antifreeze is added to the aqueous heat‐transfer fluid is necessitated by the need for increased operating temperature range in most internal combustion engines. Because most parts of the world achieve temperatures below freezing during some time of the year, an antifreeze fluid is required to keep equipment operational in these subfreezing temperatures. The colligative properties of antifreeze fluids dictate the freeze point depression of the antifreeze solution. For ethylene glycol solutions, the maximum freezing point is achieved by using a 68% aqueous solution; this concentration is known as the eutectic concentration. Boiling point elevation may also be beneficial in heat‐transfer applications. As engine power increases, the heat given off by the engine also increases. This increase in engine operating temperature increases engine efficiency. However, if too much heat remains in the engine, overheating of both the engine and the oil may occur and engine efficiency decreases. Enabling the coolant to remove more heat can be achieved by increasing the system pressure, and thus the boiling point of the coolant. Today, most engines operate slightly pressurized to increase the maximum use temperature of the coolant and thus eliminate boiling of the coolant. Other important properties of antifreeze solutions are the corrosion protection they provide and the compatibility of ethylene glycol with the various elastomers of the gaskets and hoses in the system. Before initial charging of a cooling system with fresh coolant, the system should be free of dirt, oil, and other contaminants. Similarly, the antifreeze itself must be prepared properly to extend the life of the fluid. In addition to the hardness ions, poor quality water of dilution can result in the introduction of corrosive ions, chloride and sulfate, to the system. Despite all these safeguards to extend the service life of the antifreeze, fluid replacement is required periodically. The most critical requirement for an antifreeze is that it provide adequate freeze protection. Because of evaporative losses of water and system makeup, it is necessary to monitor the freezing point of the antifreeze to ensure adequate protection. The performance characteristics of all antifreeze solutions are governed by fluid specifications that have been developed over the years by industry standards committees, such as the American Society for Testing and Materials (ASTM) and the Society of Automotive Engineers (SAE). Antifreeze chemicals, because of their characteristic freeze point depression, find additional applications in the deicing of aircraft and runways. Because of the stringent requirements of corrosion protection of aircraft surfaces and the necessity of a low flash point, aircraft deicing fluids use glycols exclusively. Runway deicing is commonly achieved with either urea or glycol‐based fluids that contain urea. The fluids must also contain corrosion inhibitors to minimize the corrosive effects on aircraft metals of construction, and must not adversely affect the acrylics and polycarbonates used for aircraft windows. The performance criteria for deicing fluids have traditionally been governed by specifications of the Aerospace Division of Society of Automotive Engineers (SAE). The toxicity of antifreeze and deicing fluids is predominantly a function of the main component, the freezing point depressant. For ethylene glycol‐based fluids, the toxicity is well‐defined; ethylene glycol is acutely toxic to humans and animals by oral exposure. Because of these toxicity concerns, propylene glycol is sometimes used instead of ethylene glycol in these applications. Studies on propylene glycol have shown no evidence of teratogenic effects and it has a lower oral toxicity. Another important consideration in the use antifreezes and deicing fluids is the rate and extent of biodegradation, or the length of time the chemical persists in the environment. Complete biooxidation of ethylene glycol‐based fluids, occurs within 20 days. The rate of biooxidation is steady throughout this period and thus exerts no undue oxygen demand on waterways (causing fish kill). Spent fluids contain varied contaminants that can drastically affect the toxicity and environmental impact of the fluid. Most pronounced is the impact of heavy‐metal contaminants in spent antifreeze. When antifreeze becomes unsuitable for use, recycling and reuse of the antifreeze rather than disposal may be considered. Two methods of recycling spent antifreeze, filtration or distillation, have been reported.