Antifreeze is a cryoprotectant used in internal combustion engines, and for many other heat transfer applications, such as HVAC chillers and solar water heaters. The purpose of antifreeze is to prevent a rigid enclosure from undergoing physical stresses and catastrophic deformation due to the expansion that occurs when water turns to ice. Compounds are added to the water to reduce the freezing point of the mixture below the lowest temperature that the system is likely to be exposed to, and to inhibit corrosion in cooling systems, which often contain a range of electrochemically incompatible metals (aluminum, cast iron, copper, lead solder, etc.).
The term colligative agent may better describe the benefits of these compounds in warm climates, since they not only achieve freezing point depression in the winter when mixed with water, they coincidentally achieve boiling point elevation of water. Colligative agents are properly referred to as both antifreeze and “anti-boil” when used for both properties. The term engine coolant is widely used in the automotive industry, which covers its primary function of convective heat transfer.
Methanol, also known as methyl alcohol, carbinol, wood alcohol, wood naphtha or wood spirits, is a chemical compound with chemical formula CH3OH (often abbreviated MeOH). It is the simplest alcohol, and is a light, volatile, colourless, flammable, poisonous liquid with a distinctive odor that is somewhat milder and sweeter than ethanol (ethyl alcohol). At room temperature it is a polar liquid and is used as an antifreeze, solvent, fuel, and as a denaturant for ethyl alcohol. It is not very popular for machinery, but it can be found in automotive windshield washer fluid, de-icers, and gasoline additives to name a few.
Ethylene glycol solutions became available in 1926 and were marketed as “permanent antifreeze,” since the higher boiling points provided advantages for summertime use as well as during cold weather. They are still used today for a wide variety of applications, including automobiles. Being ubiquitous, ethylene glycol has been ingested on occasion, causing ethylene glycol poisoning.
Propylene glycol, on the other hand, is considerably less toxic and may be labeled as “non-toxic antifreeze”. It is used as antifreeze where ethylene glycol would be inappropriate, such as in food-processing systems or in water pipes in homes, as well as numerous other settings. It is also used in food, medicines, and cosmetics, often as a binding agent. Propylene glycol is “generally recognized as safe” by the Food and Drug Administration (FDA) for use in food. However, propylene glycol-based antifreeze should not be considered safe for consumption. In the event of accidental ingestion, emergency medical services should be contacted immediately.
Propylene gycol oxidizes when exposed to air and heat. When this occurs lactic acid is formed. If not properly inhibited, this fluid can be very corrosive. Protodin is added to propylene glycol to act as a buffer, preventing low pH attack on the system metals. It forms a protective skin inside the tank and pipelines which helps to prevent acid attack that cause corrosion.
Besides cooling system breakdown, biological fouling also occurs. Once bacterial slime starts the corrosion rate of the system increases. In systems where a glycol solution is maintained on a continuous basis, regular monitoring of freeze protection, pH, specific gravity, inhibitor level, color and biological contamination should be checked routinely.
Propylene glycol should be replaced when it turns reddish in color.
Most commercial antifreeze formulations include corrosion inhibiting compounds, and a colored dye (commonly a green, red, or blue fluorescent) to aid in identification. A 1:1 dilution with water is usually used, resulting in a freezing point in the range of −35 °C to −40 °C, depending on the formulation. In warmer or colder areas, weaker or stronger dilutions are used, respectively, but a range of 40%/60% to 60%/40% must be maintained to assure corrosion protection and optimum freezing prevention.
Glycol antifreeze solutions should generally be replaced with fresh mixture every two years.
Organic acid technology
Certain cars are built with Organic Acid Technology (OAT) antifreeze (e.g., DEX-COOL), or with a Hybrid Organic Acid Technology (HOAT) formulation (e.g., Zerex G-05), both of which are claimed to have an extended service life of five years or 240,000 km (150,000 miles).
DEX-COOL specifically has caused controversy. Litigation has linked it with intake manifold gasket failures in GM’s 3.1L and 3.4L and with other failures in 3.8L & 4.3L engines. Class action lawsuits were registered in several states, and in Canada, to address some of these claims. The first of these to reach a decision was in Missouri where a settlement was announced early in December, 2007. Late in March 2008, GM agreed to compensate complainants in the remaining 49 states.
There are rumors that mixing DEX-COOL with standard green (non-OAT) coolant causes a chemical reaction that produces sludge in the cooling system. According to the DEX-COOL manufacturer, however, “mixing a ‘green’ [non-OAT] coolant with DEX-COOL reduces the batch’s change interval to 2 years or 30,000 miles, but will otherwise cause no damage to the engine.”
According to internal GM documents, the ultimate culprit appears to be operating vehicles for long periods of time with low coolant levels. The low coolant is caused by pressure caps that fail in the open position. (The new caps and recovery bottles were introduced at the same time as DEX-COOL). This exposes hot engine components to air and vapors, causing corrosion and contamination of the coolant with iron oxide particles, which in turn can aggravate the pressure cap problem as contamination holds the caps open permanently.
Typically OAT antifreeze contains a orange dye to differentiate it from the conventional glycol-based coolants (green or yellow). Some of the newer OAT coolants claim to be compatible with all types of OAT and glycol-based coolants; these are typically green or yellow in color (for a table of colors, see