Antifreeze

Antifreeze is a freeze preventive used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters. Antifreeze, also called coolant, is the colored fluid (usually green or red) found in your radiator. Antifreeze serves a few purposes. The most important and known is keeping the water in your radiator and engine from freezing in cold temps. It also keeps that same water from boiling over in the summer. Radiators are normally filled with a 50/50 mixture of antifreeze and water. The third function of antifreeze, or coolant is lubrication -- it lubricates the moving parts it comes in contact with, like the water pump.

An antifreeze mixture achieves freezing point depression for a cold environment and also achieves boiling point elevation to enable higher liquid temperatures. This is described as the action of a colligative agent, which can properly be referred to as both antifreeze and "anti-boil" when used for both properties. Careful selection of antifreeze can enable a wide temperature range in which the mixture remains in the liquid phase, which is critical to efficient heat transfer and the proper functioning of heat exchangers.

Ethylene glycol

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 used today for a variety of applications, including automobiles.

Ethylene glycol is toxic to humans, as well as to other animals,[3] and should therefore be handled and disposed of properly. It has a sweet taste that can contribute to its accidental ingestion or its deliberate use as a murder weapon, as attributed by sensational media reports concerning it.[4][5][6][7] Such poisoning is difficult to identify without specialized testing, as it often mimics other illnesses, and various symptoms can result from such poisoning, including severe diarrhea and vomiting.[8][4][5][6][7] Some ethylene glycol antifreeze contains an embittering agent such as denatonium to help discourage either accidental or deliberate consumption.

Propylene glycol

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 where incidental ingestion may be possible.

Propylene glycol oxidizes when exposed to air and heat, forming lactic acid.[10][11] If not properly inhibited, this fluid can be very corrosive,[citation needed] so pH buffering agents are often added to propylene glycol, to prevent acidic corrosion of metal components.

Besides cooling system breakdown, biological fouling also occurs. Once bacterial slime starts, the corrosion rate of the system increases. Maintenance of systems using glycol solution includes regular monitoring of freeze protection, pH, specific gravity, inhibitor level, color, and biological contamination. Propylene glycol should be replaced when it turns a reddish color.