Antifreeze is a freeze preventive used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters. The purpose of antifreeze is to prevent a rigid enclosure from undergoing catastrophic deformation due to expansion when water turns to ice. Antifreezes are chemical compounds added to water to reduce the freezing point of the mixture below the lowest temperature that the system is likely to encounter. Either the additive or the mixture may be referred to as antifreeze.
Most automotive engines are "water-cooled" to remove waste heat, although the "water" is actually antifreeze/water mixture and not plain water. The term engine coolant is widely used in the automotive industry, which covers its primary function of convective heat transfer for internal combustion engines. When used in an automotive context, corrosion inhibitors are added to help protect vehicles' cooling systems, which often contain a range of electrochemically incompatible metals (aluminum, cast iron, copper, solder, et cetera). Water pump seal lubricant is also added.
The most common water-based antifreeze solutions used in electronics cooling are mixtures of water and either ethylene glycol (EGW) or propylene glycol (PGW). The use of ethylene glycol has a longer history, especially in the automotive industry. However, EGW solutions formulated for the automotive industry often have silicate based rust inhibitors that can coat and/or clog heat exchanger surfaces. The use of PGW as a coolant is becoming more common primarily because it is environmentally friendly and non-toxic. Ethylene glycol is listed as a toxic chemical requiring care in handling and disposal.
Most antifreeze is made by mixing water (preferably distilled or demineralized) with some kind of alcohol.
Methanol (also known as methyl alcohol, carbinol, wood alcohol, wood naphtha or wood spirits) is a chemical compound with chemical formula CH3OH (sometimes abbreviated MeOH). It is the simplest alcohol, and is a light, volatile, colorless, flammable, poisonous liquid with a distinctive odor that is somewhat milder and sweeter than ethanol (ethyl alcohol). At room temperature, it is a polar solvent and is used as an antifreeze, solvent, fuel, and as a denaturant for ethyl alcohol. It is not popular for machinery, but may be found in automotive windshield washer fluid, de-icers, and gasoline additives.
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, 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. 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. Some ethylene glycol antifreeze contains an embittering agent such as denatonium to help discourage either accidental or deliberate consumption.
Once used for automotive antifreeze, glycerol has the advantage of being non-toxic and could be used in the future for some applications.
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.
Once antifreeze has been mixed with water and put into use, it periodically needs to be maintained. If engine coolant leaks, boils, or if the radiator needs to be drained and refilled, the antifreeze's freeze protection will need to be considered. In other cases a vehicle may need to be operated in a colder environment, requiring more antifreeze and less water. Three methods are commonly employed to determine the freeze point of the solution.
Most commercial antifreeze formulations include corrosion inhibiting compounds, and a colored dye (commonly a green, red, orange, yellow, or blue fluorescent) to aid in identification. A 1:1 dilution with water is usually used, resulting in a freezing point of about −34 °F (−37 °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% is frequently specified to ensure corrosion protection, and 70%/30% for maximum freeze prevention down to −84 °F (−64 °C).
Traditionally, there were two major corrosion inhibitors used in vehicles: silicates and phosphates. American made vehicles traditionally used both silicates and phosphates. European makes contain phosphates and other inhibitors, but no silicates. Japanese makes traditionally use phosphates and other inhibitors, but no silicates.
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).
HOAT coolants typically mix an OAT with a traditional inhibitor, such as silicates or phosphates.
All automotive antifreeze formulations, including the newer organic acid (OAT antifreeze) formulations, are environmentally hazardous because of the blend of additives (around 5%), including lubricants, buffers and corrosion inhibitors. Because the additives in antifreeze are proprietary, the material safety data sheets (MSDS) provided by the manufacturer list only those compounds which are considered to be significant safety hazards when used in accordance with the manufacturer's recommendations. Common additives include sodium silicate, disodium phosphate, sodium molybdate, sodium borate, and dextrin (hydroxyethyl starch). Disodium fluorescein dyes are added to antifreeze to help trace the source of leaks, and as an identifier since some different formulations are incompatible.
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