Antifreeze
An antifreeze is an additive which lowers the freezing point of a water-based liquid. An antifreeze mixture is used to achieve freezing-point depression for cold environments. Common antifreezes also increase the boiling point of the liquid, allowing higher coolant temperature. However, all common antifreeze additives also have lower heat capacities than water, and do reduce water's ability to act as a coolant when added to it.
Because water has good properties as a coolant, water plus antifreeze is 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 bursting due to expansion when water freezes. Commercially, both the additive and the mixture are called antifreeze, depending on the context. Careful selection of an 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. Most if not all commercial antifreeze formulations intended for use in heat transfer applications include anti-corrosion and anti-cavitation agents.
Principles and history
Water was the original coolant for internal combustion engines. It is cheap, nontoxic, and has a high heat capacity. It however has only a 100 Kelvin liquid range, and it expands upon freezing. To address these problems, alternative coolants with improved properties were developed.Freezing and boiling points are colligative properties of a solution, which depend on the concentration of dissolved substances. Salts lower the melting points of aqueous solutions. Salts are frequently used for de-icing, but salt solutions are not used for cooling systems because they induce corrosion of metals. Low molecular weight organic compounds tend to have melting points lower than water, which makes them suitable for use as antifreeze agents. Solutions of organic compounds, especially alcohols, in water are effective. Alcohols such as methanol, ethanol, ethylene glycol, etc. have been the basis of all antifreezes since they were commercialized in the 1920s.
Use and occurrence
Automotive and internal combustion engine use
Most automotive engines are "water"-cooled to remove waste heat, though the "water" used is actually a mixture of water and antifreeze. 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' radiators, which often contain a range of electrochemically incompatible metals. Water pump seal lubricant is also added.Antifreeze was developed to overcome the shortcomings of water as a heat transfer fluid.
On the other hand, if the engine coolant gets too hot, it might boil while inside the engine, causing voids, leading to localized hot spots and the catastrophic failure of the engine. If plain water were to be used as an engine coolant in northern climates freezing would occur, causing significant internal engine damage. Also, plain water would increase the prevalence of galvanic corrosion. Proper engine coolant and a pressurized coolant system obviate these shortcomings of water. With proper antifreeze, a wide temperature range can be tolerated by the engine coolant, such as to for 50% propylene glycol diluted with distilled water and a 15 psi pressurized coolant system.
Early engine coolant antifreeze was methanol. Ethylene glycol was developed because its higher boiling point was more compatible with heating systems.
Engine coolant standards
The Volkswagen Group has been particularly committed to the development of coolants and their standards in collaboration with Haertol Chemie from Magdeburg. VW standards include: G11, G12, G12+, G12++, G13 and G12evo.Another company involved in the development is BASF, whose standards are: G30, G40, G48, G05, G33, and G34.
Volkswagen Group:
- G11: VW TL 774 C
- G12 / G12+: VW TL 774 D/F
- G12++: VW TL 774 G
- G13: VW TL 774 J
- G12evo: VW TL 774 L
- Glysantin G48: matches VW TL 774-C
- Glysantin G30: matches VW TL 774-D/F
- Glysantin G40: matches VW TL 774-G and VW TL 774-J
- Glysantin G64: matches VW TL 774-L
Other industrial uses
Ethylene glycol has desirable thermal properties, including a high boiling point, low freezing point, stability over a wide range of temperatures, and high specific heat and thermal conductivity. It also has a low viscosity and, therefore, reduced pumping requirements. Although EGW has more desirable physical properties than PGW, the latter coolant is used in applications where toxicity might be a concern. PGW is generally recognized as safe for use in food or food processing applications, and can also be used in enclosed spaces.
Similar mixtures are commonly used in HVAC and industrial heating or cooling systems as a high-capacity heat transfer medium. Many formulations have corrosion inhibitors, and it is expected that these chemicals will be replenished to keep expensive piping and equipment from corroding.
Biological antifreezes
refer to chemical compounds produced by certain animals, plants, and other organisms that prevent the formation of ice. In this way, these compounds allow their host organism to operate at temperatures well below the freezing point of water. Antifreeze proteins bind to small ice crystals to inhibit growth and recrystallization of ice that would otherwise be fatal.Cryoprotectants are commonly used in cryobiology to prevent or inhibit freezing in sperm, blood, stem cells, plant seeds, etc. Ethylene glycol, propylene glycol, and glycerol are commonly used as biological cryoprotectants.
Primary agents
Ethylene glycol
Most antifreeze is made by mixing distilled water with additives and a base product, usually MEG or MPG. Ethylene glycol solutions first 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, but there are lower-toxicity alternatives made with propylene glycol available.When ethylene glycol is used in a system, it may become oxidized to five organic acids. Inhibited ethylene glycol antifreeze mixes are available, with additives that buffer the pH and preserve alkalinity of the solution to prevent oxidation of ethylene glycol and formation of these acids. Nitrites, silicates, borates and azoles may also be used to prevent corrosive attack on metal.
Ethylene glycol has a bitter, sweet taste and causes inebriation. The toxic effects of ingesting ethylene glycol occur because it is converted by the liver into 4 other chemicals that are much more toxic. The lethal dose of pure ethylene glycol is 1.4 ml/kg but is much less lethal if treated within an hour..
Propylene glycol
is considerably less toxic than ethylene glycol 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. For example, the U.S. FDA allows propylene glycol to be added to a large number of ultra-processed foods, including ice cream, frozen custard, salad dressings, and baked goods, and it is commonly used as the main ingredient in the "e-liquid" used in electronic cigarettes.Propylene glycol oxidizes to lactic acid.
Besides cooling system corrosion, biological fouling also occurs. Once bacterial slime starts to grow, 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. When an aqueous solution of propylene glycol in a cooling or heating system develops a reddish or black color, this indicates that iron in the system is corroding significantly. In the absence of inhibitors, propylene glycol can react with oxygen and metal ions, generating various compounds including organic acids. These acids accelerate the corrosion of metals in the system.
Other antifreezes
is used as an antifreeze in diesel engines. It is more volatile than glycol.Once used for automotive antifreeze, glycerol has the advantage of being non-toxic, withstands relatively high temperatures, and is noncorrosive. It is not however used widely.
Glycerol was historically used as an antifreeze for automotive applications before being replaced by ethylene glycol. Volkswagen introduced G13 antifreezes containing glycerol in 2008, marketed as better for the environment due to its low toxicity and reduced emissions. However, since 2018, they have moved on to G12EVO which no longer contains glycerol.
Glycerol is mandated for use as an antifreeze in many sprinkler systems.
Measuring the freeze point
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 cooling system 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 by measuring the concentration:- Specific gravity—,
- Refractometer—which measures the refractive index of the antifreeze solution, and
- Test strips—specialized, disposable indicators made for this purpose.
The boiling point can be similarly determined by a concentration given from one of the three methods. Datasheets for glycol/water coolant mixtures are commonly available from chemical vendors.