Anantifreeze is an additive which lowers the freezing point of a water-based liquid. An antifreeze mixture is used to achievefreezing-point depression for cold environments. Common antifreezes also increase the boiling point of the liquid, allowing higher coolant temperature.[1] However, all common antifreeze additives also have lowerheat capacities than water, and do reduce water's ability to act as acoolant when added to it.[2]
Becausewater has good properties as a coolant, water plus antifreeze is used ininternal combustion engines and other heat transfer applications, such asHVACchillers andsolar water heaters. The purpose of antifreeze is to prevent a rigid enclosure from bursting due to expansion whenwater freezes. Commercially, both theadditive (pure concentrate) and themixture (diluted solution) are called antifreeze, depending on the context. Careful selection of an antifreeze can enable a wide temperature range in which the mixture remains in theliquid phase, which is critical to efficientheat transfer and the proper functioning ofheat exchangers. Most if not all commercial antifreeze formulations intended for use in heat transfer applications include anti-corrosion and anti-cavitation agents (that protect the hydraulic circuit from progressive wear).
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 arecolligative properties of a solution, which depend on the concentration of dissolved substances. Salts lower the melting points of aqueous solutions.Salts are frequently used forde-icing, but salt solutions are not used for cooling systems because they inducecorrosion of metals. Lowmolecular weight organic compounds tend to have melting points lower than water, which makes them suitable for use as antifreeze agents. Solutions of organic compounds, especiallyalcohols, 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.[1]
Most automotive engines are"water"-cooled to removewaste heat, though the "water" used is actually a mixture of water and antifreeze. The termengine coolant is widely used in theautomotive industry, which covers its primary function ofconvective heat transfer forinternal combustion engines. When used in an automotive context,corrosion inhibitors are added to help protect vehicles'radiators, which often contain a range ofelectrochemically incompatible metals (aluminum,cast iron,copper,brass,solder, etc.). Water pump seal lubricant is also added.
Antifreeze was developed to overcome the shortcomings ofwater as a heat transfer fluid.
On the other hand, if the engine coolant gets too hot, it might boil while inside the engine, causingvoids (pockets of steam), 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 ofgalvanic 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 −34 °F (−37 °C) to +265 °F (129 °C) for 50% (by volume)propylene glycol diluted with distilled water and a 15psi pressurized coolant system.
Early engine coolant antifreeze wasmethanol (methyl alcohol).Ethylene glycol was developed because its higher boiling point was more compatible with heating systems.
The Volkswagen Group has been particularly committed to the development of coolants and their standards (VW TL 774 ) 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 (Glysantin), whose standards are: G30, G40, G48, G05, G33, and G34.
Volkswagen Group:
BASF:
The most common water-based antifreeze solutions used inelectronics 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. Ethylene glycol is listed as a toxic chemical requiring care in handling and disposal.
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 inHVAC and industrial heating or cooling systems as a high-capacityheat transfer medium. Many formulations have corrosion inhibitors, and it is expected that these chemicals will be replenished (manually or under automatic control) to keep expensive piping and equipment from corroding.
Antifreeze proteins refer to chemical compounds produced by certainanimals,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 smallice crystals to inhibit growth andrecrystallization of ice that would otherwise be fatal.[5][6]
Cryoprotectants are commonly used incryobiology to prevent or inhibit freezing in sperm, blood, stem cells, plant seeds, etc.[7][8] Ethylene glycol, propylene glycol, and glycerol (all used in automotive antifreeze) are commonly used as biological cryoprotectants.[7][8]
Most antifreeze is made by mixing distilled water with additives and a base product, usually MEG (mono ethylene glycol) or MPG (mono propylene glycol).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, includingautomobiles, but there are lower-toxicity alternatives made withpropylene glycol available.
When ethylene glycol is used in a system, it may become oxidized to five organic acids (formic, oxalic, glycolic, glyoxalic and acetic acid). 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 andazoles 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 (3 US fluid ounces (90 ml) is lethal to a 140-pound (64 kg) person) but is much less lethal if treated within an hour.[9] (seeEthylene glycol poisoning).
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 allowspropylene glycol to be added to a large number ofultra-processed foods, includingice cream,frozen custard, salad dressings, andbaked goods, and it is commonly used as the main ingredient in the "e-liquid" used inelectronic cigarettes.Propylene glycoloxidizes tolactic acid.[10]
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 (e.g., formic, oxalic, acetic). These acids accelerate the corrosion of metals in the system.[11][12][13][14]
Propylene glycol methyl ether is used as an antifreeze in diesel engines. It is more volatile than glycol.[1]
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.[1]Glycerol was historically used as an antifreeze for automotive applications before being replaced byethylene glycol.[15][16]Volkswagen introducedG13 (TL 774-G) antifreezes containing glycerol in 2008, marketed as better for the environment due to its low toxicity and reducedCO2 emissions.[17] However, since 2018, they have moved on toG12EVO (TL 774-L) which no longer contains glycerol.[18]
Glycerol is mandated for use as an antifreeze in many sprinkler systems.[citation needed]
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:[19]
Both specific gravity and refractive index are affected by temperature, although the former is affected much less catastrophically. Temperature compensation is nevertheless recommended for RI measurement.[19] Propylene glycol solutions cannot be tested using specific gravity because of ambiguous results (40% and 100% solutions have the same specific gravity),[19] although typical uses rarely exceed 60% concentration.
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.[20]
Most commercial antifreeze formulations includecorrosion inhibiting compounds, and a coloreddye (commonly afluorescent green, red, orange, yellow, or blue) to aid in identification.[21] A 1:1dilution 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).[22]
In the absence of leaks, antifreeze chemicals such as ethylene glycol or propylene glycol may retain their basic properties indefinitely. By contrast, corrosion inhibitors are gradually used up, and must be replenished from time to time. Larger systems (such asHVAC systems) are often monitored by specialist firms which take responsibility for adding corrosion inhibitors and regulating coolant composition. For simplicity, most automotive manufacturers recommend periodic complete replacement of engine coolant, to simultaneously renew corrosion inhibitors and remove accumulated contaminants.
Traditionally, there were two major corrosion inhibitors used in vehicles:silicates andphosphates. American-made vehicles traditionally used both silicates and phosphates.[23] European makes contain silicates and other inhibitors, but no phosphates.[23] Japanese makes traditionally use phosphates and other inhibitors, but no silicates.[23][24]
Most modern cars are built with organic acid technology (OAT) antifreeze (e.g., DEX-COOL[25]), or with a hybrid organic acid technology (HOAT) formulation (e.g., Zerex G-05),[26] both of which are claimed to have an extended service life of five years or 240,000 km (150,000 mi).
DEX-COOL specifically has causedcontroversy. Litigation has linked it with intake manifold gasket failures inGeneral Motors' (GM's) 3.1L and 3.4L engines, and with other failures in 3.8L and 4.3L engines. One of the anti-corrosion components presented as sodium orpotassium 2-ethylhexanoate andethylhexanoic acid is incompatible withnylon 6,6 andsilicone rubber, and is a knownplasticizer.Class action lawsuits were registered in several states of the US, and in Canada,[27] 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.[28] Late in March 2008, GM agreed to compensate complainants in the remaining 49 states.[29] GM (Motors Liquidation Company) filed for bankruptcy in 2009, which tied up the outstanding claims until a court determines who gets paid.[30]
According to the DEX-COOL manufacturer, "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".[31] DEX-COOL antifreeze uses two inhibitors:sebacate and 2-EHA (2-ethylhexanoic acid), the latter which works well with thehard water found in the United States, but is aplasticizer that can cause gaskets to leak.[23]
According to internal GM documents,[31] 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.[31]
Honda and Toyota's new extended life coolants use OAT with sebacate, but without the 2-EHA. Some added phosphates provide protection while the OAT builds up.[23] Honda specifically excludes 2-EHA from its formulas.
Typically, OAT antifreeze contains an orange dye to differentiate it from the conventional glycol-based coolants (green or yellow), though some OAT products may contain a red or mauve dye. Some of the newer OAT coolants claim to be compatible withall types of OAT and glycol-based coolants; these are typically green or yellow in color.[21]
HOAT coolants typically mix an OAT with a traditional inhibitor, usually silicates.[32]
An example isZerex G05, which is a low-silicate, phosphate free formula that includes thebenzoate inhibitor.[23]
A HOAT coolant can have a life expectancy as high as 10 years / 180,000 miles.[32]
P-HOAT coolants mix phosphates with HOAT.[32] This technology is typically used in Asian makes and is often dyed red or blue.[32]
Si-OAT coolants mix silicates with HOAT.[32] This technology is typically used in European makes and is often dyed pink.[32]
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.[33] Because the additives in antifreeze are proprietary, thesafety data sheets (SDS) 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 includesodium silicate,disodium phosphate,sodium molybdate,sodium borate,denatonium benzoate, anddextrin (hydroxyethyl starch).
Disodium fluorescein dye is added to conventional ethylene glycol formulas to visually distinguish leaked amounts from other vehicle fluids, and as a marker of type to distinguish it from incompatible types.[21] This dye fluoresces bright green when illuminated by blue orUV light from daylight or testing lamps.
Automotive antifreeze has a characteristic odor due to the additivetolyltriazole, a corrosion inhibitor. The unpleasant odor in industrial-use tolyltriazole comes from impurities in the product that are formed from thetoluidine isomers (ortho-, meta-, and para-toluidine) and meta-diamino toluene which are side-products in the manufacture of tolyltriazole.[34] These side-products are highly reactive and produce volatile aromatic amines which are responsible for the unpleasant odor.[35]
Antifreeze is a poisonous substance[36] and is considered to be very dangerous to ingest. The main ingredient which makes it dangerous isethylene glycol, which, when ingested, ismetabolized in the liver into various intermediate substances, which then get turned intooxalic acid.[37] Oxalic acid is incredibly dangerous as it can cause, among other ailments,kidney failure, which is why a major treatment for antifreeze poisoning isdialysis to combat said kidney failure.[37] Antifreeze is commonly consumed due to its sweet taste caused by the ethylene glycol,[38] and is also commonly consumed as asurrogate alcohol due to its high alcohol contents. To prevent consumption due to taste, many brands have bitter additives, but many[39][40][41] studies do not support the idea bitter additives reduce ingestions. Common symptoms of poisoning arevomiting,confusion,abdominal pain, agitation,ataxia andhematuria. Long term damage such as kidney damage,brain damage, central nervous system damage, andblindness are common.[42][43]
