The Two Molecules at a Glance
Ethylene glycol (ethane-1,2-diol, CAS 107-21-1) and 1,2-propanediol (propylene glycol, CAS 57-55-6) are neighbouring members of the same chemical family: short-chain diols that are completely miscible with water and that depress the freezing point of water in proportion to the amount dissolved. The difference of a single methyl group changes almost everything that matters commercially, namely freezing performance, heat transfer, toxicity, the range of regulatory grades available and price.
| Property | Ethylene glycol | 1,2-Propanediol |
|---|---|---|
| CAS number | 107-21-1 | 57-55-6 |
| Molecular formula | C2H6O2 | C3H8O2 |
| Molar mass | About 62 g/mol | About 76 g/mol |
| Atmospheric boiling point | About 197 C | About 188 C |
| Freeze point of a 50/50 water mixture | Around minus 37 C | Around minus 29 C |
| Acute toxicity by ingestion | High, metabolised to toxic acids | Low, metabolised like a food-type alcohol |
| Food and pharmaceutical grades | Not normally offered | Widely available |
Freeze Protection and Boiling Point Elevation
Both glycols protect a system in two ways: they lower the temperature at which the coolant solidifies and they raise the boiling point of the mixture above that of water, which increases the margin against vapour formation and cavitation. Ethylene glycol is the more efficient of the two on a per-volume basis, which is why a 50/50 ethylene glycol and water mixture reaches roughly minus 37 C while the equivalent propylene glycol mixture stops at about minus 29 C. In both cases the freeze point curve passes through a minimum and then rises again, so simply adding more glycol beyond the optimum, broadly in the 60 to 70 percent by volume region, makes protection worse rather than better. Formulators therefore work from a measured freeze point curve instead of assuming that more glycol always means more protection.
Heat Transfer and System Design
Ethylene glycol and water mixtures have lower viscosity and higher thermal conductivity than propylene glycol and water mixtures at the same temperature, so they transfer heat more effectively for a given pump power and produce lower wall temperatures. Propylene glycol systems compensate with slightly higher flow rates, larger heat exchanger surfaces or a larger concentration margin, which is a design consideration rather than a barrier. At low temperature the viscosity difference widens and pumpability becomes the limiting factor, particularly in outdoor chiller loops and cold-start conditions.
Toxicity: the Decisive Difference
Ethylene glycol itself is only moderately hazardous, but it is oxidised in the body by alcohol dehydrogenase to glycolaldehyde, glycolic acid and oxalic acid. The acids cause a severe metabolic acidosis, and the oxalate binds calcium, producing calcium oxalate deposits in the kidneys and potentially acute kidney injury. Ingestion of a small volume, on the order of 1.4 mL per kilogram of body weight, which can be a few tens of millilitres for a child, may be life-threatening, and treatment relies on blocking the enzyme with ethanol or fomepizole together with supportive care. Because the liquid is sweet-tasting, accidental ingestion by children and animals is a genuine risk, and many jurisdictions require it to be sold with a bittering agent.
1,2-Propanediol follows a different metabolic path: it is oxidised to lactaldehyde and then to lactic acid and pyruvate, both of which are ordinary intermediates of carbohydrate metabolism. As a result it is regarded as a low-toxicity material and is permitted in food-contact, pharmaceutical and cosmetic applications, including as a carrier in oral medicines. This does not make it harmless, since large doses act as a laxative and it should still be kept away from children, but the safety margin is measured in grams rather than millilitres.
For industrial supply, the practical consequence is that glycols intended for food, beverage, dairy, brewery, pharmaceutical or medical circuits must be propylene glycol based, and that diethylene glycol must never be substituted for propylene glycol in any of those duties.
Grades, Cost and Supply
Ethylene glycol is a very large-volume commodity chemical produced by hydrolysis of ethylene oxide, and it is available in technical and industrial grades at the lowest cost per litre of protection. 1,2-Propanediol is produced by hydration of propylene oxide and is made in smaller volumes; it is offered in technical, food-grade including the Food Chemicals Codex monograph, and pharmaceutical USP and Ph. Eur. qualities, each carrying additional testing and certification cost. Propylene glycol consequently carries a clear price premium over industrial-grade ethylene glycol, and that premium grows sharply once food or pharmaceutical certification is required.
Specifications and Regulatory Frameworks
Automotive and industrial coolants are usually purchased against recognised specifications rather than on glycol content alone. Engine coolant based on ethylene glycol is commonly specified to ASTM D3306, while propylene glycol base products are covered by the corresponding propylene glycol coolant specification; in China the national standard for motor vehicle engine coolant is GB 29743. Fully formulated coolants add an inhibitor package, for example phosphate, silicate, organic acid technology or a hybrid of these, to control corrosion of aluminium, copper, brass, solder, cast iron and elastomers, and it is the inhibitor package rather than the glycol that determines coolant service life. For food and pharmaceutical duty, buyers specify the Food Chemicals Codex, USP or Ph. Eur. monograph grade and expect batch documentation that traces the material through production and packing.
How to Choose
Choose a propylene glycol base when the circuit can touch food, drinking water, beverages, pharmaceuticals, dairy or medical devices, or where leakage into a sensitive environment is credible.
Choose an ethylene glycol base when the priority is the coldest possible protection, the best heat transfer per unit of pumping power and the lowest cost per litre of installed system.
Use measured freeze point curves for both chemistries, because exceeding the eutectic concentration degrades protection instead of improving it.
Specify the inhibitor package against the metals actually present in the loop, and refresh the fluid on the basis of reserve alkalinity rather than appearance.
Frequently Asked Questions
Q: Is 1,2-propanediol the same as antifreeze?
It is one of the two common antifreeze bases. Propylene glycol antifreeze is chemically the same family as 1,2-propanediol, but a finished coolant also contains water and an inhibitor package.
Q: Which glycol protects against lower temperatures?
Ethylene glycol, on an equal volume basis. A 50/50 mixture reaches about minus 37 C compared with roughly minus 29 C for propylene glycol, so less glycol is needed for the same protection.
Q: Why is ethylene glycol more dangerous if swallowed?
It is metabolised to glycolic and oxalic acid, causing metabolic acidosis and kidney damage. Even small ingested volumes can be lethal, whereas propylene glycol is metabolised to ordinary metabolic intermediates.
Q: Can propylene glycol be used in food processing?
Yes. Food-grade and pharmaceutical monograph grades are widely used in food, beverage, dairy, brewery and medical circuits where contact with the product is possible.
Q: Is propylene glycol worth the higher price?
Where low toxicity or regulatory acceptance is required, the premium is unavoidable. For closed industrial cooling where toxicity is not a concern, ethylene glycol usually offers the better cost per degree of protection.
Q: Can the two glycols be mixed in one system?
They are mutually miscible, but mixing makes the freeze point harder to verify and complicates inhibitor selection. Choose one base chemistry and dose it against a measured freeze curve.
