What Sets Propylene Glycol and Ethylene Glycol Apart
Propylene glycol and ethylene glycol are both simple diols used as antifreeze, coolants and industrial fluids, yet they diverge sharply in toxicity, thermal performance and the applications for which they are accepted. Ethylene glycol is acutely toxic to humans and animals and remains the workhorse coolant for closed industrial and automotive systems, while propylene glycol has low acute toxicity and is accepted for food, cosmetic and pharmaceutical contact in many jurisdictions. Choosing between them is therefore a question of where the fluid can leak, not simply which one transfers heat better.
Ethylene Glycol: Identity and Key Properties
Ethylene glycol, also called monoethylene glycol (MEG), has the formula C2H6O2, a molecular weight of 62.07 g/mol and the CAS registry number 107-21-1. It is a clear, colourless, odourless, viscous liquid with a slightly sweet taste, a density near 1.11 g/cm³ at 20 °C, a boiling point of 197.3 °C and a freezing point of about -12.9 °C in the pure form. It is fully miscible with water in all proportions, which is what allows aqueous glycol coolants to be formulated across a wide operating range. MEG is also a primary raw material for polyester and PET production, where it reacts with terephthalic acid to build polymer chains.
Propylene Glycol: Identity and Key Properties
Propylene glycol has the formula C3H8O2, a molecular weight of 76.09 g/mol and the CAS registry number 57-55-6. It is a colourless, nearly odourless, water-miscible liquid with a boiling point of about 188.2 °C and a freezing point near -59 °C. Its low acute toxicity makes it acceptable in food flavour carriers, cosmetics, pharmaceutical formulations, personal care products and aircraft deicing fluids, and it is also used in heat transfer circuits where the fluid could come into contact with food, drinking water or occupied spaces. Technical grades are not intended for direct consumption even though food and pharmaceutical grades of the same chemistry are permitted in controlled amounts.
Toxicity, Heat Transfer and Comparison Data
| Property | Ethylene glycol (CAS 107-21-1) | Propylene glycol (CAS 57-55-6) |
|---|---|---|
| Molecular formula | C2H6O2 | C3H8O2 |
| Molecular weight | 62.07 g/mol | 76.09 g/mol |
| Boiling point | About 197.3 °C | About 188.2 °C |
| Freezing point (pure) | About -12.9 °C | About -59 °C |
| Acute toxicity | High; ingestion can cause metabolic acidosis and kidney failure | Low; accepted for food and pharmaceutical use in controlled amounts |
| Typical primary role | Engine and industrial coolant, polyester feedstock | Food-contact heat transfer, deicing, cosmetics, pharmaceuticals |
The safety gap is the decisive difference. Ethylene glycol is metabolised into acidic species that cause metabolic acidosis, central nervous system depression and kidney damage, so products containing it must be labelled and stored away from children and animals. Propylene glycol has a far higher oral toxicity threshold, which is why it is preferred wherever leakage into food, drinking water or living areas is possible. On the thermal side, ethylene glycol has lower viscosity and higher thermal conductivity, giving better heat transfer efficiency at a given flow rate; systems running propylene glycol may need higher circulation rates, larger heat exchangers or slightly higher pump duty to achieve the same heat removal. Both fluids are usually supplied as inhibited grades containing a corrosion inhibitor package that must be matched to the metals in the circuit, such as copper, brass, steel or aluminium.
How to Select Between Propylene Glycol and Ethylene Glycol
Choose ethylene glycol for closed circuits where the fluid cannot reach food, drinking water or people, and where maximum heat transfer efficiency and lower pumping cost are the priority.
Choose propylene glycol when leakage risk exists near food processing lines, breweries, dairies, hospitals, solar thermal systems or any occupied space.
Confirm the CAS number, the concentration of the aqueous blend and the freeze protection point required for the local climate.
Verify the inhibitor package against the system metals and request the fluid's maintenance interval and reserve alkalinity data.
Check compatibility of seals, gaskets and hoses with the selected glycol before commissioning.
Frequently Asked Questions
Q: What is the main difference between propylene glycol and ethylene glycol?
The main differences are toxicity and thermal performance: ethylene glycol is toxic but transfers heat more efficiently, while propylene glycol is much less toxic and is accepted for food and pharmaceutical contact.
Q: What is the CAS number of propylene glycol?
The CAS registry number of propylene glycol is 57-55-6.
Q: What is the CAS number of ethylene glycol?
The CAS registry number of ethylene glycol is 107-21-1, and it is also known as monoethylene glycol.
Q: Why is ethylene glycol toxic?
It is metabolised into acidic metabolites that cause metabolic acidosis, central nervous system depression and kidney failure, so it must never be ingested.
Q: Can propylene glycol be used in food applications?
Yes. Propylene glycol has low acute toxicity and is accepted in food, cosmetic and pharmaceutical applications in controlled amounts, although technical grades are not for direct consumption.
Q: Which glycol gives better heat transfer?
Ethylene glycol generally gives better heat transfer because of its lower viscosity and higher thermal conductivity, while propylene glycol is selected where safety and leak location matter more.