Ethylene glycol and propylene glycol based coolants and antifreeze have some fundamental differences that can determine the best product for the application.

The main difference between propylene glycol and ethylene is the level of toxicity and efficiency of performance. Ethylene glycol possess superior heat transfer properties due to a higher viscosity and a high boiling point whereas Propylene Glycol carries a low toxicity.    

Ethylene glycol is commonly used in engines and industrial high temperature heat transfer applications. Propylene glycol as an engine coolant is common in applications where environmental contact and toxicity may be a concern. Propylene glycol as a coolant does not have the same density and therefore heat transfer capacity as ethylene glycol, as it pertains to the transfer of heat from the engine to the cooling system when mixed properly with pure water.   

Other key difference in these product include:

• Ethylene glycol as a coolant is more effective than propylene glycol. More antifreeze is needed of a propylene glycol based product to achieve the same freeze point.
• Ethylene glycol has superior heat transfer efficiency due to lower viscosity, but more fluid must be circulated to transfer the same amount of energy since propylene glycol has a higher specific heat.
• Since propylene glycol has a higher viscosity it increases pump head loss in circulating systems.  
• Even though propylene glycol has a lower toxicity than ethylene glycol it requires more time to biodegrade.
• Ethylene glycol should not be used where there is any chance of contact with drinking water or food processing system, or environmentally sensitive areas.
• Both products have low flammability and are not considered a carcinogen.

Application requirements for cooling systems can help determine the best product. There are a number of benefits using ethylene glycol over propylene glycol, in closed loop systems where risk of environmental or food contact is minimal. The use of propylene or ethylene glycol depends on the application in hand and the risk of accidental contact with food, potable water or human ingestion.

While propylene is slightly more expensive to produce, it offers a combination of safety and effectiveness which would make it a superior choice for marine use. Both products can be mixed if additive formulations are compatible. Additive compatibility is key when mixing any type of coolant or antifreeze formulation. If additives are compatible mixing the products would not cause any harm to cooling systems, but it does slightly change the heat transfer properties.

One other type of ethylene glycol is Triethylene Glycol (TEG), which is different from normal ethylene glycol. Triethylene Glycol (TEG) is mainly used for its hygroscopic properties. TEG is commonly used as a liquid desiccant for dehydration in natural gas processing, as a dehydrating agent for lubricants, finishing agent for textiles, used as a constituent in brake fluids, and some antifreeze formulations. Generally caution should be exercised when mixing or topping off a system using TEG with other glycol based products.