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What is the difference between ethylene glycol and propylene glycol?

Coolant Monitoring

Propylene glycol and ethylene glycol are both glycol-based coolants, but there are some fundamental differences between them that can determine which is the best solution to use in a specific application.

The key differences between ethylene glycol and propylene glycol are related to performance efficiency and levels of toxicity. For instance, ethylene glycol is recognised for having superior heat transfer attributes because of its high boiling point and higher viscosity. However, Propylene Glycol excels in terms of toxicity, presenting exceptionally low values.

In this blog post, we’ll take a closer look at these two different solutions, explaining what they are along with what makes them similar and different. Read on for an in-depth understanding of ethylene a propylene glycol.

What exactly are glycols?

The term glycol is given to organic chemical compounds which belong to the alcohol family. Within a glycol molecule, there are two hydroxyl groups which are attached to various carbon atoms. The term diol can also be used inter changeably with glycol and as mentioned these chemical compounds are included in the chemical group known as alcohols.

Both ethylene glycol and propylene glycol have a vast number of uses throughout a wide range of industries and applications. For example, propylene glycol is used for preservatives and cosmetics while ethylene glycol is important manufacturing inks, resins and polyethylene terephthalate, the most common type of thermoplastic polymer employed in making containers for liquids and clothing among other products.

However, both share a common use in coolant and antifreeze mixes. Antifreeze can have base product of propylene glycol or ethylene glycol. In certain formulas, a rust inhibitor may be included to protect system parts from corrosion.

Antifreeze is multi-functional. It lowers the freezing point of liquids in a cooling system, but also raises water’s boiling temperature. Effectively, antifreeze solutions help keep cooling systems flowing freely while clearing icy build-up and working under cold weather conditions. It also prevents any potential overheating issues.

Understanding the key difference between ethylene glycol and propylene glycol

As touched upon earlier, a core difference between ethylene glycol and propylene glycol is the levels of recorded toxicity. Propylene glycol has extremely low toxicity levels, which is why this chemical compound is often found in personal care and beauty products. In stark contrast, ethylene glycol is poisonous and as a result, must be handled with all necessary caution to mitigate any animal or human exposure.

You might be wondering, if this is the case, why isn’t propylene glycol deployed for every application to remain safe? The answer is simple, ethylene glycol use can offer several advantages that propylene glycol cannot. This is especially true when it is deployed within a closed loop system that presents only minimal risk of contact with products for ingestion or topical use.

For instance, freeze point depression is far more effective utilising ethylene glycol. To achieve the same freezing point obtainable with ethylene glycol, far more propylene glycol would be needed, increasing operational costs. Furthermore, due to its lower viscosity, ethylene glycol also possesses exceptional heat transfer qualities.

The deployment of ethylene glycol or propylene glycol often depends on the specific application involved and the risk of any accidental contact with potable water, food, or human ingestion. An example is the de-icing procedure for aircraft. Propylene glycol is employed to remove ice and other contaminants from aeroplanes but is also used over winter and in periods of heavy snow to actively mitigate accumulation of ice and snow on the vehicles. However, propylene glycol is also an active ingredient in many antifreeze products which are sold off the shelf in supermarkets and garages.

Because of its toxic nature, ethylene glycol is deployed in controlled industrial applications where it is mechanically pumped around closed systems and will not contact human operators or enter water and air.

Ethylene glycol sees common use in the closed systems of engines, bus also in industrial environments that involve high-temperature heat transfer. In comparison, propylene glycol is also used as an engine coolant, but is adopted in applications where toxicity and environmental contact are an important concern.

Propylene glycol lacks density when serving as a coolant, making it a less effective heat transfer fluid compared to ethylene glycol. For propylene glycol to be as effective as ethylene glycol as a coolant, more antifreeze is required.

Ethylene glycol has greater heat transfer efficiency because of its lower viscosity. However, more fluid is required to circulate in order to transfer the same energy as propylene glycol has a much higher specific heat.

Although propylene glycol has much lower toxicity levels than ethylene glycol, it still requires longer time to biodegrade. The similarity between the two glycols is that neither are classed as carcinogenic, and both have low levels of flammability.

Users employing glycols must always ensure they use the best option for their application. OEMs and expert lubricant distributors can be a reliable source of information when selecting products.

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