Propylene Glycol (C3H8O2) is a synthetic organic compound that absorbs water. Chemically recognised as a diol because of its 2 hydroxyl (-OH) groups, it is a liquid alcohol that finds use in a range of applications.
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Properties
Propylene glycol (PG) is extremely hygroscopic, which is why it used as a humectant in pharmaceutical formulations, food products and cosmetics. It is also miscible with solvents like water, acetone and chloroform.
There are a number of properties attributed to propylene glycol. These include its:
- Colourless appearence
- Viscous consistency
- Odourless smell
- Faintly sweet taste
- Non-corrosive nature
- Low volatility
- Non-toxicity
Although it is closely related to ethylene glycol, a very toxic substance mostly used in antifreeze, propylene glycol is generally considered as a non-toxic chemical. It even has an E-Number (E1520) in the food industry.
Uses of Propylene Glycol
Its non-toxic nature and miscibility with solvents makes propylene glycol suited to a wide range of applications. From e-cigarettes to antifreeze, it is used in many different industries. Its uses include, but are not limited to:
- Pharmaceuticals: Propylene glycol is often used in oral, topical and even intravenous medicines where it helps the body absorb chemicals
- Food Products: PG is used as a humectant, solvent and preservative in the food industry. It is often used in baked goods and has the E-Number E1520
- Polyester Resins: A large percentage of all PG produced is used in the manufacture of high-performance unsaturated polyester resins
- Non-Toxic Antifreeze: Its non-toxicity allows PG to provide a safe substitute to antifreeze solutions that contain ethylene glycol
- Artificial Smoke: Propylene glycol is one of the main chemicals used in smoke and fog machines, which are often used in theatre and fire-fighting training
- E-Cigarettes: Along with glycerol, PG is the main ingredient in e-cigarette liquid cartridges. It is responsible for the taste and smoothness of the vapour
- Veterinary Use: PG can also be used to treat animals. It is most commonly used as an oral treatment and is even found in some animal feeds
- De-Icing Fluid: PG is used in non-toxic antifreeze because, like ethylene glycol, it can lower water’s freezing point. This also why it is used as a de-icing fluid in aircrafts
How is Propylene Glycol Made?
Propylene glycol is manufactured in different grades because of its many applications. These can be grouped into 2 main categories:
- Industrial Grade: PG is converted from glycerol to produce industrial grade propylene glycol. This is used in industrial applications, like creating non-toxic antifreeze or as a solvent in the paints and plastics industry. Industrial grade propylene glycol is often high purity
- Commercial Grade: This grade of PG is obtained when propylene oxide is hydrolysed. Pharmaceutical/ food/ commercial grade PG is used in food applications, cosmetics and a variety of medicines. This is because it is lower purity and generally safer
Is Propylene Glycol Toxic?
Whether propylene glycol is toxic or not is a much-debated topic. While there have been reports of negative effects on humans, this is most commonly a result of large amounts of PG being consumed over a short period of time.
Propylene glycol has been “recognised as safe” by the Food and Drug Administration (FDA) and, according to the Environmental Working Group assessment, has a low hazard rating.
Many qualms surrounding the toxicity of propylene glycol are based on the fact that an ingredient in antifreeze is also being used in food products. While this does sound scary, it’s important to remember that the PG in antifreeze and the PG in food products are different grades, the former being of much higher purity.
Even so, antifreeze containing propylene glycol is considered non-toxic. It was introduced because of fatal accidents involving pets drinking spilt antifreeze that contained ethylene glycol. In non-toxic formulations, propylene glycol behaves like salt by simply lowering the freezing point of water.
Propylene glycol is also not bio-accumulative. This means that it does not accumulate inside the body to become toxic. A normal dose of PG in a healthy body will be broken down within 48 hours.
Fireball Whisky Controversy
As with all chemicals, there are still concentration limits that products containing PG must adhere to, despite its non-toxicity. In Europe, for example, the maximum concentration of propylene glycol in a product is 3g per kilogram.
In the US, the maximum concentration of PG is much higher at 50g per kilogram. The difference in concentrations resulted in uproar when the wrong formula of Fireball Cinnamon Whisky was sent to Europe.
In 2014 Finland, Norway and Sweden pulled Fireball Cinnamon Whisky from their shelves after the North American formula was shipped to them by accident.
The US recipe contained six times more propylene glycol than European laws allow. Because the product contained an illegal amount of this ingredient, it was quickly removed from stock. If not just a funny story of mix-ups, this shows just how controversial the topic of propylene glycol’s toxicity is.
