Propylene glycol is a colourless, practically odourless chemical compound with a high capacity to retain water, widely used in the food, pharmaceutical, cosmetic and refrigeration industries. Its chemical formula is C₃H₈O₂ and its CAS number is 57-55-6. It is completely miscible in water, which, together with its low toxicity, makes it one of the most versatile glycols on the industrial market.
In this article we explain exactly what propylene glycol is, what its physical and chemical properties are, and what it is used for in the different industrial sectors where CH Química supplies this product.
What is propylene glycol?
Propylene glycol, also known as 1,2-propanediol or monopropylene glycol, is an organic diol, that is, an alcohol with two hydroxyl groups in its molecular structure. It is obtained mainly from propylene oxide through a hydrolysis process, although variants produced from renewable sources also exist.
It is a viscous, transparent, hygroscopic liquid, able to absorb moisture from the environment. This property, combined with its low freezing point and its compatibility with water in any proportion, is the reason it is used in applications as different from one another as food preservation, cosmetics formulation or heat exchange systems.
Physical and chemical properties of propylene glycol
Knowing the properties of propylene glycol is key to understanding why it is chosen over other glycols in certain industrial applications.
| Property | Value |
|---|---|
| Chemical formula | C₃H₈O₂ |
| CAS number | 57-55-6 |
| Molecular mass | 76.09 g/mol |
| Density (20 °C) | ≈1.036 g/cm³ |
| Boiling point | ≈188 °C |
| Freezing point | ≈-59 °C |
| Solubility in water | Miscible in any proportion |
| Appearance | Colourless, viscous liquid, practically odourless |
These properties explain two of the most frequent questions about this product: propylene glycol is an alcohol from a chemical point of view, since it belongs to the diol group, and it is soluble in water in any proportion, which makes it easier to handle, dilute and apply in industrial processes.
What is propylene glycol used for in industry?
The uses of propylene glycol span very different sectors thanks to its combination of low toxicity, humectant capacity and chemical stability. These are its most common industrial applications.
Food industry
Food-grade propylene glycol is authorised as an additive under code E1520 and is used as a humectant, a solvent for flavourings and colourings, and an antifreeze agent in ice cream and frozen products. Its purity grade for food use must meet strict specifications, which is why it is marketed in USP or certified food quality.
Pharmaceutical and cosmetics industry
In pharmaceutical formulations, pharmaceutical-grade propylene glycol acts as a vehicle for active ingredients, a preservative and a solvent in syrups, creams, gels and injectable solutions. In cosmetics it is used as a humectant and formula stabiliser in creams, shampoos and hair products, always within the percentages set by cosmetic safety regulations.
Refrigeration systems and antifreezes
One of the most relevant industrial uses of propylene glycol is its role as an antifreeze and heat transfer fluid in refrigeration, air-conditioning and heating circuits. Unlike ethylene glycol, propylene glycol has a much lower toxicity, so it is preferred in installations where there is a risk of contact with food or where regulations require low-toxicity products, such as in the food industry and in building air-conditioning systems.
Chemical and process industry
Propylene glycol is also used as a raw material in the manufacture of unsaturated polyester resins, plasticisers, industrial lubricants and as a base fluid in low-toxicity hydraulic systems. Its thermal stability and its compatibility with most metallic and plastic materials make it a versatile component in chemical manufacturing processes.
USP propylene glycol and technical propylene glycol: what is the difference?
Not all propylene glycol on the market has the same purity grade. USP propylene glycol complies with the specifications of the United States Pharmacopeia, with a minimum purity of 99.5%, and is the grade required for food, pharmaceutical and cosmetic applications where the product may come into direct contact with people.
Technical-grade propylene glycol, for its part, has less demanding specifications and is intended for industrial applications where there is no such direct contact, such as certain process fluids or technical formulations. Choosing the right grade is not a matter of general quality, but of regulatory suitability for each specific application.
At CH Química we offer USP propylene glycol with the traceability and technical documentation needed to guarantee its safe use in any of these applications.
Where to find industrial-quality propylene glycol
If your company needs propylene glycol for food, pharmaceutical or cosmetic use, or as a refrigeration fluid, it is essential to work with a supplier that guarantees the right purity grade and the corresponding technical documentation, including the safety data sheet.
At CH Química we have a complete line of industrial glycols, including propylene glycol and other solutions for refrigeration, air-conditioning and industrial process systems. Contact us to receive technical advice on which grade and format best suits your application.
Frequently asked questions about propylene glycol
Is propylene glycol an alcohol?
Yes, chemically propylene glycol is an alcohol, specifically a diol, since its molecule contains two hydroxyl groups (-OH). This does not mean it has the same properties as commonly used ethyl alcohol: its chemical behaviour, toxicity and applications are completely different. It is this diol structure that gives it its ability to retain water and act as a humectant.
Is propylene glycol soluble in water?
Yes, propylene glycol is completely miscible in water in any proportion, with no solubility limit. This characteristic greatly facilitates its dilution, handling and use in aqueous formulations, in industrial as well as food and cosmetic applications.
What is propylene glycol used for in the food industry?
In the food industry, propylene glycol is used as an authorised additive (E1520) with the functions of humectant, solvent for flavourings and colourings, and antifreeze agent in frozen products such as ice cream. It must always be used in USP or food grade, which guarantees the purity and safety required for contact with food.
What is the difference between USP propylene glycol and technical propylene glycol?
The main difference lies in the purity grade and the regulations each one meets. USP propylene glycol has a minimum purity of 99.5% and meets the specifications of the United States Pharmacopeia, which makes it suitable for food, pharmaceutical and cosmetic use. Technical propylene glycol has less strict specifications and is intended for industrial applications without direct contact with people or food.
Is propylene glycol the same as car antifreeze?
Not exactly. Propylene glycol can be part of the composition of an antifreeze, but commercial antifreeze fluid is usually a mixture of glycol with water, anticorrosion additives and colourings. In addition, many automotive antifreezes use ethylene glycol instead of propylene glycol, as the latter is mainly reserved for applications where low toxicity is a requirement, such as the food industry or building air-conditioning.