What Is Propylene Glycol?
Propylene glycol, CAS 57-55-6, is a clear, colorless and almost odorless liquid that absorbs moisture from the air. It is produced from propylene oxide and is used across food, pharmaceutical, cosmetic and industrial applications because of its very low toxicity profile at intended use levels. In personal care, it is one of the most widely used ingredients, appearing in a large share of registered cosmetic products.
Key physical and chemical profile
| Property | Typical value |
| CAS number | 57-55-6 |
| Molecular formula | C3H8O2 |
| IUPAC name | propane-1,2-diol |
| Appearance | clear, colorless, almost odorless liquid |
| Hygroscopicity | absorbs water from the air |
| Miscibility | miscible with water, ethanol and many polar solvents |
| Main manufacturing route | hydration of propylene oxide |
| Typical cosmetic function | humectant, emollient, solvent and viscosity modifier |
Propylene Glycol as a Humectant
As a humectant, propylene glycol attracts water to the skin and holds moisture in the formulation, which hydrates and conditions the skin. This function is important in creams, lotions, serums and other products designed for dry skin. By slowing water loss from the product itself, the humectant also helps to extend shelf life and keep the texture stable during storage.
Emollient and Skin-Feel Benefits
An emollient is an ingredient that softens and soothes the skin. Propylene glycol acts as an emollient because it forms a thin, oily layer on the skin surface that reduces moisture loss. This makes it valuable in products for dry and sensitive skin, where it works alongside other emollients to improve skin softness and comfort.
Viscosity Control, Solvency and Preservative Support
Propylene glycol reduces the thickness of cosmetic formulations, which makes them easier to apply and helps active ingredients spread evenly. It also supports preservation, often together with other approved preservatives, because it helps prevent the growth of mould and bacteria in creams, lotions and liquid products. In addition, it is a solvent that improves the texture and consistency of foods, drugs and paints, and it is used to create artificial smoke in stage and training applications.
Formulation and Quality Considerations
Formulators value propylene glycol for its versatility and mildness. It is compatible with most cosmetic ingredients, is stable across a wide pH range, and is easy to incorporate into both oil-in-water and water-in-oil systems. Typical use levels depend on the product type and the function required. Suppliers should provide a certificate of analysis and a safety data sheet with every batch, and formulators should confirm that the grade selected meets the requirements of the target market, such as cosmetic-grade specifications.
Frequently Asked Questions
Q: Why is propylene glycol used in cosmetics?
It acts as a humectant, emollient, viscosity controller, solvent and preservative support ingredient in cosmetic and personal care products such as creams, lotions and serums.
Q: What is the CAS number of propylene glycol?
The CAS number of propylene glycol is 57-55-6; it is a clear, colorless, almost odorless liquid produced from propylene oxide.
Q: Is propylene glycol safe for use in cosmetics?
Propylene glycol has a very low toxicity profile at intended use levels and is widely accepted for cosmetic use when applied at appropriate concentrations in finished products.
Q: How does propylene glycol moisturize the skin?
As a humectant it attracts water to the skin and holds moisture in the formulation, and as an emollient it forms a thin layer on the skin surface that reduces moisture loss.
Q: How common is propylene glycol in personal care products?
It is one of the most frequently listed cosmetic ingredients and appears in a large share of registered personal care formulations, typically at low single-digit percentages.
Q: Can propylene glycol be used as a preservative?
It is often used together with other approved preservatives to help prevent the growth of mould and bacteria in creams, lotions and liquid products.