What it is
PEG-100 Stearate is the ester of stearic acid (a fatty acid) with a polyethylene glycol chain averaging about 100 ethylene oxide units. The long water-loving PEG chain at one end and the oily stearate at the other let it sit at the boundary between water and oil and hold the two together as a stable emulsion. It is one of the most common emulsifiers in lotions, creams and sunscreens, and it is frequently paired with glyceryl stearate, a duo that works across a wide pH range and tolerates salts and acids. Industry references describe its role as a surfactant and emulsifying agent.
Safety & warnings
The Cosmetic Ingredient Review Expert Panel has assessed PEG stearates, including PEG-100 Stearate, as safe for use in cosmetics, reporting that they were neither irritants nor sensitizers in clinical testing and showed only minimal irritation potential on healthy skin. The panel did advise that products containing these PEG ingredients not be applied to damaged or broken skin, because penetration and irritation potential can increase where the skin barrier is compromised. On intact, healthy skin it is considered well tolerated by most people.
Origin & sourcing
It is semi-synthetic. Stearic acid, a fatty acid that can come from vegetable or animal sources, is reacted with polyethylene glycol; the PEG itself is made by polymerizing ethylene oxide, a petroleum-derived feedstock. The '100' indicates the average number of ethylene oxide units in the chain.
Questions everyone asks
What is PEG-100 Stearate used for?
It is an emulsifier that keeps the oil and water parts of a sunscreen or moisturizer mixed together, giving the product a stable, smooth, uniform texture.
Is PEG-100 Stearate safe?
The Cosmetic Ingredient Review panel concluded PEG stearates are safe for cosmetic use on healthy skin and found them non-irritating and non-sensitizing in testing, while advising against use on broken skin.
Is it the same as stearic acid?
No. It is stearic acid joined to a polyethylene glycol chain; that PEG chain is what gives it its water-loving, emulsifying behavior.