Comparatively Speaking: Cosmetic Preservatives

By: Anthony J. O'Lenick, Jr., Siltech, LLC

Posted: April 20, 2010

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Formaldehyde Donors
Formaldehyde derivatives are the next most common preservative. These compounds interfere with membrane proteins, which kills microbes. They are effective against bacteria, fungi and mold. Bad press and real safety concerns have led cosmetic chemists to stop using formaldehyde. Instead, ingredients that dissociate into formaldehyde when put in a water solution are used. These are compounds like DMDM hydantoin, imidazolidinyl urea and gluteraldehyde. They are most often used in surfactant systems.

Phenol Derivatives
Phenol derivatives have been used in cosmetics for many years and can be effective against a range of microbes. Unfortunately, they are not as effective as the previous ingredients so their use is limited. The most common example is phenoxyethanol.

Quats
Compounds that contain nitrogen and have a positive charge when placed in solution are called quaternary compounds (or quats). Many of them demonstrate an ability to kill microbes. This includes ingredients like benzalkonium chlroide, methene ammonium chloride and benzethonium chloride. Their cationic nature makes them less compatible with anionic surfactants, which limits their application and use.

Alcohol
Ethanol is a great preservative but it must be used at high levels and therefore faces significant environmental restrictions. Other compounds like benzyl alcohol, dichlorobenzyl alcohol and even propylene glycol all have some antimicrobial effect. At lower levels, these compounds are less effective at preserving products.

Isothiazolones
Synthetic compounds like methylchloroisothiazolinone and methylisothiazolinone are effective at incredibly low levels. They have been shown to work at a wide range of pH levels and in many different formulas. Their use has been stymied, however, by at least one study that suggested it could cause skin sensitization.