The Anatomy of a Formula—Antiperspirant Sticks, Soft Solids and Gels

By: Eric S. Abrutyn, TPC2 Advisors Ltd., Inc.

Posted: May 4, 2009, from the May 2009 issue of Cosmetics & Toiletries.

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Aqueous extrudable gels are based on w/o emulsion technologies. To minimize the amount of water introduced into an extrudable gel formula, the concentration of the active solution is important since it relates to the flexibility of adjusting the refractive index with glycolics for the clarity of the gel. Ideally, a high concentration active solution (45–50%) is recommended; however, during the manufacturing process, it is important to watch for destabilization of these solutions because with their high active concentrations comes the risk of gelation as the formulation ages.

Chemistry: AP actives exist in two distinct classifications-aluminum and a combination of aluminum and zirconium. Aluminum AP actives historically have been based on aluminum chlorohydrate, a salt that is considered to provide economical performance. Specifically, this active provides the required efficacy as prescribed by the FDA monograph at a lower cost than aluminum-zirconium AP actives.

Aluminum AP actives are divided into three classifications: aluminum chlorohydrate, aluminum sesquichlorohydrate, and aluminum dichlorohydrate. Aluminum chloride, also a part of this category, requires the observance of more stringent regulatory requirements and therefore is typically used in prescription-strength APs.

Aluminum-zirconium AP actives provide superior wetness-inhibiting performance (WIP)a to aluminum AP actives. The FDA monograph divides aluminum-zirconium AP actives into the following four types: aluminum-zirconium tetratchlorohydrate, aluminum-zirconium trichlorohydrate, aluminum-zirconium pentachlorohydrate and aluminum-zirconium octachlorohydrate.⁵ These AP actives generally are buffered with glycine-aluminum-zirconium tetrachlorohydrex GLY and aluminum-zirconium trichlorohydrex GLY being the most popular; however, there is growing interest in the others.

Degree of polymerization: Controlling the degree of AP actives polymerization (activation) results in differing atomic configurations and purity, which can enhance the WIP. Historically, high molecular weight, oxo-hydrated aluminum oxide polymers have widely been used. Proprietary-processed AP actives, typically referred to as enhanced or activated salts, are based on reducing the percentage of these high molecular weight polymers while increasing the concentration of the Al-13mer polymer, thus reducing peaks 1 and 2 as denoted in high pressure liquid chromatography/gel permeation chromatograph analytical analysis.