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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Therefore, to best optimize the performance of AP sticks, formulators must understand their production process as well as the ingredients best suited for them.

AP Actives
The ingredients necessary to produce a consumer-acceptable AP solid with optimized performance can be divided into four categories (see Table 1), the first of which includes AP actives. For the most part, the AP actives used are aluminum-based cationic salt chloride complexes and complexes with zirconium acid salts. These materials are referred to as “actives” on the back label of commercial AP products. There are numerous other AP actives listed in the US Food and Drug Administration (FDA) monograph,² as well as in the US Pharmacopeia (USP).³

AP actives block sweat expulsion by forming temporary plugs within sweat ducts, thus stopping or slowing the flow of sweat to the surface of the eccrine glands.4 Plugging the sweat glands causes an osmotic pressure and prohibits transportation of sweat to the skin surface. If application of an AP active is ceased, plugs eventually are pushed from the eccrine glands, allowing normal sweat release to resume.

Selection of AP actives for solids requires careful consideration of the physical composition, chemistry and degree of polymerization to optimize efficacy and aesthetic performance.

Physical composition: Anhydrous stick and soft solid systems require a powdered active and particle sizes can vary from macro-sized (up to 75 microns) to micro-sized (below 10 microns). If the particle size is too large, consumers will detect its presence when rubbing the hard particle in the underarm area (axilla). Ideally, an active with a particle size below 45 microns ensures a non-perceptible skin feel. In formulating, however, to optimize the particle suspension, smaller particles (below 10 microns) or superfine powders aid in minimizing settling during the cooling of molten wax matrices.