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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Carrier Systems
The second category of ingredients to produce consumer-accepted, efficacious APs includes carrier systems. AP solids must incorporate carrier systems that work synergistically with a product’s solidification system. In addition, the carrier systems should completely or at least partially evaporate so as not to leave white residue or a hydrophobic film on the skin, which can inhibit the active’s transport to the eccrine gland. Historically, carrier systems for AP solids have widely been based on volatile cyclic siloxanes because they evaporate quickly and do not leave residue on the skin. These are considered non-volatile organic compounds (non-VOCs) in the United States, which meet federal and state regulatory requirements. VOCs themselves, as opposed to non-VOCs, do not meet these regulatory requirements and may thus not be acceptable for use in AP solids. Since there is a current movement to avoid the use of some types of volatile cyclic siloxanes, alternatives include isohexadecane, a lower volatility, non-VOC; and C13-15 isoalkane, a non-VOC.

Solidification Systems
 The third category of ingredients of interest to AP formulators consists of solidification systems. These systems are critical to developing solid sticks that do not melt under typical storage or consumer conditions but provide an elegant skin feel and allow for easy transfer of formula to the axilla vault.

The dynamics between solidification and volatile carrier ingredients have been investigated^6,7 and reveal that the stability and aesthetics of AP sticks are critically related to the ratio of ingredient in products and the way in which the formulas are processed. Today, most solid sticks represent a combination of cyclopentasiloxane and stearyl alcohol with varying degrees of additional waxes such as hydrogenated castor wax, hydrogenated vegetable oils and polyethylene. The cyclopentasiloxane: stearyl alcohol ratio is approximately 50:18.

In the case of extrudable soft solids, higher amounts of cyclopentasiloxane and lower levels of waxes typically are used and the pour temperature is not a critical consideration; however, the pour temperature does need to be controlled to avoid the settling of suspended AP actives before a formulation is set.

Extrudable gel solids are w/o emulsions that result from a two-phase process-a continuous oil phase and a dispersed aqueous phase; the external continuous oil phase is present at greater than 70% w/w, and the internal dispersed aqueous phase represents no more than 25% w/w. The ratio of the two phases is critical to achieving optimum viscosity and spreadability. Typically the phase ratio is in the range of 80:20, external:internal.⁸