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The Anatomy of a Formula—Antiperspirants

By: Eric S. Abrutyn
Posted: August 11, 2009, from the August 2009 issue of GCI Magazine.

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The ingredients necessary to produce a consumer-acceptable AP solid with optimized performance can be divided into four categories, the first of which includes actives that block sweat expulsion by forming temporary plugs within sweat ducts, thus stopping or slowing the flow of sweat to the surface of the sweat glands.2 Plugging the sweat glands causes an osmotic pressure and prohibits transportation of sweat to the skin surface. If application of an active ceases, plugs eventually are pushed from the glands, allowing normal sweat release to resume.

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. Ideally, an active with a particle size below 45 microns ensures a non-perceptible skin feel. In gels, the concentration of the active solution is important, and, ideally, a high concentration active solution (45–50%) is recommended.

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 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. Historically, carrier systems for AP solids have widely been based on ingredients that evaporate quickly and do not leave residue on the skin.

Solidification systems are the third category of ingredients of interest in the formulation of AP products. 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 armpits. The dynamics between solidification and volatile carrier ingredients have been investigated3, 4 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.

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.