342 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.0- 0,5- propionic iso-valeric, caproic acid aqueous solution • ZnO filtration evaporation residue I I I I I I 4000 3000 2000 1700 1500 1000 400 wavenumbers (cm Figure 5. FT-IR spectrum of zinc oxide-treated short-chain fatty acids. The strong absorption band around 1600 cm- • is assigned as the carboxylate anion of short-chain fatty acid zinc salt. The absence of an absorption band at 1700 cm-• shows that no free short-chain fatty acids are present. FORMATION OF A ZINC OXIDE/NYLON HYBRID POWDER As can easily be predicted from the microscopic photograph shown in Figure 6, spher- ical nylon powder has a smooth texture. A photograph of a composite or a hybrid powder of zinc oxide and nylon powder is shown in Figure 7. A uniform layer of zinc oxide is clearly observed around the nylon core powder. Some attractive characteristics of the hybrid powder, in comparison with zinc oxide alone, are summarized below. 1. Increases the surface area of zinc oxide 2. Improves the rough texture of zinc oxide 3. Prevents aggregation of zinc oxide 4. The specific gravity of the hybrid powder is controllable 5. Improves the transparency of zinc oxide By forming a hybrid powder, the surface area of zinc oxide should increase considerably, and hence it should react faster with short-chain fatty acids. The texture of zinc oxide was improved so much that it was indistinguishable from nylon powder alone. Since zinc oxide is uniformly wrapped around nylon powder, the particle size of the hybrid powder should be almost identical with that of nylon powder. This should prevent the clogging of aerosols considerably. As mentioned above, the specific gravity of the hy- brid powder can be controlled by changing the amount of zinc oxide to be coated on top of the nylon powder. The optimum amount to form a single layer was found to be

NOVEL POWDER DEODORANT 343 Figure 6. Photomicrograph of spherical nylon powder. The smoothness of the surface is readily observed. around 20%. Amounts above 20% would overload the nylon surface, which conse- quently would result in rough texture, and amounts below 20% would leave some portions of the surface naked. When applied to the skin, the hybrid powder was more transparent than zinc oxide alone. EFFICACY OF QUENCHERS FORMULATED WITH HYBRID POWDER ON FOOT ODOR Odor assessment results of formulae 1 and 4 are shown in Figure 8 and Figure 9, respec- tively. The horizontal axis is taken as the time in hours after the first application. The vertical axis is taken as the mean foot odor intensity self-evaluated by the subjects. Out of the four formulae, formula 1 was the most efficacious, due to the high content of zinc oxide, but its texture was the worst, and several aerosols were clogged by it. The efficacy of formula 4, i.e., a conventional formula with no zinc oxide, was the lowest. Formula 2, containing 30% of hybrid powder, was comparable in efficacy to formula 1 but with a better texture. Formula 3 was found to be more effective than formula 4 but