NOVEL POWDER DEODORANT 341 signed as the carboxylate anion of short-chain fatty acid zinc salt. The absence of an absorption at 1700 cm-•, which should be observed in the presence of free fatty acids, convinced us that the expected reaction as shown below was actually proceeding: 2RCOOH + ZnO-- (RCOO)2Zn q- H20 (R: alkyl group) 3000 .o_ 2000 (-) Figure 4. Isovaleric acid consumption values of various ingredients. The larger the value, the greater the efficacy of the ingredient to quench isovaleric acid odor.

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