GAS BUBBLE FORMATION MIXER TIP • CAVITY Figure 3. Cavity formation in anchor-type mixer 327 if a high-consistency cream is made in such a kettle, the uncovered mixer tips can create cavities behind them as they rotate in the cream. As these trailing cavities are filled, air is often trapped and dispersed into the product. For a similar reason, a planetary-type mixer can often in- corporate a considerable amount of air into a viscous product during its operation. The above problem in an anchor-type mixer can be solved simply by increasing the batch size or shortening the mixer tips so that they are completely covered by the product. But in other cases, it may be necessary to redesign the mixer so that such cavities are not created dur- ing the mixer operation. Paddle mixers are also widely used in the cosmetic industry to process creams and pastes. In using these mixers, the fluid level should be suf- ficiently high to cover the paddle completely so as to avoid excessive surface turbulence. A processing kettle equipped with a paddle mixer fitted with scraper blades plus a built-in high-speed homogenizer is quite ideal in processing cosmetic emulsions and suspensions. If neces- sary, such a kettle can be designed to operate under vacuum for a com- pletely air-free operation.* In some instances, bubble entrainment in a mixing process can be minimized by revising the manufacturing procedure. For example, the emulsifiers used in making a cream can often encourage bubble forma- tion during the emulsification. Since cosmetic emulsions are usually made at a high temperature at which the viscosity of the emulsion is usually low, most of the entrained bubbles can escape during the initial stage of emulsification. To obtain a good emulsion, mixers or homoge- * Eppenbach Agi-Mixers made bv Giffo}'-Wood Co. of New York, N.Y., have these features.

328 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS nizers should be turned on at high speeds during this stage. After the cooling water is turned on, the temperature of the batch will drop and the viscosity as well as the yield value of the emulsion will increase greatly. Since any entrapped air will have difficulty in escaping after this period, further mixing should be done with extreme care to avoid permanent aeration. If possible, the thickeners used to increase the viscosity of the emulsion should not be added at the beginning but should be added after emulsification to allow the entrained air bubbles to escape (3). Pumping Considerable air can be entrained sometimes when a cosmetic preparation is pumped from one kettle to another. A common source of aeration in this type of operation is a leakage in some section of the line. This can be due to a faulty gasket, a poor connection, or pinholes in the line. The entire line should be inspected for the source of leak- age. Since air incorporation is most likely to take place in the suction side of the pump, it is generally advisable to connect the pump as closely to the originating tank as possible to reduce the number of connections in the suction side and thus reduce the chance of air entrainment. Jetting and Pouring Operations The jetting operation discussed here is a discharging of a fluid through a nozzle, tube, or hose, into atmosphere or through atmosphere into another fluid. In this sense pouring may be considered as a special kind of jetting. These operations are very common in cosmetic process- ing as in the filling of a hand lotion or liquid shampoo into bottles, transferring of a batch of cream from a kettle to a storage tank through a rubber hose, or pouring of a drum of liquid into a batch in a process tank as illustrated in Fig. 4. These jetting or pouring operations can often be the source of the troublesome air bubble entrainment. In some cases the solution may be fairly simple. For example, the problem of air entrainment in transferring into a storage tank (Fig. 4, B) can be corrected by pumping the material through the inlet located near the tank bottom. A more careful pouring or use of a deflective plate to force the fluid to fall along the kettle wall can also minimize the prob- lem of pouring in Fig. 4, C. However, sometimes the problem is much more complex and requires thorough analysis before an intelligent solu- tion can be prescribed.