GAS BUBBLE FORMATION 335 If a very large quantity of powdered material must be incorporated in a batch and if the resulting mixture has a very high consistency, the use of a vacuum mixing kettle may be necessary. For example, a tooth- paste is most commonly made in a vacuum mixing kettle or it must be passed through vacuum deaerating equipment afterwards to remove the entrapped air bubbles. A molten lipstick bulk is sometimes processed in an evacuated kettle to free entrapped air bubbles. The use of such a vacuum kettle for a lipstick would be unnecessary if the formula can be revised to lower the yield value of the bulk at its molten state. In some applications, the presence of air is undesirable only because it contains oxygen. In such instances, nitrogen or other inert gases can be used to purge the air from the enclosed system. One possible way of mixing a batch of a fluid without air entrainment is to recirculate the fluid with a pump. An in-line mixer or homogenizer can be added in the line to achieve effective mixing or dispersing. To be effective, however, care must be taken to make sure that the circulation is not localized. Such a system is quite ideal for many applications but the consistency of the batch cannot be too high. Internal Generation of Bubbles All of the mechanisms discussed so far involve entrainment of the external air, i.e., the air which was not originally present in the liquid bulk. However, in addition to these mechanisms, it is also possible to form bubbles through internal generation of gases. Chemical Reaction As in a fermentation process, many gases can be generated through chemical reactions. If a viscous product undergoes a chemical reaction during the manufacturing operation or shelf life resulting in a liberation of a gas, the product can obviously contain gas bubbles. Although there are many chemical reactions which can lead to the liberation of gases, most cosmetic ingredients are relatively inert and this type of generation is relatively uncommon. Physical Change In the absence of a chemical reaction, gas bubbles can sometimes form as a result of a physical change. For example, if the change in en- vironment is such as to produce a decrease in the solubility of the dis- solved air in a product, the excess air can be liberated as air bubbles.

336 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Such a solubility decrease can occur not only as a result of temperature increase or pressure decrease, but can also occur in the mixing ot5 two or more liquids. If the mixing results in a decrease of the solubility ot5 the dissolved gas, a liberation of the gas will take place until a new equilibrium is established. According to the experiments conducted by the author, this mech- anism is believed to be responsible for air bubble formation in the preparation of hydroalcoholic Carbopol©* gels (6). Mixing of ethanol and water, both saturated with air, will result in a decrease of the air solubility and liberation of air bubbles. The liberated air can escape from an ethanol-water solution readily. However, if an alcohol dis- persion of a Carbopol resin is neutralized with an aqueous solution of triethanolamine, the liberated free air bubbles may be trapped in the newly formed gel because of the high viscosity and yield value of the Carbopol gel. Thus, no matter how much care is taken to avoid ex- ternally entrained air, it is still possible to form air bubb!es by this mech- anism. To obtain a bubble-free gel, one can disperse Carbopol in a preblend of alcohol and water and neutralize the dispersion only after all the excess bubbles are freed. Since unneutralized Carbopol dispersions have very low viscosities, air liberated before neutralization can readi!y escape from the system. To hasten the bubble liberating process, the un- neutralized dispersion should be agitated with a mixer. A more rapid way to remove the excess bubbles is to pass the unneutra!ized dispersion through an ultrasonic machine. By a cavitation process, the excess air can be removed quickly and the gel obtained by neutralizing such a dispersion can be completely bubble-free, provided that no external air is entrained during the neutralization process. Finally, another possible mechanism of bubble formation as a result of a physical change is due to a density change. As some melted waxes are solidified, a density increase may occur and result in a product shrink- age. Sometimes, the wax mixture may harden before the full shrinkage takes place and result in a gradual formation of bubbles. An example would be the solidification of paraffin-based hair pomades. In some formulations, the full shrinkage may take several days resulting in the pulling away of the product from the sides of the jar or formation of many bubbles. Sometimes a gradual cooling rather than a rapid cooling * Carboxy vinyl resins, B. F. Goodrich Chemical Co., Cleveland, Ohio.