I. Soc. Costttel. Chem., 22, 323-337 (May 27, 1971) Mechanisms and Control of Gas Bubble Formation in Cosmetics TONG JOE LIN, Ph.D.* Presented September 20-2_5, 1970, Sixth IFSCC Congress, Barcelona, Spain Synopsis--The presetace of GAS BUBBLES iu cosmetic and pharmaceutical preparations is generally undesirable not only from the appearance vicwpoint, but also from physical, chem- ical, and biological stability considerations. The air bubbles in a product can affect emul- sion stability, promote oxidation, and, in some instances, encourage microbial growth. EX- TERNAId ENTRAINMENT of gas bubbles can result from mixing, pumping, pouring, jet- ting operation, or addition of powdered materials. INTERNAL GENERATION of gas bub- bles may be due to chemical reactions or physical changes. Each MECHANISM of bubble formation is analyzcd and some possiblc solutions are suggested. INTRODUCTION Control of dissolved and undissolved gases in a product is a diffi- cult but extremely important problem in cosmetic and other industries. Some cosmetic preparations, such as shampoos and aerosol shaving creams, are designed to produce many gas bubbles or foam during their usage. In some food preparations the presence of air bubbles is quite essential. However, in most cosmetic and pharmaceutical preparations, the presence of air or other undissolved gases as bubbles is generally undesirable [or several reasons. The presence of air bubbles in cosmetic emulsions not only makes the texture appear coarse, but it can also affect the emulsion stability by adsorbing the emulsifier molecules at the air- liquid inter[aces (1). The trapped air bubbles in an emulsified cream * Shen Hsiang Tang Chetnical Works, 19 Fn Shin Road, Section 4, Taichung, Taiwan. 323

324 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS or fluid make-up can sometimes encourage microbial growth or undesir- able oxidation. The pinholes in a lipstick or make-up stick can create poor appearance and weaken the stick. Excessive incorporation of air in clear gel-type products can be unsightly or create an under-fill ap- pearance upon standing on the shelf. Air bubbles in cosmetics are usually introduced during the manu- facturing processes. If the viscosity of the product is low, the incor- porated air can escape into the atmosphere in a short time and the product will become bubble-free. If, on the other hand, the product is very viscous, the rate of escape may be very low. From the theological viewpoint, if the product has an appreciable yield value, the entrained bubbles may not escape at all and the product can remain aerated during the entire shelf life. Control of gas bubbles is thus particularly im- portant in plastic or thixotropic products. However, since there are many ways by which gas bubbles can be incorporated during numerous manufacturing steps, control of gas bubbles in finished products is not always easy. The purpose of this paper is to examine in detail the common sources and mechanisms of bubble incorporation into cosmetic prepara- tions during various manufacturing processes, and to suggest preventive methods. DISCUSSION External Entrainment The mechanisms of bubble formation often encountered in cosmetic preparations may be divided into two main categories: bubble incor- poration due to the entrainment of the external gas and bnbble forma.- tion due to the internal generation of gases. There are many variations but four important operations which are frequently the sources of ex- ternal entrainment of air will be discussed. Mixing Operations Broadly speaking, mixing covers agitation, blending, homogenizing, and milling it is probably the most common source of air incorporation into products such as facial creams, hand lotions, and liquid make-ups. Propeller or turbine mixers are widely used to process flowable liquids or emulsions. Depending on the mixer speed, impeller size, and loca- tion, a vortex such as the one illustrated in Fig. 1 can form. If the vortex