Foam stabilizers

In the presence of oily soils such as sebum, the stability of shampoo foam can be drastically reduced. The so-called foam boosters act as stabilizers and also modify the foam structure to give a richer, dense foam with small bubbles. The alkanol amides are well known for this behavior. The most important types are the monoéthanolamides, which are obtained by amidation of fatty acids with monoethanolamine. Diethanolamides are usually obtained by amidation of fatty acid methylester, or triglycerides such as coconut oil, with diethanolamine. The latter are liquid products with a typical glycerol content.
The monoethanolamides are the most effective foam boosters but are difficult to incorporate due to their high melting points (approx. 80⁰ C). The diethanolamide based on coconut oil is the most popular one, although the thickening effect is reduced, due to the glycerol. The price is relatively low and the product is easy to handle compared with pure amides based on methyl ester. Today, the diethanolamides are under discussion due to possible formation of carcinogenic nitrosamines. Consequently, EC regulations allow only alkanolamides with free diethanolamine below 4%.
Other well-known foam boosters are the amine oxides and betaines described previously. Protein hydrolysates and cellulose derivatives such as CMCs are recommended as foam stabilizers. Ethylene glycol mono- and distearates (EGMS, EGDS) are most often used as pearlescent agents in surfactant formulations. They have to be incorporated at high temperatures (approx. 70-75⁰C), therefore ready-made liquid pearlescent bases are now very popular. A wide range of bases with different appearances, from turbid to real pearlescent, is offered on the market. Very effective opacity without pearlescent effect can be achieved with polystyrol dispersions. These are already highly effective at very low concentrations. Direct contact with perfume oils can result in coagulation of the polystyrol, and must be avoided.