Foam Stability Foam stiffness (body) is an indicator of foam stability. It is generally recognized and preferred in aerosol shaving creams and mousse products as a signal of concentration, richness and value. The method of making an emulsion can significantly affect foam stability. For instance, when making a mineral oil in water emulsion using Triethanolamine Myristate, the best approach is the prepare a solution of mineral oil and excess myristic acid and add it to a solution of TEA and water with good agitation. The resulting super-fatted emulsion has dispersed phase droplets averaging about three microns in diameter, a creaming time of up to 23 hours and long-term foam stability at 70°F—actually over 30 times as long as emulsions and foams made by alternative procedures. The free myristic acid acts as a solid interfacial stabilizer. In some cases, it imparts a pleasing pearlescence. Increased stiffness and stability can be gained by simply adding about 1.0% to typical shave cream formulations. Other, less widely used additives include gums, peptides and 30 micron (max.) diameter zinc oxide powder. They form wettable solids at the water interface. Super-fatting is quite frequently used. Lauryl Ether.4EtO emulsion and foams have low stability, but this is increased at least 32 times by adding an equimolar amount of Lauric Acid. In aerosol work, the measurement of foam viscosity is important. One marketer has inserted a wooden match-stick into 20 millimeters of foam, releasing it at about 20° from the vertical. Ideally it will not move, but if it does, the time to fall over is noted. More sophisticated instrumental methods are available. A Brookfield Viscometer (20 rpm, large spindle) can be used, as well as a Haake Rotoviscometerr, a Curd Tension Meter and a method for placing foam on the pan of an electric balance and pressing a 25 millimeter disc into it at a fixed velocity. The apparent weight increase rises with relative viscosity (M.A. Johnsen, 1961). For aerosols, the choice and concentration of propellant can be critical. With n-butane A-17 and iso-butane (A-31) the initial foam undergoes a “secondary expansion” for up to a minute or so at 70°F and still longer at below 70°F. This is unsettling to consumers and is corrected by using such higher pressure propellants as A-46, HFO-1234ze and HFC-152a. Also, the concentration of propellant controls liquidity or foam density. In general, at least 4.0% of hydrocarbon propellants are required—more for HFC-152a and HFOs, since their molecular weights are greater. Since propellants move into the expanding head-space during aerosol use, enough must be added to prevent “soupiness” when the dispenser is nearly empty. On the other hand, if too much is added some June 2017 Spray 37
Spray June 2017
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