will escape as the foam is extruded. This can be illustrated for an emulsion of 25% light mineral oil in water (see Table 5). Several methods are in use for measuring foam density. One of the simplest is to attach a short extension tube to the actuator and fill a 30mL Coors crucible to slightly over capacity from the bottom up. The small excess is removed by a spatula after one minute. The crucible is then reweighed. The weight gain is then divided by the volume of the crucible. Some foam containers are in the order of 200–300 mL. Unfilled volumes (foam voids in the crucible) lead to false-low results. Foam densities as low as about 0.04 g/mL have been reported, but they are too low in substance and viscosity to be of any commercial interest. Most aerosol foams are in the 0.08 to .11g.mL range. The versatile foams can extend from about 1–98% by volume of the continuous phase. One of these, requiring three surfactants, was an o/w type where the dispersed phase consisted of deodorized kerosene and about 10% propane. It was designed to extrude a spiral of foam onto charcoal briquettes. The 3.6% of water stretched around the oil phase, distorting the particles into close-packed cubes, rectangles and other flat-sided shapes with slightly rounded corners. The propane was encapsulated by the foam, so that the spiral deposit could be safely lit with a match of lighter. Once ignited the foam slowly collapsed, soaking into the charcoal and igniting it as well. Products such as Charlite, Siz and Aerofuel were commercially successful, although higher priced than a can of charcoal lighter fluid. One marketer decided to offer a yellow butter or cheese colored version, made by adding about 0.005% of an FD&C Yellow dye. The product lasted about three months in a plain, tinplate can, after which sufficient detinning occurred that the resulting polyvalent metal ions broke the emulsion, releasing the propane. A valuable lesson was learned. The leuco (white) emulsion lasted for years. Generally based on about 4% Polawax (thought to be mostly ethoxylated stearyl alcohol, plus auxiliary emulsifiers) it was found that the emulsions were ruined by storage in tinplate cans, but quite stable in lined aluminum cans, glass and BOPET (biaxially oriented polyethylene terephthalate) plastic dispensers. Some of this work was done in conjunction with P.A. Sanders (DuPont), who found that “quick-breaking” foams could also be made with mixtures of water and methanol, acetone and n-propanol, but not with isopropanol (IPA). It was necessary to use about 6.5 to 7.5% A-46 to assure that enough propellant remained in the emulsion. This technology was later applied to hydro-alcoholic (35/65) hand sanitizer aerosol developments, precipitating as the dispenser was used up. Lastly, it was found that hot-tanking these products at typical 132°F temperatures caused de-foaming. This confirmed early observations by K. Klausner that some hydro-alcoholic emulsion de-stabilized permanently at temperatures as low as 105°F. Emulsions and their foams constitute a huge field of important products. They are used in such diverse areas as industrial cleaning, petroleum extraction, water-based paints, environmental protection, mining, food preparation and (of course) billions of aerosols. Many mysteries remain to be resolved. While this brief overview touches only on some of the more salient points, perhaps it will inspire more research into this challenging arena. Spray DEDICATION The author humbly dedicates this article to the memory of Dr. Paul A. Sanders of DuPont, one of the great technical geniuses of our aerosol industry and a close friend. His many speeches, articles and books are treasured reminders of his remarkable personality and intellectual prowess. He will long be remembered. Threepiece 202 x 314 lined can. The new TRESemmé Tres Two Extra Hold Mousse offers all-day humidity resistance. 38 Spray June 2017 TABLE 5. PHYSICAL ATTRIBUTES OF A BABY OIL MOUSSE (25% USP MINERAL OIL) HFC-152a (%) PRESSURE (psi-g 70°F) FOAM DENSITY (g/mL)* 0 0 0.97 1 2 0.56 2 40 0.23 3 54 0.16 4 59 0.14 5 62 0.13 6 63 0.125 7 64 0.12 8 65 0.12** 9 65 0.12** 10 66 0.115** * Determined 1.0 minutes after dispensing the foam ** Gas escapes from the foam.
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