Chapter 13 435 The propellant, usually comprised of liquefied gases such as propane and isobutane, is added to dispense the product and to produce efficacious foam. The levels of propellant and the net vapor pressure of the propellant blend are two important factors in how the foam forms during actuation and its breaking behavior. With the advancement in package design, non-aerosol mousses can be found on the market that utilize mechanical breakup instead of a propellant to produce the foam. Hydrophobically modified cationic polymers, also referred to as a polysoaps, can produce non- aerosol foams without the use of added nonionic surfactants. Mechanistic Understanding of Performance The basic properties of a polymer film can be studied by simply drawing a solution of the polymer down on a glass plate and then letting it dry. The resultant film can be tested for properties such as clarity, hardness, adhesion to the glass surface, and reactivity to water. Some of these properties can be translated to the behavior of the film on human hair. However, evaluation of these film properties and performance benefits on hair is more complex because the added dimensions of film adhesion to the substrate and the behavior of the fiber assembly due to film-fiber assembly interaction are also involved. To test the added dimension of the polymer’s behavior on hair as opposed to the film itself, mechanical properties of polymer treated hair were studied on a texture analyzer by utilizing hair tresses formed into what Jachowicz described as “omega loops.”28-31 The physical features of polymer treated hair were characterized by such mechanical properties as stiffness, stiffness durability, durability at high humidity, elasticity, and plasticity. Conclusions could then be made as to the benefits of polymer treatments to hair especially with respect to styling. Examples of two stress-strain curves are depicted in Figure 6 demonstrating a brittle polymer vs. a polymer that is flexible in nature. The brittleness is indicated by the reduction in force as the probe of the texture analyzer continually depresses the polymer treated omega loop. The elastic polymer portrays a