Advances in Hair Styling 430 system requires the propellant to be considered as much an integral component as the product concentrate. Aerosol propellants can be separated into two basic categories: liquefied and compressed gases.19 Liquefied gases used in personal care products include hydrocarbons, dimethyl ether, and hydrofluorocarbon 152a. A hydrofluoroolefin (HFO) product has been introduced in other industries though is not yet used in personal care.20 Hydrocarbons include pentane, isopentane, n-butane, isobutane, and propane often blends of the butanes and propane are used to achieve various pressures. Under typical conditions, these propellants exist in liquid/gas equilibrium exerting a constant pressure, enabling a theoretically consistent delivery throughout a product’s use. Compressed gases used in personal care include carbon dioxide, nitrogen, nitrous oxide, and air. These gases are compressed and exert pressure relative to the volume. Therefore as a product is dispensed and the volume inside the package increases, the resultant pressure exerted decreases. Boyle’s law, p1V1 = p2V2, can be used to demonstrate this relationship (p = pressure, V = volume, 1 and 2 represent two distinct points within the product use i.e., initial and final). In general, most hair styling aerosol products employ liquefied gases and are developed as traditional aerosol products (a product in which the propellant interfaces directly with the concentrate). Depending on the product type, this combination of concentrate and propellant can form a homogenous or heterogeneous mixture. In a homogenous system such as a hair spray, the propellant exists in equilibrium between the liquid and vapor phases. A heterogeneous product such as a mousse has multiple phases that require shaking before use. Determining the compatibility/interaction of a propellant with a complete system is critical in product development and can be achieved through the use of clear plastic coated aerosol bottles or tubes, such as those shown in Figure 4.21 As demonstrated, the interaction of the propellant and concentrate resulting from stepwise solvent changes can be observed the bottle labeled “D” represents a