Advances in Hair Styling 432 Using these various clear coated bottle techniques not only provides a means to evaluate compatibility and propellant/concentrate interaction but also helps to identify the potential for clogging and serves as an indication for longer term stability and exposure to extreme environmental conditions once in the finished package. In addition to system compatibility, it is necessary to ensure compatibility with the components and packaging as well. There are two basic can types that are used in styling formulations: aluminum and steel. Although the choice of can type for a product is often based on aesthetics such as shape, size, and appearance, the interaction of the system should be the primary focus. Often aluminum can linings can be chosen based on the pH of the system 22 however, cationic surfactants or polymers along with propellant interaction may present incompatibilities. Dimethyl ether, for example, is only compatible with the polyamide-imide lining. Compatibility within tin-plated steel systems can also be controlled Figure 5. Fisher porter tubes demonstrating the effect of headspace and system compatibility image on left, 50% head space image on right, 20% head space