Advances in Hair Styling 438 Wade-Rafferty explained the hardening effect through ionomeric cross-linking of ionizable groups in the polymer, in this case carboxylate groups. The conclusion drawn from this experiment was that polymer cohesion depends on the relative levels of ionomer and plasticizer effects caused by the neutralizer. Hair fixative polymers form on the hair both seam welds and spot welds after the product containing the polymer has dried, as illustrated in Figure 7.5 The wettability of the fiber surface is a factor in the formation of these interconnections. Polymers are always applied to hair as a solution, whether they are thickened aqueous solutions or low viscosity aqueous or hydroalcoholic solutions. The polymer will exhibit its own surface tension when in solution and its own behavior as it wets out a fiber based on its chemistry 34 however, in built formulations there usually will be such ingredients as glycols and nonionic surfactants that will reduce the surface tension of the liquid promoting wetting. The state of the surface of the hair is also an important factor in wetting due to the presence of the external lipid layer on the surface of the cuticle and its removal when damaged. Removal of this lipid layer, or F layer, changes the surface energy of the hair making it more hydrophilic and more wettable to a liquid, especially with a low surface tension. In a fiber assembly, the solution will preferentially reside between the fibers through capillary force. This force is dependent on both the curvature of the fibers and the surface tension of the liquid and is termed the Laplace Pressure.5 Since the performance of a styling polymer is dependent on the polymer interacting with the hair fiber, it can be deduced that the adhesion of the polymer on the hair is as important a physical parameter as the polymer’s cohesive properties. Because of the seam welds, the polymer therefore must be able to have good adhesion to the hair so that it does not detach from the surface of the hair during stress, besides the cohesive strength necessary, as discussed above. The seam and spot welds show that the importance of the interaction of a polymer film is not just with one individual fiber but rather with the hair fiber assembly. No doubt the polymeric film