Chapter 13 439 on an individual fiber contributes to the stylability to hair, but it is the inter-fiber connections that provide for an exponential degree in stiffness properties. The polymer-hair fiber assembly structure is a composite type of structure where the strength of the final system is not a mere addition of its components parts. Therefore both the physical properties of adhesion and cohesion are important for the fixative to have good performance. The importance of considering both adhesive and cohesive properties in the stiffness of polymer-hair fiber assembly composites was demonstrated by Wade-Rafferty.35 The polymer under study in this case was a quaternized natural polymer based on cassia gum with a range in charge densities. The stiffness of these polymers was measured using a three-point bending apparatus on polymer treated hair tresses using a texture analyzer. It was found that the peak force was indirectly related to the charge density, i.e. the higher the quaternary groups on the molecular backbone of the natural polymer, the lower the stiffness. Rafferty concluded from this that the higher the charge density, the lower the cohesive strength that the polymer will have, and thus the lower the stiffness. Interestingly, the trend in stiffness for the quaternized polymers reverses when tested at a higher humidity. Here the lower charged polymer becomes plasticized by the high humidity with a subsequent Figure 7. Seam welds and spot welds from a hair fixative polymer on hair