The mechanics of fixatives as explained by polymer composite principles

J. Cosmet. Sci., 60, 251–259 (March/April 2009) Synopsis Polymer composite principles are shown to explain the mechanism and performance of fixative-treated hair tresses. This concept is illustrated using cassia and experimental cassia hydroxypropyltrimonium chloride derivatives at two charge densities. Correlations are drawn between polymer film and fixative performance properties, and the primary mechanisms behind the performance of each polymer are suggested. The cationic charge density affects the adhesion and cohesion of these polymers, and the contributions of these two properties to performance are shown. It is also shown that the relationship between cationic charge density and fixative stiffness of these polymers is dependent on the relative humidity (RH) of the test. The lower charge density polymer yields higher tress stiffness than the higher charge density polymer at 50% RH, but this trend is reversed at 90% RH. A hypothesis is offered in explanation of this phenomenon, relating adhesion and cohesion to the performance of fixative-hair composites. At high humidity, moisture can plasticize the polymers, reducing the cohesive strength, so electrostatic attraction and thus adhesion becomes the dominant force. Evidence to support this hypothesis is given.