286 JOURNAL OF COSMETIC SCIENCE 25 0 23 • •o 21 • 19• 15 • ................... -•- ................................... •- 0.42 0.44 0.46 0.48 0.5 Specific Viscosity Figure 3. Stiffness of a hairset as a function of specific viscosity of butyl ester of PVM/MA copolymer applied from 100% VOC formulations. sprays, in terms of stiffness and mechanical resilience, are determined to a large extent by the interfacial contact between polymer and keratin surface (6,7). In particular, it has been demonstrated that the failure of hair-polymer-hair junctions is adhesive rather than cohesive. It has also been shown that resins of low-solution viscosity give the greatest adhesion because of their ability to remain in intimate contact with the fiber throughout the drying process. In addition to this, higher adhesive strength was ob- served for polymers with lower glass transition temperatures, which are expected to more effectively dissipate the mechanical stress resulting from the dimensional changes in polymer films resulting from solvent evaporation. Although there is no rigorous data in the literature in regard to fixative polymers, polymer-hair interactions should be influenced by the chemical nature of a polymer and the type of its functional groups. For example, cationic polymers are expected to interact more strongly with keratin than anionic, amphoteric, or nonionic polymers because the net charge on the hair surface is negative. Also, because of the protein nature of hair, amide- or urethane-containing polymers should produce a stronger contact with the keratin substrate than ester-based materials (8). The use of polymers with reactive functional groups, capable of forming covalent bonds with the keratinous fibers (mainly in the context of wool treatments), has also been explored in the patent literature (9,10). The bonding of a polymeric material to hair can be also affected by the state of the

DYNAMIC HAIRSPRAY ANALYSIS 287 keratin fiber surface. Chemical modifications, such as bleaching or waving, alter hair by increasing its wettability and porosity (11,12). As a result, a polymer solution can spread on the hair surface more effectively and the polymer can penetrate into the surface layers of hair cuticles, producing a more intimate polymer-hair contact. At the same time, because of an increase in the hair surface energy, the adhesive forces between polymers and hair should also be higher in damaged hair. Figure 4 illustrates the effect of hair structure on the stiffness of a hairset by comparing the stiffness ratios obtained for virgin brown and triple-bleached hair treated with the same hairspray compositions. The polymers used in these treatments were anionic co(vi- nyl pyrrolidone-methacrylic acid-lauryl methacrylate), cationic co(vinyl caprolactam/ PVP/dimethylaminoethyl methacrylate, copolymer), and nonionic polyvinylpyrrolidone, formulated in a 55% VOC EtOH-water system. In all cases, bleached hair showed a significantly higher stiffness ratio than virgin brown hair, supporting the hypothesis that damaged, bleached hair produces stronger interfacial interactions with hairspray polymers. Another aspect of hair characteristics that has an influence on the performance of fixatives is hair diameter, and consequently, its specific surface area. Fine hair, or hair with a relatively small diameter, has a higher specific surface area than normal caucasian or oriental hair and, thus, should interact more strongly with a fixative polymer. As- 6O 5O o 40 •) 30 • 20 10 Co(VP-MA-LM) Co(VCL-VP-DMEMA) PVP BVirgin Brown Hair B Bleached Hair Figure 4. Stiffness of virgin brown and bleached hair treated with 55% VOC compositions.