Moisture as a Plasticity Switch in Blowouts: Glass-Transition Behavior, Viscoelastic Response, and Tension-Defined Shape

Human hair is a hierarchical α-keratin composite whose mechanical response is governed by the interplay between crystalline intermediate filaments and a moisture-sensitive amorphous matrix. This review synthesizes evidence from polymer physics, thermal analysis, and cosmetic science to reframe the blowout as a controlled glass-transition event. Water acts as a potent plasticizer for the keratin matrix, depressing the effective glass-transition temperature (Tg) from approximately 144 °C in the dry state to near ambient temperature at full hydration. When Tg falls below the working temperature, the matrix transitions from a rigid glassy state to a compliant rubbery one, opening a "shaping window" in which hydrogen bonds can be disrupted and reformed under applied tension. As moisture leaves the fiber during blow-drying, the matrix re-vitrifies, locking the imposed curvature through a reorganized hydrogen-bond network. The temporary set is therefore not a function of heat alone but of the time-dependent moisture gradient across the fiber cross-section. This framework connects glass-transition theory, viscoelastic relaxation models, and bond reorganization kinetics to provide a unified, materials-science account of everyday hair shaping.