JOURNAL OF COSMETIC SCIENCE 400 as samples are heated to decomposition. Unfortunately, overlapping protein denatur- ation (TD) and decomposition endotherms confound the results. The issue is overcome by presoaking the fi ber snippets in water and subsequently charging into sealed stainless steel, high-pressure DSC capsules. By exploiting the water-plasticization behavior of the matrix, which shifts the hair denaturation peak from 230–250°C to 120–150°C, the TD and pyrolysis events are opportunely resolved. More importantly, the area of ΔHD is unaf- fected by prehydration, meaning that the extent of crystallinity is measurable since the helical structures are not tainted by excess water. EFFECT OF THERMAL HISTORY ON THE EXTENT OF ALPHA KERATIN CRYSTALLINITY As indicated in the trends in ΔHD and TD in the thermogram in Figure 16, the process of high-temperature fl at ironing (232°C) affects the stability of the IFs. Diminishing magnitudes of ΔHD, which is defi ned by the area of the endothermic DSC peak, result from prior exposure of the hair tress to the rigors of hot fl at ironing. Changes in the de- naturation enthalpy reveal that excessive temperatures infl uence the viscoelasticity of the amorphous matrix and subsequently destabilize the embedded alpha helices. In addition, Figure 16. Overlay of HPDSC thermograms for virgin, thermally styled virgin, polymer-treated virgin, and thermally styled and polymer-treated virgin European dark brown hair assemblies. The top curve (open red squares) shows a very faint endothermic peak at ~115°C, whereas the untreated virgin sample (red circles) displays a characteristically sharp endotherm at 140°C. The bottom curves show the results of polymer- treated fi bers, demonstrating that the protective polymer appears to preserve the endotherm for the heat- treated fi bers (open black squares). The individual endotherms have been offset on the ordinate axis for visual clarity.

HAIR SHAPE AND DAMAGE FROM RE-SHAPING HAIR 401 the diminution in TD, which is marked by a shift to the left on the abscissa, is related to a decrease in the matrix cross-link density and/or scission of the nonhelical terminal do- mains of the IFs—where one functional culprit is an increase in the number of thermally induced disulfi de bond scissions (35,38). Figure 16 portrays a high-pressure DSC (HPDSC) overlay data for virgin, fl at-ironed virgin, polymer-treated virgin, and polymer-treated and fl at-ironed virgin European dark brown hair tresses. Relative to the characteristic endotherm for the untreated virgin fi bers, the fl at-ironed virgin thermogram demonstrates the deleterious consequences of thermal insult on the cortical proteins, wherein the profi les of ΔHD and TD are not apparent. Extending the technique to evaluating protective polymer treatments, sam- ples represented by the bottom two curves were treated analogously, with the exception that each was also equivalently treated with poly(methylvinyl ether-alt-maleic anhy- dride) prior to challenging in HPDSC experiments (4.5 mg polymer/g hair). McMullen and Jachowicz had previously concluded that thermally styled fibers treated with poly(methylvinyl ether-alt-maleic anhydride) protected hair fi bers against protein damage, as evidenced by the attenuation of tryptophan decomposition (39). In contrast to the untreated virgin and polymer-treated virgin samples, the fl at-ironed and poly- mer-treated sample had a substantial ΔHD peak and only a small shift in TD hence, the HPDSC results align with the protein-protective conclusions presented in the earlier fl uorescence work. TENSILE PROPERTIES OF HAIR The mechanical strength of hair is one of its most important attributes. It describes the bending and/or tensile properties of hair, and is important not only for general hair health, but also plays a major role in hair’s adaptability to grooming conditions. Hair strength is a term often used to describe the health state of hair when exposed to various stresses and strains. It can be weakened due to harsh cosmetic treatments, such as permanent waving, bleaching, and relaxing as well as exposure to UV radiation and physical insult with ther- mal styling appliances. Measurement of the tensile strength of hair specifi cally refl ects the health state of the hair’s innermost cortical cells that comprise the cortex (40). This is one of the most commonly used techniques in industry to assess damage incurred by treat- ments employed to change the shape of hair as well as protective effects afforded by hair protection agents. Therefore, in the remainder of this section, we will review the funda- mental aspects of techniques to measure tensile properties and examine the infl uence of damaging and protective treatments. MEASUREMENT OF TENSILE STRENGTH Tensile strength measurements of hair are frequently carried out using an instrument equipped with a load cell that is able to monitor stress as a function of strain or vice versa. As illustrated in Figure 17, a fi ber is mounted with crimps on both sides, which allows it to be grabbed by mechanical or pneumatic clamps. The instrument is programmed to deform the fi ber along its longitudinal access by stretching the fi ber at a programmed strain rate. Historically, instruments designed by Instron were used to carry out such