J. Cosmet. Sci., 62, 139–147 (March/April 2011) 139 Thermal styling: Effi cacy, convenience, damage tradeoffs DON HARPER, JIA CATHERINE QI, and PETER KAPLAN, TRI/Princeton Synopsis We introduce a simple method to explore the effi cacy of thermal styling, By using a temperature gradient curling iron we rapidly explore a range of thermal treatment conditions. The thermodynamic literature on the glass transition in keratin fi bers explains the surprisingly limited role of elevated temperature in improve- ments in the effi cacy of holding the styled curvature of the fi bers. The onset of damage, however, is strongly temperature dependent. This combination of measurements of damage and effi cacy shows the range of con- ditions over which thermal protection products must be functional. INTRODUCTION The age-old practice of using heat to style hair has been growing in popularity and, with this growth, the interest in protecting hair from heat damage has risen. Of course, the easiest way to avoid heat damage is to minimize heat. But how much effi cacy, and how much convenience is sacrifi ced when a style is set without heat? There is little literature on the effi cacy of heat in setting and holding hairstyle. To understand the need for heat-protection from cosmetic products, we undertook the project described in this paper to better document the tradeoffs between convenience, effi cacy and damage in heat styling. Effi cacy has at least three distinct, measurable attributes. First is the initial set S0, the degree to which the hair attains the desired shape. Second is the hold time describing the initial fading of style. This is a fairly rapid process typically taking one to two hours. After the initial fade time, the rate of fading slows dramatically and remains fairly con- stant approaching a fi nal value of the retained shape Rf, which can hold all day at constant humidity. Convenience, in these laboratory tests, refers simply to the duration of heat treatment. Consumer convenience, of course, also includes other factors such as ease of use, packaging and storage these are beyond our scope. There are many measures of damage caused by heat, and a good literature describing them (1–3). We will focus on single fi ber tensile properties including break stress σb, elastic modulus E, and shear modulus G = G´ + iG˝ and loss tangent tan(δ) = G˝/G´. We describe one measures of chemical damage, the contact angle, which quantifi es oxidative damage to the surface of the fi ber.

JOURNAL OF COSMETIC SCIENCE 140 METHODS HAIR The hair used in the thermal studies was from a stock of virgin European medium brown 4-inch wide tresses supplied by De Meo Brothers, Inc. Bleached hair was prepared from these tresses soaking in pH of 10.2, 6% hydrogen peroxide solution for 40 minutes at 40°C. Smaller and thinner tresses were made from these stock tresses. They are approxi- mately one fi ber layer thick, 10 cm wide, and 8 cm long below the tape securing the tress. The 8-cm length wrapped exactly once around our 1 inch diameter curling rod without overlap (2.54π = 8.01 cm). The tresses are prepared in one of three states. Wet, is soaked in water for at least 10 minutes. Dry is made wet and then equilibrated for at least 24 hours at either 20% or 65% relative humidity. THERMAL TREATMENT, IMAGING, AND EFFICACY MEASUREMENT To increase the throughput in the screening of curling conditions, we constructed a tem- perature gradient curling iron (Figure 1), which holds a fi xed temperature at each end and has a linear temperature gradient along the iron. The linearity of the gradient was checked using thermocouples along the length of the iron. The iron was allowed to equilibrate for at least 20 minutes prior to use. After curling, hair was quickly transferred to a chamber set to 50% RH and imaged using a computer controlled digital camera (Fuji S5 pro). The camera fi eld of view is large enough to image multiple tresses simultaneously. Figure 1. Temperature gradient curling iron. (a) Schematic. (b) Sample image of hair some time after curling. (c) Geometric construction relating the visible curl length to the diameter of curvature.