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© 1998 Society of Cosmetic Chemists
Journal of the Society of Cosmetic Chemists, Vol. 49, No. 4, 223-244


Thermal degradation of hair. I. Effect of curling irons
R. McMullen , J. Jachowicz

ABSTRACT

Synopsis

The effects of thermal treatments on human hair induced by conventional curling irons, operating in the temperature range from 130°C to 164°C, have been investigated. The fibers were thermally exposed by continuous heating for extended periods of time (5–15 min) or by short (15 s) intermittent heating cycles. The model calculations of heat transfer through a fibrous assembly, based on heat conduction through a semi-infinite solid, were performed. The calculated data have shown that near-uniform temperature distributions are reached in the hair samples within a few seconds of thermal exposure, suggesting that continuous and intermittent modes of treatment are equivalent. The resulting damage to the fibers has been investigated and quantified by the use of fluorescence spectrophotometry, Hunter colorimetry, and combing analysis. The fluorescence analysis has shown that thermal treatment results in a decomposition of hair chromophores, specifically tryptophan (Trp) and its oxidation products (kynurenines). The calculated first-order rate coefficients of Trp decomposition were in the range from 0.03 to 0.12 (min −1 ), with an estimated activation energy of 6.6 kcal\/mol. Hunter colorimetry was employed to quantify thermally induced color changes in hair, such as an increase in the yellowness of white and Piedmont hair or simultaneous yellowing and darkening of bleached hair. Combing analysis has revealed a gradual increase, as a function of exposure time, in combing forces that were measured in the tress sections exposed to curling irons. The extent of the combing increase was found to be dependent on the mode of thermal treatment in which intermittent heating cycles, separated by rinsing, resulted in a higher degree of fiber damage.

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© 1998 Society of Cosmetic Chemists
Journal of the Society of Cosmetic Chemists