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]. Cosmet. Sci., 58, 621-627 (November/December 2007) High-pressure differential scanning calorimetry of colorant products J.M. MARSH, C. J. CLARKE, K. MEINERT, and R. M. DAHLGREN, Procter & Gamble Technical Centers, Rusham Park, Egham Surrey, TW20 9NW1 UK U.M.M., CJ.C.), and Wella Service GmbH, Berliner Allee 65, D-64274, Darmstadt, Germany (K.M. 1 R.M.D.). Accepted for publication July 16, 2007. Synopsis High-pressure differential scanning calorimetry (HPDSC) can be used to gain information on both the degree of crystallinity in the intermediate filaments (IFs) and the structural rigidity of the surrounding matrix or intermediate filament associated proteins (IFAP) of the hair cortex (1-3). We have used HPDSC to measure changes in the denaturation temperature (T 0 ) and enthalpy (LlH0) of the crystalline components after multiple treatments with permanent hair colorant products. We have observed that after three repeat treatments both the denaturation enthalpy and peak temperature are significantly decreased vs the untreated starting substrate. However, on dialysis of the fibers in deionized water this decrease is shown to be completely reversible, returning the enthalpy and temperature to that of the untreated hair. It is proposed that the decrease is due to the incorporation of formulation components such as the alkalizer and surfactants etc. and metal ions such as calcium and magnesium from the tap wash water. These components are predicted to have a non-permanent effect on the salt bridges and hydrogen bonds and hence the rigidity or viscosity of the matrix. We have compared the denaturation temperature with the tensile properties of the fiber after treatment both before and after removal of actives from the fiber. INTRODUCTION In a previous article (4), the authors investigated the denaturation performance of hair fibers that had been subjected to a number of treatments with bleach products. That study was conducted by applying high-pressure differential scanning calorimetry (HPDSC) for temperatures up to 180°C in excess water and under conditions of equi­ librium water vapor pressure. W ortmann and Deutz (5) have proposed that the dena­ turation enthalpy (L�H 0 ) is dependent on the structural rigidity of the ex-helical material in the intermediate filaments of the keratin fiber and that the peak temperature (T 0) is dependent on the cross-link density and viscosity of the matrix in which the interme­ diate filaments are embedded. Our original studies of the effect of multiple bleach treatments on these two parameters showed a decrease in both �H 0 and T 0 as the treatment number was increased. The work by the present authors confirmed the de- 621