622 JOURNAL OF COSMETIC SCIENCE crease in the denaturation parameters bur also showed that this decrease could, at low levels of bleaching, be reversible upon dialysis of the hair fibers in deionized water. At higher levels of bleaching, both reversible and irreversible changes were observed. The hypothesis as to the causes of these two types of behavior is illustrated in Figure 1. The permanent changes are postulated to be caused by an irreversible oxidation of the covalent bonds in the fiber, likely the cystine disulphide bonds. The reversible changes are postulated to be caused by the incorporation of formulation components and metal ions such as calcium and magnesium from the tap water into the fiber after bleaching. These components are likely to cause a change in the environment of the intermediate filaments due to changes in the fiber's electrostatic interactions (e.g., salt bridges, hydrogen bonding, Van der Waals forces, etc.) that will alter the viscosity of the matrix and thus the denaturation properties of the fiber. This hypothesis was tested by treating the fiber with one formulation component, the alkalizer, with no oxidant. As predicted, a drop in the peak temperature and peak enthalpy was observed and this drop was reversed on dialysis in deionized water, i.e. the denaturation properties returned to those of the starting untreated hair. The purpose of this present study was to extend and compare the HPDSC investigation of hair bleach products to permanent hair colorants. Hair that was treated with multiple treatments of a permanent hair colorant was studied to observe the changes in dH0 and TO as the number of treatment cycles increased and before and after dialysis in deionized water. The oxidative power of a bleach product is typically significantly greater than that R R R R R Bleach Product Dialysis , WHhlng ➔ Figure 1. Proposed mechanism: Left is virgin hair fiber. Middle is bleached fiber with compromised covalent cross-linking and residual formulation components. Right is bleached and dialyzed fiber with covalent bonds disrupted but with formulation components removed.

HPDSC OF COLORANT PRODUCTS 623 of a permanent hair colorant and thus the change in the fiber properties would be expected to be lower. EXPERIMENT AL Caucasian untreated mixed hair (medium brown), obtained from a commercial source (IHIP, New York), was formed into swatches (16 cm, 1.5 g). The hair swatches were subjected to a total of three repeat treatments with a commercial colorant product. Four grams of the mixed colorant were thoroughly applied to the hair and then kept for 25 minutes at a controlled temperature of 30°C. After rinsing for one minute, two hair swatches were removed for HPDSC and tensile strength analyses. One swatch was measured immediately after coloring and one was dialyzed in the deionized water. The remaining swatches were subjected to a series of shampoo and conditioner treatments (24 shampoos plus 12 conditioning treatments). After the washing cycles, a swatch was removed for HPDSC and tensile strength analyses. Measurements were made after one and three cycles. Four commercial colorant products were used, two blonde shades and two brown shades: • Product A-a commercial medium blonde shade containing ammonium hydroxide and hydrogen peroxide (3% on head) at pH 10. • Product B-a commercial medium blonde shade containing ammonium hydroxide and hydrogen peroxide (3% on head) at pH 10. • Product C-a commercial medium brown shade containing ammonium hydroxide and hydrogen peroxide (3% on head) at pH 10. • Product D-a commercial medium brown shade containing ammonium hydroxide and hydrogen peroxide (3% on head) at pH 10. Dialysis of the hair was achieved by soaking the hair in repeated changes of one liter of deionized water for a total of 144 hours. For the first seven hours the water was changed every hour. For the remaining time the water was changed every 12 hours. The pH of the dialyzed water was monitored at each change of deionized water. HPDSC investigations were carried out on samples for all four products using a Dynamic Difference Calorimeter DSC7 calibrated with indium supplied by Perkin Elmer. The heating range was 70 ° -l 70°C at a rate of 10°C/minute. Hair tresses were conditioned for 24 hours at 55% RH and 22°C before samples were removed and cut into 0.5-mm lengths. Four to seven milligrams of hair was placed into a Perkin Elmer pan. Fifty microliters of distilled water was added to the crucible, which was then sealed. At least three replicates were used for each determination. The tensile properties of the fibers were measured using a Diastron miniature tensile tester (MTT 675) equipped with a laser micrometer in a water-saturated environment. Only samples from product C were measured for tensile strength. RESULTS AND DISCUSSION The results below detail the effect on the HPDSC denaturation temperature and en­ thalpy following treatments with commercial colorant products A-D. Tables I-IV summarize the results of the HPDSC analyses.