136 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Measurements of Hair Damage The most widely used measure for reporting hair damage is the 20 per cent Index, which is the ratio of work required to stretch the fiber 20 per cent after treatment to the work required before treatment. Twenty per cent Indexes were obtained with a Constant Elongation Tester. The instrument was made by G. F. Bush Associates, Princeton, N.J., to meet the requirements of Evans Research and Development Corporation. After a calibration stretch, in distilled water at room temperature, the 12 human hair bundles were attached to the two-gram strands and treated for various lengths of time. The strands were rinsed for two minutes in tap water, after which the bundles of 12 hairs were carefully removed from the strands and re-stretched in distilled water. The 20 per cent Indexes were calculated from the load-extension curves. The organic solvents, methyl-, ethyl alcohol and ethylene glycol, were previously dried and distilled over calcium hydride. The water content was determined and controlled during the stretch process by means of Karl Fischer titrations. The dry fibers, which had been in a vacuum desiccator for twenty-four hours over anhydrous calcium sulfate, were immersed for seven days in the organic solvents and calibrated in these anhydrous solvents. After the bleach treatment was performed (32.2øC.), the procedure as mentioned above was carried out, the fibers re-stretched, where possible for 20 per cent of their original length, and again the 20 per cent Index calculated from the load-extension curves. However, the results obtained by using only the 20 per cent Index lead to the use of other tests, particularly the extension-at-break, alkali solu- bility and copper-uptake tests. The results of these tests, as well as infor- mation obtained by photomicrographs are described in following sections. In order to understand how these procedures came to be used, it is nec- essary to turn to the tests themselves. T•E TEsTs 20 Per cent Index The 20 per cent Indexes obtained in water on hair bleached for various periods of time are shown in Table 2. A blank run on the fibers treated with the same solution (without the H202) is also shown in Table 2 and Fig. 3. It can be seen that NH4OH, sequestrant and water have a minor effect on the physical properties of the fiber. The table shows a 20.9 per cent drop in the 20 per cent Index during the first two-hour period and a 8.1 per cent drop during the second two-hour period. This change in rate may be caused by a combination of factors, such as the drop in H20= concentration with time, the possible formation of new cross linkages, and the partial breakdown of disulfide bonds.

PROPERTIES OF PEROXIDE-BLEACHED HAIR 137 The close relation between damage, which results in a change of the physical properties, and the disulfide bonds in wool has been well estab- lished. It has been reported that the wet tensile strength is related mostly to the disulfide linkages, whereas the dry tensile strength depends largely on the peptide bonds (6). Therefore, a possible partial breakdown of peptide bonds might not show up when determining load extension in water. Therefore, it may be impossible to evaluate the over-all damage of bleached hair by means of 20 per cent Indexes. Thus the results given in Table 2 may not represent true values of bleached hair in its dry state. It was not possible to use methyl-, TABLE 2.--20% INDEXES OBTAINED IN DISTILLED WATER min. Bleach Blank 15 O.944 ... 30 0. 911 ... 45 0.89O 60 0.866 015•2 120 0.741 0.955 180 0.759 240 0.660 015}4 or ethyl alcohol, or ethylene glycol to obtain 20 per cent Indexes on bleached or virgin hair without breakage of some of the 12 single hairs. The fact that breakage does occur is very interesting and shows how human hair can differ in its physical behavior from wool. Ac- cording to Speakman's studies (7), "the capillary spaces in wool, which must be penetrated in order to alter its physical properties, are of the same order of size as the n-propyl alcohol molecule." In spite of the fact that the conditions were similar to those described by Speakman, it was not possible to stretch virgin hair 20 per cent from its original length in ethyl alcohol without breaking approximately 14 per cent of the hair fibers, and in ethylene glycol, which is even more reactive than ethyl al- cohol and only slightly less reactive than water, approximately 21 per cent. These results (Table 3) would indicate that alcohols of molecular weight greater than methyl alcohol are not capable of penetrating the hair, which may indicate that pore-sizes of human hair and wool are different. A calibration at 50 per cent and 75 per cent relative humidity could not be achieved because most of the hairs in the bundles broke before 20 per cent Extension was reached. It was therefore decided to measure the effect of hydrogen peroxide by the extension-at-break test. Extension-at-Break Extension of the fibers to the breaking point was determined with a Scott Tester employing the constant rate of loading method. For each test ten hair bundles containing 12 single hairs were processed in the bleach solution for different lengths of time, air-dried and transferred to a desic- cator containing a saturated sodium chloride solution (75 per cent relative humidity). After seven days the fibers were removed and immediately extended to the breaking point with the Scott Tester. Ten unbleached