PERMANENT WAVING OF HUMAN HAIR: THE COLD PROCESS, 117 treatment is also shown in Fig. 11, curve C. This curve was obtained from the same fiber which gave ELONGATION Figure 11.--Stress strain curve, 20% elongation in 50% monochloracetic acid (wt./vol.). •/---original water curve, B--in monochloracetic acid, C--water curve 24 hours after B. curve B after the chloroacetic acid had been washed out. As shown, the fiber has nearly regained its former state, indicating that the co-ordinate valences were reformed as soon as the chloroacetic acid was removed. Bull and Gutmann (9) have ad- vanced a very plausible theory to explain the shape. of the load-elonga- tion curve which is obtained when a human hair fiber is deformed. They visualize the hair protein in the form of a thixotropic gel which under the influence of a stress induced by elongation will be converted into a sol. This transformatioh is brought about by the severance of salt link- ages and hydrogen bonds. The ob- vious implication is that if these bon'ds are ruptured by chemical means, less elongation is required to change the keratin to the sol state and thus induce plastic flow. Hysteresis loops such as shown in Figs. 10 and 11 may seem far re- moved from practical cold wave problems. However, such is not the case, for it will be recalled that one of the steps in giving a' cold wave consists of wrapping the hair around a rod, that is, inducing a strain in the hair fiber. It is immaterial that this hal•pens to be differential strain in which one side of the fiber is stretched more than the other. From a practical standpoint it would seem important, therefore, to de- sign a process which would co-ordi- nate the effect of the lotion as a whole on the secondary valence forces with the effect of the thio- glycolate on the covalent disulfide bonds. It is of interest in this con- nection that Jones and Mechan, (10) studying the dispersion of kera- tins, stress the view that the split- ting of the disulfide bonds and of the co-ordinate bonds in keratin are processes which can take place inde- p.endently. A direct result of the severance of bonds in the keratin molecule, be they primary valence bonds or co- ordinate bonds, is the swelling of the hair fiber. It is true that hair fibers will swell to a degree when placed in pure water, yet this is very small compared to the swelling which oc- curs when hair fibers are placed in a medium which is known to sever bonds, such as an alkaline thio- glycolate solution or a concentrate•l solution of monochloracetic acid.

118 JOURNAL OF THE SOCIETY OF COSMETIC cI-rEMISTS Since in swelling the cross-sec- tional diameter of the hair fiber in- creases, it is evident that individual polypeptide chains of the keratin molecules are separated by a dis- tance greater than exists in dry fibers. This separation, the result of the rupture of the various bonds, will allow a greater degree of move- ment of the side chains, both polar and non-polar, permitting their uncoiling or reorientation with re- spect to the "backbone" or main peptide chains. The final step in any process of cold waving consists of fixing the undulation produced by the wrap- ping of the hair around the curling rod and the subsequent action of the alkaline thioglycolate solution. Offhand, this would seem to be a very simple step since the oxidation of the sulfhydryl groups alone may produee the desired effect.' In actual practice this reoxidation is pro- duced by the so-called neutralizing or fixing lotions. Essentially these are dilute solutions of oxidizing agents such as potassium bromate, hydrogen peroxide, potassium io- date, etc. If the fundamental as- pects of this step are examined carefully, it soon becomes evident that it is far from being simple, and in fact it is as involved as the other steps in cold waving. One purpose of this step is to reorient the amino acid side chains in positions com- parable to those which they had had before the ammonium thio- glycolate was applied. Then, too, the swelling of the hair fiber must be reversed, the sulfhydryl groups of the hair must not only be oxidized but reoxidized to disulfides, and salt linkages and hydrogen bonds must be reformed in such a manner that the hair will retain its curled state even when exposed to the action of repeated shampoos and the ele- ments. As was pointed out before, keratin which has been swelled and re- duced by the action of ammonium thioglycolate shows a good deal more randomness in the arrange- ment of the amino acid side chains than untreated hair. When such hair is treated with an oxidizing agent such as bromate or peroxide, disulfide bonds will be reformed. The amount of rebuilding of these bonds will be a function of several variables, such as concentration of the oxidizing agent, time of contact, and temperature. In Table 3 the ef- fectiveness of a commercial 3% po- tassium bromate solution in oxi- dizing reduced hair is shown. In this particular experiment a sample of hair was reduced with ammonium thioglycolate to a cystine content of 2.5-3.0%. The hair was then reoxidized by immersing it in a beaker of 3% KB.rOa at room tem- perature for the time stated. It can be seen from this table that the reaction is nearly complete within a short time and further oxidation did not increase the amount of cys- tine significantly. It should be pointed odt, however, that the method for evaluating the degree of reoxidation (6) determined only the amount of cystJne existing at any time and does not tell if any