372 JOURNAL OF COSMETIC SCIENCE PERMANENT WAVING AND DEPILATION R. Randall Wickett, Ph.D. University of Cincinnati, College of Pharmacy, 3223 Eden Ave, Cincinnati, OH 45267 Introduction Permanent waves and depilatories reply on the breaking of hair keratin disulfide bonds by mercaptans. Salts of thioglycolic acid (TGA) are most often used to achieve this purpose. From the standpoint of the chemistry involved the main differences between depilatories and perms are pH and counter ion and the fact that perms require reforming the s�s bond through an oxidative step(1 ). Perms using thioglycolate are typically formulated between pH 9 and 9.6. The so-called acid perms that utilize glycerol monothioglycolate (GMT) are typically adjusted to pH 6.8-7.0(2). The ammonium salt is most often used with TGA to provide swelling of the hair. Hydrogen peroxide is most commonly used for neutralization. Depilatories are typically formulated at pH 12-12.5 and utilize calcium or sodium salts or a mixture of the two(3). Chemistry and Physics of Hair Reduction for waving and depilation Wolfram and Underwood described the reactions between reducing agents and keratin disulfide bonds in detaiI(4). With TG the active species is the thiolate ion, RS-(1 4). With monothiol reducing agents the reaction proceeds in two steps converting two moles of the reducing agent to disulfide for each reduced S-S bond. 1. Ker-5-5-Ker + R-5- � Ker-5-5-R + Keratin Mercaptan Mixed disulfide 2. Ker-5-5-R + R-5- � R-5-5-R + Mixed disulfide Mercaptan disulfide Ker-5- Reduced Keratin Ker-s· Reduced keratin Reduction of the S-S bonds leads to structural changes that are keys to the action of perms and depilatories. Depilatory action is achieved when the hair is weakened to the point that it may be rubbed off of the skin. With perms the key is to mobilize the structure while the hair is under stress so that alterations in the polymeric structure can be locked in by the neutralizer treatment to produce permanent set. This is achieved by breaking a fraction of the disulfide bonds and assisted by the presence of free SH groups in the hair which can participate in the sulfide­ disulfide interchange reaction(1 5). It can be argued that the key bonds that must be broken for either depilation or permanent waving are those that support tensile stress when the hair is under load{6-8). This makes chemical stress relaxation methods very useful for studying reduction kinetics related to either process(6 8-12). To use these methods a hair is placed under tension in buffer and stress relaxed until it supports a constant level of force. The buffer is then switched for the reducing agent and the decay of the force with time is followed.· This method, also called Single Fiber Tensile Kinetics (SFTK) has been used to study the effects of pH, temperature and hair type on the reaction kinetics(6) and to study the effect of reducing agent structure. (6 13) on reduction rates. I have postulated that the reaction kinetics follow at least two different mechanisms depending on pH and reducing agent used(7 11 ). For example with TGA reaction kinetics apparently follow a pseudo first order mechanism at low pH while at higher pH reaction proceeds by a sharp front or "moving boundary". The SFTK method has been applied to the study of depilatories as well as perms. For depilatories the time required for the tensile force to decay to 5% of its original value was found to be a convenient indicator of depilatory activity.(11) that correlated well with depilation times measured in vivo. Effects of pH, counter ion and pretreatment were quantified. Beidman(14) used a thermomechanical analyzer to determine the time required for the stretching of a hair bundle in depilatory solution to begin and also reported good correlation with in vivo depilatory efficacy.

2005 ANNUAL SCIENTIFIC SEMINAR 373 We have also compared SFTK data to data obtained using amino acid analysis for TGA and cysteamine(9) and concluded that while SFTK kinetics appear somewhat faster than kinetics determined by amino acid analysis the rank order of the reaction is consistent between the two methods. Wortmann and Souren(12) used an innovative combination of chemical stress relaxation and intermittent strain pulses to try to correlate changes in tensile properties with the level of permanent set developed. The reaction was followed during both the reduction and oxidation steps. While the relaxed tensile force will not increase during oxidation the increase in modulus measured from the small strain pulses will be dependant on the number of S-S bonds reformed. The set recovery of hair subjected to the same treatment protocol was measured by treating the hair on a small cylinder of known diameter De, then measuring the diameter, D to which the hair loop opened in water. They reported that recovery, R, the opposite of set, is directly proportion to the ratio of the fraction tensile strength lost during the reduction step to the fraction of tensile strength regained during the oxidation step. The efficacy of perms may also be evaluated using the pegboard (15) or the test tube curl method(15-17). While direct kinetic data can't be obtained from these methods they are very useful for laboratory evaluation of permanetlt waves. Unfortunately the waving process does do some damage to hair. A small but significant reduction in tensile strength occurs(2 18 19). Recent results by Nishikawa et al indicate that the content of a-helical protein is reduced from 23.7% to 20.4% and that the resultant conformational change is to random coil. Reference List 1. Albrecht, L. and Wolfram L.J. (1982) J Soc Cosmet Chem 33, 363-366 2. Wickett, R. R. and Savaides, A. (2001) Permanent Waving of Hair. In Schlossman, M., editor. Chemistry and Manufacture of Cosmetics, Volume II, Formulating, Allured Publishing, Carol Stream II. 3. Shea, F. T. (2001) Depilatories and Epilatories. In Schlossman, M. , editor. Chemistry and Manufacture of Cosmetics, Volume II, Formulating , Allured Publishing, Carol Stream, II 4. Wolfram L.J. and Underwood, D. L. (1966) Textile Res J 36, 947-953 5. Feughelman, M. (1990) J Soc Cosmet Chem 41, 209-212 6. Wickett, R.R. (1983) J Soc Cosmet Chem 34, 301-316 7. Wickett, R.R. (1991) Cosmetics and Toiletries 106, 37-47 8. Wortmann, F. J. and Souren, I. (1986) J Soc Cosmet Chem 37, 461-473 9. Manuszak, M.A., Berish, E.T., and Wickett, R. R. (1996) J Soc Cosmet Chem 47, 213-228 10. Manuszak, M.A., Berish, E. T., and Wickett, R.R. (1996) J Soc Cosmet Chem 47, 49-58 11. Wickett, R. R. and Mermelstein, R. (1986) J Soc Cosmet Chem 37, 461-4 73 12. Wortmann, F. J. and Kure, N. (1990) J Soc Cosmet Chem 41, 123-139 13. Wickett, R.R.and Barman, B. G. (1985) J Soc Cosmet Chem 36, 75-86 14. Beideman, F. E. (1987) J Soc Cosmet Chem 38, 287-294 15. Kirby, D. H. (1956) Proc Sci Sect Toilet Goods Assoc 26, 12-15 16. Haefele, J. W. (1955) Hair Waving Lotion .. United States Patent 2,719,814) 17. Marti, M. E. (1990) Cosmetics and Toiletries 105, 113-120 18. Robbins C.R. (1994) The physical properites and cosmetic behavior of hair. In Robbins C.R., editor. Chemical and Physical Behavior of Human Hair, Springer-Verlag, New York 19. Wickett, R. R. (1995) Measuring the Mechanical Strength of Hair. In J.Serup and B.E.Jemec, editors. Handbook of Non-Invasive Methods and the Skin, CRC Press, Boca Raton