MOLECULAR FORCES IN PERMANENT WAVING By HERMAN BOGAT¾* Presented May 12, 1960, New York City THE PERMANENT WAVING industry has continued to grow since the advent of the cold waving process some twenty-five years ago so that today and every day this year, an average of roughly one-quarter million women will wave their hair and will spend 11/a million dollars in the process. As a consequence of its increasing economic importance, many innovations and improvements have accompanied the development of the industry, and a number of these have contributed to the growth in the patent litera- ture. Contributions to the basic scientific understanding of hair and hair waving have been fewer. This is due not only to the relatively few labora- tories that have done fundamental work in this field, but the inherent com- plexities are very great requiring the utilization of insights from the diverse scientific disciplines of chemistry, physics and biology. While initially ideas were borrowed from the field of wool chemistry, there is at least by now the beginning of a more mature scientific knowledge of hair and hair waving which can stand by itself. As time goes on, however, new thinking and ideas will be desired and needed by the industry for further progress to be made. Relevant to the present paper is the extensive research that has been done on the phenomena of set and "supercontraction" in wool. This has concerned itself with the observations of the changes in the length of wool fibers exposed in various ways to steam, to boiling water, to many other reagents and to the effects of chemical modification of the wool on these changes. These studies have produced a large body of literature with some differences in viewpoint among the workers in the field. Without attempting a detailed review, it may be sufficient to note that Speakman, his co-workers (1, 2, 3) and Phillips (4) initially emphasized the formation of new types of covalent cross links from the keratin disulfide in setting reactions. Some objection to the mechanisms proposed arose from the work of Blackburn and Lindley (5), and these authors suggested that secondary forces could also play a part in setting and in supercontraction. * The Toni Company, Chicago 54, Ill. 333

334 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The role of hydrogen bonding has been emphasized by the work of Rudall (6), of Alexander (7) and by studies of supercontraction induced by hydro- gen bond breaking reagents which do not attack the disulfide bond (8). A recent paper by Farnworth (9) gives a concise review of the older work and proposes a convincing mechanism in which hydrogen bond rupture and reformation following disulfide scission accounts for the experimental observations. Present-day views of permanent waving are well summarized by Ger- shon et al. (10). This review and other papers of the past decade or so (11), describe the complex nature of hair keratin and discuss the several kinds of molecular forces that exist. These are often shown schematically as in Fig. 1. This simplified representation makes clear that a variety of types POLYPEl)TIDE CHAIN • ,c,- (•- NH-CO-iFH-N-,,C-C ?_,m. o POLYF'EPTIDE .,,,,,,,#_c•CO_Nl.l_CH_Co / CHAIN " H SALT C¾STINE I.-I BOND LINK LINK BOND of cross links, i.e., covalent cystine cross links, ionic or polar salt links and hydrogen and other short range bonds, must be ruptured if geometrical rearrangement of the protein chains in the structure is to take place. The earlier workers referred to and recognized this requirement for waving. Yet the reactions of the disulfide bond continued to be the focus of most of the studies in the field. Several factors account for this concentration on disulfide chemistry. First, virtually all commercial permanent waving involves treatment with mercaptans or sulfites which are known to react with the cystine disulfide linkages. Secondly, the cystine sulfur of hair can be determined easily so that the reactions of the covalent bonds can be more readily studied in a quantitative manner than those involving noncovalent or secondary forces. As the point of departure in this paper, it is intended to show how second- ary molecular bonds of all kinds affect the waving of hair and its per- manence in a very material fashion. This emphasis should, however, not obscure the basic underlying role of disulfide chemistry in the day-to-day commercial practice of permanent wavi•g.