350 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Dry Fiber After 2 Minute in Solution After 5 Minutes in Solution After 16 Minutes in Solution Figure 1. Changes in curvature and fiber diameter of curly hair fiber immersed in 1 N sodium hydroxide solution (grid circle is used as reference). Table I Relationship of Hair Swelling and Fiber Supercontraction to Permanent Hair Straightening Degree of hair % Hair % Hair Reagents pH straightening swelling supercontraction NaOH (1 N) 14.0 Complete/permanent 40 5.7 NaOH (0.1 N) 13.0 Partial/temporary 40 0 DTT (0.8 M) 3.5 None 50 0 THP (1 M) 8.5 Complete/permanent 50 6 Urea (50%) 7.0 None 30 0 TGA ( 1.2 M) 9.6 Partial/temporary 80 [ 11] 2.0 LiCl (40%) 7.0 Complete/permanent 60 [ 12] 11.5 Resorcinol (40%) 7.0 Complete/permanent 50 10.0 Boiling water* Complete/permanent 15 [ 13] 6.0 Cuprammonium hydroxide 12.0 Complete/permanent 50 [ 14] 9.4 * Under tension. straightening can only be achieved when the hair fiber has supercontracted 5 % or more of its own length. Reagents such as urea, dithiothreitol, and thioglycolic acid are all excellent swelling agents for the hair fiber, and yet none of these reagents would be able to impart any degree of permanent hair straightening. In fact, sodium hydroxide at a concentration as high as 0.1 N is not a particularly effective hair straightener, since at this concentration, sodium hydroxide is not producing a significant amount of fiber supercontraction. Boiling water, on the other hand, will produce effective hair straight- ening if the hair fiber is strained to about 20%, since under these conditions hair fiber is believed to undergo supercontraction (8). It thus appears that fiber swelling alone will

HAIR STRAIGHTENING 351 not be sufficient without also the presence of fiber supercontraction. Indeed, hair fiber supercontraction is perhaps the dominant requirement. The importance of fiber supercontraction is further demonstrated by the observations that the time required to achieve permanent hair straightening tends to coincide with the onset of supercontraction, as data in Table II clearly indicate. For example, in the case of 1 N sodium hydroxide, it takes about 15-20 minutes to achieve effective hair straightening. On the other hand, it takes as long as 45 minutes for THP. Closer examinations of the rate of supercontraction show that in sodium hydroxide solution, hair fiber will start to supercontract in 10 minutes, and supercontract completely in about 20 minutes. For THP, it takes much longer for fiber supercontraction to start, and it takes about 40-45 minutes for completion. What this seems to suggest is that radial swelling of the hair fiber is only the initiation step that helps to achieve the necessary uncoiling of the curly fiber into a straight configuration. The primary driving force to impart permanent straightening to the hair is the subsequent supercontraction of the fiber. THE ROLE OF CYSTINE AND LANTHIONINE The formation of a new crosslink such as lanthionine is generally believed to be required in hair straightening to stabilize the fiber in the new straight configuration. However, the observations described in Table I would suggest that the formation of lanthionine may not be as critical. Lithium chloride, for example, is not expected to produce any significant amount of lanthionine in the hair, and yet it is able to achieve effective hair straightening under appropriate conditions. On the other hand, 0.1 N sodium hydrox- ide produces only partial hair straightening, but its action on hair is expected to produce a substantial amount of lanthionine. On analyzing the treated hair samples described in Table I for cystine and lanthionine contents, we were indeed able to show that neither the formation of lanthionine nor the reduction of cystine will always correspond to the efficacy in hair straightening. In addition to lithium chloride, for example, other reagents such as resorcinol, tris(hydroxymethyl)phosphine, and boiling water are all found to be able to achieve effective hair straightening without any evidence of lan- thionine formation, as shown in Table III below. Also, a reagent such as dithiothreitol is found to be totally ineffective even though it would cause substantial breakdown of cystine in hair. The assumption that permanent hair straightening requires the cleavage of cystine and the subsequent formation of lanthionine does not appear to be entirely valid. Table II Treatment Time Required for Fiber Supercontraction and for Hair Straightening Fiber Permanent hair Reagents supercontraction straightening Sodium hydroxide, 1 N 15-20 min 20 min THP 40-50 min 50 min LiC1 120 min Over 120 min Cuprammonium hydroxide 70-80 min 90 min