210 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 2. Illustration of set relaxations where T5os are similar and Hps dissimilar. There are circumstances, however, when the use of either parameter by itself is inadequate. Two such cases, shown in Figures 2 and 3, can easily be visualized. In Figure 2, for example, we have a situation in which the two values of T50 are similar, and yet the two values of Hp could be far apart. On the other hand, in Figure 3, we have the exact opposite: there, the values of Hp are close, but the T50 values are considerably different. Therefore, in both situations, neither Hp nor T•0 by itself would be sufficiently accurate enough for evaluative purposes, while together they provide an informative answer. Even though these cases are hypothetical, they do occur in real life, as illustrated by the data in Table IV, which show the effect of hairspray on set relaxation. In this experiment, a hairspray is applied to a water set, which is then allowed to relax at (T50) 1 (T50) 2 •4 TIME (HRS) • Figure 3. Illustration of set relaxations where Hps are similar and T50s dissimilar.

SET RELAXATION OF HAIR 211 Table IV Effect of Hairspray on Water Set of Hair Amount of Resin on Hair* (mg/g hair) T50 (Hrs.) Hp (%/Hrs.) 0 O.O7 85 1.6 O.O8 252 2.7 0.51 325 3.4 O.87 339 5.8 1.32 353 *Different amounts of resin on the hair are achieved by using hairspray solutions of varying concentrations. The number of sprays for each hair sample remains constant. 90% relative humidity. The amount of resin deposited on the hair is also determined. Based on experimental data for set relaxation, the values of T50 and Hp are calculated according to Eq. (7) and (9), respectively. Data in Table IV clearly indicate that the sensitivities of the two parameters are quite different depending on the amount of resin on the hair curl. At very low levels of resin deposition, T50 is definitely less sensitive. For example, the value of T50 remains practically unchanged even when the amount of resin on the hair is increased from 0 to 1.6 mg/g hair. The corresponding change in Hp, however, is more appreciable. It appears that here is a situation of fast relaxation so that the values of T•0 are falling on the part of the curves with the most abrupt changes. This is a situation similar to the hypothetical case described earlier in Figure 2. Data in Table IV also illustrate the other hypothetical case shown in Figure 3. One can easily see that when the set relaxation is slow, such as when a lot of resin is deposited onto the hair, Hp then becomes less responsive. For example, responding to an increase of resin deposition from 3.4 to 5.8 mg/g hair, Hp changes only slightly. In contrast, the corresponding change in T50 is much more extensive. Here we are facing a situation that resembles the case described in Figure 3. In this case, the use of T50 would be more meaningful. CONCLUSIONS Set holding behavior of water and heat set hair tresses can be adequately described by a power function of the form: Y = AT B, where Y is the percent set retention T is the time after the set is released and A and B are constants of the function. From this power function, two parameters, T50 and Hp, can be derived and have been found to be particularly useful for comparative evaluations. REFERENCES (1) J. R. Caldwell, S.J. Leach, and B. Milligan, The mechanism of setting and the release of set in water, Tex. Res. J, 35,245 (1965). (2) H. Bogaty, Torsional properties of hair in relation to permanent waving and setting, J. Soc. Cosmet. Chem., 18, 575 (1967). (3) A. L. Micchelli and F. T. Koehler, Polymer properties influencing curl retention at high humidity,J. Soc. Cosmet. Chem., 19, 863 (1968).