142 JOURNAL OF COSMETIC SCIENCE 0.9 0.8 0.7 .... 0.6 - 0.5 a: -... LO 0.4 - 0.3 0.2 0.1 0 0 5 10 15 t I min Figure 1. Change of the corrected hair-set index plotted against the time left, t, in the box controlled at 26 ° C, 90% RH. 0: untreated. L,., □, e: treated by an aqueous solution of malic acid (MA)/ benzyloxyethanol (BOE)/ethanol for 20, 40, and 60 min, respectively. ♦: treated by an aqueous solution of HO/BOE/ethanol for 60 min. untreated hair, the initial curl set rapidly relaxed and returned to the original, almost straight, shape. Such change is seen as a rapid decrease of the hair set index in Figure 1. This means that the water set of hair is broken. For the MA/BOE/ethanol-treated hairs, the set durability improved with treatment time. It is again worth noting that this measurement was done on single fibers, not on a hair bundle. This suggests that the durability improvement comes from changes in the internal properties of the hair fiber. In Figure 1 the result for the HCl/BOE/ethanol-treated hair is also plotted, showing that setting durability did not improve. This means that the improvement of the set dura- bility comes from MA, not from BOE. NIR SPECTRUM OF THE UNTREATED HAIR Figure 2 displays an NIR spectrum from 7000 to 4000 cm- 1 of the untreated hair after smoothing and baseline correction. Five main bands are observed. Three bands in the regions of 6800-6400 cm-1, 6000-5600 cm,- 1 and 4950--4750 cm- 1 have been as- signed to be a first overtone of NH stretching, first overtones of CH stretching for CH2 and CH 3 groups, and a combination of NH stretching and amide II, respectively (9,10). The band in the 4700-4500 cm- 1 region has two possibilities (10): one is due to the combination of NH stretching and amide III, and the other is due to the ternary

Q) (.) C: .0 0 .0 INTERACTION BETWEEN HAIR PROTEIN AND ORGANIC ACID 143 1.7 1.6 1.5 1.4 1.3 1.2 7000 6000 5000 4000 Wavenumber/ cm-1 Figure 2. NIR spectrum of chemically untreated Chinese hair in nitrogen atmosphere at 25 ° C after being dried under vacuum overnight. combination of 2 x amide I + amide III. The bands in the 4500-4200 cm- 1 region seem to be combinations of CH stretching and CH deformation for CH2 and CH 3 (9). Since the band of the first overtones of CH stretching for CH2 and CH 3 seems not to be changed by the interaction with acids, this intensity of the band was used for the correction of band intensity. The combination band of NH stretching and amide II was used for the following analysis. NIR SPECTRA OF THE TREATED HAIR Figure 3 shows NIR spectra in the 5 300 - 4700 cm - l regions for the untreated hair and that treated with the acid solution for 20, 40, and 60 min. In addition to the combi- nation band of NH stretching and amide II around 4890 cm - 1 , two other bands can be seen in this figure. The band at around 5170 cm - l is due to the combination of OH stretching and OH deformation of water (8), and the other small band around 5050 cm- 1 is due to the side-chain amide group (9,10,17). INTENSITY OF THE WATER BAND It can be seen that the intensity of the water band decreases with treatment by the acid solution. The trend of the decrease in water is clear in Figure 4, where the relative absorbance at 5170 cm -l is plotted against the treatment time, T. The relative absor- bance is the highest for the untreated hair. This means that the untreated hair contains