{•OSMETIC KNOWLEDGE THROUGH INSTRUMENTAL TECHNIQUES allowed to contract again and is rested for a day. After a waving treat- ment, the hair is restretched. The load-elongation curves for the fibers before and after a waving treatment are shown in Fig. 7. The 20 per cent Index is calculated from the ratio of the areas under the curves and is taken as a measure of the effect of the waving process. A low value means that damage was done during the waving process. Naturally, the same test can be used to measure the effect of any other type of treatment of hair, such as bleaching or hair dyeing. The machine is also used to study stress-relaxation. The hair is stretched 20 per cent at 85 per cent relative humidity, and kept there for ten minutes. The effect of various reagents on the hair can then be measured. The effects of mineral oil, water and waving lotion are shown in Fig. 8. Mineral oil has no extra effect on the hair. Addition of water, however, allows the stress in the hair to decay as links within the fiber, due to hydrogen bonds and salt links, are broken. The waving lotion produces even more dra- matic effects. These curves are suited for mathematical analysis and, ac- cording to work in progress, the relaxation in a waving lotion (like ancient Gaul) can be split mathematically into three parts: that due to water, that due to the pH and that due to the mercaptan concentration. lOO 8o •6o 40 20 o 0.1 mineral oil • water •.• 0.25 0.5 1 2 3 4 5 10 20 log ! Figure 8.--Stress-relaxation curves for human hair treated with mineral oil, water and waving lotion.

136 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS TGA ^•D DTA METHODS Two instrumental methods that will probably see increased use in the cosmetic industry are those of thermogravimetric analysis (TGA) and differential thermal analysis (DTA). In the first method, the weight of a sample is continuously recorded as the temperature is increased. In the second, the temperature of a sample in a heating block is continuously re- corded as the block temperature is gradually increased. The temperature is not identical with that of the block, as endothermic or exothermic reac- tions often occur at increased temperatures resulting in appreciable nega- tive or positive temperature differentials. Figure 9 shows how the TGA of sodium perborate tetrahydrate can be used to obtain information about the ease of loss of water of hydradon and of its decomposition. Other cosmetic chemicals where similar heat sta- bility problems are involved should prove amenable to this kind of treat- ment. The DTA method is not concerned directly with weight changes but only in physical and chemical changes that involve heat loss or gain. There- fore, phase changes are observed as easily as decarboxylations. A com- parison of TGA and DTA curves is a powerful method, since physical changes can then be distinguished from chemical changes such as are in- volved in oxidations and decompositions. O- '-•-- minus 0 ":, 50- /-- minus H20 • minus H202 m 100- o _• . ,,, 150- -- mi• minus (4H20 ,0) 200 - 100 200 300 400 500 600 700 800 900 SAMPLE TEMPERATURE (*C) Figure 9.--Thermogravimetric analysis of sodium perborate tetrahydrate. Sample weight, 316.6 rag.