ELECTROSTATIC PROPERTIES OF HAIR 565 100 10 m SBAC: 300 SEC. RINSE SBAC: 20 SEC. RINSE SBAC: UNRINSED 0 10 20 30 40 50 RELATIVE HUMIDITY, PERCENT Figure 11. Charge mobility half-life rR•ls of hair tresses. Charging potential: 21 O0 V 6O CHARGE DISTRIBUTION ALONG THE HAIR FIBERS The variation of charge density q(x) along the length x of a dry hair tress as it is combed was measured with the apparatus shown in Fig. 5. Some results are shown in Fig. 12 (upper graph). A clean untreated tress develops some charge all along its length, but there is a substantial increase in charge generated as the comb passes through the final few centimeters of the hair fibers. Hair treated with a commercial creme rinse formula- tion which was rinsed 20 sec before drying shows a similar curve, but the end peak in q(x) is much lower in this case (note the different ordinate scale for this curve). When the creme rinse is not rinsed before drying, the charge generated is essentially zero. The load p(x) required to pull the hard rubber comb through the dry tress shows a pronounced end peak force, which is likewise reduced substantially by the creme rinse

566 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 2.5 2.0 uJ • i I I I I .25 I ! I UNTREATED CREME RINSE A: li _ -- ---- RINSED 20 SEC. -.15 ß ß - ,,••//•I 1 ß o o 3.0 6.0 9.0 12.o 15.o ooo! , , , , ,00% __.__ UNTREATED •-•--• CREME RINSE A: 300 ..... RINSED 20 SEC. UNRINSED / .x 0 3.0 6.0 9.0 12.0 15.0 COMB TRAVEL ALONG TRESS, CM. Figure 12. Variation of charge density q(x) (upper graph) and of combing force p(x) (Dower graph) along the length x of a hair tress as it is combed. Relative humidity 50 per cent. Hard rubber comb. Combing velocity 1.7 cm/sec treatments (Fig. 12, lower graph). The end-peak force has been described by Tolgyesi (19) and he has attributed this to an accumulation of tangling between the fibers near the free fiber ends. The effect is not eliminated by precombing or by parallelizing the fibers. The explanation for the rise in generated charge in the final portion of the comb pass is not hard to find. It has been shown by many workers that the amount of charge generated when two bodies are rubbed together increases with the normal contact force between them (4,11,20,21,22). The effect arises from the increase in real area of