CHLORINATION OF HAIR AND pH 373 of soaking the samples for one hour in the chlorine solution. The samples were then transferred to fresh chlorine solution for the next cycle. After each ten cycles of such treatment, the samples were rinsed in deionized water. FRICTION The twist method of Lindberg and Gralen (7) for measuring friction was used. In this method the coefficient of friction, pt, is given by pt = ([•'rrn)-• (In T 2 - In T•). In this equation, T• is the entering tension, T 2 iS the withdrawing tension, • is the angle between the fiber axes in the twisted assembly, and n i• the number of turns of twist. An apparatus developed by Gupta (8), which adapts the twist method of mea- suring friction for use on a constant-rate-of-extension tensile tester, was used. The con- ditions selected for making measurements of friction were as follows: T• = 3 gf on each fiber, n = 2 turns of twist, crosshead speed = 0.5 in/min, chart speed = 10 in/min, full-scale load = 20 gf, and data sampling rate = 30 pts/sec. All tests were performed under standard conditions (65% RH, 20øC) after equilibration of fibers for at least 24 hours. Using new fibers for each test, measurements of friction were made in the "with" scale direction, five pairs of fibers being used. Real time data acquisition was used for collection of force values. The value of • was measured in each test. From these values, the average coefficient of friction for each test was determined by the following equa- tion: N where N is the number of observations (approximately 1500 per test). The percentage of total time involved in the sticking (positive slope) portion of a stick slip profile was evaluated by determining the percentage of the number of observations (taken at equal time intervals) whose coefficients of friction values were larger than those of the corresponding observations. If N was the total number of data points pti recorded in a stick-slip test and X was the difference pti - pti - •, then using only the positive values of X, percent stick could be calculated as follows: Percent stick = •X x 100. -- N SURFACE MORPHOLOGY Three pairs of fibers from each treatment were chosen for examination in the scanning electron microscope. Samples were mounted onto the specimen holder using copper- conducting adhesive tape. The specimens were sputter-coated with gold/palladium to an approximate thickness of 50 nm. All specimens were examined using an accelerating voltage of 20 kV and a 45 ø tilt angle, with the tip of the fiber pointing towards the source of the electron beam.

374 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS WEIGHT LOSS Changes in the weight of samples were evaluated by determining dry weights before and after treatments. Samples were dried in a vacuum dessicator for 16 hours at 50øC, allowed to cool in weighing bottles, and weighed on an electrobalance to q- 0.01 mg accuracy. The percent weight loss was determined by the following equation: rn o -- rn 1 Weight loss (%) - X 100, ITI o where m o is the initial weight and m• is the weight after treatment. FORCE AND WORK REQUIRED FOR 20% EXTENSION The force and the work required to extend wet fibers 20% of their original length was measured on a constant-rate-of-extension tensile tester using a crosshead speed of 5 mm/min. Samples were extended to 20% and relaxed before any treatment was given. They were reextended after being subjected to 10, 20, and 30 cycles of chlorination and the respective cosmetic treatment. This test method has been recommended for the study of the effect of chemical modifications on the tensile properties of wool fibers (9). The percent reduction in force required to extend a fiber 20% after treatment was calculated using the following equation: F o - F 1 Force reduction (%) - X 100, Fo where F o is the initial force required to extend a fiber 20%, and F 1 is the force required to reextend the same fiber to 20% after treatment. The percent reduction in work required to extend a fiber 20% after treatment was calculated using the following equa- tion: W o - W• Work reduction (%) = x 100, Wo where XV o is the initial work required to extend a fiber 20%, and XX71 is the work required to reextend the same fiber to 20% after treatment. KNOT STRENGTH Knot strengths were measured to assess the change in flexibility of fibers with chlorina- tion. Knot strengths are generally lower than the straight strengths of fibers due to the initiation of breakage by the high extension of the outside layers of the fibers in the knot. Increased knot strength (approaching the straight strength) indicates an increase in the ductility of the fiber, allowing the fiber to adjust to the know configuration (10a). Two samples were cut from each blond hair fiber to be tested, one for fiber tenacity and the other for knot-breaking tenacity. The samples were brought to standard conditions, and the linear density was determined for each sample using an Insco Vibroscope ©. The knotted samples were prepared by forming a single, overhand knot in the center of each sample. Fiber tenacity and knot-breaking tenacity were measured on a constant-rate-of- extension tensile tester, using a crosshead speed of 10 mm/min and a gauge length of 20