PERMANENT WAVING AND PERM CHEMISTRY 109 Figure 5. Custom-designed SFTK cell.

JOURNAL OF COSMETIC SCIENCE 110 hook located in the base of the cell. The upper hook is then attached to the Instron load cell by means of pneumatic jaws. The cell is fi lled to a mark with deionized water, and the fi ber is allowed to sit for 5 min. Using a fi ne adjust dial, the Instron cross-head is raised to take up the slack in the hair fi ber until a very slight static force is recorded by the load cell. At this point, the load cell and the gauge length of the device are tarred. Unless otherwise stated, experiments were performed by cycling a 2% intermittent strain at 30-s intervals. Initially, this strain profi le is imposed on fi bers suspended in deionized water and, as described earlier, is intended to remove/minimize viscous relaxation. An exponen- tial decrease in stress occurs during this cycling in water, with a stable baseline typically being obtained within around 10 min. At this point, the water is rapidly drained from the cell via the tap shown on the front of the cell. The test chamber is quickly refi lled to the same level with the test solution, and the experiment is ready to begin. The strain profi le is restarted, and tensile properties of the hair are recorded as a function of time. Figure 6 gives an example of a typical experimental output which shows the progressive decrease in tensile properties over time on exposure to a perm solution. In actuality, the previous graph shows a normalized decrease in force in which the scale has been adjusted such that the initial force becomes unity and the decrease in tensile properties can then be equated to a percentage. Therefore, if we accept all the assumptions described earlier, this y axis is taken to represent the progression of the perm reaction. Figure 6. Typical output from an SFTK experiment.