412 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 01- o o ,t IO I, I 2 3 4 5 6 7 8 I/2 (MINUTES) Figure 3. A•-o• •. po]ymeHz•tio• •me tm•e• co•st•t •e•ucfio• co•t•o•s Lengthy rinses prior to the polymerization step were not required with either TGA or bisulfite as reducing agent. A short 2-rain rinse under the tap was employed, providing reproducible results with a minimum of solution polymerization. Steps 4-9, Additional Aspects of Diffusion Rate Control Experiments were conducted measuring polymer add-on at different poly- merization times. Results from these experiments are plotted in Fig. 3. The initial part of this curve shows a retardation effect, possibly an inhibition time, followed by a reaction linearly related to the square root of time, consistent with diffusion rate control into a cylinder of infinite length (6). Considering the entire reaction sequence, only steps 1, 4, and 6 involve diffusion. The previous section describes how step 1 affects the rate of poly- merization i•l thc fibers. The roles of CHP and MMA concentrations on this reaction are illustrated graphically in Fig. 4 and 5. These plots show that the

POLYMERIZATION INTO HUMAN HAIR 413 113- 15- I0- 5- 0 I I I I I 2 3 4 % CHP Figure 4. Add-on at different CHP concentrations amount of add-on at 30 min is influenced by both oxidizing agent and vinyl monomer concentrations. These data in con]unction with diffusion rate con- trol suggest that the diffusion of these species into the fibers (steps 4 and 6) is rate limiting, therefore step 1 can influence the rate of polymerization by con- trolling the density of crosslinks in the fibers and thereby controlling the rate of penetration of CHP and MMA into the fibers. Both steps 4 and 6 can be rate limiting but do not permit a further distinction. Assuming that both steps 4 and 6 are rate limiting permits the following conclusion. Anything that can accelerate diffusion of oxidizing agent and monomer into the fibers will in- crease the rate of add-on. The converse also holds anything that retards dff-