EFFECT OF POLYMERS AND SURFACTANTS 249 9O 8O 75 7O 65 [] PVP/DMAPA Acrylates Copolymer [] Quaternium 70 [] Hydrolyzed Wheat Protein [] untreated 12 minutes Figure 2. Trp degradation as a function of time for light-brown hair treated with the indicated compounds and subjected to a curling iron temperature of 132øC. 8o :r• PVP/DMAPA Acrylates Copolymer [] Quaternium 70 [] Hydrolyzed Wheat Protein [] untreated 6o 55 5o -4 •.•==•..•.• .:• .. .-.-v•........• .:•: ... 4 minutes 8 minutes 12 minutes Figure 3. Trp degradation as a function of time for light-brown hair treated with the indicated compounds and subjected to a curling iron temperature of 152øC. ments on Piedmont hair, which resulted in 40% and 65% Trp decomposition after 10 rain of exposure at 132øC and 152øC, respectively. In addition to polymers, which provided a 10-20% thermal protection effect, we also administered pretreatment with

250 JOURNAL OF COSMETIC SCIENCE sodium bisulfite, a strong reducing agent and antioxidant. The use of this reagent resulted in 15% and 23% protection of Trp after 10 min of hair exposure to hot irons at 132øC and 152øC, respectively. The mechanism for the thermal decomposition of Trp most likely involves oxidation, which could be inhibited by using sodium bisulfite. Similarly, it has previously been reported that the photodecomposition of Trp in hair can be affected by reducing agents such as ascorbic acid. In contrast to this, the thermal-protective effect of such chemically diverse materials as a cationic polymer, cationic surfactant, or protein hydrolyzate cannot be explained by an oxidation-prevention mechanism. The retardation of heat conduction is a possibility, although our earlier calculations suggest that heat propagation through a fiber assembly is so fast that steady-state conditions are attained within a fraction of a second (5). A very thin layer of surface treatment, with a thickness on the order of a fraction of a micron, could result in a multilayer heat barrier in a bundle of hair possibly affecting the temperature distribution and consequently the rate of Trp decomposition. Conceivably, another factor could be that an intervening layer of surface treatment prevents direct contact between the hot surface of an appliance and the fiber surface, thus eliminating local overheating effects. COMBING ANALYSIS In order to monitor the surface damage or surface modification induced by thermal exposure, we have performed combing analysis on thermally treated hair. This tech- nique, which measures frictional forces corresponding to the combing process, has previously been found to be a very sensitive tool for detecting changes in the fiber surface as a result of thermal treatment (5), photo-irradiation (16), and reactive chemical treat- ments (17). Figure 4 contains combing curves, representing force difference as a function of distance (or tress length), for modified and unmodified hair that was thermally exposed to a curling iron at 152øC for a total of 12 min. The values of force difference were obtained by subtracting untreated control traces from the curves obtained after the treatment. In agreement with our previous publication (5), untreated hair exhibits an increase in combing force values in the thermally exposed region of the tress, resulting in a peak (maximum) on the force-vs-distance curve presented in Figure 4. This may be attributed to the thermal-oxidative damage of the lipid layer present on the fiber surface. For fibers treated with hydrolyzed wheat protein, there is an even greater increase in fiber friction than in untreated hair. This result has been obtained not only for neat solutions of protein hydrolyzates, but also for commercial formulations based on this raw material. This effect, perhaps caused by heat-activated grafting of wheat protein fragments to hair keratin, could lead to an increase in the hydrophilicity and surface energy of hair, resulting in increased combing forces. In contrast, hair treated with PVP/DMAPA acrylates copolymer exhibits a depression (minimum) in the combing curve, correspond- ing to the thermally exposed region of the tress. This suggests preferential binding of the cationic polymer to the thermally treated section of the fiber assembly and either an actual reduction in surface damage or its masking by a deposited layer of polymer capable of reducing the combing forces. Finally, with a cationic surfactant (quaternium 70), the changes in the combing curve as compared to the untreated control are minimal and provide evidence of only a slight increase in the region where thermal treatment was administered.