EFFECT OF POLYMERS AND SURFACTANTS 253 properties of the hair fiber assemblies as a result of polymer or surfactant pretreatments prior to curling iron application. The most perceptible effect was increased binding between fibers, evident after allowing them to dry undisturbed while in close contact with each other. This was most evident after pretreatment with a polymer, such as PVP/DMAPA acrylates copolymer, at high-temperature conditions ( 152øC). The con- dition of hair after such a procedure resembles that commonly observed after application of a leave-in styling product in which the fibers are linked together and the stiffness of the fiber assembly is significantly increased. The bonds between fibers are produced by a residual amount of polymer, whose affinity to hair was enhanced by thermal treatment and could not be removed by rinsing. This effect can often be diminished by a single shampooing, although we observed that certain experimental polymers, not discussed in this paper, were able to resist several shampooings. We should also mention that the thermally induced interfiber bonds can easily be broken by dry combing, which leaves individual hair fibers separated from each other. However, subsequent hydration and drying results in reformation of bonds between fibers and increased stiffness of the hair tress. As described in the Experimental section, we have employed a texture analyzer and a dual-cantilever bending apparatus to measure the variation in mechanical properties of thermally treated hair. Figure 7 presents the variation in stiffness ratio as a function of treatment time at 152øC for intact hair and hair pretreated with a polymer, a protein hydrolyzate, and a surfactant. The data in Figure 7 were obtained for hair subjected to thermal treatment followed by rinsing (a) and shampooing (b). After rinsing, an increase in the stiffness ratio of hair pretreated with PVP/DMAPA acrylates copolymer is evident, reaching a value of approximately 2.5 after 12 min of exposure. The values obtained for untreated hair are also elevated, especially at shorter treatment times. On the other hand, fibers treated with protein and surfactant are characterized by values that are only slightly higher than 1.0. After shampooing, the binding effect of the polymer disap- pears, presumably due to removal of the polymer by a detergent. Differently, the stiffness ratio values for unmodified fibers remain elevated, ranging from 1.4 to 1.7. The increase in hair stiffness, observed after thermal treatment, could be related to the previously reported toughening transition observed at 140ø-170øC by differential scanning calo- rimetry (1). The reason for this transition, appearing as a decrease in sample heat capacity (opposite to glass transition), is the removal of tightly bound water and an increase in the content of the crystalline or-helical phase in hair. It has also been demonstrated by Milczarek et aL (1), in agreement with the results of the present work, that an increase in hair crystallinity as a result of high temperatures (150øC) is stable and remains unchanged even after fiber exposure to high humidity or liquid water. The increased stiffness effects are not evident in all of the data obtained for hair thermally exposed at 132øC. For example, the stiffness ratios for untreated and polymer- or protein-treated hair fall within the range of 1.0-1.4 (Figure 8). However, fibers pretreated with quater- nium 70 are generally characterized by the lowest stiffness ratios, suggesting that the surfactant may play the role of a plasticizer within the keratin structure. CONCLUSIONS The use of a cationic polymer, protein hydrolyzate, and a cationic surfactant as a hair pretreatment (i.e., prior to the use of curling irons) was found to affect the processes that lead to (a) damage of hair amino acid (e.g. Trp), (b) damage of the fiber surface, and (c)

254 JOURNAL OF COSMETIC SCIENCE 3.5 2.5 1.5 0.5 o 2.5 f I ..... 4 minutes 8 minutes B PVP/DMAPA Acrylates Copolymer ß Quaternium 70 [] Hydrolyzed Wheat Protein ß untreated •'s...'.- •, :•8 • .i'i 12 minutes .o_ 1.5 f,o 1 0.5 ß PVP/DMAPA Acrylates Copolymer ß Quaternium 70 [] Hydrolyzed Wheat Protein ß untreated I [ •:• :i 4 minutes 8 minutes :•:• :::::s::.•:::: :• i:i:....... :'• i .. : ..... ::': -•::• Vz•i•.i*•.....!• •i::ii::. . ". ........ 12 minutes Figure 7. Stiffness ratio as function of time for light-brown fibers treated with the indicated compounds and subjected to a curling iron temperature of 152øC after rinsing (a) and after shampooing (b). modification of the fiber mechanical properties reflected by a change in the stiffness of hair assemblies. All analyzed materials, which were employed as leave-in treatments, were found to reduce the extent of thermal decomposition of Trp by 10-20% as a result of 4-12 min of curling iron application at 132øC and 152øC. Combing measurements have demonstrated that the use of PVP/DMAPA acrylates copolymer or a cationic