SUNLIGHT AND PRETREATED HAIR 83 25 L ID bleoched 1 / 1 I •.,,• dyed ! . I . •' .. © permed I 1 • I 1 •. ......' o untreoted 0 2 /+ Irrodiofion time in weeks Figure 2. Increase in lightness of untreated, permed, dyed, and bleached human hair after irradiation with VIS. I, Aa). Untreated hair becomes significantly more green (Aa* _- 2.8). Cosmetically pretreated hair becomes slightly more green than untreated hair (Aa* bleached -- 3.8 units permed = 3.9 units dyed -- 4.1 units). In comparison to permanent wave or dyeing, chemical bleaching promotes additional photobleaching of hair the most, which can be detected both after irradiation with UV-A and VIS. This photobleaching effect cannot be linearly increased by irradiation for four weeks, but instead it approaches asymptotically, just as the yellow value from bleached hair, a saturation value. This effect serves as a hint that the color pigments of hair (melanin) are largely degraded by the combination of chemical and photooxidative bleaching. There either remains a light yellow residual color or, in analogy to photo- chemically yellowed wool, a photochemically newly formed yellow product (6,7) is formed. The subjective observer notices a hair sample faded to a yellowish white strand. Photochemical brightening of dyed hair occurs to a lesser extent than that observed for chemically bleached hair but to a larger extent than that observed for permed hair. After

84 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 5 3 2 1 •111 dyed ß bLec[ched ..•"' ........... .••o perreed unfreafed I' o 2 Irradiafion lime in weeks Figure 3. Shift in color on the blue yellow axis of untreated, permed, dyed, and bleached human hair after irradiation with UV-A. irradiation for four weeks with VIS, hair dyed red is faded most heavily in comparison to chemically bleached, permed, or untreated hair. This phenomenon is accompanied by a visual assessment of a photochemically induced color change from red via orange to yellowish blonde. We assume that the red pigment introduced by the dyeing process into the hair fiber acts in addition to the natural pigment melanin as an additional photoreceptor, which is photochemically degraded parallel to the brown melanin by UV-A and especially by VIS. Thus, a more pronounced color change occurs than ob- served for untreated, bleached, or permed hairs. An increase in the photochemical brightening or yellowing by a permanent-wave process is significant only after irradiation with VIS for four weeks and occurs in comparison to chemical bleaching or dyeing only to a lesser extent. TENSILE STRENGTH TESTS For the examination of photochemically induced changes in tensile strength, the hair bundle strength (breaking force) and the tensile stress-strain properties of the hair samples were determined. Both methods give information about the mechanical stability of the hair cortex and, to a lesser extent, of that of the cuticle (8). In the following only statistically certain reductions in tensile strength will be discussed (comparison of mean values, 95% certainty). BR•^•:INO FORC• The results from the breaking force determinations are shown in Table II. A significant