J. Cosmet. Sci., 53, 219-228 (July/August 2002) Investioations of cosmetically treated human hair by differential scannino calorimetry in water F.-J. WORTMANN, C. SPRINGOB, and G. SENDELBACH, Deutsches Wollforschz•ngsinstitut e.V., Veltmanplatz 8, D-52062 Aachen (F.-J.W.), and Wella AG, Berliner Allee 65, D-64274 Darmstadt (C.S., G.S.), Germany. Accepted for publication March 15, 2002. Presented at the 21st IFSCC International Congress, Berlin, 2000. Synopsis By applying differential scanning calorimetry (DSC) on human hair in water, the thermal stability of hair's major morphological components is determined. Against the background of the two-phase model for o•-keratins, these components are identified as the partially helical, fibrous intermediate filaments (IF) and the intermediate filament associated-proteins (IFAP) as a cross-linked, amorphous matrix. DSC yields the denaturation enthalpy AH• which depends on the amount and structural integrity of the o•-helical material, and the temperature To, which is kinetically controlled by the cross-link density of the matrix. To assess the effects of cosmetic treatments, hairs were investigated that had undergone either multiple bleaching or perm-waving treatments. The respective dependencies between denaturation temperature and enthalpy show that both morphological components are similarly affected by bleaching, while reductive damage, in comparison, is more pronounced in the IFs. For both types of treatments, changes in enthalpy follow apparent first-order kinetics with respect to the number of treatments as well as treatment time (perm-waving), yielding characteristic reaction rate constants. It appears that DSC in water is an especially suitable method to determine the kinetics of damage formation in human hair resulting from cosmetic treatments. INTRODUCTION Materials made from hard o•-keratin, including human hair, exhibit a complex, mor- phologically fine structure (1). The study of the mechanical properties (2) has led to the two-phase filament-matrix model for o•-keratins, originally proposed by Feughelman (3). In this model, axially oriented, crystalline filaments, traditionally called microfibrils are embedded in an amorphous matrix. One main component of the model can be identified as being comprised of the partly g-helical intermediate _filaments (IF), with their helical fraction as the crystalline fila- mentous phase. The matrix in consequence comprises summarily the rest of the mor- Dedicated to Prof. Dr. H. Zahn on the occasion of his 85 •h birthday. 219

220 JOURNAL OF COSMETIC SCIENCE phological components, with the intermediate _filament associated l•roteins (IFAP) as the major component, but also including minor morphological components such as cuticle, cell membrane complex, nuclear remnants, etc. (4). The two dominant components, IFs and IFAPs, largely determine the mechanical prop- erties of human hair (2) and, according to their molecular structure, play specific roles for the performance and effects of hair cosmetic treatments (5). The thermal analysis of keratins in water by applying differential thermal analysis (DTA) (6-8) or differential scanning calorimetry (DSC) (9-11) was found to be especially suited to investigate the denaturation performance of the o•-helical IF structures, the role of the cystine cross-linked IFAPs, and the effects of physical and chemical treatments. Spei and Holzem (12) and others (13) have shown that the denaturation peak can usually be detected adequately and evaluated also for dry fibers (approx. 240øC). It is obvious from their DSC curves, however, that the effect is always secondary in size compared to a large background peak, due to general keratin pyrolysis (14). By measuring in water, the helix denaturation peak shifts for human hair down to around 150øC and exhibits no background effects (9). This paper deals specifically with the effects that bleaching and permanent waving leave in the DSC curves of hair fibers measured in water. These effects, as reflected in changes in denaturation temperatures and enthalpies, are discussed in terms of the thermal properties of filaments and matrix and lead to a kinetic approach for the description of the influences of the cosmetic treatments. EXPERIMENTAL All investigations were conducted on a power-compensated DSC instrument (DSC-7, Perkin Elmer) using pressure-resistant (25-bar), stainless steel, large-volume capsules in the temperature range of 50ø-190øC (heating rate: 10øC/rain sample weight: 4-7 rag). The samples consisted of short fiber snippets (approx. 2 mm in length) cut from the middle sections of hair swatches. Prior to measurement the samples were stored under standard room conditions (20øC, 65% RH) to ensure invariant water content. Under these conditions a given material was weighed into the sample container, 50 lal of water was added, and the container was sealed and stored over night to achieve equilibrium water content and distribution. Caucasian mixed hair, untreated, medium brown, from a commercial source (Kerling, Backnang) was used in the form of swatches (16 cm long, 100 hairs). Bleaching was done with a commercial preparation (Wella) based on an alkaline solution (pH 8.3) of hydrogen peroxide (9%) and ammonium persulfate applied for 30 rain and at room temperature. Eight hair swatches were prepared, namely the unbleached start material and swatches bleached repeatedly and at intervals of 24 hr up to seven times. DSC tests on these samples were conducted five times. Perre-waving was done on hair swatches at room temperature with a commercial prepa- ration (Wella) based on thioglycolate (8%) as reducing agent at pH 8, adjusted with ammonia, for various times between 10 and 30 min. Hydrogen peroxide solution (2.5%) (pH 3) was used as neutralizer. This process was repeated up to five times at 24-hr