j. Cosmet. sd., 49, 223-244 (July/August 1998) Thermal degradation of hair. I. Effect of curling irons R. McMULLEN and J. JACHOWICZ, International Specialty Products, Wayne, NJ 07470. Accepted for publication July 15, 1998. Synopsis The effects of thermal treatments on human hair induced by conventional curling irons, operating in the temperature range from 130øC to 164øC, have been investigated. The fibers were thermally exposed by continuous heating for extended periods of time (5-15 min) or by short (15 s) intermittent heating cycles. The model calculations of heat transfer through a fibrous assembly, based on heat conduction through a semi-infinite solid, were performed. The calculated data have shown that near-uniform temperature distri- butions are reached in the hair samples within a few seconds of thermal exposure, suggesting that continuous and intermittent modes of treatment are equivalent. The resulting damage to the fibers has been investi- gated and quantified by the use of fluorescence spectrophotometry, Hunter colorimetry, and combing analysis. The fluorescence analysis has shown that thermal treatment results in a decomposition of hair chromophores, specifically tryptophan (Trp) and its oxidation products (kynurenines). The calculated first- order rate coefficients of Trp decomposition were in the range from 0.03 to 0.12 (min-•), with an estimated activation energy of 6.6 kcal/mol. Hunter colorimetry was employed to quantify thermally induced color changes in hair, such as an increase in the yellowness of white and Piedmont hair or simultaneous yellowing and darkening of bleached hair. Combing analysis has revealed a gradual increase, as a function of exposure time, in combing forces that were measured in the tress sections exposed to curling irons. The extent of the combing increase was found to be dependent on the mode of thermal treatment in which intermittent heating cycles, separated by rinsing, resulted in a higher degree of fiber damage. INTRODUCTION The deleterious effects of chemical treatments on hair, such as permanent-waving, bleaching, relaxing, or oxidative dyeing, have received significant attention in cosmetic literature (1). Also, physical changes produced as a result of grooming operations have been thoroughly discussed (2). In addition to this, the photodegradation of human hair has been of growing interest to scientists and has warranted considerable attention (3,4). On the other hand, the literature reflects a limited amount of research focusing on the irreversible chemical or physicochemical changes occurring in hair as a result of thermal treatments applied to hair in conjunction with the use of curling irons or hairdryers, i.e., in the temperature range from 100øC to 170øC (5-9). This area has been investigated primarily in the context of reversible thermally induced effects such as hair drying (5,6) or hair softening at elevated temperatures (96øC) in aqueous solutions (7). Also, high-temperature decomposition of keratin at temperatures greater than 200øC has been 223

224 JOURNAL OF COSMETIC SCIENCE studied by using differential scanning calorimetry (DSC) (5,8), thermomechanical analy- sis (8), and thermogravimetric analysis (8). The key structural elements found in hair and wool that could undergo thermal deg- radation on contact with hot curling irons or hot air include the cuticle and its outermost layer (epicuticle), which is constructed of fatty acids covalently bound to the protein the cortex, which accounts for the major portion of the fiber's dry mass and holds most of the water intercellular binding material, known as the cell membrane complex, which provides adhesion between cortical cells and the crystalline phase, which is responsible for the mechanical strength of the fibers (10). DSC was used to identify the thermal transitions in hair when subjected to temperatures ranging from 30øC to 250øC (5,8). This study characterized three processes: removal of water (50ø-120øC), which occurs during drying a "toughening transition" in the amorphous matrix (140ø-170øC) and denaturation of the crystalline phase (233øC) (5). A detailed study of water desorption/ absorption curves related to heat drying of hair at temperatures ranging from 50øC to 110øC, followed by equilibration at 55% relative humidity and 22øC, has shown a reduction in moisture regain (6). Based on this, it was concluded that heat-dried hair becomes more susceptible to static charge accumulation and flyaway during subsequent grooming procedures. The processes occurring at temperatures ranging from 100øC to 170øC are of interest to cosmetic scientists, since conventional curling irons typically operate in this range. One of the physical transformations in hair structure, occurring as a result of annealing between 70øC and 180øC, is an increase in fiber crystallinity, demonstrated by Milczarek et •l. (5). This effect is similar to wool fiber strengthening, which has been observed after short-term (6-30 mid) heat treatments ranging from 130øC to 150øC (11). According to the same paper, longer heating times can also cause destabilization of the o•-helical component, as detected by mechanical stress-strain or relaxation measurements. Furthermore, using low-angle X-ray diffraction, Lee was able to postulate the formation of amide cross-links in wool heated at temperatures ranging from 170øC to 235øC (12). Earlier work, completed by Asquith and Otterburn (13) and Medefee and Yee (14), also provides evidence for the formation of cross-links as a result of heat application. Crosslinking could also be responsible for a decrease in urea-bisulfite solubility and a loss in moisture regain (15), observed as a result of short thermal treatments of wool at temperatures ranging from 110øC to 230øC, for as little as 30 seconds. Chemical reactions in thermally treated keratin fibers were investigated by analyzing the gaseous product of wool degradation at 160øC. Identified products include H20 , CO2, CH4, CO, H2S , and COS, suggesting decarboxylation and other decomposition path- ways for keratin protein (16). The effects of temperature on hair were also investigated using electron spin resonance (ESR) spectroscopy to monitor the signal produced from melanin (9). The yellowing of wool represents another important aspect of thermally induced keratin degradation, which has been of great interest within the textile industry. This phenom- enon is also significant from a cosmetic scientist's point of view, since hair yellowing is commonly perceived as undesirable, especially in the discoloration of unpigmented grey hair. In wool, yellowing can be produced by irradiation and by thermal treatments exceeding 100øC. Several papers were published on this topic, but the mechanism of color formation has not yet been firmly established. Decomposition of cysteine and tyrosine and oxidation of tryptophan (Trp) to kynurenine were proposed as likely path-