1. Soc. Co, met. Chem., 22, 667-678 (September 17, 1971) Elasticity and Tensile Properties of Human Hair. II. Light Radiation Effects RICHARD BEYAK, M.S., G. S. KASS, B.S., and C. F. MEYER, M.S., M.B.A. • Synopsis--The effects on the TENSILE PROPERTIES of HAIR by LIGHT RADIATION in the form of ULTRAVIOLET and SUNLIGHT are reported. These effects are inter- preted from the stress-strain measurements made on single fiber tests and the indcx used is the force at 15% elongation. The ultraviolet light effects were studied by exposing hair samples to a carbon-arc lamp for increasing pcriods of time, while the solar effects were examined by exposing hair to natural daylight for periods of ten wceks. The daylight ex- posures were made throughout the year to study seasonal variation in the solar radiation. Effects of other environmental factors such as temperature and humidity were monitored by te•ting unexposed control sample groups. Results indicate a correlation between the total light radiation on the hair and the loss of tensile properties as exprcssed by thc yield point at 15% elongation. In addition, the correlation appears to be the same for UV and sunlight suggesting the effects are due to the total inci:tent radiation and not the charactcr of the light. INTRODUCTION There is a growing interest in the field of physicomechanical test- ing of hair and wool. These studies may be classified into three cate- gories: chemical and morphological characterization of keratin (1-3), chemical and environmental effects on the fiber (4-9), and diagnostic in- terpretations in the medical literature (10-12). Sunlight and ultraviolet light have long been known to affect the mechanical properties of hair (6, 7, 13), but very little quantitative data are available in the literature. The purpose of this study is to determine the environmental effects of sunlight and ultraviolet radiation on the tensile properties of human hair. Alberto-Culver Co., Melrose Park, Ill. 60160. 667

668 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS / SLOPE / / / / YIELD POINT @ •5% STrmrN / / SLOPE / / / / / STRAIN Figure 1. Typical stress-strain curve of a hair fiber Interpretations of tensile and mechanical changes to the hair fiber are based upon changes in the stress-strain curve. A typical stress-strain curve is shown in Fig. 1. The yield region of the curve has been reported to represent the unfolding of the a-helices of keratin (2), while the major restoring forces to the extension when performed in water are the disul- fide linkages (1). The yield point at 15% elongation was shown to be independent of cross-sectional aberrations (3) and responsive to chemical effects (14) thus, it serves as a convenient reference point for detecting tensile changes to the hair as a result of disulfide rupture. EXPERIMENTAL Apparatus The Instron Tensile Tester, * Model TM, with a Model A load cell and a 50-g full scale chart response was used. The gage length taken was 2.5 in. with a 0.2-in./min or 8c•o/Inin rate of elongation. The total elon- gation was 0.5 in. and a 2-in./min rate of unloading. All extensions were run in deionized water at 20 ø ñ 2øC. Instron Model G-51-1A sample mount was used to mount the hair fibers which were wound around the The Instron Corp., Canton, Mass.