j. Soc. Cosmet. Chon., 36, 87-99 (January/February 1985) Torsional behavior of human hair LESZEK J. WOLFRAM and LINDA ALBRECHT, Clairol Research Laboratories, 2 Blachley Road, Stamford, CT 06922. Received October 31, 1984. Presented at the Annual Meeting of the Society of Cosmetic Chemists, New York, December 6-7, 1984. Synopsis The unique tensile properties of keratin fibers in water led Speakman to develop the technique of fiber calibration which has become one of the mainstays of hair testing methodology. We have followed a similar approach in studying the torsional rigidity of hair. A simple modification allows for the torsion pendulum technique to be used both in air and in liquids. The results of torsional measurements on fibers of different diameters suggest that the hair cuticle, while tough and resilient in the dry state, undergoes water plasticization to a much greater extent than the hair cortex. The change in the torsional moduli of fibers exposed to chemical modification can be related to the configurational stability in water and be of sonhe utility in predicting the setting behavior of hair. INTRODUCTION Bogaty (1) was the first to point to the relevance and importance of torsional defor- mations both in impartation and in maintenance of hair styles. By setting or waving of hair, helical coil configurations are formed and the performance of these can be related to the engineering spring theory in respect to the effects of fiber diameter, coil radius, and torsional stiffness. The latter is highly moisture sensitive, a behavior that can be accounted for by the two-phase model for keratin structure developed by Feugh- elman (2). In this model, the hair fiber is viewed as a composite made up of two phases: the water unpenetrable and axially oriented filaments embedded in a water absorbing, cystine-crosslinked matrix. In the dry hair, the mechanical properties of both phases are similar. The fiber acts as a homogeneous, isotropic material, the tensile or torsional deformations being resisted by the total structure. On exposure of hair to increasing levels of humidity, the absorbed water progressively softens the matrix, thus lowering its mechanical modulus while the filament phase remains relatively unchanged and highly resistant. The water brings to the fore the latent anisotropy of keratin fibers. In the tensile mode of deformation, the filaments are the primary load-bearing elements of the fiber as the swollen and highly weakened matrix contributes little to the overall resistance. The reverse occurs in torsion. In this deformation mode, the water-pene- trated and soft matrix phase takes up virtually all shear stress imposed on the fiber, with the stiff filaments merely tilting as the matrix deforms. Since the early work of Speakman (3), the measurements of tensile properties of keratin fibers have been utilized in studies of their behavior in a diversity of media, in mon- itoring the course of cosmetic modification of hair by various reagents, and in evaluation of fiber damage. The calibration approach (work or force index in the wet state) which 87

88 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS has been frequently the technique of choice yields, with a minimal number of fibers tested, a reproducible assessment of changes in hair properties. There is, however, a potential drawback in relying exclusively on the tensile calibration approach. Cosmetic treatments of hair such as waving, coloring, bleaching, straightening, etc., are carried out in aqueous media. The water-accessible phase of the fiber, the matrix, thus appears to be the principal locus of these reactions, and it is the change in the properties of the matrix that may underlie subsequent behavior of the treated fibers. Relatively little information of such changes can be gained from the tensile properties of hair which is dominated by the filament phase. Clearly, measurement of torsional rigidity appears to be the most sensitive and direct way to evaluate these effects and thus to augment our knowledge of both the hair fiber reactivity and its rheology. This study has a two-fold objective: to develop a single-instrument technique for measurement of torsional rigidity of hair, both in air and in liquids, and to explore the torsional behavior of hair from the point of view of its practical relevance. EXPERIMENTAL MATERIALS AND METHODS Human Hair. Two prime sources of hair were used: (a) blended dark brown European hair commercially available from De Meo, New York and (b) authentic hair fibers from subjects of various racial origins whereby it was known that the hair had not been subjected to any cosmetic modification. Prior to use, the hair was cleaned by sham- pooing, thoroughly rinsed, and stored in a room maintained at 65% RH, 70øF. Instrumentation. The torsion instrument* measures damping and the force-to-twist of single hair fibers utilizing a free swinging torsional pendulum. The pendulum is brought into motion using a rotating electromagnetic field. The output signal is in- stantaneously fed to a strip chart recorder and to a data station consisting of a Cyborg 41A A/D converter and an Apple lie microcomputer. Fiber Calibration. Using the torsion pendulum method, one can compute either the rigidity of a fiber which is given by: I • = 8 •r 3 I L / T 2 ............ (I) where T is the period of oscillation, L is the length of the fiber, and I is the moment of inertia of the pendulum or, if the diameter of the fiber is known, the torsion modulus G, which is given by: I • 128 •r I L G - •r 2 D4 - T 2 D4 ............ (II) 16 where D is the diameter. The decay in the amplitude of successive oscillations gives rise to a very useful quantity defined as the logarithmic decrement (8): _ al 8 = 1 In--. ........... (III) (III) n a n * The instrument, similar in design to that described by Bogaty (1) was built by Mr. Marion DenBeste of DenBeste Products, Chicago, Illinois.