J. Cosmet. Sci., 63, 81–92 (March/April 2012) 81 Advantages of a high-throughput measure of hair fi ber torsional properties F. I. BELL, P. CARPENTER, and S. BUCKNELL, Unilever Research and Development, Port Sunlight, Wirral, UK, CH63 3JW (F.I.B., P.C.), and Dia-Stron Limited, 9 Focus Way, Andover, Hampshire, UK, SP10 5NY (S.B). Accepted for publication September 21, 2011. Synopsis While the tensile response of fi bers of human hair is the most extensively studied mode of mechanical defor- mation, the properties of hair in different deformation modes remain of interest and can provide valuable insight into the effects of chemical treatments. Previously reported methods for the measurement of fi bers in torsional deformation have inherent systematic errors, are low-throughput, and are operator-intensive. This paper presents a new method for the measure- ment of fi ber torsional properties developed to reduce these errors and to improve the effi ciency of the tech- nique. This method was designed to be fully automated, requiring no operator input during an experiment, and affording higher sample throughput while improving the ease of use in variable climatic conditions. The new method is compared to a conventional torsional pendulum method for measuring fi ber shear modulus, and an evaluation of experimental reproducibility is made using hair and nylon fi bers. It was found that the new method provides absolute values for shear modulus similar to those of the pendulum technique, with reduced run-to-run variability between fi bers, while enabling larger sample numbers to be explored in shorter times. INTRODUCTION The mechanical properties of hair fi bers have been widely reported, but the focus of the majority of the literature has been their tensile characteristics (1–8). Only a small number of papers exist in which the mechanical behavior of hair in bending and torsional defor- mation modes is described (9–12). The complex composite structure of the hair fi ber yields anisotropy in response to external stimuli, and in order obtain a full understanding of the material properties of hair, measurement of the mechanical properties in modes other than only tensile deformation is vital. With signifi cant relevance to the personal care industry, when consumers interact with their hair, the manner by which the fi ber responds will infl uence the perception of the hair condition and infl uence the choices made in care and styling products. The resistance to deformation is a consequence of composition and structural morphology due to the number and strength of the chemical and physical bonds within the hair fi ber. For this

JOURNAL OF COSMETIC SCIENCE 82 reason, changes in the forces generated upon deformation can also be related to changes in the molecular structure of the hair, and infl uencing these characteristics is desirable for the optimization of hair products. The stiffness and strength properties of hair are impressive when compared to other natural and man-made materials, but due to the small cross-sectional area of a hair fi ber, an area of the order of 10−9 m2, the absolute values of the restoring forces generated by a hair under deforming loads are small. At small deformations within the Hookean region, tensile stresses of up to 980 MPa (13) are measured, but small deformations in bending and torsion generate much lower forces. Furthermore, tensile measurement requires a simpler experimental confi guration than required for the determination of bending or torsional properties and few commercially available instruments are currently available. Consequently, the scant literature relating to the bending and torsional properties of the hair fi ber can largely be attributed to diffi culties associated with making accurate deter- minations of these characteristics for such microfi brous materials (14,15). The tensile stiffness of hair is dominated by the crystalline fi brillar regions within the microstructure, which are aligned along the direction of the fi ber axis, and by the strength of the inter- and intramolecular interactions in the amorphous matrix that connects these regions. As the tensile stiffness of hair along the fi ber axis is a function of these molecular interactions in the direction normal to the fi ber axis, the Young’s modulus can be assumed to be almost entirely associated with the fi ber cortex. Any contribution from the cuticle will be small due to its low area fraction of the fi ber cross section in the direc- tion normal to the fi ber axis (2,16). The fi ber’s mechanical response in deformation by bending is complex and will be infl uenced by both the cuticle and the cortex. In bend- ing, a fourth-power relationship exists between the fi ber diameter, and the restoring force generated upon deformation, and for this reason material at greater distances from the center of the cross-section has a larger infl uence on this property than material close to the center. It has been predicted that the cuticle will have a major infl uence on bend- ing stiffness (17,18). Elsewhere, it has been reported by modeling of the hair fi ber structure that the cuticle will have a signifi cant but non-dominating infl uence on fi ber- bending properties, with a signifi cant contribution from the fi ber cortex (19). A wide range of methods has been applied to characterize bending forces, including application of a cantilever load to one end of the fi ber, loaded loop, pendulum, and vibrating rod methods (20–24). The torsional storage modulus is a function of the shearing forces experienced in the amorphous matrix of the fi ber cortex as the fi ber is deformed around the long axis, and it may be signifi cantly infl uenced by the cuticle that surrounds the hair fi ber. Although the cuticle generally accounts for a small fraction of the total area of the hair fi ber cross sec- tion, as with deformations arising from bending, a fourth-power relationship exists be- tween torsional stiffness and the fi ber diameter. To date, oscillatory pendulum methods have largely been used (11,15,25,26) to determine the torsional storage modulus, and they can also be used to measure loss modulus characteristics. These pendulum methods observe the frequency and magnitude of the oscillatory movement of a bob that is sus- pended from the fi ber and excited into motion by an applied force or torque. There are several factors that limit the utility of the oscillatory method, particularly regarding the testing of large numbers of samples. The torsional pendulum method re- quires manual attachment of the pendulum bob and loading and unloading of samples