j. Soc. Cosmet. Chem., 38, 341-350 (September/October 1987) Triboelectric charge distributions generated during combing of hair tresses G. WIS-SUREL, J. JACHOWICZ, and M. GARCIA, Clairol Inc., 2 Blachley Road, Stamford, CT 06922. Received February I2, I987. Synopsis Triboelectric charge distributions generated by combing of hair tresses were correlated with combing force curves by an experimental set-up comprising a load cell and a static detector probe interfaced with a computer. The charge-density distribution profiles showed three distinct peaks corresponding to the upper, middle, and tip-end sections of a hair tress. The upper section peak was usually the most pronounced, and a hypothesis of its existence is discussed. The comb-work function and hair-surface modification effects were explained qualitatively in terms of the band model of the electronic structure of polymers and metals. Hair-surface modifications by a cationic surfactant or a cationic polymer were demonstrated to affect both the magnitude and the distribution of comb-generated static charges along the length of a hair tress. INTRODUCTION Triboelectric charging of hair in the rubbing mode with metals and polymers has been the subject of two reports (1,2). In experiments detailed in these articles, hair fibers were subjected to tangential rubbing, and charge density was measured as a function of time (or the number of rubs) in a selected and small area of contact between the fibers and the probe. In each contact event, the fibers underwent similar elongation and stress since the distance between the rubbing element and the plane formed by the fibers was constant. The charge generation, under these simplified conditions of rubbing, was demonstrated to be controlled by the work function of the contact probe (1), direction of rubbing (1), hair surface modification with polymers (2), surfactants (2), and oils, as well as the mechanical and/or electrostatic history of a tress (3). There is, however, very little information on the relationship between these data and real-life combing electrifi- cation. Therefore, we have investigated the process of triboelectric charging of hair tresses during combing. It was expected that combing triboelectrification is a more complex phenomenon than rubbing since such parameters as fiber elongation and stress, as well as the magnitude of frictional forces between the comb and fiber, vary during the movement of a comb from the upper part of a tress towards the fiber tips. To correlate mechanical combing data and charge distribution (charge density as a function of time or the position along the length of a fiber tress), we have constructed a special apparatus which allows for the measurements of these two quantities simultaneously, 341

342 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS EXPERIMENTAL EXPERIMENTAL SET-UP A scheme of the device used for the simultaneous measurement of combing force and triboelectric charge density is shown in Figure 1. Load cell (Sensotronic) and static detector probe (Keithley Model 2501) were interfaced with an IBM PC by means of an analog-to-digital converter (Model DT2801, Data Translation, Inc.). Acquisition of data was performed by Labtechnotebook software (Laboratory Technologies Corpora- tion), and all subsequent calculations were done with a Lotus 1-2-3 spreadsheet (Lotus Development Corporation). A typical experiment consisted of passing a hair tress in the root-to-tip direction at a rate of 1 cm/s through a comb, with continuous monitoring of force and potential arising from generated charge. The signal voltage values were then corrected for drift and recalculated into charge density from calibration curves. Total transferred charge and combing work were obtained by numerical integration of charge density-distance or mechanical force-distance plots, respectively. After each charging cycle, discharging was done using a polonium discharging element. Each experiment was performed on two different tresses, and the reproducibility in terms of charge densities, combing forces, and integrated charge density values was within 20%. The entire set-up was housed in a dry box maintained at 25-30% relative humidity under a positive pressure of air passed through several columns filled with Drierite. All mea- surements were performed at room temperature. PREPARATION OF HAiR SAMPLES FOR TRIBOELECTRIC MEASUREMENTS Virgin brown hair, purchased from deMeo Brothers, New York, was used throughout this work. It was washed with sodium dodecyl sulfate (SDS), rinsed with a large amount of deionized water, and dried at room temperature. Further purification was conducted by extraction with a mixture of methanol and chloroform (1:1) overnight • 'tlllrr,tqllll I lllllllllll IIIIillllll Computer Figure 1. A device to study combing forces and distribution of triboelectric charges. 1, motor 2, dis- charging element 3, comb (four teeth, 1.5-mm thick, per cm) 4, holding frame 5, static detector probe (Keithley, Model 2503) 6, hair tress 7, load cell (Sensotronics, Model 60036) A and C, power supply (+ 10V, q-15V) B, operational amplifier (Analog Devices, Model 2B31J) D, electrometer (Keithley, Model 616).