j. Soc. Cosmet. Chem., 38, 341-350 (September/October 1987) Triboelectricharge distributions generated during 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 of hair tresses were correlated with force curves by an experimental set-up comprising a load cell and a static detector probe interfaced with a computer. The charge-density distribution profile 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 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 Triboelectri 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 electrification. Therefore, we have investigated the process of triboelectric charging of hair tresses during. It was expected that 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 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 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 Corporation), 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 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, 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 measurements 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 Computer 'tlllrr,tqllll I lllllllllll IIIIillllll Figure 1. A device to study forces and distribution of triboelectric charges. 1, motor; 2, discharging 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).
TRIBOELECTRIC CHARGE DISTRIBUTIONS ON HAIR 343 (solvent/hair ratio of about 50). The tresses were then dried and conditioned at 30% RH prior to use. The extraction is essential to obtain consistent results. Hair tresses were prepared by gluing 2 grams of 17-cm-long hair fibers to plastic tabs while distributing the fibers evenly over the width of 3 cm. To modify the surface properties of the fibers, hair swatches were placed into a large excess of 10 g/l poly(1,1-dimethylpiperidinium-3,5,-diallyl methylene chloride) (PDMPDAMC) or stearalkonium chloride aqueous solution (solution/hair ratio of about 50) for a few hours at room temperature and stirred occasionally. The fibers were then rinsed under running deionized water, exposed to an excess (2-3 liters) of deionized water for 2-4 hours, dried, and conditioned at 30% RH. RESULTS AND DISCUSSION CHARGE DISTRIBUTION AND COMBING FORCE PATTERNS Figure 2a shows a typical example of charge density distribution and force curves for untreated hair with a nylon comb. The charge density distribution profile shows three distinct peaks, an intense and fairly sharp one in the upper portion of the tress, a broad and structureless peak in the middle region of the trees, and a small o 0 Combing time 0 - force 86a 0 0- force tsa 0 O- force First (O),second ( ) and third ( ) Figure 2. Triboelectric charge distributions and force curves generated with nylon and aluminum combs. a, time-adjusted force and charge curves; b, length-adjusted force and charge curves; c, multiple s of discharged hair with a charged nylon comb (length-adjusted force and charge curves); d, length-adjusted force and charge curves generated with an aluminum comb.
344 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS one at the fiber tips. In contrasto this, Lunn and Evans reported the existence of only disentanglement peaks on tribocharge distribution curves (4). The static detector probe and the comb are located at different positions in the experimental set-up. Such an arrangement of sensors was necessary to minimize interference of the static detector probe from the electrical field of the charged comb. The data can thus be reported in graphs showing force and charge as a function of time of, as in Figure 2a, or as a function of distance along the tress length as in Figure 2b; the latter allows for comparison of charge and force at the same point on the tress. Mechanical measurement of forces, represented by the lower curves in Figures 2a and 2b, reveals that the force is relatively constant for most of the tress length, rising significantly near the tip end. This is related to disentangling of "cross-over" hairs. High forces, arising from disentangling of tip ends, lead to better contact and increased friction between the comb and the fibers. These factors might contribute to the presence of a distinct tip-end peak in the charge distribution curves which corresponds to the maximum of the force curve shown in Figure 2b. The origin of the disentanglement peak in charge distribution curves was proposed earlier by Lunn and Evans (4). An explanation for the large charge peak in the upper portion of the tress may be related to the insulation characteristics of plastic combs and their ability to acquire a high electrical potential from charge accumulation. Combing of a hair tress causes continuous contact of an increasingly charged comb surface with fresh, uncharged portions of hair fibers. The conditions for electron transfer are most favorable in the immediate vicinity of the comb insertion point, where an uncharged or low potential comb contacts the keratin surface. As the process of electron transfer continues during the movement of the tress, the comb becomes charged to high potential and its surface states available for electron exchange are depleted. This should lead to an equilibrium in terms of electrochemical potentials of contacting surfaces and inhibition of electron migration (5,6). The charge density should, thus, be highest in the area close to the point where starts and drop to lower values in the middle section of a tress. The tribocharge density might rise again in the tip-end portion of a hair tress because of increased contact and friction associated with disentangling of "cross-over" hairs. This interpretation of experimental curves obtained with insulator combs is upheld by qualitatively different charge density distributions recorded in metal-comb electrification experiments depicted in Figure 2c; the prominent peak in the upper portion of the tress is not present, and the charge distribution generally parallels the force curve. This result with the aluminum comb is consistent with the constancy of the comb and hair surface potentials throughouthe whole test. Only when the entanglement at the tip end is reache does the increased friction lead to peaks in force and charge. This analysis of the charge distribution curves is, however, hard to reconcile with data from repeated with charged nylon comb. Figure 2d shows the distribution of charge after three consecutive s which involved a discharged tress and a charged comb. After the first, prior to which both the comb and the hair were discharged, the intensity of the peak in the upper portion of the tress was relatively low. The second and the third resulted in an increase in the intensity of the first peak. The charge densities corresponding to the lower portions of the hair tress were
TRIBOELECTRIC CHARGE DISTRIBUTIONS ON HAIR 345 similar after each, independent of the comb potential. This effect of enhanced charging by the probe loaded to high potential of the opposite sign is difficult to explain within the framework of existing models of polymer-polymer or metal-polymer electrification. According to the band model of contact charging proposed by Davis (5,7) and Lewis (8), the charge would be transferred until either all the surface states are filled, or until a sufficient surface potential is created to prevent further charge transfer. Repeated contact usually leads to accumulation of transferred charges, which is explained by slow diffusion from surface states into bulk states, creating surface vacancies which can be subsequently refilled. This additional charge transfer is, thus, related to the concentration gradient of charged species within the bulk and to the rate of transport of the surface states to the bulk states. For insulators such as comb materials used in this study, and keratin with low moisture content, the dielectric relaxation times are long, and consequently rapid filling of the surface states and slow filling of bulk states can be expected. This representation of contact charging justifies the enhanced electron transfer in the upper part of a tress but fails to explain why a charged comb contributes to the further intensification of this process. Another theory of triboelectric charging, the Duke and Fabish (9) sampling/non-communicating state model of polymer-polymer contact, does not account for the surface potentials developed after charge injection and does not predict how the state energies might be modified by the existence of electrical fields created by the excess charge. It cannot be thus used to analyze the effects reported in this paper. The middle peak in the charge distribution profiles shown in Figures 2a and 2b is sometimes undetectable in charge distributions, or it becomes merged with the entanglement peak. Some data suggesthat it might be an artifact which we believe may be related to the change in geometry of the tress as the hair clears the comb. However, the measurements of charge distribution without on previously charged tresses also demonstrate the existence of this peak. EFFECT OF COMB WORK FUNCTION AND MULTIPLE COMBINGS Teflon (4.26-6.71 ev), polyethylene (4.9-6.04 ev), nylon (4.08-4.5 ev), polycarbonate (3.85-4.8 ev), and aluminum (3.38-4.25 ev) combs were used to assess the effect of comb material on charge distribution profiles. The range of work function values (the work required to remove electrons from the Fermi level to the surface) given in brackets and reported in the literature serves only as a general indication of relative positions of these materials in the triboelectric series (3). As mentioned in our previous paper, there is a considerable discrepancy between various sources, mainly due to the use of different experimental procedures and materials with varying degrees of purity. Figures 3a-e shows charge density distributions on hair tresses combed with nylon, polyethylene, teflon, polycarbonate, and aluminum combs. The same figures illustrate the gradual buildup of static charge on hair as a result of consecutive s. The numbers assigned to each distribution curve represent'the charge densities integrated over the length of the tresses (expressed in C/cm). Figure' 3a presents the charge distributions obtained with a nylon comb, including the force curve corresponding to the first cycle. The shape of the force curve as well as the values of the forces are representative for all comb materials studied. This is in accord with the
346 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.7' 0.6 0.4, 0.5, 0.2 0.1 0 a) nylon First (0), seoond (D) andthird (o)- force 120 no % 100 90, 1.5 80 ø 1.( 70 } 6O 40 0. 30 } 2O 10 0 b) olyethylen Tress length First (o),second (a) and third (V) c) teflon ress length First (o),second (9) and third (V) -5.0 e) alumin. 1.6 First (O), second ( 1) a l third (V) First (O), s 3nd (f7) and third ( ) Figure 3. Triboelectri charge distributions on untreated hair after multiple s with combs made of various polymers or aluminum; no discharging between s. finding that comb material has no effect on both work and maximum force (10). Teflon, nylon, and polyethylene generate a high density of positive charges with a characteristic bimodal or trimodal distribution pattern. The polycarbonate comb, on the other hand, produced a high-intensity negative charge which probably indicates that piezoelectric potential (which we believe determines the sign and the magnitude of transferred charges in rubbing triboelectrification experiments (3)) does not influence the direction of electron transfer under low-stress conditions of comb-keratin contact in experiments. In general, the charge density in the tip-end and upper portions of hair tresses increases to a similar extent after multiple s. This contrasts with the selective increase of the insertion peak in distributions produced by multiple of discharged tresses with a charged comb, as shown in Figure 2d.
TRIBOELECTRIC CHARGE DISTRIBUTIONS ON HAIR 347 The characteristic apportionment of transferred charges and the progressive increase in the charge density is probably related to the fact that multiple s produce an increased number of contacts between uncharged sections of contacting surfaces. The gradual increase of potential after multiple s can continue until equilibrium surface charge density is reached. Also, the electrical breakdown of the surrounding atmosphere might limit surface charge density to less than 7 ø 109 C/cm 2 (2), the value never reached after a few s. THE EFFECT OF HAIR SURFACE MODIFICATION Adsorbed long-chain alkyl quaternary ammonium salts and cationic polymers significantly modify the electrochemical potential of the fiber surface and affect the process of electron transfer (2). Adsorbed long-chain alkyl quaternary ammonium salts cause a considerable decrease in the electrochemical potential of hair (2). In consequence of this, the probes characterized by the work function lower (poly(methyl methacrylate)) and close to keratin (polycarbonate) generate negative charges on quat-treated hair. Charging against teflon, which has a high work function value, produced positive charges on hair because the reduction of the electrochemical potential of the fiber sur- face was not sufficient to match that of the teflon surface. An opposit effect, increased electron-donating character of hair, is caused by adsorption of the cationic polymer PDMPDAMC. Triboelectrification, in an experimental setup similar to the one described earlier (1,2), revealed a consistent increase in the electrochemical potential of PDMPDAMC-modified hair surface using polycarbonate, aluminum, and poly(methyl methacrylate) probes. Triboelectrification results from of hair treated with cationic substances are given in Figures 4a-d and Figures 5a-c. The polycarbonate comb generated trimodal distributions of high positive-charge density for PDMPDAMC-modified hair (Figure 5c) and high negative-charge density for fibers treated with the cationic surfactant (Figure 4d). Reduction of the electrochemical potential gap between the comb and quat-modified hair, accomplished by polyethylene, nylon, or teflon combs results in a reduced overall density of positive charge on hair. Correspondingly, an increase in the electrochemical gap between the comb and PDMPDAMC-modified hair produced an enhancement in the overall charge density of the hair. This indicates that the electrochemical surface-potential gap between the rubbing element or comb material and keratin is a decisive factor in determining the magnitude and sign of the generated charge. Apart from the overall magnitude of the charge density produced by, the quat and PDMPDAMC treatments exert an influence on the charge distribution profiles. In the case of quat-treated hair, the charge concentrates mainly in the upper portion of the tress. Selective increase of the intensity of the peak in the upper portion of the tress after consecutive s is similar to that observed in of discharged tresses with a charged comb. On the other hand, the middle and tip-end peaks are considerably reduced. Since the adsorbed stearalkonium chloride reduces the tip-end peak force by about 10-30% as compared to untreated fibers (compare force curves presented in Figures 3a and 4a), this is probably related to a combined effect of reduced electrochemical potential gap and friction (lower combining force should result in a decreased number of fiber-comb contacts and consequently produce less triboelectric charge). Triboelectri charge distributions, similar to those shown in Figures 4a-d,
,,. 348 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.7 0.6' 0.5 0.2 Tress length First (0), seoond ( ) and third ( ) (o) - 1.2 5.8.1..0 force {.5\ C) teflon 0.8-120 0.8- ß 100 v o 0.6 80 ø 60 = 0.4 40 o 0.2 20 o 0 b) polyeth Tress length First (o),second (D) and third mo 0-8.4 0.6 0.4 0.2 0 First (o),second (s) and third (V)!-2 d) polyc -3 First (o),second (D) and third (V) Figure 4. Triboelectric charge distributions on stearalkonium chloride-treated hair after multiple s with combs made of various polymers; no discharging between s. with the highest charge density located in the upper portion of a hair tress, should not result in "fly-away" hair. This observation is compatible with the well-documented antistatic nature of cationic surfactants which are widely used as static electrification and friction-lowering agents in hair-conditioning products. Combing of PDMPDAMC-modified hair yields high-intensity charge distributions centered around the tip-end portion of a hair tress (Figures 5a-c). Following the previous line of reasoning, this has to be attributed to a widened electrochemical gap between contacting materials as well as an increased force observed for PDMPDAMC-modified tresses (Figure 5a). Such charge distributions result in the "fly away" phenomenon which is known to plague PDMPDAMC-containing formulations. CONCLUSIONS Simple single-peak charge density profiles during the of hair at 50% RH, as reported by Lunn and Evans (4), are not observed in our results obtained under lower RH conditions. Measurements performed using insulator combs indicate that the typical charge distribution for clean hair fibers consists of two or three distinct peaks, with the one from disentanglement of tip ends contributing only to a small extent to the total generated charge density. The origin of a large peak at the upper portion of the tress was not unequivocally ascertained. We speculate that the initial contacts between polymer-comb surface and keratin result in electron transfer and produce a very high
TRIBOELECTRIC CHARGE DISTRIBUTIONS ON HAIR 349 120 : 4' loo ' B0 3 2. 60 8. b) t flon tress length O - oc bing force First (0), second (0) and third ( 7) 0 o 0 tress len First (O), s oond (1 ) third ( ) mbmg 1. c) poly z tress lergth First (O), second ( ) and third Figure 5. Triboelectric charge distributions PDMPDAMC-modified hair after multiple s with combs made of various polymers; no discharging between s. potential on the comb surface, inhibiting the charge-exchange process in the lower part of the tress. This hypothesis finds support in metal- charging characteristics which do not exhibit the initial peak but is, however, not consistent with the observed enhancement of this peak by insulator combs loaded to the potential of the opposite sign. The influence of comb work function and hair surface modification on charge distribution profiles are generally similar to the ones observed in the experiments involving surface rubbing electrification and can be rationalized in terms of the band model of the electronic structure of polymers and metals, assuming certain characteristic values of work functions for each material. Superimposed frictional effects, introduced by surface treatments, distorthe division of transferred charge representative of untreated hair by altering the intensity of friction-dependent middle and tip-end peaks. REFERENCES (1) J. Jachowicz, G. Wis-Surel, and L. J. Wolfram, Directional triboelectric effect in keratin fibers, Text. Res. J., 54(7), 492 (1984). (2) J. Jachowicz, G. Wis-Surel, and M. L. Garcia, Relationship between triboelectric charging and surface modifications of human hair, J. Soc. Cosmet. Chem., 36, 189 (1985). (3) G. Wis-Surel and J. Jachowicz, unpublished results. (4) A. C. Lunn and R. E. Evans, The electrostatic properties of human hair, J. Soc. Cosmet, Chem., 28, 549 (1977).
350 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (5) D. K. Davis, Charge generation on dielectric surfaces, Brit. J. Appl. Phys. (J. Phys. D.), 2, 1533 (1969). (6) D. A. Seanor, Triboelectrification of polymers--a chemist's viewpoint, Physicochem. Aspects Polym. Surf., Proc. Int. Symp., 1, 477 (1983). (7) D. K. Davis, The examination of the electrical properties of insulators by surface charge measurement,j. Sci. Instrum,, 44(7), 521 (1967). (8) T. J. Lewis, "The Movement of Electrical Charge Along Polymer Surfaces," in Polymer Surfaces, D. T. Clark and W. J. Feast, Eds. (John Wiley and Sons, Chichister, 1978). (9) C. B. Duke and T. J. Fabish, Contact electrification of polymer: A quantitative model, J, Appl. Phys., 49(1), 315 (1978). (10) A. Hambidge and L. Wolfram, unpublished results.