2006 ANNUAL SCIENTIFIC SEMINAR 421 THE EFFECT OF TREA TMENTS ON THE SHEAR MODULUS OF HUMAN HAIR CORTEX AND CUTICLE LAYER MEASURED BY THE SINGLE FIBER TORSION PENDULUM Don Harper and Yash Karnath, Ph.D. TRI/Princeton Previous studies with the single fiber torsion pendulum have alluded to the ability of this device to selectively measure different regions of a fiber, namely, the core and the sheath. This selective ability of the torsion pendulum was explored further as a means of better understanding treatments effects. First, the relationship between shear modulus and fiber circularity was investigated and, secondly, the effect of moisture on shear modulus was evaluated. The measurements were made using a single fiber torsion pendulum on untreated and bleached hair fibers. The bleached fibers were subsequently treated with either Polyquatemium-10 or cetyl trimethylammonium bromide (CET AB) and measured again. The effect of moisture was evaluated by varying the humidity inside a chamber surrounding the sample mounted in the torsion pendulum. Shear Modulus and Circularity It was discovered that a correlation exists between the circularity (defined as minor axis/major axis) of hair fiber cross-sections and the shear modulus such that more circular fibers have higher shear moduli and fibers which deviate from circularity have proportionally lower shear moduli. This relationship was amenable to linear regression and yields the following equation for untreated hair: Shear Modulus (in GPa) = (0.52 x Circularity)+ 0.62 By extrapolation, the equation produces a shear modulus of 0.62 GPa when circularity equals 0 (i.e., the fiber is least circular) and a shear modulus of 1.14 GPa when circularity equals I (i.e., the fiber is the most circular). Since it is known that the cuticle layer is more rigid than the cortex and that torsion measurements are dominated by the outer surfaces of a sample, it is speculated that the shear modulus of the cuticle layer is in the vicinity of I. I GPa and that of the cortex is about 0.6 GPa. These predictions were supported by measurements on decuticled (abraded) hair fibers which yielded moduli between 0.5 and 0.8 GPa with no dependence on circularity whereas the same fibers that were unabraded yielded moduli between 0.9 and I. I. The shear modulus of over-processed bleached hair (see figures below) also lacked dependence on circularity and yielded shear moduli in the range of 0.6 to 0. 7 GPa [I]. This indicates that the cuticle layer was softened by bleaching and resulted in a shear modulus similar to that of the cortex. The bleached hair treated with CET AB showed a fortifying effect on the cuticle layer as the torsional behavior approached that of the untreated fibers. The bleached hair treated with Polyquaternium-10 showed only a slight increase in modulus indicating little fortification of the fiber. It has been shown, by independent measurements, that the difference between the two treatments is that CET AB penetrates into hair, whereas, Polyquaternium-10 remains mainly on the surface. c:, CETAS 1.4 �-------------� 1.2 08 0.6 04 0,2,__ ____ ----- - 04 05 06 07 Circularity 0 8 0.9 Polyquatemium-1 O 1◄ �---------------, Unt 12 0 8 c:, � 06 "=l 04 0.2 +-------- --------- - 0.5 0.6 07 Circularity 08 09

422 JOURNAL OF COSMETIC SCIENCE Shear Modulus and Moisture Water is one of the simplest and best known "treatments" which softens hair fibers. The dosage effect of water, in the form of water vapor, was investigated by measuring the torsional behavior at different humidity levels from less than 10% relative humidity to 80% or higher. The humidity effect on the shear modulus of untreated hair (solid lines) and of bleached hair (dashed lines) is shown in the figure on the right. For untreated hair, the effect is sizeable with shear moduli in excess of 1.5 GPa at low humidity levels (10% RH or less) and drops steadily to about 0.7 GPa at 90% RH. The OS--- ,o 5G -------- ��----·1 I I .. 70 IO effects are even more extreme for bleached hair with shear moduli in the vicinity of 2.0 GPa at I 0% RH and dropping below 0.5 GPa in the 80% to 90% RH range. These responses were very rapid when the humidity was changed indicating that changes to the cuticle layer were involved and dominated the response. It is speculated that the more extreme response of bleached hair is due to the more open structure and the increase in salt linkages from the bleaching process. The hysteresis effect of water vapor on an untreated hair fiber was investigated by first thoroughly drying the fiber and then measuring the shear modulus at incrementally higher humidity levels. Upon reaching almost 90% RH, the shear modulus was measured as the humidity was incrementally reduced (see figure on the right). The hysteresis between the desorption curve and the sorption curve is due to moisture retained in the fiber during desorption which is most likely in the cortex and is typically only 0.06 GPa or less. On the other hand, larger responses (in many cases, double) are L . \ /� .______.,,___ - -�- - - �- � -- -· -·- -- - � .. SORPTION . --- ··-· - - - �. 211 5G IO 70 caused by each 10% change in humidity above 50% RH. These larger responses to rapid changes in humidity are evidence that these effects are occurring primarily at the cuticle layer. The data presented shows that the single fiber torsion pendulum can be useful in better understanding where in a fiber structure treatments are having their effect. The relationship between shear modulus and fiber circularity for untreated fibers allows one to estimate the shear modulus of the cuticle layer and that of the conex separately. When treatments are evaluated in this way, the effect and location of the effect can be approximated. The response of hair fiber shear modulus to changing humidity was found to be very large and it occurred rapidly indicating that the effect was dominant on the cuticle layer. [I] D. Persaud and Y. Karnath, J. Cosmet. Sci., 55 Supplement, (2004).