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).

2006 ANNUAL SCIENTIFIC SEMINAR 423 UNDERSTANDING THE MICRO-PHYSICAL AND MECHANIC AL PROPERTIES OF THE HAIR CUTICLE VI A DAMAGE AN ALYSIS Manuel Gamez-Garcia, Ph.D. Ciba Specialty Chemicals Corporation) 540 White Plains Road) Tarrytown) NY 10591 Abstract: This paper presents a systematic analysis of various patterns of hair cuticle damage. The analysis suggests that as the hair grows from root to tip the cuticle envelope gradually ages by weathering and grooming stresses losing its natural visco-elasticity and its ability to dissipate shear, compression, and extension stresses. The results show that patterns of cuticle damage such as cuticle lifting. de-cementation, buckling, bulging, cracking, and breakage do not only occur by mechanical stresses imposed during combing, but also as the hair cuticle expands and contracts naturally during wetting and drying. The analysis also suggests that most types of cuticle damage occur because the weathering stresses induce physical-chemical changes in the cuticle protein structure. These changes appear to compromise the cuticle's protein ability to bind water and consequently affect negatively the cuticle visco-elastic behavior. The role of conditioners will be discussed Discussion and Analysis: Changes in the aesthetic properties of hair are of great concern to cosmetic formulators. Damage to the cuticle at the hair surface is the main cause for hair aesthetic properties degradation. In order to prevent this type of damage it is critical to understand the various mechanisms leading to cuticle damage during grooming practices. In this paper an analysis of patterns of cuticle damage is presented. The analysis shows that various common patterns of cuticle damage produced during grooming occur because the cuticle protein structure loses its natural softness and visco-elastic characteristics, thereby becoming gradually rigid and brittle. This rigidization of the cuticle occurs either because the cuticle cell components lose transiently or irreversibly their hydration water that acts as protein plasticizer, or because the protein structure is irreversibly changed. For instance, Fig. 1 a shows a SEM micro-graph of a severe pattern of cuticle buckling resulting from cyclical tension stresses on a hair fiber when stretched cyclically at low moisture conditions (IO % RH). This pattern of damage is fully reversible and results from a transient lack of water in the cuticle cells. As the cuticle cells are devoid of its natural plasticizer "water" there is a mismatch in the visco-elastic response of the various inner layers of a cuticle cell and the whole cuticle cell buckles under Poisson compression stresses ( 1 ). The proof that this type of damage is fully reversible is sho'Nn in Fig. I b, where the same hair fiber shown in Fig. 1 a can be seen after it has been allowed to absorb water. Observe in Fig. I b that after the proteins in the cuticle cells are hydrated, the latter are able to recover from their distorted buckled state into their normal shape. Note that the fiber depicted in Fig. I a, is the same as that one shown in Fig. I b, but before hydration. Fig. 2 shows, on the other hand, a form of irreversible damage that often occurs in hair when its cuticle surface is devoid of water and builds up enough static charge to produce a discharge. The electric charge is produced tribo-electrically on the cuticle surface by the