400 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS distinguishing mechanical events which occur in each of the two phases during the extension of the fiber. These are the events which are compatible with the physical evidence available and lead to an understanding of the mechanical properties of ce-keratin fibers. SET IN c•-KERATIN To obtain set in a fiber it is necessary not only to reduce the forces tending to return the fiber to its native configuration, but also to create new bonds in equilibrium with the setting strain. These latter bonds will oppose on release of the fiber any tendency for the fiber to return to its original state. In the case of "wet setting" of hair and wool fibers, the setting strain is applied to the fiber in a wet state, which assists in the rapid breakdown of weak interactions (hydrogen bonds, Coulombic interactions, etc.). If the fiber is dried while held strained, new interactions are formed between components of the keratin structure by the removal of water and the reduction of swelling. On release dry, the greater part of the fiber strain is retained, the fiber is set. However, if placed in water this set is released, and the fiber returns to its native configuration. This set is referred to as the Cohesive set, and is dependent on the breakdown and reformation of the Cohesive forces within the fiber. In commercial practice, where set in hair or wool textile material must be stable in the presence of moisture, it is necessary for the breakdown and reformation of bonds at the setting strain to involve covalent bonds, which will not be affected by the presence of moisture in the fiber. The disulphide bond formed by the diamino acid cystine present in c•-keratins is important in stabilizing the native configuration of the fiber. The breakdown and reformation of this bond via the mechanism of sulfhydryl-disulphide interchange (74) forms the basis of most techniques of "permanent" setting of c•-keratin fibers. Disulphide bonds in neutral or alkaline aqueous solutions can undergo a breakdown and reformation mechanism referred to as sulfhydryl-disulphide interchange in the presence of thiol groups by the following scheme: --S•- + --SbS•--•--Sb- + S•Sc-- This reaction results in the removal of disulphide bonds opposing the setting strain, the interchange favoring the breakdown of strained bonds, and their reformation in equilibrium, with the set configuration. In normal wool and hair about 0.5% of the amino acid residues are thioIs and the above mechanism proceeds in water at temperatures above 50-60øC. If more rapid interchange at lower temperatures is required to obtain this bond breakdown and reformation, it is necessary to form more thioIs by reduction of disulphide groups. The review of the chemistry involved, and the various techniques of setting of hair and wool, is beyond the scope of this consideration of the physical properties of ce-keratin fibers. However, it is necessary to note the effect of setting under certain conditions on the mechanical properties of the ce-keratin fibers and what this reveals with regard to.fine structure of the fibers. If a fiber is set by heating in distilled water at a strain corresponding to the Yield Region, the mechanical properties of the fiber are modified to a varying degree (see Figure 4). The stress-strain curve of the fiber is modified up to a definite strain level dependent on the setting strain, and beyond that level the stress-strain curve reverts to that of the unset fiber (75). The whole behavior of the fiber suggests that only zones

PHYSICAL PROPERTIES OF ALPHA-KERATIN FIBERS 401 C ! 0 LOAD '--'=' Figure 4. The typical load-extension curve of a wool fiber in water at 20øC after having been held strained 10% for one minute in boiling, distilled water and then released for one hour in water at: (i) 20øC, (ii) 53øC, (iii) 100øC. Also shown is the curve in an environment of 63% relative humidity and 20øC for a fiber set and released as for case (iii). In all cases the modification due to setting mechanical properties of the fiber is limited to the range ce to 15, a range defined by the setting strain. within the microfibrils of the fiber have been modified by the setting treatment, and the zones affected relate to the structure unfolded by the setting strain. The presence of these affected zones has been confirmed by torsional experiments on hair fibers set at various strains within the Yield region (76). These torsional experiments showed a linearly proportional relationship between setting strain and zones present in the set keratin-structure. SERIES-ZONE MODEL As a further refinement in our understanding of the mechanical properties of ot-keratins, the series-zone model (77) was proposed based on results obtained for longitudinal stress-strain behavior in water and in concentrated lithium bromide of unmodified and of set fibers (75). The model proposes the existence of two varieties of alternating zones along the microfibrils called X and Y, differing in their stability. The ot-helices within the X zones are the 30% that unfold in the Yield region of the stress-strain curve of a keratin fiber, and the Y zone ot-helices are unfolded with ,extension into the Post-Yield region. The opening up of the X zones is quite recoverable with no covalent bond breakdown involved, whereas the opening up of the Y zones as the fiber is extended into the Post-Yield region involves the breakdown of covalent interactions (disulphide bonds) which stabilize these zones. This series-zone