Hair Damage 376 Young’s modulus of the fibers increases slightly, while the breaking strength decreases.38,39 An increase in radiation doses to 20 Mrad or greater leads to breaking of keratin chains and the complete degradation of the fiber.40 Effect of Temperature The key structural elements found in hair that could undergo thermal degradation on contact with hot surfaces (or air) include the cuticle and its outermost layer, the epicuticle, which is constructed of fatty acids covalently bound to the protein the cortex, which accounts for the major portion of the fiber’s dry mass and holds most of the water the intercellular binding material, known as the cell membrane complex, which provides adhesion between cortical cells and the crystalline phase, which is responsible for the mechanical strength of the fibers. Specific temperature ranges encompass processes respective to each, including: the removal of water (50-120°C), which occurs during drying an intermediary stage of the amorphous matrix (140‑170°C) and denaturation of the crystalline phase ( 220°C). Moreover, temperature is also a factor that amplifies the speed of chemical reactions. Therefore, all the previously described processes are enhanced by increased temperature increases. A general rule of thumb is that a 10 degree increase in temperature roughly equates to a doubling of the reaction rate. On the other hand, the temperature increase is equally distributed across all chemical bonds in hair, and subsequent increases of vibrational energy can lead to the breaking of bonds and the initiation of new reactions. Temperatures below 120°C: A study of water desorption/ absorption curves related to heat drying of hair at temperatures ranging from 50°C to 110°C, followed by equilibration at 55% relative humidity and 22°C, has shown a reduction in moisture retention.41 Based on this, the authors concluded that heat-dried hair becomes more susceptible to static charge accumulation and flyaway during subsequent grooming procedures.
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