Void and pore formation inside the hair cortex by a denaturation and super-contraction process occurring during hair setting with hot irons.

An analysis of hair fibers from donors that frequently use hot irons for hair straightening showed the presence of multiple pores and voids (φ approximately 0.1-1.5 μm) that extend from the cuticle sheath to regions inside the hair cortex. Pore formation in the cortex was found to be confined at its periphery and could be reproduced in the laboratory with virgin hair fibers after the application of various hot-iron straightening cycles. The appearance of pores and voids in the cortex was found to be associated to the production of hot water vapor while the fiber is undergoing mechanical elongation or contraction. The number of pores was seen to rapidly increase with temperature in the range from 190 to 220°C and also with the number of straightening cycles. Larger hair voids (φ approximately 2-5 μm) were also detected in the cortex. The small pores found at the cortex periphery appear to occur by the simultaneous occurrence of rearrangement of hair proteins, fiber mechanical contraction/expansion, and the flow of super-heated steam. Hot irons create, thus, the conditions for the onset of pore formation as the high temperatures produce superheated steam and soften the native state of hair proteins by a process involving denaturation and changes in the crystalline regions.