Structure and Its Extensional Properties of Straightened Hair Prepared by Heat Treatment with Glyoxylic Acid

Abstract

The hair straightening method applying heat-treatment with glyoxylic acid (GA) at about 180 ̊C was studied. Straightening effect was estimated by shampooing test. The stress-strain relationship of GA/heattreated hair was also studied. It was revealed that from the WAXS, DSC study and stress-strain characteristics of treated sample, new crosslinking is introduced into the rod region of the intermediate filament (IF), a slight decrease in the degree of crystallinity but an increase in the thermal stability of the α-crystallites. The initial modulus and extensional work of the treated sample were markedly increased as compared to untreated sample. It was demonstrated that such large internal stress accumulated in the keratin network chain disappeared by relaxation treatment in water at 52 ̊C for 1 h. A part of the new cross-linking is destroyed when the treated hair is extended, but residual stable cross-linking to extension contributes to increase of initial modulus of the hair. Study of SAXS showed that the interspace distance between IFs increased about twice as much as the value of untreated sample. SEM observation shows that oblateness of the hair decreased by 10%, and cross-sectional shape changed from elliptical shape to circular shape which followed by increase of cross-sectional area. This phenomenon was considered to be due to the stress relaxation occurring in shampooing process throughout the cleavage of SS bonds located interfacial region between hard IF and soft KAP materials in water. Shampooing test of GA/heat-treated sample showed excellent set-ability. It was concluded that introduction of GA crosslinks into IF rod region resulted in a modified IF with higher modulus of elasticity than that of untreated hair, which means that the increase of resilience maintains the straight conformation exhibiting excellent straightening effect.