Achieving Highly Efficient Repeatable Hair Perming by Bunte Salt-Thiol Exchange Reaction

Abstract Conventional chemical perming typically suffers from a gradual decrease in styling efficiency due to the cumulative degradation of original disulfide bonds. To address this issue, this study proposes a novel perming technology based on the Bunte salt–thiol exchange reaction. A bifunctional Bunte salt, sodium hexamethylene-1,6-bisthiosulfate (SHB), was successfully synthesized, with its high purity and structure validated by X-ray diffraction (XRD) and Raman spectroscopy. The perming performance of this Bunte salt method was evaluated and compared against the conventional oxidative perming method on Chinese virgin hair. X-ray photoelectron spectroscopy (XPS) was utilized to calculate the relative content of disulfide bonds and to elucidate their reconstruction mechanism. Furthermore, the hair morphology, surface properties, and mechanical performance were systematically evaluated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile testing, and other auxiliary experiments. The results demonstrated that after six repetitive perming cycles, the SHB-permed hair maintained a high perming efficiency of 920%, whereas that of the H2O2-permed hair declined to 730%. Notably, the disulfide bond content in SHB-permed hair increased by 15.5%, whereas it decreased by 10.7% in the H2O2 group. Additionally, the Bunte salt perming successfully preserved both the surface and internal structural integrity of the hair fibers. After six cycles, the mechanical and surface properties of SHB-permed hair showed only a marginal reduction (13–16%) compared to virgin hair, in stark contrast to the approximately 45% degradation observed in the oxidative perming group. In conclusion, while repeated conventional oxidative perming causes extensive cleavage of disulfide bonds and severe fiber damage, the Bunte salt–thiol exchange technology effectively mitigates hair damage and maintains disulfide bond density. This methodology offers a robust alternative to conventional protocols and demonstrates significant potential for application in repetitive hair styling products.