Studies On The Prepraration And Dyeing Properties Of Melanin-mimetic Safe Hair Dyes

Graying of hair is a visible sign of aging.With the increasingly aging of the population structure in China,people’s demand for age protection is increasing,and the demand for permanent hair dyes is growing rapidly year by year.However,the main components of traditional permanent hair dyes are aniline-based small molecules,which have the potential to cause carcinogenic and sensitization.In addition,these compounds can cause serious damage to the hair by penetrating the inside of the hair.Therefore,it is highly desirable to develop green and safe permanent hair dyes.Polydopamine（PDA）,as the main composition of eumelanin,has advantages such as excellent adhesion ability and good biocompatibility,which has great potential being applied in hair dyes.However,the polymerization of DA is slow,and the interactions between PDA and hair surface are weak.Therefore,dyeing hair with PDA only exhibit poor coloration and poor washing resistance.To address these problems,the present thesis proposes a high-efficiency dyeing strategy for PDA,that is,DA monomers are pre-immobilized on the hair surface through physical or chemical interactions,followed by polymerization and dyeing,to enhance the interactions between PDA and hair surface and thereby improving the hair dyeing efficiency.The main contents of the thesis are as follows:Using cystine（Cys）as the reaction intermediate,DA-grafted Cys（DA-Cys）was synthesized by Schiff base and Michael addition reactions,and then DA was covalently pre-grafted on the hair surface by disulfide bond exchange reaction.The reaction between DA and Cys was characterized by X-ray photoelectron spectroscopy and UV-Vis absorption spectroscopy.By studying the changes of elements and thiol content on the hair surface before and after pre-grafting DA,it was proved that DA was successfully grafted on the hair surface.By examining the hair dyeing properties of PDA with or without pre-grafting,it is demonstrated that the DA-Cys pre-grafting strategy can significantly improve the coloration and color fastness of PDA.By investigating the effects of different oxidation systems,p H,temperature and other preparation conditions on the dyeing properties of PDA,the optimal coloration conditions could be using Na IO₄ oxidation system,both Na IO₄ and DA concentration were being 5 mg/m L,at p H 5,at 40°C and dyeing for 20 min.The microscopic morphology of the hair surface after dyeing was studied,and the results showed that the distribution of PDA nanoparticles on the surface of the hair pre-grafted with DA was more uniform.In addition,PDA is coated on the surface of the hair and will not penetrate the inside of the hair damaging the hair.The effect of UV on the mechanical properties of hair before and after PDA dyeing was studied,and the results showed that PDA has a good UV protection effect on hair.DA was grafted onto hyaluronic acid（HA）by Schiff base reaction,and the products were coded as DAHA.DAHA was used to pretreat hair to strengthen the interactions between PDA and hair surface.The chemical structure of DAHA was characterized by infrared spectroscopy,UV-Vis absorption spectroscopy,and nuclear magnetic resonance.The results showed that the grafting rate of DA was as high as 34%.The microscopic morphology of the hair after dyeing was observed,and the results showed that the PDA nanoparticles on the surface of the DAHA-assisted dyed hair were more uniform than those of the hair without DAHA.The results of the study on the structure and properties of dyed hair illustrated that a pretreatment with DAHA not only improved the color fastness of PDA,but also increased the tensile strength of hair due to the hair care function of HA.