Hair Colouring by Utilising Chemical Oxidation of (+)-Catechin and Bio-Catechols with Oxygen

In this study, the authors investigated hair colouring by utilising chemical oxidation of biocatechols, which are obtained from organisms and have catechol (o-dihydroxybenzenes) groups such as (+)-catechin (Cat), (-)-epicatechin (EC), L-3,4-dihydroxyphenylalanine (DOPA), hematoxyline (HX), braziline (BZ), rosmarinic acid (RA), caffeic acid (CA) and chlorogenic acid (ChA). The three types of dyeing methods for the chemically decolourised white human hair were tried as follows: the dyeing by using the solution of the oxidised bio-catechols at 30 °C and pH = 7 (redissolution dyeing), the dyeing by using the oxidation solution of bio-catechol reacting at 30 °C and pH = 10.8 with the introduction of O2 gas continuously (simultaneous oxidation dyeing) and the oxidation by O2 at 30 °C and pH = 11.6 after the treatment of hair with the bio-catechol solution at 30 °C (post-oxidation dyeing). The order of the hair dyeability of the methods using Cat as the bio-catechol was found to be post-oxidation>redissolution>simultaneous oxidation dyeing. Moreover, the resulting colour of hair dyed with Cat by post-oxidation dyeing was reddish brown. The results demonstrate that EC, HX and BZ are also available for hair dyeing by using post-oxidation dyeing and the colours of the dyed hair are yellowish brown, deep brown and reddish brown, respectively. It was found that the bio-catechols having a chroman (3,4-dihydro-2H-1-benzopyran) structure with the catechol part work effectively as dye precursors for colouring hair by the post-oxidation dyeing technique.