Hair Coloring 128 structure of the coupler there can be a second reaction with an additional primary intermediate to form a tri-nuclear dye.14 Thus the different primary and coupler structures can give a variety of final dye structures that have different colors. Figure 3 shows the range of colors delivered on white unpigmented hair from a selection of binary combinations of primary intermediates and couplers. Note that in final colorant shades there are often multiple primary intermediates and couplers giving an even wider possible range of colors to the consumer. Figure 3 Color delivered on white hair for a range of primary intermediate/coupler couples Primaries: PPD = p=phenylenediamine HDAP = 1-hydroxyethyl-4,5-diamino pyrazole sulfate HED = N,N-Bis(2-hydroxyethyl)-p-phenylenediamine sulphate Couplers: AHT = 2-methyl- 5-aminophenol PAOX = 2-methyl-5-(hydroxyethylamino)phenol MAP = 3-aminophenol, NAP = 1-napthol RES = resorcinol In addition, the formulator making a final shade will mix a number of different primaries and couplers to form the desired color, rather like an artist mixing his paints. The rate of dye formation, and specifically rates of coupling, have been extensively studied by Corbett et al.15 The initial oxidation of the primary intermediate is a kinetically slow step followed by a faster reaction with the coupler. The kinetics need to be carefully controlled so the dye precursors can diffuse into the hair before coupling to form the final dye. If the rates are too fast, color is formed outside the hair and is easily washed out yet if the rates are too slow, not enough color forms during the treatment time.