Potential alternatives for 3,3′-dichlorobenzidine as tetrazo components for diarylide yellow and orange pigments, Part 1: p-Phenylenediamine and its derivatives

A series of diarylide yellow pigments based on selected p-phenylenediamine derivatives as tetrazo components have been synthesised and their application performance assessed. Among the most interesting products were a range of new metal-salt disazo pigments synthesised from 2,5-phenylenediamine-1,6-disulphonic acid, which is of potential interest as an alternative to 3,3-dichlorobenzidine, as it is likely to be of low toxicity, non-carcinogenic, non-mutagenic and potentially low cost. The synthesis of disazo metal-salt pigments is in this case complicated by the requirement for a two-stage diazotisation/azo coupling process, although this offers a versatility advantage with an easy route to unsymmetrical derivatives. The colouristic properties, overpaint fastness, lightfastness and heat stability of the metal-salt pigments were comparable in some cases, inferior in others, to commercial dichlorobenzidine-based pigments selected as standards. Lightfastness was, in some cases, comparable in full strength although inferior in tint; solvent resistance was superior to that of the commercial pigments, as expected for metal-salt type pigments.