StudieS on the dyeing of Chitin and ChitoSan with pre-metalliSed dyeS at neutral ph

Both 1:1 pre-metallised acid dyes and pre-coppered direct dyes, but not 1:2 pre-metallised acid dyes, have been shown to readily dye chitin and chitosan from neutral dyebaths. With the 1:1 pre-metallised acid dyes the addition of neutral electrolyte increases the uptake of dye, indicating that electrostatic forces of attraction are not involved in the adsorption process. A mechanism of dyeing is proposed involving complex formation between the metal ion of the pre-metallised dye and the amine groups of the substrate. The wet fastness properties of the 1:1 pre-metallised acid dyeings carried out at neutral pH are at least equal to, and in many cases greater than, those obtained by the conventional dyeing process at pH values of 4.5 - 5. In the case of precoppered direct dyes a rigorous washing-off process is required in order to remove dye adsorbed on the substrate, but not complexed with the amine groups, if the maximum wet fastness is to be achieved.