Dyeing of Anionically-Modified Polyamide Fibers with Cationic Dyestuffs

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Abstract
The physicochemical principles of the dyeing of two polyamide-6 fibers modified with the monosodium salt of 5-sulphoisophthalic acid at pH 1.8 and 4.5 with an oxazonium dye (Astrazon Blue BG, C.I. 51004) and a thiazonium dye (Methylene Blue, C.I. 52015) are described. Measurements of the zeta potential by the flow-potential method have shown that the fibers have a very highly negatively-charged surface as compared with unmodified types. This decisively promotes the adsorption of the dyestuff cations on the fiber. The uptake of the dye can be described by means of the Langmuir adsorption isotherm as an ion-exchange reaction. It follows from the agreement of dyestuff saturation figures and comonomer contents that the dyestuff is bound stoichiometrically by the sulpho groups (pH 4.5; 80°C). Carboxyl groups probably take no part in the dyestuff binding at pH 1.8 and 4.5. The standard values of the heat of reaction have been determined from the temperature dependence of the equilibrium constants for reactions responsible for the dyestuff binding. They show that the dyestuff/fiber bond contains a covalent component, which is probably due to interactions between the sulphonate group and the dyestuff cation.

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