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A study has been made of the viscosity and iodine sorption of cellulose dyed with cold‐dyeing (Procion) and hot‐dyeing (Procion H) reactive dyes. The fluidity in cuprammonium hydroxide of the dyed cellulose decreases linearly with increasing dye concentration on the substrate and abnormal viscosity behaviour is observed at high dye concentrations. The extrapolated viscosity‐average molecular weights of the dyed substrate do not show a significant difference from that of undyed cellulose. The energy of activation for viscous flow increases and iodine sorption decreases linearly with increasing dye concentration on the substrate. The dyed cellulose absorbs more copper ion as the dye concentration increases. Formation of copper‐dye complexes has been observed with hydrolysed purified Procion dye in neutral and ammoniacal solutions, by means of Job's method. The abnormal viscosity behaviour of Procion‐dyed cellulose can be largely attributed to such complex formation. A method of purifying Procion dyes is described.

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