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A sample of commercial nylon 6 filament was dyed to equilibrium with C. I. Acid Red 88, 13 and 18 at 100°C at pH 3.5-2.3. Tg reduction of the dyed filament in air or in water from that of the control was measured.The results are: (1) Tg does not decrease until the dye concentrations in the filament exceed certain thresholds. (2) This threshold is close to the amine-end group content of the sample, 41-43 meq/kg, in the case of the monosulfonated dye, Acid Red 88. However, in the cases of Acid Red 13 and 18, which have two and three sulfonic acid groups, respectively, with the same nucleus structure as that of Acid Red 88, the threshold values are 1/2 and 1/3 of the amine-end group content, respectively. (3) Above these thresholds Tg decreases linearly with increasing the dye concentration in the filament. The number of sulfonic acid groups in a dye molecule seems not to affect the mode of Tg reduction. (4) The Tg reduction cannot be observed when measured in water.Summarizing, Tg of nylon 6 is not reduced when the dye molecules principally bind to amine end groups as it happens for the dyeing at pH 3.5, while the dye molecules associated with amide groups by overdyeing contribute effectively to the Tg reduction. This reduction is not observed for the Tg measurement in water, because it is covered by an overwhelming effect of water imbibed in the filament.

EFFECT OF DYEING WITH ACID DYE UPON GLASS TRANSITION TEMPERATURE IN AIR OR IN WATER OF NYLON 6