Polyamide 66 fibers were thermoset in a torsion-bending deformation at various temperatures up to 240 degrees C. Some of the fibers were heat-set at constant length prior to the deformation at presetting temperatures of 150 degrees C and 200 degrees C to vary the structural state of the starting material. Fractional recovery was measured after various combinations of temperature and time. It was found that heat setting of PA66 is dominated by time-dependent stress relaxation exhibiting time-temperature equivalence. Increased crystallinity, and/or other molecular rearrangements occurring during presetting, impose additional constraints on molecular mobility, which delay onset of the flow regime and increase the time constant of relaxation at a given temperature. The thermosetting characteristics of PA66 fibers are very similar to those of poly(ethylene terephthalate) fibers. For both polymers, superposing the curves of fractional recovery vs. setting time at different temperatures produce satisfactory master curves, without the need for vertical shifting of the data. Arrhenjus plots yield approximate activation energies for the thermosetting flow process of 35-65 kcal/mol in PA66 and 95-115 kcal/mol in PET. (C) 2009 Elsevier Ltd. All rights reserved.