Tension effects during a commercial heat setting process were investigated through the analysis of the thermomechanical behavior of heat set nylon 6 carpet yarns. The stress history of these yarns was modified by altering the yarn tension level at various positions along the Superba continuous heat setting process. Changes in fiber structure and morphology were characterized by mechanical testing, shrinkage measurements, differential scanning calorimetry, and thermal stress analysis.
A model based on the hypothesis that there are two domains of molecular order that respond differently to the effects of stress, such that structural and morphological changes proceed via two distinct mechanisms, was fit to the experimental shrinkage force-temperature curves and described the experimental curves very well. Changes in the calculated curve parameters were highly correlated with the observed changes in shrinkage and initial moduli. Each of these changes was a function of the tension state of the nylon 6 yarns during the heat setting process. Analysis of the shrinkage force curves and their characteristic parameters showed that this technique is capable of identifying subtle variations in fiber microstructure and therefore has significant applications to the identification of stress history differences in heat set fibers and yams.
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