Effect of strain rate and temperature on the crystallization behavior of amorphous poly(ethylene terephthalate) fibers during superdrawing

Abstract

The melt spinning process is the most common process to produce poly(ethylene terephthalate) (PET) fibers for textile and technical applications. The process involves extruding molten polymer through a spinneret followed by a drawing process. In the conventional drawing process, there appears to be an upper limit for draw ratio somewhere in the range of 3–9. Overcoming this limitation, superdrawing-based process offers a route to produce very fine denier fibers economically from as-spun fibers of ‘normal’ textile deniers, and it offers a unique method for quick-response manufacturing of fibers with various finer diameters (deniers) from a single spun supply. During superdrawing, PET fibers undergo large extension without significant crystallization or increase in molecular orientation. This study is directed at understanding the limitations of processing conditions for PET superdrawing process with temperatures ranging from 90 °C to 120 °C and strain rates from 0.008 s⁻¹ to 0.425 s⁻¹. Evolution of crystallinity and molecular orientation of the drawn samples are evaluated using differential scanning calorimetry (DSC) and birefringence measurements. The results indicate that superdrawing is not possible for any strain rate at or below 90 °C because of significant strain-induced crystallization. Between 90 °C and 105 °C, superdrawing occurs only when the strain rate is below a threshold value, which increases from less than 0.333 s⁻¹ to below 0.425 s⁻¹ as temperature increases from 105 °C to 110 °C. Results also show that between 115 °C and 120 °C, superdrawing is only possible at strain rates higher than 0.016 s⁻¹. Increasing temperature from 110 °C to 120 °C leads to more crystallization at low strain rates (0.001 s⁻¹), and less crystallization at high strain rates (0.1 s⁻¹). The change of strain rate limitation from an upper bound to a lower bound around 110 °C for superdrawing suggests a change in molecular deformation mechanism based on a combination of factors such as the temperature, entanglement dynamics, and strain rate. The study also shows that the mechanism of crystallinity development in PET undergoes a transition at drawing temperature near 113 °C and strain rate of 0.17 s⁻¹.

Keywords:

• Poly(ethylene terephthalate)
• crystallization
• superdrawing

Disclosure statement

No potential conflict of interest was reported by the author(s).