The effect of heat treatment conditions using the drawing process on the properties of PET filament sewing thread

For the production of drawn poly(ethylene terephthalate) (PET) filament threads it is useful to determine the relationship between the drawing process conditions and the resulting product properties. Drawing at an elevated temperature over a defined time usually causes changes of the polymer structural parameters that influence the mechanical properties of the filaments. In order to elucidate the physical background of the drawing process for developing the production process of PET filament sewing thread, the thread was drawn by varying the treatment temperature and contact time. The effect of the treatment conditions on the structural and mechanical properties of the drawn threads was investigated, such as the birefringence, crystallinity, and dynamic mechanical properties of the threads’ filaments, as well as the mechanical properties and shrinkage of the treated threads. Using the drawing process, higher temperature and contact time up to the critical values ( T ₂ = 220°C, n ₂ = 9 turns) change the structural parameters and, consequently, any modifications influence changes in the mechanical properties of the threads. Namely, a higher treatment temperature and contact time increases the degree of crystallinity, the birefringence, and the maxima of the loss tangent and loss modulus temperature, and a significant increase in the breaking tenacity, elastic modulus and the thread’s tension at the yield point, and a decrease of the breaking extension is achieved. The observed deterioration of the thread’s mechanical properties above the critical treatment conditions is attributed to the destructive phenomena of the supra-molecular structure, resulting in an imperfect structure of the thread’s filaments.