Tensile Mechanical Properties
The tensile mechanical properties of the fibers are reported in Table 2. The break stress of the virgin PET fiber was higher than the value obtained for the recycled PET fiber; while its Young’s modulus was lower (almost 50 %). The break elongation values found in this study are very near to the values reported in other studies [29,30]; while the breaking stress and Young’s modulus are lower than the values reported by Wróbel and Bagsik [30] and Frounchi et al. [31] but in the same order of magnitude. To explain these differences, the average molecular weights of virgin and recycled fiber were calculated using the intrinsic viscosity approach. The average molecular weights of virgin and recycled fiber were 19342 g/mol and 15813 g/mol, respectively. According to literature, the presence of smaller polymer chains, due to the polymer degradation, may be are not accommodated than the larger chains. This can cause a decrement of the crystallinity, which has a significant influence on the mechanical properties. Therefore, the fibers with lower average molecular weight may show lower break elongation, higher Young’s modulus [29,30] and a lower breaking stress [30]. Flow index of raw materials used in the extrusion process was obtained, see Table 3. The difference in flow index is mainly caused by the hydrolytic degradation that the extrusion process produced to the recycled PET; therefore, an increase in the melt flow is an indicator of the extent of the thermo-mechanical degradation [32]