Currently, polyester fibers are ranked first among synthetic fibers in terms of production level, and their manufacturing is increasing worldwide. These fibers are dyed by the application of disperse dyes at high temperature or in boiling dyeing solutions in the presence of carriers which is due to the difficulty of dyeing this fibers because of absence of proper chemical groups in the polymeric chains and compacted structure of the PET fibers. The color depth of polyester yarn produced in Iran is not comparable to the imported PET fibers produced. In the present research, polybutylene terephthalate (PBT) and caustic soda (NaOH) were used, separately, during melt spinning of PET fibers. Pure PBT was applied at two different amounts of 2.5% and 5%. NaOH was prepared in two different methods: in the first method, polyester granules were immersed in a 3% NaOH solution for 1 hour at ambient temperature, and then was fed at different amounts of 1%, 3% and 6% in the melt spinning process. In the second method, masterbatch containing 0.2% NaOH were prepared and was fed during melt spinning with feeding rate of 3% and 5%. The effects of these additives on the structural and mechanical properties of the fibers, and their dyeability were evaluated. Mechanical properties, crystalline and chemical structures and dyeing properties of yarns were evaluated. The results showed that the addition of PBT was found to slightly improve the mechanical properties. Also, the addition of NaOH, was found to have slightly reverse effect on mechanical properties, which is likely to be due to the chain scission of the polyester polymer. No significant differencese were found between FTIR and SEM images and viscosity of different produced fibers, which is likely to be due to the low amount of additives. The X-ray diffraction (XRD) results showed that the crystallinity index for samples is about 35%, and the related difference between samples can be neglected. The crystals' growth in the samples was higher along with the plate at an angle of 17.5 degrees than other crystalline plates. After dyeing at 100 °C, PBT-containing yarns showed slightly higher color depths, compared to pure polyester samples. Also, the addition of NaOH was found to slightly increase the dyeing depth. Although it was found different color depth of PET yarn with and without additives, these differences were in significant which is due to low amounts of additives. It is believed that using optimized amount of PBT and/or NaOH may improve some mechanical and dyeing properties of the PET fibers and further investigation should be followed.
