Ali Keshavarzian - Iran polymer & Petrochemical Institute, Pazhoohesh Blvd., Km 17, Tehran-Karaj Hwy Tehran, I.R. Iran, P.O. Box: 14965/115 , 14185/458.
Mehdi Rafizadeh - Department of Polymer Eng. & Color Tech., Amirkabir University of Technology, P. O. Box 15875-4413.
Faramarz Afshar-Taromi - Department of Polymer Eng. & Color Tech., Amirkabir University of Technology, P. O. Box 15875-4413.

Polyesters, mainly poly(ethylene terephthalate) (PET), are now widely used for textile fibers, technical fibers, films, and bottles because of their good mechanical properties, thermalstability, and low production cost [1]. Several parameters are effective on the PET synthesis process, such as temperature,pressure, catalyst and etc [2,3]. In this research, PET synthesis is studied in the presence of titanium tetrabutoxide (TBT) ascatalyst in a homemade laboratory setup. The effect of postesterification step and mixing rate on the intrinsic viscosity (IV) of final produced PET has been investigated. In postesterificationstep, after the esterification step that reaction was conducted at 240-250oC, 4.5 bar., and 60 rpm of mixingrate, mixture was stirred at 260oC under atmospheric pressure for one hour to complete esterification and remove of water residue