Steady‐state flow properties of poly(ethylene terephthalate) melts

Steady‐state flow properties of poly(ethylene terephthalate) melts as a function of Temperature (T), shear rate (γ), and inherent viscosity (I.V.) were determined. An Instron capillary rheometer was used to obtain data on wall shear stress (τW) vs. γ for poly(ethylene terephthalate) (PET) at temperatures of 265 to 295°C. The polymer samples investigated covered an inherent viscosity range of 0.35 to 0.82, corresponding to a weight‐average molecular weight range of 23,000 to 70,000. Capillaries used in this investigation covered the range of 10 to 67.5 in length/diameter (L/D) values. End‐effect corrections (n) estimated by Bagley's method were small with n < 1 over the entire shear rate range covered (50–24,000 sec.−1). Flow curves showed (1) that over the shear rate range of about 50 to 1000 sec.−1, the flow behavior of PET was nearly Newtonian, i.e., log‐log plots of τW vs. γ were linear with slopes of about 1, and (2) that over the shear rate range of 1000 to 24,000 sec.−1, the flow behavior of the polymer was slightly pseudoplastic. Using Mendelson's shear rate‐temperature superposition scheme with an additional shifting to a single reference I.V. level, we were able to shift all the data to a single master curve of log (aTb τ γ), where aT and b are the shift factors for temperature and molecular weight, respectively.