Effect of screw configuration using extensional flow on dispersion of PET/ZnO nanocomposite

Polyethylene terephthalate (PET)/ZnO nanocomposites were made by melt compounding in a 24 mm co-rotating twin-screw extruder to clarify how screw configurations that combine conventional kneading disks (KD) with a non-rotating extensional-flow element (Fixed Blister Disk, XBD) govern dispersion and optical performance. Four screw configurations (KD and XBD1-XBD3) were prepared by changing the number and placement of KD sections upstream and downstream of XBD, aiming at pre-dispersion, extensional breakup, and suppression of re-agglomeration. Dispersion was quantified from SEM and TEM images using equivalent circular diameter statistics, including the percentage of agglomerates below a target size of 50 nm, while pressure drop and specific mechanical energy (SME) were monitored during extrusion. XBD containing configurations reduced coarse agglomerates relative to KD configuration, whereas KD shear mixing was more effective for increasing the submicron (<1000 nm) fraction; XBD2 configuration, despite the largest pressure drop, showed limited submicron dispersion because of low SME. TEM analysis further demonstrated that pre-dispersion upstream of XBD increased the <50 nm fraction, showing the synergy between shear and extensional elements. UV-Vis measurements confirmed UV blocking (<400 nm) for ZnO-filled sheets; however, visible transmittance was substantially lower than that of neat PET and did not improve with the configuration that achieved the finest nanoscale dispersion. DSC and haze showed no pronounced differences among configurations, whereas color (b*) correlated strongly with the short-wavelength transmittance ratio (T500/T600), indicating that PET yellowing due to increased thermal history dominated the transparency loss. These results provide screw configuration guidance for balancing dispersion enhancement with thermal-history control in PET nanocomposite extrusion.