The residence time distribution (RTD) was measured on-line using light transmission in a twin-screw extruder with the detector signal being sensitive to the dispersed phase that was used as tracer. Various tracers were used in different concentrations: two pigments (TiO2 and phthalocyanine) and two polymers (polystyrene PS and polyamide-6 PA6) added as a pulse in a polypropylene PP flow. The time characteristics (delay t i, average t n, and normalised variance, <FONT FACE=Symbol>s q</FONT>²) measured at the same processing conditions are very close, inside the experimental error, indicating that the axial dispersion in this type of extruder is independent of the tracer. On the other hand, RTD curves measured using a polymer tracer (PS) dispersed in a polymer flow (PP) are shifted in time when the materials are inverted, i.e. using PP as tracer in a PS flow. The time parameters depend on the rheological characteristics of the flowing polymer, thus different polymer types show RTD curves with changes in shape and shifted in time. The detector signal intensity is dependent not only on the tracer concentration but also on the optical characteristics of the polymer/tracer pair. Reactive (PP/PA6/PP-g-AA) and non-reactive (PP/ PA6) polymer blends show differences in the RTD signal intensity due to the grafting reaction and its influence on the morphology and rheological behaviour. These results were corroborated via off-line infrared spectroscopy.
Twin-screw extruder; residence times of distribution; reactive processing
