The time that a polymeric material spends in the barrel of an extruder is known as its residence time. In an ideal plug flow reactor, all elements of the fluid travel at the same velocity, and the residence time of each fluid element is the same. However, the velocity profile developed in twin-screw extruders is nonideal because of back-mixing, as well as the slow exchange of bulk material to and from a polymer film on the barrel wall or screw surfaces. Consequently, a distribution of residence times is produced for the polymer, and this distribution is known as the exit age distribution or residence time distribution (RTD). The measurement of the RTD in an extruder usually involves a stimulus-response method. The extrusion process is subjected to a pulse input of a tracer at the feed port. The concentration of the tracer in the extrudate is then recorded as a function of time. A graphical plot of the tracer concentration against extrusion time produces the RTD curve. Some tracers which have been employed in the literature and the relevant property measured are given in Table I.

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