Extrusion residence time distribution

In a final RTD experiment, a sheet of dye was frozen as before and positioned in the feed channel perpendicular to the flight tip. The sheet positioned the dye evenly across the entire cross section. After the dye thawed, the extruder was operated at five rpm in extrusion mode. The experimental and numerical RTDs for this experiment are shown in Fig. 8.12, and they show the characteristic residence-time distribution for a single-screw extruder. The long peak indicates that most of the dye exits at one time. The shallow decay function indicates wall effects pulling the fluid back up the channel of the extruder, while the extended tail describes dye trapped in the Moffat eddies that greatly impede the down-channel movement of the dye at the flight corners. Moffat eddies will be discussed more next. Due to the physical limitations of the process, sampling was stopped before the tail had completely decreased to zero concentration. 

Another scale-up variable that can be easily controlled is the length to diameter ratio L/D). Scale-up with an equivalent L/D ratio is beneficial in avoiding non-linear scale-up issues, especially at production scale. Residence time distribution (RTD) is a useful term for understanding scale-up of hot-melt extrusion processing. The RTD is used to attempt to quantify the average amount of time a material spends in the processor. The RTD depends on screw speed, screw element design, and material characteristics. The preference during scale-up is to maintain an equivalent ... 

The reactive extrusion is a process demanding precise operation by well-trained personnel. The knowledge of the extruder, compounding and mixing principles, design of screw and temperature profile, local residence time distribution, etc., is required. For optimization of the reactive extrusion in- or on-line process monitoring, control, and data logging are recommended — this imposes further demands on the extruder operator. 

A peroxide-induced reactive compounding of PP/GRT blends in a corotating twin-screw extruder was carried out to prepare thermoplastic elastomer alloys (Wiessner et al., 2012). The extrusion experiments showed that the width of the residence time distribution was the key parameter determining the mechanical properties of elastomer alloys, whereas the mean residence time had little effect. This was confirmed by performing both quasi-static and cyclic tensile tests. 

The extrusion products typically contain 8%-10% of flavor oil, but the final washed surface makes the product very stable (Rish and Reineccius, 1988, Qi and Xu, 1999). Carbohydrate matrices in the glassy state have very good barrier properties (i.e., atmosphere gases). Dripping and jet breakup are the methods to form microcapsules (Whelehan and Marison, 2011). Parameters are screw temperatures and speed, and residence time distribution (Yuliani et al., 2006). 

Changes in screw geometry and operating parameters can influence both the mean residence time and the residence time distribution. For the purpose of reactive extrusion it is important to know these influences not only for creeping flow conditions but also in case of low-viscosity liquids. Jongbloed did extensive experiments, both with model liquids of different viscosities (0.8, 5.6 and 15.0 Pas) and with a reactive polymerization of butylmetha-crylate (BMA) (7) ... 

In the discussion on residence time distribution it was mentioned that the RTD of a static mixer tends to be narrower than the RTD of a narrow pipe. However, in addition to the RTD the actual residence times are important as well. Static mixers always add extra volume between the extruder and the die. Therefore, when a static mixer is used, the mean residence time will always increase when the entire extrusion system is considered, i.e., extruder, static mixer, and die. If the static mixer section is long, which it should be to give good mixing, then the increase in mean residence time can be considerable. This should be a concern in thermally less stable polymers. 

Keywords peroxide, molar weight distribution (MWD), rheology, crystallization, extrusion, melt flow index (MFI), controlled rheology (CR), peroxide-degradation, residence time distribution (RTD), halflifetime of peroxides, melt elasticity, die swell, viscosity curve, shear rate, elongational viscosity, melt fracture, heterophasic PR... 

Fig. 7.6 Residence time distribution (RTD) monitoring of extrusion operations...

M. Wetzel, C.K. Shih, U. Sundararaj, Determination of Residence Time Distribution during Twin Screw Extrusion of Model Eluids , SPE ANTEC 1997, 3 3707. 

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