Fully filled length

The large variations in viscosity during reactive extrusion can lead to process instabilities as a result of competition between die effects and pressure build-up abilities of the screws. In general, two stable working points can be distinguished - one at low conversion with a short, fully filled length, and one at high conversion with a... 

An increase in die resistance demands for a larger pressure build-up in the extruder, which leads to a longer fully filled length. 

Hydrodynamic considerations lead to the contention that there are two stable operating points in reactive extrusion - one with a large, fully filled length and a high conversion, and one with a small, fully filled length and a low conversion. Severe... 

As a consequence the fully filled length before the die in axial direction is... 

Additionally, expressions for the fully filled length, the kinetics and the heat transfer are needed to complete the model. The fully filled length is determined by the leakage flows, the viscosity profile of the reacting material and the die pressure the expressions for leakage flows and pressure buildup are derived in Chapter 2. The viscosity expressions can be found in Chapter 4, using the kinetics of an addition polymerization with gel effect, as given in Chapter 3. For the heat transfer coefficient one of the expressions from Chapter 6 can be used. 

Figure 7.6 Influence of fully filled length on the conversion and molecular weight for the extruder polymerization of w-BMA (+ experiments, — model).

All simulations above were performed with an open die, i.e., a die pressure equal to zero. The die pressure has a major influence on the reaction process because it influences the fully filled length and therefore the residence time. Figure 7.11 shows this influence. Two simulations are given, the zero... 

The influence of the die resistance and screw rotation rate on the conversion is shown in Fig. 8.7. A reduction of the die resistance results in a decrease in residence time, and therefore a decrease in conversion can be expected. Experimentally, a very sharp decrease in conversion was observed at a certain die resistance. The sharp decrease in conversion indicates a transition from one regime with a high to another with a low residence time. This transition is due to the occurrence of multiple steady states and will be dealt with in more detail in Chapter 13. After the drop in conversion, the die pressure was almost zero, which indicates that hardly any fully-filled length was present anymore. An increased screw speed resulted also in a decreased residence time and therefore in a decreased conversion. 

For hydrodynamic similarity, it is also necessary that the fully filled length remains unchanged (r = 0). Equation (12.10) simplifies to... 

For equal average residence times it is not necessary that the filling of the extruder remains constant equal average residence times can also be achieved if the fully filled length changes. Flowever, this will affect the... 

As the fully filled length varies proportional to the screw speed [Eq. (12.21)], the maximum speed is restricted by the screw length available. 

We will restrict ourselves to the second situation. For equal residence times we need similarity in the fully filled length the important equation is Eq. (12.10)... 

Because this fully filled length is expressed as the number of fully filled chambers or the number of pitches, it follows for constant fully filled... 

The important factors when scaling up a twin-screw extruder are the fully filled length and the back flows. 

In high speed TSEs, the fully filled length is relatively short, typically 20 to 40% of the length of the extruder. The fully filled regions occur where restrictive elements are placed along the screw and at the end of the screw where the diehead pressure has to he developed. Restrictive elements are often placed just upstream of a vent port to create a melt seal. In a starve fed extruder, a melt seal is necessary to be able to draw a vacuum at the vent port. This is illustrated in Fig. 10.65. 

The influences of the reaction and extruder parameters on the reaction process in the extruder were investigated by doing experiments with n-butylmethacrylate in a counterrotating twin screw extruder [5]. Several parameters were changed, such as the rotation rate of the screws, the throughput, the die resistance, and, as a consequence, the fully filled length of the extruder. 

The average molar mass of the product also shows a drastic increase with an increase of the fully filled length of the extruder (Fig. 10). When the reaction is in the gel stage in the extruder, an increase of the residence time, due to the increase of the fully filled length, leads to a longer reaction time at this stage. Therefore, the average molar mass of the polymer will increase... 

Table 6 Influence of the Fully Filled Length on the Polydispersity of the Polymer...

An increase of the die resistance and, consequently, the fully filled length of the extruder has a positive effect on the conversion of the reaction, caused by an increase of the residence time. 

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