Predicting the reaction rate in a twin screw extruder

Twin screw extruders (TSEs) are widely used in chemical polymer modification or polymerization. Indeed, TSEs are continuous chemical reactors proposing an important mixing capability, even with highly viscous polymers. Their flexibility in the screw design and control of temperature is an asset for processing chemical reaction as they allow to control and favor residence time and thermal exchange.

Nevertheless, reaching the desired product using TSEs could be time-consuming and requiring too much experimentations. In this context, 1D-simulation is a good predictive tool able to compute rheological and thermal evolution with complex kinetics reactions.

This study focus on the reaction of chain extension of a carboxyl terminated polyamide 12 (CTPA), using dioxazoline (OO) as coupling agent. The marker of the reaction chosen is the residual acid ratio (this ratio is as low as the reaction is efficient).

The objective of the study is to highlight the influence of screw speed on reaction progress. The screw profile holds two mixing zones, including respectively 25 and 15 kneading blocks. The throughput is fixed at 2 kg/h and the barrel temperature at 230°C.

Ludovic® computes the main flow parameters, and the concentration of residual carboxyl acid functions. The computation of the residual acid concentration starts in the melting zone. The acid ratio is then computed in each sub-element. Its evolution along the screw profile is ploted for a screw speed of 68 rpm in figure 1.

Article #6 fig1

In those conditions, the comparison between experimental and calculated carboxyl acid ratio reveals a good agreement.  As illustrated on the figure 1, the computation shows that the carboxyl acid ratio drops abruptly in the first mixing block. Afterwards, a second drop is observed in the second mixing block, followed by a weak decrease until the die exit. These results are explained by the high residence time spent in these sections of the extruder (which are the only one to be totally filled).

On a second step of the study, different screw speeds are tested (between 50 and 400 rpm).  For all those configurations, Ludovic® computes the residual acid ratio (figure 2).

Article #6 fig2

We can see that the order of magnitude of the residual acid ratio (0.10 to 0.15) is well captured by the model.

The global trend is also well identified : the increase of the screw speed leads to a decrease of the residual acid ratio.

Some exceptions are identified (in the experiments and also in the simulation) for the lowest screw speed : as the residence time is shorter, the process has not enough time to “consume” the residual acid.

On the other hand, the decrease of this residual acid ratio is strongly connected to the thermal history of the material. The figure 3 shows the computed material temperature along the screw at different screw speeds.

Article#6 fig3

The impact on the screw speed increase on the residual acid ratio can be roughly explained this way :

  • Screw speed increases -> Residence Time descreases -> Thermal degradation decreases too
  • Screw speed increases -> Temperature increases (caused by the viscous dissipation) -> Reaction acceleration

As a conclusion, the use of flow computation software allowed to determine the influence of the screw speed and the confirmation of the importance of the thermal and mechanical degradations in the reaction process.

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