Nanoclay are nano-sized particles of layered mineral silicates. They are used as nanofillers to mechanically reinforce thermoplastic resins. The improvement of composite properties depends on the nanofillers dispersion inside the polymer matrix which is reached by melt processing. Twin-screw extrusion, as traditionally used in industrial compounding, is the preferred method for processing thermoplastic/based composites.
The objective of this study is the optimization of extrusion parameters on a small scale industrial TSE process to obtain a high degree of nanoclay dispersion so as to achieve a significant matrix reinforcement.
The polymer used in this study is a poly(ε-caprolactone) (PCL), which is a semi-crystalline biodegradable aliphatic PE and the nanoclay is based on a natural montmorillonite. The TSE dimensions are 18mm diameter and a L/R ratio of 40.
Simulation is used to better understand the impact of processing parameters (screw speed, throughput).
Screw speed has been tested from 80 rpm to 400 rpm keeping a constant throughput (2kg/h) and barrel temperature (120°C). Temperature, pressure and residence time have been ploted along the screw profile to see the impact of the screw speed variations (see Fig 1).
Figure 1 illustrates that by increasing the screw speed, the melt temperature increases, whereas the pressure and residence time decrease, even though the residence time distribution is hardly affected.
To study the influence of the throughput, the two other process parameters namely the screw speed (150 rpm) and barrel temperature (120°C) are fixed. The throughput is then varied between 0,5 and 3 kg/h. Pressure and residence time have been ploted along the screw profile for this range of throughputs (see Fig 2).
We can see that pressure increases considerably when using a higher throughput as can be expected from a higher filling ratio. Moreover, the residence time distribution shifts to higher time values, when the throughput decreases, along with a marked broadening indicating improved mixing. The correspondance between simulated and measured residence times is good.
Previous experiments show that superior nanoclay dispersion is obtained at high screw speeds and low throughputs.
Firstly, simulation confirms that conclusion:
- high screw speed leads to higher temperature, lower pressure and lower RT,
- low throughput leads to low pressure, high RT and broad RTD
On the other hand, simulation allows going further in the analysis of the influence of these 2 processing parameters (screw speed and throughput) by searching correlations between screw speed/throughput ratio and other simulation output.
It is shown in that study, that the cumulative strain from Ludovic© is linked to the nanofiller dispersion. This cumulative strain is obtained by multiplying the local shear rate by the local residence time, and is therefore proportional to the screw speed/throughput ratio (see Fig 3): the best nanofiller dispersion is achieved at the highest cumulative strain.
To conclude, Ludovic© simulations were used to evaluate the impact of changing the processing parameters on the main flow characteristics inside a TSE. The simulation output can be used to select the most relevant extrusion conditons (high screw speed and low throughput) that may generate good nanofiller dispersion. In this way, the experimental work can be significantly reduced.
*based on: “Optimization of Extrusion Parameters for Preparing PCL-Layered Silicate Nanocomposites Supported by Modeling of Twin-Screw Extrusion” by N. Watzeels, H.E. Miltner, C. Block, G. Van Assche, B. Van Mele, H. Rahier (Physical Chemistry and Polymer Science FYSC, Vrije Universiteit Brussel), K. Van Durme, B. Bogdanov (Orfit Industries), (Macromolecular Materials and Engineering Journal – 2013).