Mixing in single screw extrusion: modelling and optimization

• Author(s): Domingues, N.
• Date: 2011
• Persistent ID: https://hdl.handle.net/1822/20878
• Origin: RepositóriUM - Universidade do Minho
• Document Type: Doctoral thesis
• Language: English
• Contributor(s): Universidade do Minho; Gaspar-Cunha, A.; Covas, J. A.

Description

Given the importance of mixing in polymer processing, the aim of this work is to implement a mathematical model for quantifying the mixing behaviour in single screw extruders. The model developed considers the incorporation of solid or liquid additives into a polymeric matrix. For this purpose, the existing numerical routines capable of describing the flow in the melting and melt conveying zones of the extruder were coupled to specific programs incorporating the algorithms that quantify distributive and dispersive mixing in each system. In this way, a global modelling program for single screw extruders is developed, able to describe the flow, heat transfer and morphology development as a function of the materials properties, geometry and operating conditions. Initially, a mathematical model is developed to predict the evolution of the morphology of immiscible liquid–liquid systems. It takes into account the stretching, breakup and coalescence phenomena and computes the dimensions of the dispersed phase in the polymeric matrix. Inserting this routine in the existing process modelling software, it becomes possible to compute the evolution of the drop dimensions along the melting and melt conveying zones. The experimental data obtained generally validated the theoretical predictions. Subsequently, a model for solid agglomerate dispersion is proposed. As before, the numerical simulations of flow patterns in a rectangular channel were coupled to a Monte Carlo method of clusters, in order to predict rupture and erosion phenomena based on the value of the local fragmentation number. Mixing is characterized by the particle size distribution and Shannon entropy. In a further step, the model is used to predict the dynamics of filler size distribution in a plasticating single screw extruder. Again, the experimental results were generally in line with the predictions. The software is then used to investigate the effects of the process parameters on mixing. Finally, the models of the evolution of the morphology of immiscible liquid-liquid systems and of the dispersion of solid agglomerates are adapted to compute global distributive and dispersive mixing indices in single screw extrusion. The effect of material properties, operating conditions and geometry of screw and die are discussed. For a given polymer system, the intensity of mixing is governed by the magnitude of the hydrodynamic stresses and by the residence time in the melt. The mixing indexes are used to optimize the process.