Dispersive Mixing Consideration of Twin-Screw Compounding Scale-Up Methodologies

Twin-screw polymer extrusion has shown increased utility for creating composite materials. However, in order to achieve the desired product properties, sufficient mixing is essential. Dispersive mixing, or the breaking-up of particle agglomerates, is critical to create filled compounds with the required material properties. In a twin-screw compounding process, the Residence Stress Distribution (RSD) has been used to quantify the dispersive mixing induced by the stresses in the polymer melt. These stresses are quantified by the percent break-up of stress-sensitive polymeric beads. It was found that the amount of material that experiences the critical stress is a function of the operating conditions of screw speed and specific throughput [1]. The quantification of dispersive mixing allows for better control of a compounding process and can be used to design new processes. During the development of a new compounding process, screw geometries and operating conditions are often refined on a laboratory-scale extruder and then scaled up to a manufacturing level. Scale-up rules are used to translate the operating conditions of a process to different sizes of extruders. In a compounding process, the goal when scaling-up is to maintain the same material properties on both scales by achieving equivalent mixing. The RSD methodology can be used to evaluate the effectiveness of scale-up rules by comparison between two or more scales. This paper will demonstrate the utility of the RSD in evaluation of two unique scale-up rules. Conventional industry practice is based on the volumetric flow comparison between extruders. The proposed approach demonstrates that in order to maintain equivalent dispersive mixing between different sizes of extruders, the degree of fill, or the percent drag flow (%DF), must be kept equivalent in the primary mixing region. The effectiveness of both rules has been evaluated by experimental application of the RSD methodology. A design of experiment approach was used to generate predictive equations for each scale-up rule that were compared to the behavior of the original small-scale extruder. Statistical comparison of the two scale-up rules showed that the %DF rule predicted operating conditions on the large-scale extruder that produced percent break-up behavior more similar to the small-scale behavior. From these results, it can be concluded that the %DF scale-up rule can be used to accurately scale operating conditions between different-sized extruders to ensure similar dispersive mixing between two processes. This will allow for greater accuracy when recreating the material properties of a small-scale twin-screw compounding process on a larger, mass production machine.