Use of thermoplastic olefin elastomers (TPO) is expected to enjoy a 12% annual growth worldwide through 1996. The majority of growth is in automotive applications such as bumper fascia, claddings, exterior panels, airbag covers and instrument panels, etc. This healthy forecast is fueled by the overall cost/performance, the wide range of properties available, the ease of processing and the recyclability of this class of material. The current TPO business is dominated by compounded grades which offer: freedom in formulation to meet specific customer needs, good balance of physical properties, minimal dimensional change during processing, and flexibility to incorporate colors and other additives for tailored applications. In compounding, the dispersion and distribution of polymer phases as well as fillers and additives can be controlled through mixing. This article discusses the methodology involved in studying the effect of mixing on the performance of TPO formulations with different filler levels in a laboratory scale intensive batch mixer. Variables studied are mix time and batch size. Properties analyzed include heat elongation, tear strength, mold shrinkage, melt flow rate, density and cold temperature impact. The behavior of these compounds is graphically illustrated using a statistical software package. The results, in general, suggest an optimal mix time of around three minutes for most of the systems evaluated. It is theorized that through intensive mixing, phase morphology is changed and fillers are also preferentially incorporated into selected polymer phases to provide reinforcement. Electron microscopy and a low strain rheology technique are used to confirm some of these observations. Results on experimentation with single screw and twin screw extruders are also discussed. The variables studied in the extruders are barrel temperature and screw speed. Impact and flexural properties of different TPO formulations are compared for the three different types of mixing processes.
