Polymer blend de-mixing and morphology development during tube flow

This work is an investigation of morphology and de-mixing of polymer blends during melt flow through a tube. Morphology is the relative size, shape and location of each distinguishable phase present in a polymer blend. De-mixing is the shear-induced migration of different types of polymers away from each other during the flow. Being able to tailor de-mixing during extrusion can potentially result in a new family of plastics waste recycling processes with mixed waste entering an extruder and separate streams of different polymer types leaving it. Also, control of morphology development can lead to the formation of layered structures without the need for two or more extruders and co-extrusion. These ideas formed the basis for a U.S. Patent. However, obtaining an understanding of the phenomena is critical to improving separations to a practical level. This thesis is directed at elucidating morphology development and de-mixing of polymer blends in the most simple process design: melt flow through a tube. The work had four objectives. The first was to design a process that would enable elucidation of both morphology and de-mixing along the tube. This was done by attaching a long segmented tube to a static mixer which in turn was attached to the end of a single screw extruder. At the conclusion of a run, the tube was quenched and disassembled to provide the needed samples. The second objective was to develop analytical methods to measure polymer composition in the samples. A mid-infrared spectrometer technique and a method based on the use of a differential scanning calorimeter were developed. The third objective was to use the above accomplishments to elucidate morphology development and polymer migration. Shear-induced migration was quantitatively shown in various polyethylene-polypropylene, polypropylene-nylon6 and polyethylene-nylon6 blends. The theoretical rate of viscous energy dissipation per unit length of the tube was used to show that the observed shear-induced migration was in accordance with the principle of energy minimization. The ratio of the viscosity of the dispersed phased to that of the continuous phase greatly influenced the morphology of polypropylene-nylon6 and polyethylene-nylon6 blends: a droplet-dispersed phase structure occurred at a high viscosity ratio whereas a multi-layer structure resulted at viscosity ratios near unity. Shear-induced deformation and coalescence contributed to formation of the multi-layer structure. Finally, the fourth objective was to investigate the effect of morphology development on viscosity measurement by capillary rheometry. The extruder-tube process was used as the rheometer. Morphology had a large impact on the value of the measured viscosity and viscosity-composition data were shown to be not readily fit by two mixing rule models: Lees' model and a sheath-core model. Greatly improved results were obtained by introducing a “shear-induced interlayer slip factor” into the sheath-core model.