VISCOSITY RATIO EFFECTS IN THE COMPOUNDING OF LOW-VISCOSITY, IMMISCIBLE FLUIDS INTO POLYMERIC MATRICES

Many low viscosity, immiscible fluids are difficult to incorporate int o polymer matrices because of thermodynamic immiscibility and a large mismatch of melt viscosities. In this investigation, a model system wa s used to determine the mechanisms and kinetics of mixing in such form ulations. The model systems consisted of a series of different molecul ar weight polyethylenes in polystyrene. The viscosity ratio, eta(polye thylene)/eta(polystyrene), at 180 degrees C and 100 s(-1) was varied f rom 0.7 to 0.003. Phase inversion was observed during the compounding of these formulations. The phase inversion was associated with a trans ition from low to high mixing torque during compounding. This change w as primarily due to an increase in the blend viscosity caused by the m orphological transformation. The melting behavior during compounding d epended on the melt viscosity of the polyethylene. A critical viscosit y ratio of approximate to 0.1 exists below which softening of the poly styrene, and thus mixing of the two components, was greatly retarded. Even at very low concentrations, low viscosity polyethylene can have a significant effect on the processing behavior. Effects of mixer set t emperature, degree of fill, and polyethylene particle size were explor ed. The roles of thermal conduction and mechanical energy input were e valuated in the melting regime of the process.