Investigation of mixing in the melting regime during polymer compounding
Author(s)Ratnagiri, Ramabhadra, 1972-
Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.
Christopher E. Scott.
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Morphology evolution in the melting regime during compounding of immiscible polymer blends. where most of the size scale reduction occurs. is studied. Starting from an initial solid pellet mixture of two components. the progression to the final two-phase viscoelastic melt involves an intermediate stage where either one or both the components are melting or softening. Our focus is identifying and quantifying the factors that determine morphologies in the melting regime. We identify blend systems that exhibit a transformation in morphology from a minor-component continuous phase with dispersed major component domains to that with the major component being the continuous matrix phase. as a function of mixing time. This phenomenon of phase inversion during compounding is demonstrated to occur even in blends with a higher melting point minor component. A low solid modulus and a low melt viscosity are shown to favor the formation of the continuous phase by the minor component. Polycaprolactone/polyethylene. polystyrene/polyethylene. polycarbonate/ polyethylene, poly(ethylene-co-cyclohexane dimethylene terephthalate)/ polyethylene. and polybutylene/polycaprolactone blends were studied. These model blends were chosen based on the melt viscosity ratio and the relative softening temperatures of the two components. These two parameters were used to develop a two-dimensional framework for summarizing the compounding behavior of blends. For compounding runs with a small amount of the minor component (-1 Owt. % ) at a constant mixer temperature, phase inversion was observed for blend viscosity ratios less than 0.2. irrespective of the relative transition temperatures of the two components. Using a temperature ramping program resulted in the low melting component forming the continuous phase initially. Selective dissolution studies were used to quantify the amount of minor component present in the continuous phase at different mixing times. A polystyrene/polyethylene blend with a melt viscosity ratio of -0.001. was used to study the effect of batch size on the time required to form a continuous phase of the compounding of batch sizes ranging from 12g to 240g. Upon a five-fold increase in batch size the time to phase inversion increased by a factor of 3. This increase was explained by a combination of reduced heat conduction and reduced mechanical energy input to the batch. To enable studies at different batch sizes in the same mixing bowl, a novel mixing blade with modular elements was designed and constructed. This design was used for both radial and axial scaleup studies. The effect of changing the blade configuration on the time to phase inversion was explained using a specific relative stagger parameter, which is a measure of the effectiveness of stress transfer to the batch. Flow visualization using a glass window and blend sampling was used to develop a detailed description of the deformation steps leading to phase inversion in a model low viscosity ratio blend. Intermediate morphologies of flattened pellets, stacks of pellets, fibers and clusters were identified. Based on these observations a micro-structural model was developed to predict the time to phase inversion. The model incorporates a simplified flow-field approximation and calculates the strain in the major component. A strain-based criterion was proposed which in conjunction with the model yielded an explicit expression for the time to phase inversion. Model predictions of the dependence of time to phase inversion on nominal maximum-shear-rate in the mixer, volume fraction of the minor component and blend viscosity ratio were shown to be in excellent agreement with experimental results.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, February 2000.Includes bibliographical references (leaves 124-126).
DepartmentMassachusetts Institute of Technology. Dept. of Materials Science and Engineering.
Massachusetts Institute of Technology
Materials Science and Engineering.