A study of trace devolatization of polymers

Polymer devolatilization is one of the most important operations in the manufacturing and compounding of polymers. The reasons for devolatilization come from the concerns of improving polymer product quality and reducing health hazards. The goal of this research is to investigate the rate of approach to thermodynamic equilibrium in devolatilization associated with foaming dynamics. The main thrust of this work has been to examine the role of stripping agents in polymer devolatilization.; Batch foaming experiments have been conducted to study the foam dynamics as functions of the volatile concentration, the vacuum level, and the addition of stripper bubbles. The results showed that there is a limiting maximum foam volume for a polymer to grow and sustain regardless of the volatile concentration and the stripper bubbles. Based on the cell model for bubble growth, a mathematical model was developed to study the effect of stripper bubbles on polymer devolatilization performance, which accounts for the observation of a limiting foam volume in devolatilization. The model combines a bimodal model for foam growth with a film model for devolatilization. A case study based on polystyrene/styrene devolatilization was used to show the effectiveness of the model and how it compares with published studies.; A stripper dispersion experiment was performed to study the effect of polymer viscosity, the initial number and size of stripper bubbles on foaming and devolatilization and to compare with the bimodal model and film model for devolatilization. Viscosity was found to be the dominant factor in limiting the maximum foam volume expansion. The number and size of nitrogen bubbles showed positive and negative effects on the maximum foam volume expansion. But their effects were only secondary. The thermodynamic devolatilization efficiency calculated by the model showed good agreement with the experimental data.; Finally, a model was developed to establish an expression for design of the devolatilization section in an extruder, which is optimized for foam dynamics and process parameters. A statistical regression method and a back-propagation neural network were used to correlate the number of stripper bubbles with several important process parameters. The application of both methods in describing polyethylene/acrylic acid devolatilization was demonstrated.