(217bb) Experimental and Theoretical Study of High-Impact Polystyrene (HIPS) Devolatilization

In the high-impact polystyrene (HIPS) process, a devolatilization stage is carried out in order to remove the unreacted monomer and other volatile species (solvent, oligomers). High temperature reactions take place in the devolatilizer (DV), affecting the molecular structure and quality variables of the final product. In this work, an experimental and theoretical study of HIPS batch devolatilization is developed. The experimental work consisted on a series of HIPS devolatilizations carried out in a laboratory DV equipment. Monomer and oligomer content as well as molecular weights were determined from experimental measurements taken at different times of the process. After devolatilization, the Swelling Index (SI) of the obtained product was experimentally determined. The proposed mathematical model is based on the complex reactions kinetics for HIPS polymerization and the interfacial mass transfer phenomena. Inputs for this model are HIPS composition, molecular weight distributions of the different species (free polystyrene, residual polybutadiene and graft copolymer), crosslinking degree, and DV operating conditions and geometry. Outputs to the module are the monomer and oligomer content, molecular weight distributions and rubber molecular weight between crosslinking points. The SI is theoretically calculated by a modified Flory-Rehner equation (Karam and Tien, J. Appl. Pol. Sci., 30, 1969, 1985), considering: thermodynamic equilibrium between the swollen gel and the solvent, equivalent spherical gel, simplified strain energy function and negligible contributions from interfacial tension forces to the overall free energy. Osmotic pressure is considered in order to include the effect of polystyrene occlusions in the rubber particles. A good agreement between measurements and estimations is observed.