(181bm) Theoretical and Experimental Study of the Phase Inversion Point during the High-Impact Polystyrene (HIPS) Production Process

The high-impact polystyrene (HIPS) is a heterogeneous thermoplastic produced by styrene (St) polymerization in presence of polybutadiene (PB). It consists of a polystyrene (PS) matrix with dispersed PB particles, which often contain occluded PS. Depending on the rubber particle size and the number of occlusions, two typical morphologies are usually identified: a ‘salami morphology’ (large rubber particle with several occlusions) or a ‘core-shell morphology’ (relatively small rubber particle with only one large occlusion), which provide the material with improved mechanical properties.

The bulk pre-polymerization (first of the three main stages of manufacturing process) is carried out with intense agitation, producing free PS and a graft copolymer (PS-g-PB). The reacting system is homogeneous only at very low conversion, since the incompatibility between the PS and the PB chains forces it to undergo a phase separation mechanism, by which a dispersed, PS-rich phase is formed at the bulk of a PB-rich continuous phase. St monomer is almost evenly distributed between both phases. As the polymerization proceeds, more PS is produced and the dispersed phase eventually becomes the continuous phase, through a phase inversion (PI) process. The desired morphology is developed at this crucial stage, characterized by a sudden drop of the mixture’s apparent viscosity.

The PI process is affected by several variables, such as phase viscosity ratio, phase volume ratio, rubber cis/trans content, stirring speed, grafting efficiency, reaction temperature, solvent content, PS and PB molecular weights, etc. Given that the strong point of HIPS is its enhanced mechanical properties, and that these are the result of the in-situ morphology development during the PI stage, then the understating of this phenomenon and of the relative effect of each operating variable becomes a full chemical engineering challenge. The optimization of the polymerization recipes that provide desired material properties may be achieved by fully understanding the PI phenomenon. This holds a significant interest both from academic and industrial standpoints.

In this work, the main operating variables that affect the PI process during the bulk pre-polymerization are studied: chemical initiator concentration, stirring speed, and temperature. Throughout the reactions, samples were taken to determine: monomer conversion (by a gravimetric technique), PS molecular weights (by size-exclusion chromatography), grafting efficiency (by a solvent-extraction technique) and apparent viscosity (with a Brookfield viscometer). The phase inversion period of each reaction was identified by following the evolution of the mixture’s apparent viscosity with conversion. In order to verify that the PI had occurred, same samples were also analyzed under a STEM microscope.

Results show that the PS average molecular weight and the grafting efficiency both affect the PI point considerably. Stirring speed, while substantially modifying the apparent viscosity, has a minor effect on the location of the inversion point (at least under the examined conditions). For each reaction, the evolution of the system’s viscosity was compared to that predicted by several models available in emulsion rheology literature, very few of which include a dependence on particle size. Consequently, fitting the experimental data was unsuccessful and a new empirical model was derived, that accounts for this missing effect. This correlation is based on the operating variables of the polymerization reactor (i.e., temperature, stirring speed, copolymer concentration, PS molecular weight, rubber concentration), which makes it of interest from an industrial point of view.

The future goal of this work is to develop a comprehensive mathematical model that can be coupled with existing kinetic models and provide a useful tool for the HIPS industry. This would enable to perform engineering assessments, optimize recipes, improve reactor designs and enhance quality control