Phase Inversion Prediction during the Bulk Synthesis of High-Impact Polystyrene: A Fluid-Dynamic Approach

MAFFI, J. M.; ESTENOZ, D.

Boston

Congreso; AIChE Annual Meeting; 2021

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, etc5. 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. There are different mechanisms by which PI may occur, the most popular being the unbalance between the coalescence and breakage rate of the dispersed particles6. According to this mechanism, PI will take place when particles undergo coalescence at a much faster rate than they break up. This means that the coalescence-to-breakage ratio should tend to an infinitely large value at the onset of inversion.In this work, this mechanism is tested by means of a population balance model, coupled with a heterogeneous polymerization module. This model reproduces the particle size distribution (PSD) of the vitreous dispersed phase along the reaction. It computes the growth, coalescence and breakage rates of each particle according to models available in literature. Predicted PSDs are compared to those measured by TEM images obtained in a previous study, varying different reaction conditions: initiator concentration, temperature and stirring speed.Results show that predictions are in very good agreement with measured particle sizes. With the adjusted model, the coalescence vs breakage imbalance is assessed. Results indicate that the coalescence-to-breakage ratio never spikes at any point during the polymerizations. After careful physical analysis, it is concluded that – at least in this system – it is likely that this feature is never observed, as the inversion process does not occur in an accelerated, “instantaneous” way. Rather, it is the result of a co-continuous transition that may last several minutes. In order to test the physical reliability of the model, cumulative frequency curves are produced and compared to the expected behaviors around the inversion points. These are shown to be in excellent agreement with what would be anticipated at such points.The future goal of this work is to identify a clear phase inversion criterion that can apply in the polymerization of HIPS and to develop a comprehensive mathematical model that is able to predict both its occurrence and the morphology therein developed.