Detailed modelling of an industrial process: vinyl acetate emulsion homopolymerization

Radical emulsion polymerizations are a class of fast and exothermic reactions widely diffused around the world to produce a great variety of paints and plastics. At industrial scale, repeatability of emulsion polymerization processes within narrow limits is highly desirable; this means that final solids content, particles size, emulsion viscosity and polymer average molecular weight should vary little from batch to batch. Moreover, the process should be completed in the shortest possible time and preparing a latex at the highest possible concentration to save time in production. Because of all these critical features, a reliable modeling of such processes would be very helpful at industrial scale. In this work, a detailed model of emulsion polymerization, accounting for dosing strategies, temperature control modes, volume variations, radical diffusion (inside and outside the micelle/polymer particles) and different mechanisms of particles nucleation, is developed. A non-stationary numerical approach based onto the Smith-Ewart (SE) theory has been employed to compute the average number of radicals per particle. Finally, a series of experiments on the emulsion polymerization of vinyl acetate has been carried out in an indirectly cooled semibatch reactor (RC1, 1 L, Mettler Toledo) to validate the model. Results have shown that, when the reactor operates under conditions (e.g., temperatures and dosing times) at which the cooling system is not able to remove all the power released by the polymerization reaction, the nonstationary approach based on the numerical solution of the Smith-Ewart equations is able to predict reactor temperature vs. time and monomer conversion vs. time profiles in a more accurate way than the stationary analytical solution of the SE equations does.