(369f) Emulsion Copolymerization Process: Mathematical Modeling and Experimental Validation
Latexes with high solid contents (above 55% of polymer relative to the continuous aqueous phase) exhibit several advantages such as lower costs of transport and storage, and shorter drying and film formation times. However, in order to keep the latex viscosity at satisfactory levels, and to prevent loss of colloidal stability, the particle size distribution (PSD) should be either broad or multimodal. Therefore, to produce high solids content latexes, the tight control of the PSD is important to keep the viscosity within manageable limits and, even more important, to prevent the risk of emulsion stability. The aim of the present work was the study of the production of high solid contents latexes by emulsion copolymerization of styrene and butyl acrylate in semi-batch operation mode. Copolymerization reactions were carried out in a lab-scale glass reactor, applying recipes with solid contents higher than 50%; surfactant concentration (ionic and non-ionic polymeric emulsifiers) between 4.3 and 6.4 wt.%; and acrylic acid concentration between 1.5 and 3.5 wt.%. During each run, samples are periodically taken from the reactor, polymerization is stopped with inhibitor (hydroquinone), and analysis are performed to measure the polymer content (monomer conversion) by gravimetry, the concentrations of the residual monomers by head-space gas chromatography, the average particle size by dynamic light scattering/photon correlation spectroscopy, the particle size distribution by electronic microscopy. The viscosity of the final emulsion is also measured using a Brookfield viscosimeter. A mathematical model was developed to simulate the emulsion copolymerization process. The model is based on mass balances for the different species (monomers, polymer, emulsifier, water, etc.) and considers the classical free radical polymerization mechanism and events of radical transfer among the different phases present (aqueous phase, polymer particles, monomer droplets, micelles). The model also includes the population balance to describe the evolution of the particle size distribution during the polymerization process. The simulation results obtained from the mathematical model were successfully compared and validated with the experimental measurements for the evolution of conversion, average particle size and particle size distribution, showing that it is able to follow very well the trends of the main process variables. The developed model can be useful for further studies of process control, or to predict the effects of changes in the process on the final PSD and solids content, important variables that affect the apparent viscosity and the rheological characteristics of the final latex produced.
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