(217cx) Liquid-Liquid Dispersion Behavior and Particle Formation in Suspension Polymerization Process
In typical suspension polymerization processes, liquid monomers with oil soluble initiator are dispersed into small droplets with synergetic action of strong agitation and suspending agents. Polymerization occurs inside the small drops. With increase in conversion rate, the viscosity of dispersed phase raises, causing the rates of particle coalescence and dispersion change. Until the viscosity of dispersed phase reaches excessively high, the particle coalescence stops and the particle size growth ceases. Thus, the solid particles with constant size are formed in the later reaction stage. The particle properties of suspension polymer resin, such as particle size and its distribution, depend on the particle formation process which is affected by monomer dispersion behavior and variation of viscosity of the dispersed phase. In this work, methyl methacrylate monomer was used to study the liquid-liquid dispersion behavior in suspension polymerization by means of on-line particle size analysis system (Sequip IPAS). To be specific, the effects of operating parameters, such as agitation speed, type and concentration of suspending agents, viscosity of dispersed phase, interfacial tension, on liquid-liquid dispersion behavior were studied. Meanwhile, monomer/polymer solution with different concentrations and viscosity were used to simulate polymerization stages with different conversion. Furthermore, the effects of operating conditions on the droplet/particle size and its distribution were determined by experiments. Based on the above results, liquid-liquid dispersion and particle formation in the suspension polymerization process were comprehensively investigated from the views of turbulence forces, interfacial tension and viscous forces. Semi-empirical and phenomenological expressions were employed to describe the coalescence and breakage rates of dispersed monomer droplets in terms of agitation speed, type and concentration of suspending agent, interfacial tension, and evolution of the viscosity properties of the polymerization system. A new suspension polymerization model was proposed to describe accurately the particle formation process, and thus help to improve polymerization operation stability and to control the particle size and its distribution of polymer resin.
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