Fouling during emulsion polymerization

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The research project aims at a deeper understanding of the mechanisms that lead to the formation of deposits on the reactor walls as well as heat transfer surfaces during emulsion polymerization. Therefore, the initial phase of the deposit formation shall be tracked in-situ by two complementary measures: firstly, a quartz crystal microbalance (QCM) integrated into the reactor wall and, secondly, a heating finger that can be withdrawn vertically from the reactor in a stepwise manner. In this way, the state of deposition at the respective time is preserved for later inspection. A comprehensive time-resolved characterization of all processes in the bulk and at the surface combined with the correlation of these processes with each other provides a comprehensive view on all mechanisms contributing to the deposit formation. The characterization shall include both chemical parameters, such as the composition of the fouling material, and physical parameters, such as the particle size, the roughness and the softness of the deposited layer. The characterization shall allow to distinguish between particle fouling and reaction fouling, the former also occurring in non-reactive dispersions. Preliminary investigations have shown that under certain conditions a thin polymer layer (thickness < 1 µm) forms on the wall, which stabilizes this surface against the formation of further, thicker fouling layers. Which system parameters bring about such a passivation is poorly understood and shall be investigated thoroughly. With regard to the materials, a focus shall be on polyacrylates and polyvinyl acetate (pVAc). pVAc tends to crosslink and has a glass transition temperature below the reaction temperature. It is usually stabilized with grafted chains of polyvinyl alcohol (PVOH). Insufficient solubility of PVOH in water at high temperatures may promote fouling. Acrylates are available with different glass transition temperatures. Hard and soft particles differ in the extent to which the spheres deform into polyhedra while the coating is still forming ("wet sintering"). The use of the QCM opens special opportunities in the analysis. In particular, the growth of a planar layer can be distinguished from cluster growth. According to one of the working hypotheses, planar layers tend to passivate the surface against the deposition of thick layers more efficiently than clusters. Furthermore, the QCM can estimate an effective shear modulus. A shear modulus increasing with time indicates compactification of the layer, possibly by wet sintering. The project aims to break new ground in terms of both a description of the mechanisms involved and of quantitative modelling.