Redox-Initiated ‘Adiabatic' Emulsion Polymerization

Thursday, November 11, 2010: 12:51 PM
Alpine Ballroom West (Hilton)
Shi Wang1, Andrew Klein2 and Eric S. Daniels2, (1)Emulsion Polymer Institute, Department of Chemical Engineering, Lehigh University, Bethlehem, PA, (2)Chemical Engineering, Lehigh University, Bethlehem, PA

Emulsion polymerizations are usually carried out under isothermal conditions at relatively high temperatures (e.g., 70°C) to synthesize commercial latex polymer products. However, this process incurs significant energy costs. To reduce energy cost, redox-initiated ‘adiabatic' emulsion polymerization processes are being evaluated as a green process, where the polymerization can be initiated at a low temperature and the reaction heat can be efficiently utilized. n-Butyl methacrylate (BMA) was used as the monomer in a model system employing redox initiators (ascorbic acid and H2O2). An isothermal batch emulsion polymerization process was first carried out to study the influences of redox initiator on both particle size and molecular weight. The high radical flux resulting from the use of redox initiator leads to latexes with much smaller particle size (measured by DLS, Nicomp), lower molecular weight (measured by GPC) and faster reaction rate (measured by Mettler RC1) compared with thermal-initiated emulsion polymerizations. 7 mM of NaCl was added to control the viscosity of the latex due to the low concentration of electrolyte. In addition, small particle size and narrow particle size distribution have been proved by transmission electron microscopy (TEM) with the negative staining technique. The particle size was increased with lower surfactant concentration, higher solids content, or lower redox initiator concentration. The molecular weight was higher with higher surfactant concentration and lower redox initiator concentration. Based on the reaction rates and the relationship between particle numbers and surfactant concentrations, it has been demonstrated that the homogeneous nucleation plays an important role other than micelle nucleation in redox-initiated system. Besides, the higher reaction temperature can result in significantly larger particle size and lower and broader molecular weight with the redox initiator. ‘Adiabatic' batch emulsion polymerization was thus carried out in Mettler RC1, and the reaction heat was utilized to increase the reactor temperature and shorten the cycle time. No significant change was detected in particle size and molecular weight compared with the latex produced by redox initiators under the isothermal condition.

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