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Reaction calorimetry for the development of ultrasound-induced polymerization processes in CO2-expanded fluids
Maartje F. Kemmere, Martijn W.A. Kuijpers, and Jos T.F. Keurentjes
Process Development Group, Department of Chemical Engineering & Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB, The Netherlands; Tel: +31-40-2473673; Fax: +31-40-2446104; E-mail: M.F.Kemmere@tue.nl
A strong viscosity increase upon polymerization hinders radical formation during an ultrasound-induced bulk polymerization. Since CO2 acts as a strong anti-solvent for most polymers, it can be used to reduce the viscosity of the reaction mixture. In this work, a process for the ultrasound-induced polymerization in CO2-expanded fluids has been developed. Temperature oscillation calorimetry has been applied to study the influence of CO2 on the viscosity during the ultrasound-induced polymerization. In contrast to polymerizations in bulk, the results show that a low viscosity is maintained during polymerization reactions in CO2-expanded methyl methacrylate (MMA). As a consequence, a constant or even increasing polymerization rate is observed when pressurized CO2 is applied. Moreover, the ultrasound-induced polymer scission in CO2-expanded MMA is demonstrated, which appears to be a highly controlled process. Finally, a preliminary sustainable process design is presented for the production of 10 kg/hour pure PMMA (specialty product) in CO2-expanded MMA by ultrasound-induced initiation.
The chemical effects of ultrasound arise from cavitation, i.e. the collapse of microscopic bubbles in a liquid. Upon implosion of a cavity, locally extreme conditions in the bubble occur (5000 K and 200 bar)  and high strain rates are generated outside the bubble (107 s-1) . Monomer molecules are dissociated by the high temperatures inside the hot-spot, whereas polymer chains are fractured by the high strain rates outside the cavitation bubble[3-5]. Since the radicals are generated in- situ by ultrasound, no initiator or catalyst is required to perform an ultrasound-induced polymerization. An additional advantage of this technique is the intrinsic safe operation, because turning off the electrical power supply will immediately stop the radical formation and consequently the polymerization reaction.
Viscosity is an important factor during ultrasound-induced bulk polymerizations as the long polymer chains formed upon reaction cause a drastic increase in the viscosity of the reaction mixture, thereby hindering cavitation and consequently reducing the production rate of radicals. Precipitation polymerization forms a potential solution to this problem, because a constant viscosity and hence a constant radical formation rate can be maintained. In this perspective, high-pressure carbon dioxide is an interesting medium as most monomers have a high solubility in CO2, whereas it exhibits an anti-solvent effect for most polymers.
Up till now ultrasound is rarely studied at higher pressures, because in most cases a high static pressure hampers the growth of cavities. Recently, we have shown that cavitation is possible in pressurized CO2. Unlike ordinary liquids, carbon dioxide has a high vapor pressure, which counteracts the static pressure. Cavitation is possible
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