Monitoring monomer conversion during emulsion polymerization

1 Monitoring monomer conversion during emulsion polymerization 

Chabot et al. at Atofina Chemicals (King of Prussia, PA, USA) used in-line NIR to monitor monomer conversion in real time in a batch emulsion polymerization process. The business value of this monitoring 

Chabot s work was done in a laboratory-scale batch reactor. Experiments at lab scale are a common hrst step in process analyzer work, since they allow technical feasibility to be demonstrated relatively quickly and inexpensively without having to interfere with production targets in a commercial-scale plant. In this work, moreover, one of the two business goals (improving process understanding) could be largely accomplished without going beyond lab-scale experimentation. 

The technical challenges were dealt with as follows  

The use of a transflectance probe, rather than a transmission probe, allowed good quality spectra to be obtained despite the low light transmission due to the high solid levels. 

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In this context, the use of ACOMP for the simultaneous monitoring of the evolution of coUoid phase (monomer droplets, polymer latex particles) and solution phase (polymer and monomer) characteristics during emulsion polymerization was recently reported as a versatile characterization tool that offers absolute, model-independent determination of quantities such as conversion, composition, and molecular weight [39],... 

Other than temperature and pressure, some of the more critical state variables during emulsion polymerization are monomer conversion, particle size and molecular weight. The bulk of this paper will be organized around discussions of the continuous monitoring of the above properties, with the exception of molecular weight, since, at the present time, continuous measurement of the molecular weight of a polymer does not appear to be feasible. 

Figure 7.4 shows the comparison of instantaneous and overall conversion and free monomer during the seeded semibatch emulsion copolymerizations of n-butyl acrylate/ methyl methacrylate (BA/MMA) calculated online from calorimetric measuranents and off-line by gravimetry [29]. These results show that calorimetry predicts overall conversion well but that the prediction of instantaneous conversion and free monomer is less accurate, especially when the polymerization is carried out under starved conditions and/or the monomer concentrations are very low in the reactor, as for the MMA in Figure 7.4d. Under these polymerization conditions, other more accurate techniques (but less robust and more expensive) such as Raman spectroscopy might be better suited for monitoring free monomer concentration [29].

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