Emulsion polymerization continuous monitoring

The use of a precision digital density meter as supplied by Mettler Instruments (Anton Paar, Ag.) appeared attractive. Few references on using density measurements to follow polymerization or other reactions appear in the literature. Poehlein and Dougherty (2) mentioned, without elaboration, the occasional use of y-ray density meters to measure conversion for control purposes in continuous emulsion polymerization. Braun and Disselhoff (3) utilized an instrument by Anton Paar, Ag. but only in a very limited fashion. More recently Rentsch and Schultz(4) also utilized an instrument by Anton Paar, Ag. for the continuous density measurement of the cationic polymerization of 1,3,6,9-tetraoxacycloundecane. Ray(5) has used a newer model Paar digital density meter to monitor emulsion polymerization in a continuous stirred tank reactor train. Trathnigg(6, 7) quite recently considered the solution polymerization of styrene in tetrahydrofuran and discusses the effect of mixing on the reliability of the conversion data calculated. Two other references by Russian authors(8,9) are known citing kinetic measurements by the density method but their procedures do not fulfill the above stated requirements. 

The available data from emulsion polymerization systems have been obtained almost exclusively through manual, off-line analysis of monomer conversion, emulsifier concentration, particle size, molecular weight, etc. For batch systems this results in a large expenditure of time in order to sample with sufficient frequency to accurately observe the system kinetics. In continuous systems a large number of samples are required to observe interesting system dynamics such as multiple steady states or limit cycles. In addition, feedback control of any process variable other than temperature or pressure is impossible without specialized on-line sensors. This note describes the initial stages of development of two such sensors, (one for the monitoring of reactor conversion and the other for the continuous measurement of surface tension), and their implementation as part of a computer data acquisition system for the emulsion polymerization of methyl methacrylate. 

The need for continuous monitoring of monomer conversion in batch, semi-continuous and continuous emulsion polymerization is growing because the requested performance of polymer materials has caused higher demands on the reproducibility and fine tuning of the production processes. 

Spectroscopic Techniques for Continuous Monitoring of Emulsion Polymerization Reactions... 

Because of its importance, continuous on-line monitoring of emulsion polymerization reactors continues to be an area of considerable investment in terms of research and development. It is evident that to better understand the nucleation mechanisms, and to develop better control strategies,... 

Real time continuous monitoring of the emulsion polymerization reactions was achieved using transmission and reflectance techniques. Transmission spectra were obtained on-line using the automatic sampling and dilution system. The reflectance probes were immersed in the reactor and the spectra recorded as function of time. Figure 3 shows a schematic of the probes in the reactor. In all cases special care was taken to ensure that the tip of the probe was well immersed in the reacting mixture. The lattices prepared were also used to study the effect of particle concentration on the reflectance measurements. 

Although a lot of information can be obtained with the analysis of diluted polymer lattices samples, as shown previously, the advantages that in situ analysis of the emulsion polymerization processes represent over systems that require dilution are well documented. A dilution step implies sample manipulation and increased measurement dead times. Therefore, regardless the effectiveness of the dilution strategy, there is incentive in continuing the development of in-line and in-situ measurement probes. Of the light based techniques reported only Raman and Near Infrared probes have been used for in situ to monitor conversion." The use of light and specifically fiber optic probes as a particle concentration and particle size detectors is a concept that has become widely accepted. ... 

Monomer conversion in emulsion and solution polymerization can be determined via density of the reaction medium due to the difference in density between monomer and polymer. The availability of on-line digital densitometers manufactured by Anton Paar of Austria and others make this approach amenable to on-line application [23-26]. These instruments are capable of immediate determination of the density of any fluid, and, if equipped with a flow cell, can continuously monitor the density of a process stream. Results are displayed locally and can be transmitted digitally to a data acquisition computer. Density measurement is accomplished by introducing a test fluid into a glass U-shaped sample tube which is rigidly supported at the open ends. The tube is electronically excited to vibrate at its natural frequency. The frequency of oscillation is continuously monitored electronically, and from the change of frequency caused by the test fluid within the tube, the... 

A technique has been developed for the continuous measurement of emulsion surface tension based on the pressure necessary to form a bubble in liquid. Details of the method may be found in Schork and Ray [24]. With a laboratory prototype of the bubble tensiometer, it has been possible to measure surface tensions continuously to within 1 to 2% [24]. A commercial instrument based on these principles is now available. Figures 5.5 and 5.7 demonstrate the use of the bubble tensiometer to monitor the surface tension of methyl methacrylate emulsion during continuous and batch polymerization. It will be noted that during conversion oscillation the surface tension oscillated as well, in accordance with the discrete initiation mechanism often postulated to explain this phenomenon. 

This paper will review measurement techniques for emulsion polymerization which have been successfully demonstrated as continuous monitors, as well as those techniques which show promise of adaptability to continuous monitoring in the near future. This paper is not meant to introduce new monitoring techniques, but to review existing techniques with respect to their current or potential application to the continuous monitoring of emulsion polymerization processes. 

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. 

The resultant voltage signal is continuous and proportional to the density of the liquid under test. By placing the orifices in a small flow cell, the instrument may be used to monitor flowing streams. When applied to emulsion polymerization (4), the prototype instrument appears to be capable of monitoring monomer con-... 

Thus it is rapidly becoming possible to continuously monitor the critical states of an emulsion polymerization. The challenge, then, for the polymerization reaction engineer is to make full use of this data in designing reactor trains and open and closed-loop control policies to tailor polymer properties and end-use needs. 

A first study of reaction kinetics with ACOMP was made for the semibatch emulsion polymerization of MMA at 70 °C [50]. The use of continuous monitoring method described in the previous sections offered a robust means of determining the characteristic features of the starved and flooded monomer conditions and identifying them during the experiments. 

The formation of gel particles and coagulum during the production of inverse emulsion and dispersion polymerization require filtration and diminish productivity. Continuous monitoring of the particle size and of the change in particle properties during production might provide insight into the onset of gel formation and could allow for specific action. 

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