Emulsion conversion monitoring

For the purposes of conversion monitoring of emulsion polymerization, we have found the DMA40D with a precision of +1 x 10 g/cm capable of resolving monomer conversion to +0.2% in the absence of thermostating and sampling errors. On-line, in the presence of such possible errors, a resolution of at least +0.5% can be expected. Care must be taken to ensure a representative sample and good temperature control of the sample stream before introducing it into the instrument. Some example results with this instrument are presented below. 

Near-infrared spectroscopy (short wavelength 700-1100 nm) Near-infrared spectroscopy (wave-length 1500-1800) Stsrene emulsion poljmierization conversion monitoring 438,439... 

Canegallo S, Storti G, Morbidelh M, Carra S. Densimetry for on-line conversion monitoring in emulsion homo- and copolymerization. J Appl Polym Sci 1993 47 961-979. 

Alhamad B, Romagnoh JA, Gomes VG. Inferential conversion monitoring in emulsion co-polymerization through calorimetric measurements. Int J Chem React Eng 2006 4 A7,1-10. 

Bauer et al. describe the use of a noncontact probe coupled by fiber optics to an FT-Raman system to measure the percentage of dry extractibles and styrene monomer in a styrene/butadiene latex emulsion polymerization reaction using PLS models [201]. Elizalde et al. have examined the use of Raman spectroscopy to monitor the emulsion polymerization of n-butyl acrylate with methyl methacrylate under starved, or low monomer [202], and with high soUds-content [203] conditions. In both cases, models could be built to predict multiple properties, including solids content, residual monomer, and cumulative copolymer composition. Another study compared reaction calorimetry and Raman spectroscopy for monitoring n-butyl acrylate/methyl methacrylate and for vinyl acetate/butyl acrylate, under conditions of normal and instantaneous conversion [204], Both techniques performed well for normal conversion conditions and for overall conversion estimate, but Raman spectroscopy was better at estimating free monomer concentration and instantaneous conversion rate. However, the authors also point out that in certain situations, alternative techniques such as calorimetry can be cheaper, faster, and often easier to maintain accurate models for than Raman spectroscopy, hi a subsequent article, Elizalde et al. found that updating calibration models after... 

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... 

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. 

Monomer conversion has traditionally been determined gravitimetrically by drying emulsion samples to constant weight. The procedure is slow, requiring several hours for analysis, and precludes automated data acquisition. A new method has been developed based on the DMA-series digital densitometers manufactured by Anton Paar of Austria, and marketed in the United States by Mettier Instrument Corporation. (Very recently Dr. Kirk Abbey made us aware of his parallel work in these directions and of some initial data reported from his laboratory [1, 2]). This instrument is capable of immediate determination of the density of any test 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. 

This work has shown that by monitoring conversion curves by a computer, emulsifier metering can be varied to produce a desired particle size distribution of smalls in a seeded PVC emulsion polymerization. 

The relevant variables to be monitored in emulsion polymerization are temperature, flow rate, conversion, molecular weight, particle size, density, viscosity. 

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. 

Bon and coworkers carried out a study on the fate of the nanoparticles throughout solids-stabilized emulsion polymerization [119], A quantitative method based on disk centrifugation was developed to monitor the amount of nanoparticles present in the water phase in solids-stabilized emulsion polymerizations of vinyl acetate, methyl methacrylate, and butyl acrylate. The concentration profile of nanoparticles in the water phase as a function of monomer conversion agreed with theoretical models developed for the packing densities in these systems [120]. Noteworthy was that in the case of silica-nanoparticle-stabilized emulsion polymerization of vinyl acetate, the event of late-stage limited coalescence, leading to small armored non-spherical clusters, could be predicted and explained on the basis of the concentration profiles and particle size measurements. Adjusting the amount of silica nanoparticles prevented this phenomenon. 

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. ... 

The propagation of small pressure pulses or sound waves is a physical effect which can be a source of information for single and multiphase systems. Ultrasonic methods (between 20 kHz and 100 MHz) are easy to use, safe, non-destructive and non-invasive. MorbideUi et al. [19] used this acoustic method, with success, for the determination of the evolution of conversion in various copoly-meric systems in the dispersed phase (emulsion polymerization). The same methodology has been applied to bulk and solution systems by Gavin et al. [20, 21] and Zeihnann et al. [22] for monitoring high-soUds content polymerization of styrene and MM A. 

Other non-invasive (e.g., calorimetry and ultrasounds) and invasive (e.g., densimetry) techniques can also be used for monitoring of monomer concentration in homopolymerization reactions. However, in multimonomer formulations the individual monomer concentrations cannot be obtained with these techniques, meaning that a state estimator is required [66, 67, 69]. The use of reaction calorimetry is appealing because the hardware is very cheap and, when coupled with a state estimator, provides good estimation of the monomer concentration. The performance of calorimetry was compared with that of the Raman spectroscopy in emulsion polymerization to monitor overall and individual monomer conversions [68]. Calorimetry was as good as FT-Raman spectroscopy when monomer concentrations in the reactor were relatively high, but the performance of calorimetry was poorer when monomer concentrations were low. 

Figure 8.5 Monitoring a semibatch emulsion polymerization reaction of VAc/BA by means of an open-loop observer based on calorimetric measurements, (a) Conversion and (b) copolymer composition.

Figures. Conversion (digital densitometer) and surface tension (bubble tensiometer) monitoring during batch emulsion polymerization of methyl methacrylate.

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. 

The application of ultrasonics to the monitoring of emulsion polymerisation reactors is considered. The use of acoustic speed measurements to monitor conversion is demonstrated by its apphcation to the control of the emulsion copolymerisation of styrene and butyl acrylate. The potential of acoustic attenuation for the measurement of particle size is discussed and applied to the determination of the particle size distribution of PVC and PTFE latices. 27 refs. 

One of the issues when monitoring an emulsion polymerization reactor is selection of the most appropriate technique [124, 126]. For instance, monomer conversion and copolymer composition can be monitored on-line by means of densim-etry, refractive index, gas chromatography, calorimetry, ultrasound, fluorescence, ultraviolet reflection, and other spectroscopic methods such as Raman, mid-range infrared, and near-infrared. 

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. 

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