(749g) In Line Monitoring of Residual Monomer by NIR Spectroscopy During Styrene-Divinylbenzene Solution Polymerization Reactions

Introduction: The lack of reliable sensors for on-line, real-time measurements has long been recognized as the main bottlenecking for the implementation of advanced control strategies in polymerization processes. In this work, on-line fiber optic based near infrared spectroscopy was used to monitor styrene and divinyl benzene (DVB) residual monomer concentrations in solution copolymerization reactions aiming at the production of copolymers with pre-specified properties that are able to attend the increasingly exigent demands of the polymer market.

 Materials and Methods: Solution polymerization reactions were carried out in a jacketed glass stirred-tank reactor using different ratios between styrene and toluene (solvent).The temperature of the reactor content was controlled within 85±1 ºC and the agitation speed was kept 200 rpm. All NIR spectra were collected with an IFS 28/N Bruker spectrometer, equipped with a quartz bean splitter, using a transflectance probe with overall light pathlength equal to 1mm, immersed into the reaction medium. Off-line measurements of monomer conversion by gravimetry were previously validated with a mathematical model of the process. These information were used as reference for the development and validation of a PLS multivariate NIR calibration model developed using the software of the equipment (OPUS, program QUANT2).

 Results and Discussion: The spectral regions for residual monomer were the combination (4460-4750cm^-1) and the first overtone region (6212-6187cm^-1), wherein the bands attributed to the monomer are identified. Spectral pre-processing was performed using second derivative with 25 points smoothing and the internal validation was made by using “leave-one-out” cross-validation. The resulting calibration model uses three principal components (latent variables) and presents a high correlation coefficient (0.986). The NIR calibration model was able to follow successfully the monomer concentrations during new reactions not employed in the calibration step (external validation).

 Conclusion:  The in-line and in-situ monitoring of monomer concentration during solution copolymerization of styrene and DVB was based on a NIR calibration model developed and validated with NIR spectra and off-line gravimetric measurements and a mathematical model of the process. The results show good agreement between the experimental data and in-line monitoring predictions from the NIR spectra and the calibration model.

 Novelty statement: From the NIR calibration model for the residual monomer concentration, the global monomer conversion can be monitored on-line and in real-time, making possible the implementation of closed-loop control strategies and real-time decision-taking during the polymerization process.

 Summary: Solution polymerization of styrene-DVB residual concentration was monitored by NIR spectroscopy. Spectral changes in 4460-4750 cm¹ and 6212-6187cm^-1 were attributed as a function monomer concentration. The results show good agreement between the experimental data and in-line monitoring predictions from the NIR spectra and the calibration model.