Remote in-line monitoring of emulsion polymerization of styrene by short-wavelength near-infrared spectroscopy .1. Performance during normal runs

Near-infrared (NIR) spectroscopy in the wavelength range 700-1100 nm is used with an in-line fiber-optic diffuse reflectance probe to monitor the emulsion polymerization of styrene. A reactor and spectroscopic monitoring system is constructed that provides computer control of the spectrometer, delivery of the monomer, temperature and stir rate. Based on comparison with the spectra of pure components, the diffuse reflectance spectra obtained exhibit bands that can be assigned to water, styrene monomer and polystyrene. However, the original zero derivative spectra are distorted and suffer large variable offsets due to the effects of light scattering. By application of the second derivative transformation, improved results are obtained and a good correlation is found between the NTR spectra and percentage of solid polymer as measured by the gravimetric method. Predictions between succeeding runs result in relative standard deviations of 4.5% for linear regression at 880 nm, and 3.2% for two-latent-variable principal component regression and partial least squares, As a further check on the value of NIR spectra for emulsion process monitoring, an attempt was made to correlate spectral features with the concentrations of the three major chemical species in the reactor. Since reference analytical methods were not available for these components, a novel alternative calibration method was developed. The spectra were correlated to the concentrations of the species as predicted by a computer simulation of the process. The correlation between model predictions and spectra proved quite good, resulting in NIR methods for monomer, polymer and water with standard errors of prediction of 7-27%, 2-7% and 0.2-0.6%, respectively.