In-Line NIR Sensing of CO2 Concentration in Polymeric Foaming Extrusion Process

Abstract

An in-line sensor using near infrared (NIR) spectroscopy was developed for monitoring carbon dioxide (CO2) concentration, which is dissolved into molten polymers in extrusion foaming processes. The sensor infers the CO2 concentration from the measured NIR absorption spectra. The spectra were acquired on-line through the probes equipped at the die of the extruder, where foams were created by dissolving CO2 gas into molten polymer at the middle of the extruder and releasing pressure at the exit of the die. The experiments were conducted on polypropylene (PP), low-density polyethylene (LDPE) and polybutylene succinate (PBS), at different flow rates and at different operating temperatures. By employing a novel signal processing scheme, i.e., wavelet transform, to analyze the NIR spectra, the influence of the operating conditions such as operating temperature and flow rate onCO2 measurement could be eliminated. The regression model derived from the wavelet-treated NIR spectra could successfully infer the CO2 concentrations for all three kinds of polymer.