An in-line sensor using near infrared (NIR) spectroscopy was developed for monitoring carbon dioxide (CO_2) concentration,which is dissolved into molten polymers at the extrusion foaming processes.The sensor infers the CO_2 concentration from the measured NIR absorption spectra.The spectra were acquired on-line through the probes equiped at the die of the extruder,where foams were created by dissolving CO_2 gas into molten polymer at the middle of the extruder and releasing pressure at the exit of the die.The experiments were conducted on three different kinds of polymer,polypropylene (PP),low-density polyethylene (LDPE) and polybutylene succinate (PBS),at different low rates and at different operating temperature.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 could be eliminated.The regression model derived from the wavelet-treated NIR spectra could successfully infer the CO_2 concentrations for all three kinds of polymer.
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