Process Monitoring of Polymer Systems by using Near-Infrared Spectroscopy

Abstract

Polymers are a very important class of organic materials. Nowadays, polymers have been widely used in many fields due to their specific and excellent properties. They have become increasingly significant in our lives. With the growing demands on the quality of polymer products as well as reduction of costs in production and processing, the fast, accurate and reliable monitoring methods become more and more important. The traditional off-line monitoring methods have many limitations, one of which is lag effect. This is mainly because sampling and lab testing can be very time-consuming and for some tests a long time is required before the results are obtained. It is obviously that there is a large time gap between sampling and obtaining the results. Meanwhile, the reaction or processing of unknown quality polymer continues. Hence, the sample and end product don’t have the same properties, which can result in large amounts of wasted material (Rohe et al., 1999). Another problem is that the sample analysed is small and as a result might not have all the properties that the bulk possessed (Coates et al., 2003). Due to these limitations and problems, the off-line measuring methods are clearly unsuitable for the process monitoring. Undoubtedly, it is exciting to observe that the spectroscopic techniques can meet the above demands required by the process monitoring of the polymer systems. The vibrational spectroscopy is ideally suited to qualitative and quantitative analysis of polymers. Moreover, it can also give valuable information at the molecular level about the reactions extent and the products quality. It deserves to be specially noted that Near-infrared (NIR) spectroscopy is a promising tool among the spectroscopic techniques. Near-infrared spectroscopy lies between the visible and mid-infrared regions of the electromagnetic spectrum, whose region extends from about 12,500-4000 cm-1. This region encompasses bands that result from the harmonic overtones of fundamental and combination bands associated with hydrogen atoms, which is why compounds containing CH, OH and NH bonds lend themselves favourably to analysis by NIR spectroscopy. However, NIR spectroscopy is comparatively weaker than mid-infrared, and contains molecular structure information seriously overlapped which resulted in the silence of its research in the early stage. The modern earliest analytical applications of NIR spectroscopy were reported in the early 1960’s (Butler & Norris, 1960; Norris & Butler, 1961). Nowadays, NIR spectroscopy