Online determination of chemical and physical properties of poly(ethylene vinyl acetate) pellets using a novel method of near-infrared spectroscopy combined with angle transformation
Melt flow rate (MFR) and vinyl acetate content (VAC) are the target parameters for quality control of poly(ethylene vinyl acetate) (EVA) pellets in production processes. Standard test methods for these two properties are too time consuming to meet the actual instant needs in industry. In order to solve this problem, an online near-infrared spectroscopy (NIR) system was integrated in this study to determine the MFR and VAC of EVA pellets in process. When collecting spectra, the difference of sample loading density and optical path will cause noise and decrease the spectral and prediction precision of quantitative models of the MFR and VAC. Although routine spectral pre-treatment methods, including the first order (1st) derivative and multiplicative scatter correction (MSC), are effective in reducing noise, the standard error of prediction (SEP) values of partial least squares (PLS) models of MFR and VAC using the pretreated spectra are still much larger than the reproducibility requirement of standard methods. The key aim of this present study is to explore a novel method based on angle transform to improve the online models. Before establishing models, the original spectra are transformed to angular spectra that reflect the composition information of the sample and are not influenced by the spectral intensity. This method can effectively reduce the severe noise and improve the performance of the models. The SEPs of PLS models of the MFR and VAC using angular spectra are 0.08 and 0.21, respectively. These two values are lower than the reproducibility requirement of standard test methods for the MFR, 0.1 and VAC, 0.3. A new online method of NIR spectroscopy combined with PLS using angular spectra has been successfully established to simultaneously determine the chemical and physical properties of EVA pellets. This research solves a tough technical problem existing in online NIR analysis of pellet samples carried by conveyor belts or pipelines with diffuse reflectance mode.