Accurate determination of polyethylene pellet density using transmission Raman spectroscopy

Abstract

In this study, transmission Raman spectroscopy was explored for the direct measurement of the density of packed polyethylene (PE) pellets. A simple and direct transmission Raman measurement of packed, solid granules or pellet samples without pretreatment is greatly advantageous. Initially, the optimal packing thickness of PE pellets for transmission Raman measurement was determined by investigating the reproducibility of triplicate spectra collected by varying the thickness from 2 to 9 cm. Once determined, transmission Raman spectra were collected for 25 different grades of PE pellets and the partial least squares method was used to determine the sample density. The resulting accuracy was 0.00067 g.cm(-3), while that obtained using backscattering measurements was 0.00083 g.cm(-3). To investigate possible inhomogeneity within a pellet, Raman line mapping was performed over the face of a sectioned pellet and spectral variations among the mapped spectra were examined using principal component analysis. In addition, differential scanning calorimetry was performed on three samples prepared separately by cutting a pellet into left, middle, and right sections. Based on both studies, internal pellet inhomogeneity was found to be minute, but was clearly present. The correct sample representation of internally inhomogeneous PE pellets by the transmission Raman measurement eventually improved the accuracy for density determination. Finally sample-to-sample two-dimensional correlation analysis was used to further examine the origin of the improved accuracy.