On-Line Analyses of Molten Polymers by near Infrared Spectroscopy and Chemometrics—Development of the Flow Cell and Realisation of the Long-Term Repeatability and Continuity

Abstract

An on-line monitoring system using Fourier transform near infrared spectroscopy was introduced and applied to molten poly-olefin polymers. Molten polymers are a kind of non-elastic fluid and their characteristics are ascribed mainly to their molecular weight and/or viscosity. From this standpoint, the cell for the molten polymer analysis, including probes with sapphire windows, heaters and thermocouples for temperature control, was developed. By using the developed cell, it was demonstrated that on-line monitoring of the density of linear low-density polyethylene could be done in a real plant over three months maintaining a good standard error of performance. The reproducibility before and after maintenance periods was also evaluated. The results, using the same sample, showed that the predicted values are the same between the maintenance periods. It can be estimated that the reproducibility of the system, including the flow cell, was sufficient for the application. For the realisation of the practical calibration, keeping reliability and traceability during long term in a real plant, the changes in the performance of the on-line system before and after maintenance caused by the relocation and/or the change of parts were investigated. A method for calibration correction considering wave-number shift and path-length correction was proposed as the solution for the change in performance. The result of the evaluation, using the flow cell and polypropylene samples, showed that the proposed method was useful in spite of using only one sample. It is available as a practical calibration method for real industry, because real plants require a method of calibration correction using as few samples as possible from the view point of saving cost and labour.

Keywords

NIR optical fibre FT-NIR molten polymer cell polyethylene polypropylene calibration correction wavenumber shift path-length correction

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