Controlling the level of water residues in the reaction system for polyurethane (PU) synthesis is as crucial as ensuring the proper formulation of polyol and isocyanate components. Excessive water content, which leads to side reactions and disrupts the stoichiometry between reagents, can significantly affect the mechanical properties and performance of the final products. Conventionally, Karl-Fischer (KF) titration is the industry standard for accurate water quantification in polyols. However, this method is labour-intensive and not suited for real-time feedback, needed in continuous manufacturing, such as reactive extrusion (REX) of PUs. Although Near-Infrared (NIR) spectroscopy is a robust technique for process analysis, its capability for effective monitoring of water content and simultaneous tracking of reactive aspects of PU materials have not been determined.
In this study, we present a flexible concept for rapid quantification of water in poly(ethylene glycol) (PEG) polyol – suitable for synthesis of specialty PUs – with NIR spectroscopy. We have targeted two use-cases: (1) on-line characterisation using an optical probe integrated into an extrusion process, enabling continuous monitoring, and (2) at-line determination with diffuse reflection spectroscopy, allowing for periodic quality checks.
By using KF as a reference method and advanced chemometric analysis of the NIR spectra, Partial Least Squares (PLS) regression models to predict the residual water content of PEG were generated with high accuracy and precision. Furthermore, we explored the options for parallel determination of water and isocyanate species in the system of PU reagents, addressing the challenges posed by their overlapping characteristic peaks in the NIR range. This approach shows the potential of a single measuring technique adaptable to various processing lines, providing comprehensive insight into PU systems for enhanced production control and optimisation in large-scale applications.