Mid- and near-infrared spectroscopies for quantitative tracking of isocyanate content: Streamlined development of monitoring tools for reactive extrusion synthesis of specialty polyurethanes

Robust in-situ monitoring can expedite the development and tuning of reactive extrusion (REX) processes, especially for the synthesis of polymers requiring insight into the reaction course, such as specialty polyurethanes (PUs). In order to advance the feasibility of such analysis, our research concentrated on a spectroscopic protocol that combines at-line mid-infrared (MIR) and on-line near-infrared (NIR) measurements in a laboratory extruder, assessing both their qualitative and quantitative capabilities. To mimic the depletion of the reactive species during polymerisation, a series of premixes were formulated and characterised under non-reactive conditions using a benchmark pair of a polyol and a diisocyanate. By this means, we identified suitable spectral features for quantifying isocyanate content via multivariate regression models. This approach achieved good calibration metrics for both MIR and NIR models and enabled a robust assessment of their predictive performance through cross-validation, using only a moderate number of spectral datasets. Our work is therefore the first step towards establishing a method for in-process NIR analysis with streamlined at-line MIR verification. It provides a shortcut for designing monitored REX syntheses of PUs and other material systems amenable to quantitative calibration based on spectroscopic data from raw reagents.