Isocyanates quality control with NIRS

Summary

Analyzing NCO content in isocyanates as raw materials for polyurethane (PU) is crucial to control properties of the final polymer. Wet analytical methods such as those used in ASTM D7252 are challenging due to the reactivity of these organic species with atmospheric moisture, as well as their toxicity. Furthermore, analysis of NCO content with HPLC (high-performance liquid chromatography) involves many sample preparation steps and chemical reagents, with each measurement taking up to 20 minutes to complete.

This Application Note demonstrates that near-infrared spectroscopy (NIRS) provides a reagent-free and faster alternative to determine NCO content in isocyanates. The OMNIS NIR Analyzer Liquid analyzes isocyanates in a few seconds without any sample preparation.

Experimental equipment

Figure 1. OMNIS NIR Analyzer and a sample filled in a disposable vial.

Liquid samples of isocyanates were measured with an OMNIS NIR Analyzer Liquid in transmission mode over a wavelength range of 1000–2250 nm. Reproducible spectrum acquisition was achieved using the built-in temperature control at 30 °C. For convenience, disposable vials with a path length of 8 mm were used. OMNIS Software was used for all data acquisition and prediction model development.

**Table 1.** Hardware and software equipment overview.
Equipment	Article number
OMNIS NIR Analyzer Liquid	2.1070.0010
Holder OMNIS NIR, vial, 8 mm	6.07401.070
Disposable vial, 8 mm, transmission	6.7402.240
OMNIS Stand-Alone	6.06003.010
Software license Quant Development	6.06008.002

Configuration

OMNIS NIR Analyzer Liquid

2.1070.0010

Near-infrared spectrometer for liquid samples. Developed and produced in accordance with Swiss quality standards, the OMNIS NIR Analyzer is the near-infrared spectroscopy (NIRS) solution for routine analysis along the entire production chain. Its application of the latest technologies and its integration in the modern OMNIS Software are reflected in its speed, operability, and flexible utilization of this NIR spectrometer.Overview of the advantages of the OMNIS NIR Analyzer Liquid:Measurements of liquid samples in less than 10 seconds; Temperature control on the sample from 25–80 °C; Automatic detection of the insertion and removal of the sample vessel; Simple integration in an automation system or link with additional analysis technologies (titration); Supports numerous sample vessels with different path lengths;

Holder OMNIS NIR, vial, 8 mm

6.07401.070

Vial Holder for the OMNIS NIR Analyzer for 8 mm disposable vials (6.7402.240).

Disposable vial, 8 mm, transmission, qty. 100

6.7402.240

100 disposable glass vials (borosilicate) with an optical path length of 8 mm for analyses of liquids in transmission. The disposable vials are supplied with the associated stoppers (number of pieces = 100).Compatible with:Holder OMNIS NIR, vial, 8 mm (6.07401.070); DS2500 holder for 8 mm disposable vials (6.7492.020) ;

OMNIS Stand-Alone license

6.06003.010

Enables stand-alone operation of the OMNIS software on a Windows™ computer.Features:The license already includes one OMNIS instrument license.; Must be activated via the Metrohm licensing portal.;

Software license Quant Development

6.06008.002

Software license for the creation and editing of quantification models in a stand-alone OMNIS Software installation.

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Result

The NIR spectra (Figure 2), along with the corresponding reference values, were used to create a prediction model for isocyanate quantitative analysis. The quality of the prediction model was evaluated using the correlation diagram shown in Figure 3 which displays a very high correlation between the NIR prediction and the measured values using HPLC. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis.

Figure 2. Stacked collection of NIR spectra from isocyanate samples analyzed with the OMNIS NIR Analyzer Liquid.

Result NCO content in isocyanates

Correlation diagram and the respective figures of merit for the prediction of isocyanate content in polyurethane raw materials using an OMNIS NIR Liquid Analyzer.

Figure 3. Correlation diagram and the respective figures of merit for the prediction of isocyanate content in polyurethane raw materials using an OMNIS NIR Liquid Analyzer. HPLC was used to evaluate isocyanate group content (% NCO) lab values.

R²	SEC (%)	SECV (%)	SEP (%)
0.983	0.30	0.33	0.39

Conclusion

This Application Note shows the feasibility of using NIR spectroscopy for the quality control of isocyanates for PU production. NCO content can be determined in only a few seconds without any sample preparation.

Compared to wet chemical methods (Table 2), the time to result is a major advantage of NIR spectroscopy. Also, no chemicals are required, helping to better adhere to environmental, chemical, and health regulations.

**Table 2.** Time to result overview for the analysis of isocyanate content with HPLC.
Parameter	Method	Time to result
Isocyanate content	HPLC	∼20 min (preparation) + ∼20 min (HPLC)

Kontakt

Metrohm Nordic ApS

Lyskær 3C. 1. th.
2730 Herlev

Isocyanates quality control with NIRS

Fast, chemical-free determination of isocyanate content for polyurethane production

Summary