Simultaneous determination of multiple quality parameters of polyols using Vis-NIR spectroscopy

Importance of the Topic

Quality control of polyols is essential in polyurethane production because the presence of hydroxyl groups and other chemical properties directly influences polymer performance. Traditional titration methods for determining acid value, hydroxyl value and moisture content require extensive sample preparation, chemicals and time, raising operational costs and delaying decision making. Vis-NIR spectroscopy offers a rapid, reagent-free alternative capable of simultaneous multi-parameter analysis, improving efficiency and sustainability in industrial and research laboratories.

Objectives and Overview of the Study

This study aimed to evaluate Metrohm Vis-NIR analyzers for the simultaneous quantification of acid value, hydroxyl value and moisture content in polyol samples. By comparing NIR predictions against standard reference methods, the goal was to establish robust calibration models that deliver comparable accuracy while significantly reducing analysis time and cost.

Methodology and Instrumentation

The study analyzed 140 customer-provided polyol samples covering a wide range of quality parameters. Reference values for acid value, hydroxyl value and moisture content were supplied.

• Instrument: NIRS XDS RapidLiquid Analyzer (400–2500 nm)
• Sample holders: Disposable glass vials, 8 mm diameter
• Software: Vision Air 2.0 Complete for data acquisition and model development
• Measurement conditions: Transmission mode, 30 °C, 30 s thermal equilibration
• Calibration approach: Partial least squares (PLS) regression with optimized wavelength ranges and second derivative pretreatment

Twenty-five percent of samples were reserved for external validation to assess model performance.

Main Results and Discussion

All three quality parameters showed strong correlation between NIR-predicted and reference values:

• Acid value (0–8 mg KOH/g): PLS with nine factors over 1366–1482, 1612–1828 and 1900–2116 nm; R²=0.989; SEP=0.38 mg KOH/g
• Hydroxyl value (70–125 mg KOH/g): PLS with six factors over 1350–1500 and 1800–2100 nm; R²=0.999; SEP=0.68 mg KOH/g
• Moisture content (75–400 ppm): PLS with seven factors over 1350–1600 and 1800–2000 nm; R²=0.902; SEP=52 ppm

The prediction errors were comparable to those of classical titration methods. Excluding redundant spectral regions and applying targeted pretreatments enhanced model robustness and reduced overfitting.

Benefits and Practical Applications

Vis-NIR spectroscopy for polyol analysis offers:

• Rapid results: full multi-parameter profile in under one minute per sample
• Chemical-free workflow: eliminates solvents, reagents and waste disposal
• Cost savings: reduced labor, consumables and instrument runtime
• Scalability: suitable for routine QC, inline process monitoring and high-throughput laboratories

Future Trends and Opportunities

Extending Vis-NIR calibration libraries can enable additional quality checks such as solvent content, primary vs secondary hydroxyl distinctions, and solid content. Integration with process analytical technology and Industry 4.0 platforms may allow real-time monitoring of polymerization reactors. Advanced machine learning algorithms could further improve predictive accuracy and transferability across instruments and sites.

Conclusion

This application note demonstrates that Metrohm Vis-NIR analyzers provide accurate, rapid and cost-effective determination of acid value, hydroxyl value and moisture content in polyols. The method delivers performance on par with conventional reference analyses while offering significant operational advantages. Its multi-component capability and minimal sample preparation make it a valuable tool for modern QC workflows.

References

• Metrohm. NIR Application Note NIR-065, August 2018.
• ASTM D6342: Standard Test Method for Hydroxyl Number of Polyols by Near Infrared Spectroscopy.
• ISO 15063: Thermogravimetry — Determination of hydroxyl value of polyols by NIR spectroscopy.