Near-infrared analysis of polyols: process monitoring in a hostile environment

Significance of the Topic

Real-time monitoring of polyol polymerization through hydroxyl and acid numbers is essential for optimizing molecular weight and determining reaction end points. Near-Infrared (NIR) spectroscopy enables fast, non-destructive analysis under harsh reactor conditions, leading to cost savings and improved product quality.

Objectives and Study Overview

This application note demonstrates the implementation of NIR spectroscopy for on-line measurement of hydroxyl and acid numbers in a high-temperature polyol batch reactor. The goal is to achieve accurate real-time process monitoring to reduce sampling time and enhance control over the polymerization process.

Methodology and Instrumentation

The batch process was conducted at 260 °C with nitrogen sparging and turbulent mixing, producing suspensions of particles. In vivo sampling employed a fiber-optic interface with an anhydroguide bundle and a stainless-steel immersion probe (3 mm gap, 6 mm pathlength). Spectra were collected in transmission mode (1100–2500 nm) using the FOSS Process Analytics Polyol Analyzer and later with the NIRS XDS Transmission Optiprobe Analyzer with heater. Each sample scan comprised 32 co-added scans against an internal fiber reference. Multilinear least-squares regression models were built on second-derivative spectra to correlate spectral features with known hydroxyl and acid numbers.

Main Results and Discussion

Absorbance spectra revealed characteristic O–H overtone (around 1400–1500 nm) and combination bands (1900–2100 nm). Second-derivative processing enhanced subtle features and minimized scattering effects. Calibration for hydroxyl number using a two-term model achieved R2=0.999 and SEC=0.41; acid number calibration reached R2=0.999 and SEC=0.28. Both models delivered relative errors of 1.5% compared to laboratory titrations. Real-time monitoring over an 8-hour reaction showed clear trends in hydroxyl and acid values, enabling endpoint detection within one minute.

Benefits and Practical Applications of the Method

• Reduces sample residence time by 3–4 hours, decreasing energy consumption for reactor heating.
• Delivers rapid (under one minute) and accurate measurements comparable to laboratory reference methods.
• Enhances reproducibility and prevents overprocessing of polyol batches.
• Supports better quality control and lower production costs.

Future Trends and Potential Applications

NIR spectroscopy can be extended to analyze mixed polyols (ethylene and propylene oxides), assess methyl substitution ratios, measure moisture (down to ppm levels), detect residual oxides, and quantify primary and secondary amines in substituted polyols, further expanding its utility in polymer manufacturing.

Conclusion

Near-Infrared spectroscopy provides a robust, real-time analytical tool for monitoring hydroxyl and acid numbers in hostile polyol reactor environments. The technique improves process efficiency, product quality, and cost-effectiveness, demonstrating its value for industrial polymerization control.

References

• Application Note NIR–7: Near-infrared analysis of polyols: process monitoring in a hostile environment
• AN-NIR-6: Near-infrared analysis of polyols