Near-infrared analysis of polyols

Significance of the Topic

The determination of hydroxyl number in polyols is essential for controlling polymer molecular weight and ensuring consistent performance of polyurethane products. Traditional titration techniques are time consuming and require sample preparation. Near-infrared spectroscopy (NIRS) provides a rapid, noninvasive alternative suitable for in-process quality control.

Objectives and Overview of the Study

This application note describes the development of a NIRS method to quantify hydroxyl end groups in polypropylene glycol samples. The goal was to generate a robust calibration that matches or exceeds the accuracy of conventional titration, enabling real-time monitoring of polyol production.

Methodology

Samples of polypropylene glycol with hydroxyl numbers ranging from 26 to 280 were analyzed in transmission mode. Second-derivative spectral preprocessing was applied to correct baseline shifts. A linear least-squares regression model was built using known titration values as reference.

Instrumentation Used

• FOSS Model 6500 spectrophotometer (1100–2500 nm) for initial measurements
• NIRS XDS Transmission Optiprobe or XDS RapidLiquid Analyzer as current recommended instruments
• 4 mm quartz cuvettes with temperature control at 32 ± 0.1 °C
• 32 co-added scans per sample against an air reference

Main Results and Discussion

Key absorption features were observed near 1460 nm (O–H first overtone) and 2070 nm (O–H combination band). The regression model at 2070 nm yielded a correlation coefficient of 0.999 and a standard error of calibration of 4.20 hydroxyl number units. NIRS predictions agreed within ±5 units of titration results across the full range.

Benefits and Practical Applications

• Rapid, nondestructive analysis without reagents or sample cleanup
• Real-time feedback for manufacturing process control
• Simultaneous monitoring of primary vs. secondary hydroxyl groups
• Extension to residual oxides, moisture, amine levels, and backbone characterization

Future Trends and Potential Applications

Advancements in calibration algorithms and instrument integration could expand NIRS use for mixed polyols and complex formulations. Online process analyzers with automated temperature compensation and multicomponent models will further enhance throughput and data reliability.

Conclusion

NIRS provides a fast, accurate, and versatile tool for determining hydroxyl number and related parameters in polyols. Its nondestructive nature and minimal sample handling make it well suited for in-process quality assurance and optimization of polymer production processes.