Near-infrared analysis of polyols

Importance of the Topic

High-quality polyols are essential precursors in polyurethane production, where hydroxyl number directly influences polymer properties such as molecular weight, mechanical strength, and thermal stability. Traditional titration methods for hydroxyl number determination are time-consuming and destructive. Implementing near-infrared spectroscopy (NIRS) offers a rapid, nondestructive alternative for in-process quality control.

Objectives and Study Overview

This study aims to develop and validate a fast NIRS-based approach to accurately quantify hydroxyl numbers in polypropylene glycol samples covering a range from 26 to 280 mg KOH/g. The method leverages second-derivative spectral analysis and linear regression to align with conventional titration results.

Methodology and Instrumentation

Key experimental details include:

• Spectrometer: FOSS Model 6500 (1100–2500 nm); recommended replacement instruments are NIRS XDS Transmission Optiprobe Analyzer or RapidLiquid Analyzer.
• Sample handling: 4 mm quartz cuvettes at a controlled temperature of 32 ± 0.1 °C to stabilize the hydroxyl band.
• Spectral acquisition: 32 co-added scans per sample, referenced against 32 air scans.
• Data processing: second-derivative transformation to correct baseline drift, followed by linear least-squares regression at the 2070 nm combination band.

Main Results and Discussion

Spectral features near 2070 nm (O–H combination band) and 1460 nm (first overtone) showed a systematic increase with higher hydroxyl numbers. The regression model achieved a correlation coefficient (R) of 0.999 and a standard error of calibration (SEC) of 4.20 mg KOH/g. NIR-predicted hydroxyl numbers agreed within ±5 units of titration values, matching the inherent accuracy of the reference method. A separate calibration for samples below 100 mg KOH/g further enhanced precision.

The method also distinguishes between primary and secondary hydroxyl end groups and can monitor additional parameters such as EO/PO ratio, moisture, and residual oxide content.

Benefits and Practical Applications

• Real-time, nondestructive measurement directly on bulk samples without chemical reagents or sample preparation.
• Rapid feedback enables tighter process control in polyol manufacturing and polyurethane foam production.
• Multicomponent analysis extends to acid number, amine levels, and backbone characterization (methyl vs. methylene ratios).

Future Trends and Potential Uses

Emerging opportunities include inline NIR sensors for continuous monitoring, integration with automated control systems, and expansion to other polymer precursors. Advances in portable NIR instrumentation and machine-learning–based chemometrics will further enhance sensitivity and versatility.

Conclusion

NIR spectroscopy provides a robust, rapid, and accurate method for determining hydroxyl number and related quality attributes in polyols. Its nondestructive nature and capability to monitor multiple parameters in real time position it as a valuable tool for industrial quality assurance.

Reference

No external references cited.