Lean manufacturing of polyurethane, assisted by near-infrared (NIR) and Raman spectroscopy

Significance of the Topic

In polyurethane production, cost-intensive processes and environmental pressures drive the need for efficient monitoring tools.
Vibrational spectroscopy techniques such as near-infrared (NIR) and Raman offer real-time, chemical-free analysis that supports lean manufacturing.

Objectives and Overview

This study reviews how NIR and Raman spectroscopy can be applied across the polyurethane value chain.
It highlights key steps from polyol and isocyanate production to final polymer curing, demonstrating how spectroscopic methods reduce waste, improve quality, and lower operational costs.

Methodology and Instrumentation

The analytical approach integrates benchtop, online ATEX-certified, and handheld devices.
Online NIR systems—with dispersive optics for low noise and high sensitivity—monitor moisture content at ppm levels in epoxide and acid streams.
Benchtop NIR analyzers support atline quality control of polyols and isocyanates, determining hydroxyl and acid values, NCO content, viscosity, and other parameters.
Handheld Raman spectrometers enable rapid identity checks of incoming raw materials through packaging or observation windows.

Main Results and Discussion

NIR spectroscopy achieved real-time moisture detection down to ~10 ppm during alkoxylation, enhancing yield and product consistency.
Online monitoring of acid mixtures, solvents, and intermediates eliminated delays and reduced chemical handling.
In isocyanate production, continuous NIR ensured purity of MDI and TDI isomer streams and controlled byproduct recycling streams.
Handheld Raman devices accelerated raw material verification, cutting sample transport and lab setup times.

Benefits and Practical Applications

Implementing atline and online vibrational spectroscopy aligns with lean principles by reducing transportation, inventory, waiting, over-processing, and defects.
Single NIR measurements replace multiple wet-chemistry assays, delivering multi-parameter results within one minute and requiring minimal training.
Chemical-free protocols lower reagent costs, minimize waste, and support environmental goals.
Payback periods for spectroscopic systems typically fall below one year due to energy savings and productivity gains.

Future Trends and Potential Uses

Advances in instrumentation, such as portable NIR and integrated sensors, promise broader deployment across smaller-scale plants.
Combined with multivariate chemometric models and machine learning, spectroscopy will enable predictive control and autonomous process adjustments.
Digital connectivity may facilitate remote diagnostics, real-time performance dashboards, and cloud-based data analytics.
Broader use of process analytical technology (PAT) will support circular economy objectives by improving resource efficiency and reducing emissions.

Conclusion

Vibrational spectroscopy techniques are versatile, rapid, and cost-effective tools for polyurethane manufacturing.
Their ability to deliver immediate, multi-parameter insights without reagents fosters lean production, enhances product quality, and lowers ecological impact.
As instrumentation and data analytics evolve, NIR and Raman spectroscopy will become integral to advanced, sustainable polymer production.

References

• Ullmanns Polymers and Plastics Products and Processes Wiley-VCH 2016
• Statista Market size forecast of polyurethane worldwide from 2016 to 2021 2018
• Grand View Research Polyurethane PU Market Analysis By Product By End-Use Segment Forecasts 2017
• Metrohm Application Note NIR-007 Near-infrared analysis of polyols process monitoring in rough environments
• Metrohm Application Note NIR-006 Near-infrared analysis of polyols
• Metrohm Application Note NIR-035 Hydroxyl number in liquid polyols using Vis-NIR spectroscopy
• ASTM D6342 - 12(2017) Determining Hydroxyl Number of Polyols by NIR Spectroscopy
• ISO 15063:2011 Plastics Polyols Hydroxyl number by NIR spectroscopy
• D Randall S Lee The Polyurethanes Book Wiley-VCH 2003
• Metrohm Application Note RS-001 Identifying polymers with Raman spectroscopy
• Metrohm Application Note RS-007 Identification of various polymer master batches
• Metrohm Application Note RS-008 Identification of monomers with Raman spectroscopy
• Metrohm Application Bulletin AB-414 Polymer analyses using NIR spectroscopy
• Metrohm Process Application Note AN-PAN-1041 Inline monitoring of free isocyanate content in polyurethane
• M Holweg The genealogy of lean production Journal of Operations Management 2007