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

Significance of the topic

Polyurethanes are ubiquitous polymeric materials with a global production exceeding 20 million tons and a market value above USD 55 billion. Their manufacture is energy‐ and cost‐intensive and poses environmental challenges. Vibrational spectroscopy—near-infrared (NIR) and Raman—enables real-time, chemical-free analysis directly in production lines, reducing waste, improving product quality, and supporting lean manufacturing principles.

Objectives and study overview

This white paper illustrates how NIR and Raman spectroscopy can be applied at key stages of polyurethane production: polyol synthesis, isocyanate manufacture (MDI and TDI), and final polymer formation. It reviews analytical strategies for raw material verification, reaction monitoring, solvent recovery, intermediate quality control, and end-product testing, demonstrating cost savings and ecological benefits.

Methodology and instrumentation

• Near-infrared spectroscopy: dispersive and FT-NIR analyzers with ATEX-certified probes for inline, atline, and benchtop measurements.
• Handheld Raman spectroscopy: portable systems for rapid identification of incoming raw materials through packaging or inspection windows.
• Chemometric modeling: multivariate calibration for moisture (ppm level), hydroxyl value, acid number, isocyanate content (NCO), solids, color, monomer residuals, and curing state.
• Integration: direct connection to reactors, distillation units, and solvent recovery circuits to provide continuous feedback to operators.

Main results and discussion

Polyol manufacturing: online NIR achieved moisture detection in epoxides down to 10 ppm; monitored alkoxylation degree and hydroxyl number in real time; assessed solvent recovery composition. Quality control of finished polyols included EO/PO ratio, acid value, residual oxides, and color.
MDI production: inline NIR monitored acid mixtures (HNO3, H2SO4) and hydration to polymeric MDA; tracked phosgenation to crude MDI and final isomer purity; controlled moisture to prevent side reactions.
TDI production: continuous nitration of toluene yielded 80/20 or 65/35 DNT mixtures; online NIR quantified isomer ratios and purity of distilled fractions; phosgenation monitored for NCO content.
Polyurethane formation: NIR provided end-point detection of NCO conversion; atline benchtop NIR assessed prepolymer quality; inline analysis of curing progress and physical properties.
Lean manufacturing impact: spectroscopy reduced waste categories—transportation, inventory, motion, waiting, over-processing, defects—by bringing the lab to the process, eliminating sample prep, and consolidating multi-parameter assays into a single, one-minute measurement.

Benefits and practical applications

• Rapid turnaround (<1 min vs. >1 h) and one-step multi-parameter QC.
• Chemical-free operation; no reagents or hazardous waste.
• Simplified operator training and automated reporting.
• Return on investment in under one year through energy and material savings.

Future trends and potential applications

• Integration with Industry 4.0 and digital twins for predictive control.
• Advanced AI and chemometric algorithms for self-optimizing processes.
• Sensor miniaturization for broader inline deployment in small reactors.
• Extension to emerging polymer systems and bio‐based monomers.

Conclusion

Vibrational spectroscopy—NIR and Raman—offers a versatile, eco-friendly, and cost-effective toolset for lean manufacturing of polyurethane. By enabling real-time process monitoring and quality control, these techniques help reduce waste, lower production costs, and enhance product consistency across the entire value chain.

Reference

• [1] Ullmann's Polymers and Plastics: Products and Processes, Wiley-VCH, 2016.
• [2] Statista, Global polyurethane market size forecast 2016–2021, 2018.
• [3] Grand View Research, Polyurethane Market Analysis 2017–2025, 2017.
• [4] Metrohm Application Note AN-NIR-007: Near-infrared analysis of polyols, 2018.
• [5] Metrohm Application Note AN-NIR-006: Polyol quality control by NIR, 2018.
• [6] Metrohm Application Note AN-NIR-035: Hydroxyl number in polyols by Vis-NIR, 2018.
• [7] ASTM D6342-12(2017): Hydroxyl number by NIR, 2017.
• [8] ISO 15063:2011: Hydroxyl number determination of polyols by NIR, 2011.
• [9] Randall D., Lee S., The Polyurethanes Book, Wiley-VCH, 2003.
• [10] Metrohm Application Note AN-RS-001: Polymer identification by Raman, 2018.
• [11] Metrohm Application Note AN-RS-007: Master-batch analysis by Raman, 2018.
• [12] Metrohm Application Note AN-RS-008: Monomer identification by Raman, 2018.
• [13] Metrohm Application Bulletin AB-414: Vis-NIR polymer analysis, 2018.
• [14] Metrohm Process Application Note AN-PAN-1041: Inline NCO monitoring, 2018.
• [15] Holweg M., The genealogy of lean production, J. Operations Management, 2007.