Analysis of polymers using near-infrared spectroscopy

Importance of the Topic

Near-infrared spectroscopy (NIR) offers a rapid, non-destructive and reagent-free approach to characterize polymers and plastic materials. By exploiting the sensitivity of NIR to fundamental molecular bonds such as O–H, C–H and other functional groups, NIR enables online, atline, inline or offline monitoring of chemical and physical attributes during polymer synthesis, processing and quality control.

Objectives and Overview

This bulletin compiles over 120 application examples demonstrating Metrohm NIR analyzers in polymer analysis. It covers a wide range of target parameters including monomer and additive concentrations, hydroxyl and acid numbers, melt flow and melt index, density, viscosity, degree of cure, moisture, end‐group analysis and film thickness determination. Applications span polyols, polyesters, polyurethanes, polyolefins (PP, PE, PVC, PET etc.), polystyrene, polylactic acid, epoxy and phenolic resins, coatings on glass and other specialty polymers.

Methodology and Instrumentation

• Sample presentation modes: transmission (liquids, films) and reflectance (powders, pellets, fibers, plates, bottles, wafers).
• Temperature control where needed (e.g. heating polyols to 30–65 °C, controlling reaction vessels at 220 °C, ovens for curing studies).
• Optical configurations: fiber-optic interactance probes, fiber-optic transmission pairs, remote reflectance units, spinning and rapid content sample cups.
• Instrument platforms: Metrohm NIRS Model 5000 (discontinued), DS2500 Solid Analyzer, XDS RapidLiquid Analyzer, XDS SmartProbe and XDS Process Analyzer series.
• Data treatment: single-wavelength calibrations, divisor normalizations for pathlength correction, multiple linear regression and multivariate PLS models with tailored spectral ranges (typically 1100–2500 nm).

Main Results and Discussion

• Functional group analysis: Hydroxyl numbers in polyols, polyesters, esters and synthetic polymers were measured within ±0.1–2 OH units using bands near 1400–2100 nm.
• End‐group determination: Carboxyl and amine end‐groups in polyesters and polyurethanes demonstrated detection limits of ±0.2–5 units.
• Additive quantification: Antioxidants, plasticizers, chain extenders (MOCA), irganox/irgafos stabilizers, erucamide, vinyl pyrrolidone and other low‐level additives down to hundreds of ppm were accurately determined.
• Polymer blend and copolymer composition: Ethylene, propylene, vinyl acetate, methyl acrylate and styrene comonomers were monitored in PP, PE and EVA copolymers, with errors typically <1% (w/w).
• Physical property monitoring: Melt flow/index and viscosity of polyolefins and resins were predicted with standard errors comparable to reference methods; density and molecular weight estimation via hydroxyl content allowed inline process feedback.
• Reaction and cure monitoring: Real‐time tracking of polyurethane reactions, epoxy and phenolic resin cure via dedicated fiber probes enabled direct endpoint detection and kinetic profiling.
• Inspection and quality control: Glass coating levels, film thickness, bottle and film identification, moisture, extractables and defect detection in fibers and textiles were performed through spectral libraries and matching algorithms.

Benefits and Practical Applications

• High throughput: Measurements in seconds without sample preparation accelerate quality decisions and reduce bottlenecks in production.
• Non‐destructive and green: Eliminates reagents and waste, offering sustainable testing aligned with zero‐waste goals.
• Inline and atline integration: Fiber optic probes and process analyzers enable continuous process monitoring for endpoint control, reducing overprocessing and scrap.
• Cost savings: Minimizes laboratory analyses and operational downtime, leading to reduced operational expenses and faster time to market.
• Versatility: One NIR platform can address multiple parameters across diverse polymer types, simplifying instrument fleets.

Future Trends and Applications

• Deeper multivariate analytics: Integration of advanced chemometrics and AI-driven algorithms to improve calibration robustness across broader polymer families and process conditions.
• Process digitalization: Embedded NIR sensors in continuous production lines and smart factories for real-time data streams, predictive maintenance and adaptive control.
• Miniaturization and portability: Development of handheld and OEM-integrated NIR modules for field testing and mobile quality assurance.
• Regulatory compliance: Adoption of NIR in pharmaceutical and food-contact polymer production for rapid GMP and HACCP verification.
• Emerging polymers: Expansion into bioplastics, recycled blends and novel composites where rapid composition and property profiling is critical.

Conclusion

Near-infrared spectroscopy has proven to be a powerful tool for comprehensive polymer analysis in research and industrial settings. By enabling rapid, non-destructive and multi-parameter monitoring, NIR improves process yield, product consistency and operational efficiency. As chemometric methods and sensor technologies advance, NIR will continue to expand its role in polymer synthesis, processing and sustainability initiatives.

Used Instrumentation

• Metrohm NIRS Model 5000 with transmission, reflectance and process modules
• DS2500 Solid Analyzer and XDS RapidLiquid Analyzer configurations
• XDS SmartProbe fiber-optic bundles and XDS Process Analyzer bundles for atline and inline integration
• Coarse and fine sample cells, spinning modules and remote reflectance interfaces