Intrinsic viscosity (IV) determination of recycled PET by NIR spectroscopy

Importance of the topic

Intrinsic viscosity (IV) is a key parameter reflecting the molecular weight and quality of recycled polyethylene terephthalate (rPET). Accurate IV determination is critical for assessing polymer properties, ensuring consistent product performance, and optimizing recycling processes. Traditional viscometric methods demand lengthy sample drying, dissolution, and use of hazardous solvents, creating bottlenecks in high‐throughput quality control.

Objectives and Study Overview

This study aimed to develop a rapid, solvent‐free approach for IV measurement of rPET using visible and near‐infrared (Vis-NIR) spectroscopy. By comparing results from a Metrohm DS2500 Solid Analyzer against the standard glass capillary viscometer (ASTM D4603), the work evaluates feasibility, speed, and precision of the NIR method for routine laboratory analysis.

Methodology

48 rPET samples spanning a range of intrinsic viscosities were analyzed. Each sample was placed in the DS2500 large sample cup and scanned in rotation to average particle‐size effects. Spectral data in the 400–2500 nm range were captured in under a minute per sample. Reference IV values were obtained via glass capillary viscometry following ASTM D4603, including pre‐analysis drying to constant weight. Multivariate calibration and cross validation were conducted using Vision Air 2.0 Complete software to build predictive models correlating Vis-NIR spectra with reference IV values.

Instrumentation

• Metrohm DS2500 Solid Analyzer operating in Vis-NIR region
• DS2500 large sample cup for bulk polymer pellets
• Vision Air 2.0 Complete software for data acquisition and model development
• Glass capillary viscometer compliant with ASTM D4603 standard

Main Results and Discussion

The Vis-NIR calibration model achieved a correlation coefficient (R²) of 0.9061 against reference IV values, with a standard error of calibration (SEC) of 0.0068 dL/g and cross-validation error (SECV) of 0.0084 dL/g. These figures of merit demonstrate precision suitable for routine QC. The NIR method eliminates lengthy drying and solvent steps, delivering results in under one minute, compared to several hours required for the traditional viscometric approach.

Benefits and Practical Applications

• Chemical‐free, rapid analysis boosts laboratory throughput
• Minimal sample preparation enhances safety and reduces waste
• High reproducibility and accuracy support consistent recycled polymer quality
• Scalable to production environments for inline or at‐line monitoring

Future Trends and Potential Applications

Vis-NIR spectroscopy can be extended to quantify other additives or degradation markers in recycled plastics, such as diethylene glycol or isophthalic acid. Integration with automated sampling robots and process analytical technology (PAT) frameworks will enable real-time monitoring in recycling plants. Advances in machine learning may further refine predictive models and facilitate rapid method transfer between instruments and sites. Portable NIR devices may offer decentralized field‐testing capabilities for incoming feedstock screening.

Conclusion

The Metrohm DS2500 Solid Analyzer provides a fast, reliable, and solvent‐free method for intrinsic viscosity determination of rPET, significantly reducing analysis time and operational complexity. The demonstrated model performance and operational advantages make Vis-NIR spectroscopy an attractive alternative for quality control in polymer recycling.

References

• ASTM D4603: Standard Test Method for Intrinsic Viscosity of Polyethylene Terephthalate (PET).