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

Importance of the Topic

Recycled polyethylene terephthalate (rPET) plays a critical role in sustainable plastic reuse and quality control of recycled materials.
The intrinsic viscosity (IV) of rPET reflects chain length and molecular weight, directly influencing mechanical properties and processing behavior.
Traditional IV determination involves lengthy drying, solvent use, and manual viscometry, creating bottlenecks and chemical waste.

Study Objectives and Overview

This application note evaluates a rapid, solvent-free approach to quantify rPET IV using visible and near-infrared (Vis-NIR) spectroscopy.
The primary goals were to develop a robust calibration model correlating NIR spectra with standard viscometer values (ASTM D4603) and to demonstrate measurement in under one minute without sample pretreatment.

Methodology and Instrumentation

A set of 48 rPET samples covering a wide IV range served as calibration and validation material.
Each sample was measured in duplicate using:

• DS2500 Solid Analyzer operating from 400 to 2500 nm
• Large sample cup accessory to minimize particle size effects
• Vision Air Complete software for data acquisition and chemometric model building
• Glass capillary viscometer (ASTM D4603) as the reference method

Spectral data were preprocessed and multivariate regression models were developed to predict IV values directly from raw spectra.

Main Results and Discussion

The Vis-NIR calibration model achieved:

• Coefficient of determination (R2) = 0.9061
• Standard error of calibration (SEC) = 0.0068 dL/g
• Standard error of cross-validation (SECV) = 0.0084 dL/g

High correlation between predicted and laboratory IV values confirms model reliability.
The large sample cup provided reproducible spectra despite variations in pellet size and crystallinity.

Benefits and Practical Applications

The NIR-based method offers several advantages over conventional viscometry:

• Measurement time below one minute, eliminating lengthy drying and dissolution steps
• Chemical-free operation enhances laboratory safety and reduces disposal costs
• Minimal sample preparation enables direct analysis of bulk pellets
• Improved throughput supports routine quality control and sorting in recycling plants

Future Trends and Potential Applications

Integration of advanced chemometric algorithms and machine learning could further improve prediction accuracy.
Extension of NIR methods to additional quality parameters such as diethylene glycol and isophthalic acid content is feasible.
On-line and at-line NIR sensors may enable real-time monitoring within extrusion or pelletizing processes.

Conclusion

The Metrohm DS2500 Solid Analyzer combined with Vis-NIR spectroscopy provides a rapid, precise, and solvent-free alternative for intrinsic viscosity determination of rPET.
This approach streamlines quality control workflows, enhances safety, and supports sustainable recycling operations.

Reference

Metrohm Application Note AN-NIR-112: "Intrinsic viscosity (IV) determination of recycled PET by NIR spectroscopy"