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

Importance of the Topic

Recycled PET quality control relies on accurate intrinsic viscosity (IV) measurements to ensure polymer performance in downstream applications. Traditional glass capillary viscometry methods require hours of sample drying, use of solvents, and elaborate lab procedures.

Objectives and Study Overview

This application note demonstrates the development of a rapid, chemical-free VIS–NIR spectroscopic method for predicting the intrinsic viscosity of recycled PET (rPET) using the Metrohm DS2500 Solid Analyzer. The goal was to achieve sub-minute analysis without sample pretreatment and to validate the approach against the ASTM D4603 reference method.

Methodology and Instrumentation

Measurements were performed on 48 rPET samples with varying IV values. Key experimental details include:

• Metrohm DS2500 Solid Analyzer operating in the 400–2500 nm range
• Large sample cup to rotate and average spectra, mitigating particle size effects
• Reference IV determination by glass capillary viscometer following ASTM D4603 (including 2–3 h drying and dissolution)
• Vision Air Complete software for spectral acquisition, calibration, and prediction model development

Main Results and Discussion

The VIS–NIR calibration model achieved an R2 of 0.9061 versus reference viscosities. Figures of merit were:

• Standard Error of Calibration (SEC): 0.0068 dL/g
• Standard Error of Cross-Validation (SECV): 0.0084 dL/g

Correlation plots indicated excellent agreement between predicted and laboratory values, confirming the method’s robustness for routine analysis.

Benefits and Practical Applications

Compared to conventional viscometry, the VIS–NIR approach offers:

• Results in less than one minute without any chemical reagents or sample drying
• Elimination of hazardous solvents, improving lab safety and reducing waste
• Capability to extend calibrations for additional rPET parameters such as diethylene glycol and isophthalic acid

Future Trends and Potential Applications

Further developments may include:

• Expansion of spectral libraries for multi-component quantification in recycled polymers
• Integration of VIS–NIR modules into production lines for real-time IV monitoring and process control
• Automation with sampling robots and inline probes for high-throughput quality assurance

Conclusion

The Metrohm DS2500 Solid Analyzer combined with VIS–NIR spectroscopy provides a fast, accurate, and solvent-free alternative for intrinsic viscosity determination in rPET, streamlining QC workflows and enhancing laboratory safety.

References