Quality Control of PET

Significance of the Topic

Accurate and rapid quality control of polyethylene terephthalate (PET) is essential for ensuring consistent material performance in packaging, textiles, and engineering applications. Traditional wet chemical analyses for key parameters such as diethylene glycol content, isophthalic acid content, intrinsic viscosity, and acid number are time-consuming, require extensive sample preparation, and generate chemical waste. Near-infrared spectroscopy (NIRS) offers a reagent-free, non-destructive alternative capable of simultaneous multi-parameter evaluation in under one minute, greatly improving laboratory efficiency and production throughput.

Objectives and Study Overview

This application note aims to demonstrate the capability of the Metrohm DS2500 Solid Analyzer operating in the visible and near-infrared range (400–2500 nm) for rapid, simultaneous determination of:

• Diethylene glycol content
• Isophthalic acid content
• Intrinsic viscosity
• Acid number

Measurements were performed directly on PET pellets without any sample preparation or chemical reagents, and calibration models were developed using Vision Air Complete software.

Methodology and Instrumentation

Reflectance spectra of PET pellets were acquired in a rotating large sample cup to minimize particle size effects and ensure spectral reproducibility. Key instrumentation:

• DS2500 Solid Analyzer (400–2500 nm)
• DS2500 Large Sample Cup for automated, multi-position sampling
• Vision Air 2.0 Complete software for data acquisition and multivariate model development

No sample dissolution or reagent addition was required, enabling true at-line and laboratory-based analyses.

Main Results and Discussion

Partial least squares regression models were generated for each analyte and evaluated by calibration and cross-validation figures of merit:

• Diethylene glycol: R² = 0.931, SE of cross-validation = 0.066 % (w/w)
• Isophthalic acid: R² = 0.995, SE of cross-validation = 0.085 % (w/w)
• Intrinsic viscosity: R² = 0.873, SE of cross-validation = 0.0238 dL/g
• Acid number: R² = 0.991, SE of cross-validation = 0.143 mg KOH/g

Correlation diagrams showed strong agreement between NIRS predictions and reference methods (HPLC-MS, HPLC, viscometry, titration). Compared with conventional workflows requiring 45–90 minutes of preparation plus analysis, NIRS delivers all results in less than one minute.

Benefits and Practical Applications

• Drastic reduction of analysis time and labor
• Elimination of chemical reagents and waste
• High throughput screening in production or QC laboratories
• Non-destructive measurement enabling sample reuse
• Robust performance under varying environmental conditions

Future Trends and Opportunities

Advancements may include integration of inline NIRS sensors in production lines for real-time process control, application of advanced machine learning algorithms for enhanced calibration robustness, expansion to other polymer types and quality attributes, and cloud-based data management for remote monitoring and predictive maintenance.

Conclusion

The Metrohm DS2500 Solid Analyzer provides a fast, reliable, and sustainable solution for simultaneous determination of critical quality parameters in PET. By replacing traditional wet chemical methods with NIRS, laboratories and manufacturers can achieve significant time savings, reduce costs, and minimize environmental impact while maintaining high analytical performance.

References

• Metrohm AG. Quality Control of PET: Determination of diethylene glycol, isophthalic acid, intrinsic viscosity, and acid number within one minute with NIRS. Application Note AN-NIR-023.