Quality Control of PET

Importance of the Topic

The rapid and accurate quality control of polyethylene terephthalate (PET) is essential for industrial production, research laboratories, and quality assurance in polymer manufacturing. Traditional wet-chemical methods for determining diethylene glycol, isophthalic acid, intrinsic viscosity, and acid number are laborious, time-consuming, and require reagents. Implementing near-infrared spectroscopy (NIRS) accelerates analysis, reduces cost, and minimizes sample handling while maintaining reliable results.

Study Aims and Overview

This application note evaluates the use of a DS2500 Solid Analyzer operating in the 400–2500 nm range for simultaneous determination of four key PET quality parameters. The goal is to develop prediction models that match reference method performance, deliver results in under one minute, and eliminate sample preparation and chemical reagents.

Methodology and Instrumentation

Measurements were performed in reflection mode using a rotating large sample cup to accommodate heterogeneous pellet sizes and ensure reproducible spectra. The setup included:

• DS2500 Solid Analyzer (400–2500 nm range)
• DS2500 Large Sample Cup (rotating cup for solids)
• Vision Air Complete software for data acquisition and model development

Spectra were collected directly on PET pellets without grinding or dissolution, followed by chemometric calibration against reference values obtained by HPLC-MS (diethylene glycol), HPLC (isophthalic acid), viscometry (intrinsic viscosity) and titration (acid number).

Main Results and Discussion

Prediction models exhibited strong correlations with laboratory reference methods:

• Diethylene glycol: R² = 0.931, SECV = 0.066 %
• Isophthalic acid: R² = 0.995, SECV = 0.085 %
• Intrinsic viscosity: R² = 0.873, SECV = 0.0238 dL/g
• Acid number: R² = 0.991, SECV = 0.143

Correlation diagrams confirmed that Vis-NIR predictions closely follow primary method values across the calibration range. The chemometric approach successfully captured subtle spectral variations associated with chemical composition and polymer chain properties.

Benefits and Practical Applications

The NIRS method delivers all four quality parameters in a single, sub-minute measurement without reagents or sample preparation. Compared to conventional workflows, total analysis time is reduced from over two hours to under one minute, enabling high-throughput screening, at‐line monitoring in production, and rapid feedback for process control.

Future Trends and Applications

Advancements in NIRS instrumentation and chemometric algorithms will further enhance sensitivity and extend applicability to other polymers and composites. Potential developments include:

• Integration in process analytical technology (PAT) for real-time monitoring
• Expansion to multicomponent recycling streams and sustainability assessments
• Machine-learning models to improve robustness against sample variability

Conclusion

This study demonstrates that Vis-NIR spectroscopy with the DS2500 Solid Analyzer offers a fast, cost-efficient, and robust alternative to classical wet-chemical methods for comprehensive PET quality control. The approach supports high-throughput operations and aligns with industry demands for rapid, sustainable analytics.

References

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