Polyethylene content determination in polypropylene pellets by NIRS

Importance of the topic

Efficient recycling of polyolefins is critical to reduce plastic waste and support a circular economy.
Separation of polyethylene (PE) and polypropylene (PP) streams remains challenging, and contamination of PP with PE can degrade mechanical performance of recycled pellets.
Rapid, accurate quantification of PE in PP feedstock improves quality control and process optimization in polymer recycling.

Goals and overview of the study

This application note describes development and validation of a near-infrared spectroscopy (NIRS) method to determine PE content in PP pellets.
Primary objectives:

• Produce a calibration set of polymer blends with 0.5–35 % PE in PP.
• Acquire NIR spectra and build predictive models for full and narrow PE content ranges.
• Compare analysis time and performance against the standard DSC method.

Methodology

Fifty-four polymer blends were prepared in a compounder, covering PE contents from 0.5 to 35 % by weight.
Reference PE concentration was established gravimetrically before blending.
NIR spectra were recorded in diffuse reflectance mode under rotation to average subsample variability and mitigate particle size effects.
Two calibration models were built: one for the full PE range and one focused on low-level contamination (0.5–9.0 %).

Used instrumentation

• OMNIS NIR Analyzer Solid
• Large holder OMNIS NIR, 100 mm
• Large cup OMNIS NIR, 100 mm
• OMNIS Stand-Alone software license
• Quant Development software license

Main results and discussion

The full-range calibration (0.5–35 % PE) achieved R²CV = 0.987 with an SECV of 1.10 %.
Focusing on the 0.5–9.0 % region improved error performance (R²CV = 0.950, SECV = 0.60 %).
Correlation diagrams demonstrate excellent linearity between NIR predictions and reference values.
Comparison with DSC reveals that NIRS completes a measurement in seconds, whereas DSC requires approximately 60 minutes per sample.

Benefits and practical applications of the method

Rapid, non-destructive measurement of low-level PE contamination in PP supports real-time process control in recycling plants.
Minimal sample preparation and automated multi-position measurements ensure reproducible results.
NIRS can be extended to determine additional physical parameters, such as density, further enhancing quality assessment.

Future trends and potential applications

Integration of advanced chemometric algorithms and machine learning will further refine prediction accuracy and adaptivity.
Inline and at-line NIR monitoring can enable continuous process feedback and automated sorting of mixed polyolefins.
Miniaturized and handheld NIR devices may facilitate field-based quality checks and rapid screening in decentralized recycling facilities.

Conclusion

This study validates NIRS as a fast, accurate alternative to DSC for quantifying PE in PP pellets.
The method delivers high correlation with reference data, significant time savings, and expanded analytical capabilities.
Implementation of this approach can streamline quality control in polymer recycling and contribute to sustainable materials management.

References

Application Note AN-NIR-124: Polyethylene content determination in polypropylene pellets by NIRS