Evaluation of Microscopic Foreign Matter in Recycled Plastics Using Dynamic Image Analysis, Infrared Microscopy, and SEM-EDS

Importance of the Topic

Quantitative assessment of microscopic foreign matter in recycled polyethylene terephthalate (PET) is critical for ensuring pellet quality and reliable downstream processing. Foreign particles can cause filter clogging, increased melt pressure, and product defects such as mold bursts. Accurate measurement of particle size, concentration, morphology, and composition supports optimization of removal steps and improves overall yield and product integrity.

Objectives and Study Overview

This study compared virgin and post-consumer recycled PET pellets by:

• Dissolving pellets in hexafluoro-2-propanol (HFIP) to isolate insoluble particles.
• Measuring particle size distributions and concentrations using dynamic particle image analysis (iSpect DIA-10).
• Characterizing morphology and chemical composition via Fourier transform infrared microscopy (IRTracer-100 and AIM-9000) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS).

Methodology and Instrumentation

• Sample Preparation:
  100 mg/mL suspensions of virgin and recycled PET were prepared by adding HFIP at room temperature. Virgin pellets dissolved within four days, recycled pellets within six hours, leaving insoluble particles dispersed in solution.


• Dynamic Particle Image Analysis:
  Instrument: iSpect DIA-10
  Analysis conditions: 8 fps frame rate; 96.5 % efficiency; 250 µL pump volume; 216 threshold; background correction enabled. Control measurements with pure HFIP assessed baseline contributions.


• Particle Collection for Microscopy:
  Suspensions diluted tenfold and filtered through silver membrane filters. Particles on filters were observed with a digital microscope and selected for further analysis.


• Infrared Microscopy:
  Instruments: IRTracer-100 and AIM-9000
  Resolution 8 cm⁻¹; 100 scans; SqrTriangle apodization; 700–4000 cm⁻¹ wavenumber range; liquid nitrogen-cooled MCT detector.


• SEM-EDS Analysis:
  Instrument: ProX (Phenom World). Used to confirm elemental composition of inorganic or metallic particles identified by infrared microscopy images and spectra.

Main Results and Discussion

Dynamic image analysis revealed a dramatic difference in particle concentration:

• Virgin PET: ~286 counts/mL (5–10 μm dominant).
• Recycled PET: ~20 240 counts/mL (5–10 μm dominant, with some up to 40 μm).

Particle images displayed variable brightness and shape, reflecting refractive index contrast and orientation in the flow cell. Infrared microscopy of virgin-derived particles identified amide-type compounds and cellulose, likely introduced during handling. Recycled PET samples contained inorganic and metallic fragments:

• Carbonate-like particles (C, Ca, O peaks in EDS).
• Aluminum shards (metallic luster and Al peak in EDS).

Morphological similarity between dynamic analysis images and particles characterized by microscopy suggests potential for non-targeted composition inference directly from DIA imagery.

Benefits and Practical Applications

• Rapid, quantitative evaluation of particle size and count using minimal sample quantities.
• Data-driven selection and scheduling of filter replacements to prevent clogging.
• Identification of foreign matter origins to refine recycling and handling protocols.
• Enhanced quality control for recycled-content PET in molding and extrusion operations.

Future Trends and Potential Applications

Integration of automated classification algorithms and machine learning with dynamic image analysis could enable real-time monitoring of foreign matter during recycling. Advances in high-throughput infrared microscopy and in situ SEM-EDS may further streamline contamination profiling. Extending this combined workflow to other polymers and composites will support broader applications in plastics recycling, quality assurance, and regulatory compliance.

Conclusion

The combined use of iSpect DIA-10, infrared microscopy, and SEM-EDS provides a robust platform for quantitative and qualitative evaluation of microscopic foreign matter in PET pellets. This approach enables clear differentiation between virgin and recycled material contaminants, informs process optimization, and supports improved recycled plastic product quality.