Multifaceted Evaluation of Changes in Physical Properties of Recycled Plastics by Advanced Recycling Process and Influencing Microstructural Changes (Part 1): Example of Application to Container/Packaging-Derived Recycled Polyethylene

Significance of the Topic

Recycling of plastics has become a critical component of decarbonization and sustainable material management. Mechanical property deterioration, thermal changes and quality variations in recycled plastics can limit their reuse in high‐value applications. Understanding microscopic structural changes during advanced recycling processes is essential to restore or enhance the performance of recycled polyethylene (PE) and improve product safety and consistency.

Objectives and Study Overview

This study evaluated the effects of a novel advanced recycling process developed by Fukuoka University researchers on recycled PE derived from container and packaging waste. Key aims included:

• Comparing mechanical properties (static and high‐speed tensile behavior, hardness) of treated vs. untreated recycled PE.
• Investigating changes in crystallization behavior, phase distribution and microstructure using thermal analysis, FTIR mapping and scanning probe microscopy (SPM).
• Identifying microstructural factors (tie molecules, lamellar crystal orientation, phase separation) responsible for performance improvements.

Methodology

Recycled PE pellets (with ~20 % polypropylene contamination) were press-molded into sheets and die-cut to ISO 527-2 1B and ASTM D1822 Type L specimens. Evaluations included:

• Static tensile testing (elastic modulus, strain at break) using AGX-V2 Autograph with TRViewX digital video extensometer.
• High‐speed tensile tests (yield stress, break energy) on HITS-TX machine at strain rates of 1/s, 10/s and 100/s.
• Dynamic microhardness (HIT) using DUH-210 with a Berkovich indenter.
• Thermal analysis via DSC-60Plus to determine crystallization start temperature and heat of fusion.
• SPM-Nanoa imaging for elastic modulus distribution and lamellar crystal morphology.
• FTIR area mapping on AIRsight microscope to visualize PE/PP phase distribution.

Used Instrumentation

• Shimadzu AGX-V2 Autograph Precision Universal Testing Machine
• HITS-TX High‐Speed Tensile Testing Machine
• DUH-210 Dynamic Ultra Micro Hardness Tester
• DSC-60Plus Differential Scanning Calorimeter
• SPM-Nanoa Scanning Probe Microscope
• AIRsight Infrared Microscope (IR mode)

Main Results and Discussion

The advanced recycling process significantly enhanced ductility and toughness of recycled PE:

• Average strain at break increased from ~106 % to ~439 % under static load.
• High‐speed tensile tests showed 30 %–100 % higher break energy and 20 %–70 % greater strain at break across strain rates.
• Elastic modulus decreased by ~15 %, indicating a more compliant material.
• Indentation hardness (HIT) decreased from 34.3 MPa to 31.5 MPa, reflecting altered microstructure.
• DSC revealed a ~0.3 °C lower crystallization start temperature, suggesting increased polymer entanglements that delay crystallization.
• SPM analysis showed the proportion of intermediate tie‐molecule layers (100–200 MPa region) rose from 15.7 % to 26.8 %, while lamellar crystal orientation became less aligned.
• FTIR mapping demonstrated more uniform PE distribution and reduced localized PP segregation after the advanced recycling process.

These findings indicate two synergistic mechanisms: an increase in tie‐molecule‐rich intermediate layers enhances ductility, and improved continuity of the PE matrix reduces stress concentrators associated with PP agglomerates.

Benefits and Practical Applications

• Recycled plastics with restored or improved mechanical properties can replace virgin materials in packaging, automotive and consumer goods.
• Multifaceted evaluation using general‐purpose instruments enables rapid quality assessment in research and production laboratories.
• Knowledge of microstructural changes guides optimization of recycling process conditions to target desired performance attributes.

Future Trends and Possibilities

Advancements may include:

• Integration of Raman imaging with IR mapping for simultaneous chemical and structural analysis.
• In‐line monitoring tools based on microhardness or DSC modules for real‐time quality control during compounding.
• Application of machine learning to correlate multi‐instrument datasets and predict recycled polymer performance.
• Extension of advanced recycling protocols to multi‐polymer blends and composite materials.

Conclusion

The advanced recycling process markedly improves mechanical properties of recycled PE by modifying microstructure: increasing polymer entanglements and tie‐molecule layers while homogenizing phase distribution. A combined evaluation strategy—tensile testing, hardness, DSC, SPM and FTIR mapping—provides a sensitive, practical framework for characterizing recycled plastics and guiding process optimization.

References

• Yao S., Tominaga A. Novel Technology Development on Plastic Material Recycling. Haikibutsu Shigen Junkan Gakkaishi. 2018;29(2):116–124.
• Japan Institute of Energy. Present and Future of Waste Plastics: Plastic Resource Circulation in Sustainable Society. 2024.