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

Importance of the Topic

The growing emphasis on decarbonization and circular economy has spurred initiatives to recycle plastics. Mechanical degradation, quality variability, and cost challenges hinder broader adoption of recycled polyethylene. Understanding microscopic structural changes that drive mechanical property deterioration is critical to ensure safety and product performance.

Objectives and Study Overview

This study evaluated the impact of an advanced recycling process on mechanical and thermal properties of container and packaging-derived recycled polyethylene. Multifaceted assessments were conducted to clarify factors behind property improvements, focusing on strain at break, elastic modulus, hardness, impact behavior, crystallization, microstructure, and phase distribution.

Methodology

Recycled polyethylene pellets with and without advanced recycling treatment were pressed into sheets and die-cut into ISO 527-2 1B and ASTM D1822 Type L specimens. Static and high-speed tensile tests, hardness measurements, differential scanning calorimetry (DSC), scanning probe microscopy (SPM), and Fourier transform infrared spectroscopy (FTIR) mapping were performed to characterize mechanical performance, thermal transitions, microstructure, and component dispersion.

Used Instrumentation

• AGX-V2 Autograph Precision Universal Testing Machine with 500 N load cell and TRViewX extensometer
• HITS-TX High Speed Tensile Testing Machine
• DUH-210 Dynamic Ultra Micro Hardness Tester with Berkovich indenter
• DSC-60 Plus Differential Scanning Calorimeter
• SPM-Nanoa Scanning Probe Microscope
• AIRsight Infrared Microscope for FTIR mapping

Key Results and Discussion

Static tensile tests revealed a threefold increase in strain at break (average from ~106% to ~439%) and a decrease in elastic modulus (average from ~556 MPa to ~472 MPa) with the advanced process. Hardness (HIT) decreased significantly, indicating enhanced ductility. High-speed tensile tests showed higher break energy and strain at break at strain rates of 1, 10, and 100 s−1. DSC cooling curves exhibited a lower crystallization start temperature by ~0.3 °C, suggesting increased polymer entanglements. SPM elastic modulus mapping demonstrated a 71% increase in intermediate tie-layer area, while crystalline layer proportions remained constant. FTIR chemical imaging indicated more uniform PP dispersion after recycling treatment.

Benefits and Practical Applications

The advanced recycling process enhances toughness and impact resistance of recycled polyethylene, addressing key barriers to its use in safety-critical applications. Multifaceted evaluation techniques provide sensitive, practical screening tools for quality control, process optimization, and simulation input for design validation.

Future Trends and Potential Applications

Integration of advanced microstructural analyses with in-line monitoring and machine learning may enable real-time quality assurance. Expanding the approach to other polymers and composite systems could broaden sustainable materials adoption. Coupling recycling processes with life cycle assessment and digital twins will support optimization of supply chains and environmental performance.

Conclusion

This work demonstrates that the advanced recycling process significantly improves mechanical performance of recycled polyethylene by promoting polymer entanglements and uniform phase distribution without altering crystallinity. A combination of tensile testing, hardness, DSC, SPM, and FTIR mapping offers a robust framework for elucidating microstructure–property relationships in recycled plastics.

Reference

• Yao S, Tominaga A. Novel Technology Development on Plastic Material Recycling. Haikibutsu Shigen Junkan Gakkaishi. 2018;29(2):116–124.
• Present and Future of Waste Plastics: Plastic Resource Circulation in Sustainable Society. The Japan Institute of Energy. 2018. p 147.