Multifaceted Evaluation of Changes in Physical Properties of Recycled Plastics by Advanced Recycling Process and Influencing Microstructural Changes (Part 2): Example of Application to Simulated Degraded Polypropylene

Significance of the Topic

Recycling of polypropylene and other plastics is critical for sustainable resource management. Understanding how advanced recycling processes alter both bulk mechanical properties and microscopic physical structures enables the production of recycled materials with performance comparable to virgin polymers. Multifaceted analytical evaluations reveal structure–property relationships that guide process optimization and quality control.

Objectives and Study Overview

This study examines simulated degraded polypropylene (PP) treated with an advanced recycling process. By applying a suite of complementary techniques, the aim was to quantify changes in strain at break, impact energy, hardness, crystallization behavior, polymer conformation and orientation, and to correlate these findings with microstructural modifications induced by the recycling treatment.

Methodology and Used Instrumentation

Virgin homopolypropylene was subjected to multiple shear degradations in a kneading machine, followed by the advanced recycling protocol. Injection-molded test specimens conforming to ISO 527-2 1A and ASTM D1822 Type L were prepared.

The following analyses were performed:

• Static tensile testing (AGX-V2 Autograph universal tester with TRViewX digital extensometer)
• High-speed tensile (HITS-TX high-speed tensile tester)
• Dynamic microhardness (DUH-210 ultra micro hardness tester)
• Thermal analysis (DSC-60 Plus differential scanning calorimeter)
• Infrared mapping (IRXross and AIRsight infrared/Raman microscope)
• Scanning probe microscopy (SPM-Nanoa for adhesion/orientation imaging)

Main Results and Discussion

Static tensile tests showed that recycled PP treated with the advanced process achieved an average strain at break of 601.7% versus 570.1% without treatment, while elastic modulus decreased from 1649 MPa to 1612 MPa. High-speed tensile testing confirmed increased break energy and elongation under dynamic loading. Microhardness decreased from 74.0 MPa to 70.3 MPa. DSC revealed a slight reduction in crystallization start temperature (122.06 °C to 121.64 °C), indicating delayed crystal formation. FTIR mapping demonstrated an increased ratio of helical to parallel polymer structures in recycled samples, suggesting enhanced molecular relaxation. SPM adhesion images showed more uniform micro-orientation, implying moderated shear alignment due to the recycling treatment. These findings support a mechanism of increased polymer entanglements and formation of intermediate tie-molecule layers, leading to improved toughness.

Benefits and Practical Applications

This multifaceted evaluation framework facilitates deeper insight into how advanced recycling modifies polymer microstructure and performance. The approach can guide development of recycling protocols that restore mechanical properties, support quality assurance in industrial recycling operations, and aid formulation of recycled PP grades for demanding applications.

Future Trends and Possibilities

Emerging directions include applying similar analyses to other polymer types, integrating inline spectroscopic and thermal sensors for real-time process monitoring, and leveraging data analytics or machine learning to predict structure–property outcomes. Continued refinement of advanced recycling technologies promises higher-quality recycled plastics and broader circular economy impact.

Conclusion

The advanced recycling process significantly enhanced strain at break and impact energy of simulated degraded PP while moderately reducing stiffness and hardness. Multifaceted testing linked these improvements to increased polymer entanglements, delayed crystallization, and more uniform micro-orientation. This comprehensive evaluation methodology offers a powerful tool for optimizing recycling processes and ensuring high performance of recycled polymers.

Reference

1) Present and future of waste plastics: plastic resource circulation in sustainable society, The Japan Institute of Energy, p. 147