Multifaceted Evaluation of Changes in Physical Properties of Recycled Plastics by Advanced Recycling Process and Influencing Microstructural Changes (Part 3): Example of Application to Recycled Polypropylene Derived from Automotive Offcuts without Fillers

Significance of the Topic

Recycling of polypropylene (PP) from automotive offcuts addresses environmental and regulatory pressures in the plastics industry by enabling material circularity and reducing waste. Assessing the microscopic physical structure of recycled PP is critical for ensuring performance in end-use applications and supporting global advanced recycling initiatives.

Objectives and Study Overview

This study applies a multifaceted evaluation methodology to real recycled PP derived from automotive offcuts without fillers. The goals are to verify whether combined mechanical, thermal, and spectroscopic analyses can reveal microstructural changes induced by an advanced recycling process and to assess the impact of any foreign matter on material properties.

Methodology and Instrumentation

Sample Preparation
Recycled PP pellets processed by an advanced recycling method and control pellets (without the process) were injection-molded into ISO 527-2 1A dumbbell specimens.

Instrumentation Used

• Autograph AGX-V2 with 5 kN load cell and TRViewX 500D extensometer for tensile testing
• DUH-210 Dynamic Ultra Micro Hardness Tester for indentation hardness (HIT)
• DSC-60 Plus Differential Scanning Calorimeter for crystallization analysis
• AIRsight Infrared and Raman Microscope in transmission mode for FTIR mapping

Main Results and Discussion

• Tensile testing revealed a decrease in strain at break (≈27.7 % vs 42.8 %) and elastic modulus (≈1399 MPa vs 1421 MPa) after recycling.
• Indentation hardness decreased (≈55.6 MPa vs 58.4 MPa) in recycled PP, indicating softer microstructure.
• DSC showed a reduction in crystallization start temperature (130.66 °C vs 131.45 °C), suggesting delayed nucleation due to increased chain entanglements.
• FTIR detected polyethylene (PE) aggregates as foreign matter, which likely contributed to reduced ductility.
• Infrared mapping demonstrated an elevated ratio of helical to parallel structures in recycled samples, confirming enhanced polymer relaxation and tie-molecule formation.

Benefits and Practical Applications

• Provides a comprehensive framework to characterize the microstructure of recycled plastics for quality control.
• Enables identification of process-induced changes and contaminant effects, guiding recycling optimization.
• Supports automotive and industrial stakeholders in validating recycled PP performance for structural applications.

Future Trends and Potential Applications

• Integration of energy dispersive X-ray fluorescence (EDX) to detect inorganic contaminants alongside FTIR mapping.
• Development of inline analytical tools and AI-driven data interpretation for real-time process monitoring.
• Extension of multifaceted evaluation protocols to other polymer types and composite materials.
• Advancement of regulatory frameworks promoting certified recycled content in automotive and consumer markets.

Conclusion

The combined use of mechanical testing, thermal analysis, and FTIR mapping effectively elucidates microstructural changes in recycled PP, even in the presence of foreign aggregates. This approach supports the development and implementation of advanced recycling processes to deliver high-performance recycled plastics.

Reference

1) The Japan Institute of Energy, Present and Future of Waste Plastics: Plastic Resource Circulation in a Sustainable Society, p. 147, 2025.