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

In the context of increasing environmental regulations and the drive for circular economy, assessing the microscopic physical structures and mechanical properties of recycled polypropylene is crucial. A detailed understanding of how advanced recycling processes influence material performance enables more reliable reuse of plastics, especially in industries like automotive manufacturing.

Objectives and Study Overview

This study evaluates whether a previously developed multifaceted analysis—combining mechanical, thermal, and spectroscopic measurements—can be applied to practical recycled polypropylene from automotive offcuts without fillers. The goal is to verify changes in microstructure and correlate them with key physical properties.

Instrumentation Used

• Autograph AGX-V2 precision universal testing machine with TRViewX digital extensometer for tensile testing
• DUH-210 dynamic ultra micro hardness tester with Berkovich indenter for indentation hardness
• DSC-60Plus differential scanning calorimeter for crystallization and thermal events
• AIRsight infrared microscope for FTIR imaging and foreign matter identification

Methodology

Polypropylene pellets processed by an advanced recycling technology and control pellets were injection molded into ISO 527-2 1A dumbbell specimens. Tensile tests measured strain at break and elastic modulus. Hardness tests followed ISO/TS 19278. Thermal behavior was recorded during cooling to identify crystallization start temperatures. Infrared mapping assessed helical and parallel polymer structures and detected foreign matter.

Main Results and Discussion

Tensile tests revealed a lower strain at break and reduced elastic modulus in recycled samples, attributed partly to polyethylene contamination identified by FTIR. Indentation hardness also decreased. DSC analysis showed a decrease in crystallization start temperature, indicating delayed polymer chain motion and increased entanglements. FTIR imaging demonstrated a higher ratio of helical to parallel structures, supporting the conclusion of enhanced polymer relaxation and tie molecule formation. The multifaceted approach successfully linked microstructural changes to material properties, though mechanical strength was compromised by foreign matter.

Benefits and Practical Applications

• Enables comprehensive quality control of recycled plastics using accessible laboratory instruments
• Helps optimize recycling conditions by correlating processing parameters with microstructure and performance
• Allows early detection of contaminants that may impair mechanical properties

Future Trends and Opportunities

Future work may integrate energy dispersive X-ray spectroscopy for inorganic contaminant analysis, develop real-time monitoring of polymer structure during processing, and apply machine learning to predict material performance from multifaceted datasets. Such advances will further improve the reliability and sustainability of recycled plastic applications.

Conclusion

This study confirms the effectiveness of a combined mechanical, thermal, and spectroscopic evaluation to understand microstructural changes in recycled polypropylene. While foreign matter can offset mechanical benefits, the methodology provides a robust framework for quality assessment and process optimization in plastic recycling.

Reference

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