Evaluation of Polypropylene Reusability Using a Simple Mechanical Model Derived from Injection-Molded Products

Area of Science:

• Materials Science
• Polymer Science
• Sustainable Engineering

Background:

• Growing global plastic waste necessitates efficient recycling technologies for thermoplastics, particularly polypropylene (PP).
• Conventional mechanical property testing requires dedicated specimens, increasing material and energy costs.
• Sustainable material circulation for PP is crucial for environmental and economic reasons.

Purpose of the Study:

• To propose a simplified mechanical model for deriving Poisson's ratio and critical expansion stress from standard uniaxial tensile tests.
• To validate the model's applicability across various thermoplastics.
• To assess the impact of repeated extrusion on the mechanical properties of different PP types.

Main Methods:

• Development of a simplified mechanical model based on the true stress-true strain relationship in the small deformation region.
• Validation of the model using standard uniaxial tensile tests on polypropylene (PP), POM, PC, and ABS.
• Application of the model to Homo-PP and Block-PP after repeated extrusion cycles, supplemented by DSC and chemiluminescence analyses.

Main Results:

• The simplified model accurately derived mechanical properties for various thermoplastics, showing good agreement with existing literature.
• Both Homo-PP and Block-PP showed reduced elastic modulus and critical expansion stress after repeated extrusion.
• Block-PP exhibited a slower degradation rate due to thermo-crosslinking in its ethylene-propylene rubber (EPR) phase.

Conclusions:

• The proposed simplified model provides a practical and cost-effective method for evaluating mechanical properties of molded thermoplastics.
• This approach facilitates the assessment of recycled PP, supporting high-quality recycling initiatives.
• Understanding material degradation mechanisms, like changes in stereoregularity and radical formation, aids in advanced material design for recycled plastics.