Upcycling Post-Industrial Recycled PP for Injection Molding Applications

As industries strive to integrate recycled materials into mainstream manufacturing, researchers continue to explore ways to enhance the usability of post-industrial recycled polypropylene (PIR-PP).

Initially tailored for extrusion, PIR-PP often lacks the flow characteristics required for injection molding. In a study presented at ANTEC® 2025, Joshua Voll and his team addressed this challenge by investigating how to modify PIR-PP’s melt flow rate (MFR) to make it suitable for injection molding without sacrificing key mechanical properties.

You can also read: Post-Industrial Recycled Content Means Better Plastic.

Tailoring Flow with Additives and Processing

The base material for this study was PIR-PP, with a low MFR of 4.4 g/10 min, which is ideal for extrusion but inadequate for injection molding, as it typically requires an MFR closer to 30 g/10 min. The team tested three methods to boost flow:

1. Thermal-oxidative (TO) degradation

2. Peroxide masterbatch additive (1.2 wt%)

3. bFI masterbatch – a novel PMMA-based flow enhancer (tested at 5–15 wt%)

All materials were processed using a twin-screw extruder, and researchers evaluated the changes in rheological and mechanical properties through a series of tests, including melt flow index, amplitude and frequency sweeps, and mechanical impact and tensile strength tests.

Rheological Improvements and Flow Performance

Each method increased the MFR of PIR-PP to around 30 g/10 min, making it suitable for injection molding applications. Among them, the bFI-modified compound achieved the highest MFR at 33.3 g/10 min, indicating the most significant improvement in flow. Despite its high flow rate, bFI samples exhibited greater storage and loss moduli compared to TO and peroxide treatments.

This behavior indicates less polymer degradation in bFI-modified materials, which suggests improved stability during processing. In contrast, peroxide-treated samples showed higher tan delta values, reflecting a more viscous response and increased molecular breakdown under stress. Meanwhile, the bFI compound demonstrated greater elasticity, transitioning to solid-like behavior at lower frequencies during frequency sweep tests. Therefore, bFI’s performance implies a more stable molecular structure under shear, distinguishing it from the other two approaches.

Mechanical Trade-Offs and Material Integrity

While the rheological results were promising, the mechanical testing revealed important trade-offs. All three modified compounds showed a reduction in mechanical performance compared to the unmodified PIR-PP. However, TO and peroxide treatments preserved more of the original tensile strength and impact resistance.

The bFI-modified PP, although superior in flow behavior, experienced a 48% drop in tensile strain and exhibited brittle, delaminated fracture patterns. These issues were linked to additive overload, which likely compromised the internal bonding of the material.

Finding the Right Dose: The bFI Threshold

To better understand the limitations of the bFI additive, the researchers tested it at 5%, 10%, and 15% loading levels. The results showed:

• 5 wt% bFI: Acceptable MFR increase with minimal mechanical loss

• 10–15 wt% bFI: Severe deterioration in tensile and impact performance

These results helped identify 5 wt% as the critical threshold for using bFI without compromising the mechanical properties of PIR-PP.

A Balanced Approach to Recycling Upgrades

This study demonstrates that increasing the MFR of PIR-PP through targeted modification strategies can successfully prepare the material for injection molding applications. Both thermal-oxidative degradation and peroxide additives offer effective solutions that balance flow enhancement with mechanical integrity.

While the bFI masterbatch offers exceptional flow behavior and structural stability under rheological testing, its application requires careful dosing. When overloaded, it undermines the material’s structural integrity, leading to brittle failures. However, when used correctly, it shows great promise as a flow enhancer for recycled polymers.

As the industry pushes toward greater circularity and sustainability, solutions like these are critical to unlocking the full potential of recycled materials. Through optimized processing and additive selection, post-industrial polypropylene waste can be upcycled into high-value injection molded products, supporting both performance and environmental goals.