High Melt Strength Polypropylene via Reactive Extrusion for Injection-Molded Foams: Performance of Virgin and Recycled Blends

This research demonstrates an eco-friendly and reproducible method to synthesize high melt strength polypropylene (HMSPP) by reactive extrusion of virgin polypropylene (PP) and recycled polypropylene (rPP) blends with low-density polyethylene (LDPE). In contrast to many previous reactive extrusion studies that primarily focused on low-MFR polypropylene grades for extrusion or thermoforming-based processes, the present study targets high-MFR-HMSPP materials suitable for injection molding applications, particularly injection-molded foams. A peroxide initiator (DHBP) and a polyfunctional coagent, namely trimethylolpropane triacrylate (TMPTA), were added to the blends to induce long-chain branching (LCB) while maintaining adequate processability. Detailed analyses were conducted to evaluate the effects of LDPE (20–40 wt %) and TMPTA (2–5 wt %) on morphology, thermal behavior, rheological response, mechanical performance, and foaming characteristics. Rheological investigations revealed pronounced increases in complex viscosity, storage modulus, and melt elasticity, accompanied by strain-hardening behavior in TMPTA-modified blends, which are widely accepted rheological signatures of long-chain branching rather than extensive gel formation. SEM analyses demonstrated improved interfacial compatibility among reactively modified blends and refined LDPE domain structures following reactive modification. The recycled blend (rPP5) displayed similar foamability, indicating the potential capability of the procedure to upcycle waste PP.