From waste to performance: Reactive extrusion upcycling validated by rheology and chemical (FTIR) imaging

Importance of the topic

Mechanical recycling of commodity polymers like polypropylene (PP) often leads to molecular degradation and loss of performance, limiting reuse in high-value applications. Reactive extrusion with compatibilizers offers a scalable route to upcycle reprocessed PP (rPP) into robust blends with engineering polymers such as polyamide 12 (PA12). This study demonstrates a validated workflow combining twin-screw reactive extrusion, rheology, mechanical testing, and micro-ATR FTIR chemical imaging to quantify interfacial stabilization, recover mechanical properties, and provide objective metrics for process optimization.

Objectives and study overview

• Assess the impact of reactive extrusion using PP grafted with maleic anhydride (PP-g-MA) to compatibilize blends of PA12 with 50% rPP.
• Characterize molecular and morphological consequences of reprocessing and reactive blending via oscillatory rheology, tensile testing, DSC, and micro-ATR FTIR mapping.
• Establish quantitative chemical-homogeneity metrics from FTIR maps to correlate with rheological and mechanical performance.

Materials and methods

• Materials: virgin PP (vPP), reprocessed PP (rPP generated by high-shear reprocessing), PA12, and PP-g-MA compatibilizer.
• Blend formulations: Reactive blend = 50% PA12 / 40% rPP / 10% PP-g-MA; Non-reactive = 50% PA12 / 50% rPP.
• Reactive compounding: Thermo Scientific Process 11 co-rotating twin-screw extruder with modular screw (11 mm; 40 L/D). Typical processing: ~400 rpm, ~40 s residence time; reactive blending performed at ~185–200 °C.
• Specimen production: HAAKE MiniJet Pro piston injection molder to produce discs for rheology and dog-bone tensile bars.
• Thermal analysis: DSC to track crystallinity changes after reprocessing and in blends.

Used instrumentation

• Thermo Scientific Process 11 Twin-Screw Extruder (modular co-rotating screws).
• Thermo Scientific HAAKE MiniJet Pro Injection Molding System.
• Thermo Scientific HAAKE MARS iQ rotational rheometer (parallel plate geometry; amplitude and frequency sweeps).
• Thermo Scientific Nicolet RaptIR+ FTIR Microscope with diamond ATR for micro-ATR chemical mapping (10 × 10 µm sampling, >1000 spectra per map).

Key methodological details

• rPP produced by reprocessing vPP at 400 rpm with temperature profile from 120 °C to 200 °C (die at 190 °C), generating practical recycled feedstock showing molecular degradation.
• Rheology: amplitude sweeps to determine LVR and frequency sweeps (oscillatory) at 180 °C for PP samples and 220 °C for blends; Cox–Merz considerations applied to interpret complex viscosity.
• FTIR mapping: micro-ATR spectra collected at 8 cm⁻¹ resolution with 64 scans per point; chemical markers chosen were PA12 amide I (~1636 cm⁻¹) and PP methyl deformation (~1376 cm⁻¹). A PA12/PP peak-area ratio was computed for each spectrum (areas: 1680–1610 cm⁻¹ for PA12; 1390–1345 cm⁻¹ for PP) to quantify local composition and generate histogram/statistics.

Main results and discussion

• Reprocessing effects: DSC indicated a decrease in PP crystallinity from 63.1% (vPP) to 58.1% (rPP), consistent with chain scission under high shear. Rheology of rPP showed reduced plateau storage modulus (G') and complex viscosity relative to vPP, reflecting lower molecular weight and fewer entanglements.
• Reactive compatibilization: In reactive blends (10% PP-g-MA), in situ reaction between maleic anhydride grafts and PA12 terminal amines forms interfacial diblock copolymers. No bulk crosslinking or torque spikes were observed, indicating interfacial rather than bulk chemistry.
• Rheological signatures: Reactive blends exhibited higher G' across the LVR, increased low-frequency elasticity, and an extended LVR compared to non-reactive blends. These features indicate constrained relaxation and enhanced interfacial elasticity from copolymer anchoring.
• Mechanical performance: Both blends showed increased modulus relative to neat PA12 (attributed to semi-crystalline PP reinforcement). Tensile strength of the non-reactive blend decreased substantially (≈32.5 MPa vs 41.8 MPa for PA12), consistent with weak interfaces. The reactive blend recovered tensile strength to near-PA12 levels (~41.9 MPa) and increased yield stress (from ~15.0 to ~20.2 MPa), demonstrating effective stress transfer and suppression of interfacial debonding.
• FTIR chemical mapping: High-density micro-ATR maps (>1000 spectra per area at 10×10 µm resolution) enabled quantitative phase analysis. Reactive blend ratio values (PA12/PP area) ranged narrowly 2.6–4.8 (std dev = 0.306), implying homogeneous mixing and domain sizes below the 10 µm spatial resolution. Non-reactive blend ratios ranged widely 1–12 (std dev = 1.65) with near-pure domains detected (~1% mapped area), indicating poor mixing and large phase heterogeneity.
• Correlation across techniques: Narrower composition histograms, higher G' and extended LVR, and recovery of tensile strength converge to show that reactive extrusion with PP-g-MA stabilizes morphology and restores performance in blends with 50% recycled PP.

Benefits and practical applications

• Enables incorporation of high fractions of recycled PP into engineering-grade PA12 while retaining mechanical integrity—supporting circular economy goals.
• Reactive extrusion provides in-line compatibilization that prevents bulk gelation and targets interfacial chemistry, scalable to industrial twin-screw platforms.
• Micro-ATR FTIR mapping offers an objective, quantitative metric (ratio histograms and standard deviation) for assessing compatibilization efficacy and for process control/optimization.
• Combined rheology-mechanics-chemical imaging workflow accelerates formulation development by linking process parameters (screw design, mixing intensity, compatibilizer content) to measurable performance outcomes.

Future trends and opportunities

• Refinement of screw element design and residence-time/temperature profiles to maximize interfacial area while minimizing thermal/mechanical degradation of recycled streams.
• Extension to other immiscible polymer pairs and exploration of alternative reactive compatibilizers (tailored graft densities, multifunctional chemistries) to broaden material combinations.
• Integration of in-line spectroscopic monitoring (e.g., on-line ATR-FTIR) for real-time control of compatibilization kinetics and to reduce offline testing cycles.
• Application of higher-resolution chemical imaging (e.g., AFM-IR or synchrotron-based techniques) to resolve sub-micron domain structure and further link morphology to fatigue, impact, and long-term durability.

Conclusion

This application study demonstrates that reactive twin-screw extrusion using PP-g-MA effectively upcycles reprocessed PP into PA12-based blends containing 50% recycled content. Interfacial diblock copolymer formation yields stabilized morphologies, reflected in rheological signatures (elevated G' and extended LVR), quantitative micro-ATR FTIR homogeneity metrics, and recovery of tensile strength to near-virgin PA12 values. The combined analytical workflow provides robust, quantitative indicators for process optimization and supports scalable strategies for performance-oriented polymer recycling.

References

1. On-line ATR-MIR for real-time quantification of chemistry kinetics along the barrel in extrusion-based processes. ScienceDirect.
2. Crawford NC. Thermo Fisher Application Note AN56376 – Examining the rheological behavior of three nearly identical linear low-density polyethylene (LLDPE) samples. Thermo Fisher Scientific.
3. Cox WP, Merz EH. Correlation of dynamic and steady flow viscosities. Journal of Polymer Science, 28, 619 (1958).
4. Effect of PP-g-MAH compatibilizer content in polypropylene/nylon-6 blends. Polymer Bulletin. Springer Nature.
5. Lee HG, Sung Y-T, Lee YK, Kim WN, Yoon HG, Lee HS. Effects of PP-g-MAH on the mechanical, morphological and rheological properties of polypropylene and poly(acrylonitrile-butadiene-styrene) blends, 2009.
6. Effect of different polypropylenes and compatibilizers on the rheological, mechanical and morphological properties of nylon 6/PP blends. Journal of Materials Science. Springer Nature.