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

Significance of the topic

The global volume of plastic production (>400 million tonnes/year) and the wide use of polypropylene (PP) create urgent needs for scalable strategies that convert post-consumer or reprocessed polymer streams into value-added products. Reactive extrusion offers a route to compatibilize immiscible blends and restore mechanical performance lost through mechanical recycling, enabling higher circularity by integrating recycled PP into engineering-grade matrices such as polyamide 12 (PA12). This study demonstrates an integrated experimental workflow—reactive twin-screw extrusion, thermal and mechanical characterization, and micro-ATR FTIR chemical imaging—to validate upcycling of reprocessed PP (rPP) into PA12 while quantifying interfacial chemistry and morphology.

Objectives and study overview

Primary goals were to: (1) examine molecular degradation induced by reprocessing of PP, (2) produce and compare reactive versus non-reactive PA12/rPP blends using PP grafted with maleic anhydride (PP-g-MA) as compatibilizer, (3) evaluate rheological and mechanical signatures of interfacial stabilization, and (4) develop quantitative chemical-homogeneity metrics via micro-ATR FTIR mapping to objectively assess compatibilization efficiency. Blends targeted 50 wt% PA12 with either 40 wt% rPP + 10 wt% PP-g-MA (reactive) or 50 wt% rPP (non-reactive) to compare effects of in situ interfacial reaction under identical extrusion conditions.

Materials and methods

Materials: virgin PP (vPP), reprocessed PP (rPP, produced by high-shear reprocessing), PA12, and PP-g-MA compatibilizer.
Blend compositions: reactive = PA12 50 / rPP 40 / PP-g-MA 10 (wt%); non-reactive = PA12 50 / rPP 50 / PP-g-MA 0 (wt%).
Processing: reactive and non-reactive blends were compounded on the same co-rotating twin-screw extruder under matched conditions (Process 11, 11 mm screws, 40 L/D, ~400 rpm, ~40 s residence time, temperature zones up to ~200 °C). A modular screw configuration provided zones for feeding, plastification, distributive mixing and dispersive mixing to maximize interfacial generation while limiting excessive thermal/mechanical degradation.

Used instrumentation

• Thermo Scientific Process 11 twin-screw extruder with modular screw elements and pelletizer.
• Thermo Scientific HAAKE MiniJet Pro piston injection molding system for specimen molding.
• Thermo Scientific HAAKE MARS iQ rotational rheometer (parallel-plate geometry) for amplitude and frequency sweeps.
• Thermo Scientific Nicolet RaptIR+ FTIR Microscope coupled to Nicolet iS50 FTIR spectrometer with a built-in diamond ATR module for micro-ATR chemical mapping (10 × 10 µm spatial resolution, >1000 spectra per map).
• Differential scanning calorimetry (DSC) for crystallinity assessment.

Methodological details

rPP generation: vPP was reprocessed at high shear (400 rpm) and elevated temperature profile (120–200 °C) to simulate realistic recycled feedstock and induce chain scission. Rheology: oscillatory amplitude sweeps determined LVR and frequency sweeps (converted via Cox–Merz reasoning) characterized viscoelastic spectra and complex viscosity. Mechanical testing: injection-molded dog-bone specimens measured Young’s modulus, tensile strength, yield stress and strain. FTIR mapping: micro-ATR maps of pellet cross-sections used the PA12 amide I band (~1636 cm⁻¹) and the PP methyl deformation (~1376 cm⁻¹). A PA12/PP peak-area ratio was computed per spectrum to generate quantitative histograms, ranges and standard deviations describing compositional uniformity.

Main results and discussion

Molecular degradation of rPP: Rheology and DSC indicated chain scission in rPP relative to vPP. rPP showed lower storage modulus (G′), reduced complex viscosity across frequencies and a modest decrease in crystallinity (63.1% → 58.1%), consistent with reduced entanglement density and shorter chain lengths after reprocessing.

Rheological signatures of compatibilization: Under identical extrusion conditions, the reactive blend (with 10% PP-g-MA) exhibited a higher G′ plateau, increased low-frequency elasticity, and an extended linear viscoelastic region (LVR) compared with the non-reactive blend. These features indicate constrained relaxation dynamics and enhanced interfacial elasticity resulting from in situ formation of interfacial diblock copolymers (reaction of maleic anhydride moieties with PA12 amine end groups), which suppress droplet coalescence and increase stress transfer across phases.

Mechanical property recovery: Both blends showed higher modulus than neat PA12 due to the reinforcing semi-crystalline PP phase; however, tensile strength diverged. The non-reactive blend exhibited marked tensile strength loss and earlier yield failure consistent with weak interfaces and stress concentrators. The reactive blend recovered tensile strength to values essentially matching neat PA12 (despite 50% recycled PP), and showed increased yield stress (from ~15 MPa to ~20 MPa), indicating improved load distribution and limited interfacial debonding.

Quantitative FTIR chemical mapping: Micro-ATR maps (>1000 spectra per region at 10 × 10 µm sampling) provided direct, spatially resolved chemical quantification. The reactive blend produced a narrow PA12/PP area-ratio distribution (range ~2.6–4.8, standard deviation ≈0.306), whereas the non-reactive blend showed a broad distribution (range ~1–12, sd ≈1.65) with near-pure domains present. The narrow distribution in the reactive material implies domain sizes below the 10 µm resolution, absence of large pure-phase regions, and homogeneous mixing. These chemical-homogeneity metrics correlate directly with rheological and mechanical improvements and can be used as objective process optimization indicators.

Benefits and practical applications

• Demonstrates scalable upcycling route that integrates 50 wt% recycled PP into PA12 while restoring tensile performance—relevant for higher-value applications that demand mechanical integrity.
• Shows reactive extrusion combined with PP-g-MA is effective to stabilize morphology and enlarge interfacial area via in situ copolymer formation without bulk crosslinking.
• Establishes micro-ATR FTIR mapping and statistical analysis (ratio histograms, range, standard deviation) as actionable QC metrics for compatibilization efficiency and process robustness.
• Provides a modular processing approach: tuning screw elements (distributive vs dispersive zones) and compatibilizer loading to manage droplet size, interfacial area, and degradation trade-offs.

Future trends and potential uses

• Integration of in-line ATR-MIR spectroscopy for real-time monitoring of interfacial reaction kinetics and blend homogeneity to enable closed-loop control in extrusion lines.
• Optimization of screw design and scale-up studies to preserve interface generation while minimizing thermal/mechanical degradation of recycled streams.
• Exploration of alternative reactive chemistries or lower-temperature coupling strategies to broaden applicability across polymer pairs and reduce degradation risk.
• Deployment of automated chemometric and machine-learning analysis of FTIR chemical maps for rapid classification, anomaly detection and predictive process tuning.
• Application in circular-economy product streams where reclaimed PP is blended into engineering thermoplastics for automotive, consumer goods and industrial components requiring validated mechanical performance.

Conclusion

The combined experimental workflow—reactive twin-screw extrusion with PP-g-MA, rheological and mechanical characterization, plus quantitative micro-ATR FTIR mapping—proves an effective pathway to upcycle reprocessed PP into PA12 while restoring key mechanical properties. Rheology and mechanical tests provide indirect evidence of interfacial stabilization; micro-ATR FTIR mapping converts that evidence into quantitative compositional metrics that correlate with performance. The approach supports process optimization and provides objective QC endpoints to accelerate development of sustainable polymer blends that retain industrial functionality.

References

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