Real-time monitoring of polymer extrusion using a process Raman analyzer integrated with a twin-screw extruder

Importance of the topic

Real-time, inline chemical monitoring of polymer extrusion provides immediate insight into material state, composition and process dynamics that are otherwise only accessible through off-line laboratory analysis. Integrating Raman spectroscopy directly into an extruder enables manufacturers and R&D teams to detect phase changes (crystalline vs amorphous), follow resin changeovers, identify contamination or incomplete purging, and make rapid process adjustments that reduce waste, improve product consistency and accelerate development of new formulations.

Study objectives and overview

This study demonstrates a proof-of-concept integration of a MarqMetrix All‑In‑One Process Raman Analyzer with a Thermo Scientific Process 11 parallel twin‑screw extruder to monitor polymer processing in situ. Primary aims were to: (1) compare Raman fingerprints of virgin LDPE and PLA pellets with spectra collected during extrusion; (2) identify structural changes induced by extrusion (e.g., melting/crystallinity loss); and (3) monitor and quantify a resin changeover from LDPE to PLA using chemometrics (PCA), establishing the feasibility of real-time material identification and changeover tracking.

Methodology and instrument configuration

• Inline optical sampling: MarqMetrix threaded Process BallProbe (Hastelloy body, sapphire ball tip, >300 °C rating) installed in the extruder barrel was used for direct contact Raman sampling.
• Extrusion platform: Thermo Scientific Process 11 Parallel Twin‑Screw Extruder with eight independent barrel zones (5 L/D segments) and feeder for pellet introduction.
• Spectral acquisition: MarqMetrix All‑In‑One analyzer, 450 mW laser power, 1 s integration time, 3 accumulations per scan, automatic background; effective sampling cadence ~6 s per measurement.
• Materials and conditions: Low‑density polyethylene (LDPE) and polylactic acid (PLA) were used. Representative process conditions included temperature profiles ensuring molten states (LDPE melt ~105–110 °C, PLA melt ~150–160 °C), varied screw speeds, feed rates and torques to evaluate process sensitivity.
• Chemometric processing: Principal component analysis (PCA) applied to in‑line Raman spectra to classify materials and follow the dynamic transition during resin changeover. Potential for future PLS models for quantitative composition estimation was noted.

Used instrumentation

• MarqMetrix All‑In‑One Process Raman Analyzer (inline Raman spectrometer).
• MarqMetrix Threaded Process BallProbe sampling optic with sapphire ball tip and Hastelloy construction.
• Thermo Scientific Process 11 Parallel Twin‑Screw Extruder (multi‑zone heated barrel and feeder).
• Process monitoring: feed, screw speed, die temperature, pressure/temperature probe and display interfaces for concurrent process data.

Main results and discussion

• Characteristic spectra: Virgin LDPE and PLA pellets exhibit distinct Raman fingerprints facilitating straightforward spectral differentiation prior to processing.
• LDPE structural changes on extrusion: The methylene bending band indicative of crystalline LDPE (peak ~1416 cm−1) present in pellets disappeared after extrusion, consistent with melting and loss of crystalline order. Peaks associated with LDPE amorphous phases (≈1080 and 1303 cm−1) increased in extruded material, while C–C stretching bands assigned to all‑trans chains (≈1063 and 1123 cm−1) diminished, indicating disrupted 1D translational periodicity upon melting. Significant changes were also observed in the C–H stretching region (≈2,800–3,000 cm−1).
• Effect of process parameters: Increasing temperature and screw torque (comparing conditions A and B) produced no major spectral differences for LDPE, suggesting full melting already occurred under the milder set of conditions.
• PLA behavior: Raman spectra for PLA before and after extrusion remained largely unchanged under the studied conditions, indicating retention of its spectral features through the process conditions used.
• Resin changeover monitoring: Continuous inline sampling during the LDPE→PLA changeover showed appearance of PLA spectral features at the probe between ~30 and 60 s after PLA was introduced. PCA provided clear temporal separation: LDPE clustered distinctly early in the run; a transitional path appeared as PLA entered the barrel; a stable PLA cluster was reached after approximately 84 s. The first principal component explained the overwhelming majority of variance (~98.7%), enabling robust discrimination and monitoring of the changeover.

Benefits and practical applications

• Real‑time quality control: inline Raman enables immediate detection of incomplete purging, cross‑contamination and process excursions, reducing off‑spec production and material waste.
• Changeover verification: rapid identification of new resin presence shortens downtime and increases confidence during material switches.
• Process optimization and R&D: direct observation of phase transitions and molecular changes supports optimization of temperature, screw speed and residence time, and accelerates formulation development.
• Potential for quantitative analytics: building PLS models on inline spectra could deliver concentration or composition estimates for blends and additives in process streams.

Future trends and potential applications

• Quantitative inline analytics: extension from PCA classification to validated PLS regression models for real‑time compositional monitoring and control of blends or filled polymers.
• Reactive extrusion monitoring: deploying inline Raman mid‑barrel to follow reaction progress, conversion and by‑product formation in reactive extrusion processes.
• Integration with process control: closed‑loop control using Raman‑derived metrics to automatically adjust temperature, screw speed or feed to maintain target properties.
• Expanded materials and environments: application to filled polymers, composites, pharmaceutical hot‑melt extrusion, and battery electrode mixing where twin‑screw extruders are used.
• Higher temporal resolution and advanced chemometrics: faster acquisition, denoising and multivariate time‑series analysis to capture short transient events and improve sensitivity to minor components.

Conclusion

This work demonstrates that an inline, contact Raman probe integrated into a twin‑screw extruder can reliably detect polymer melting and structural transitions, discriminate between LDPE and PLA, and monitor resin changeovers in near real time. The approach provides actionable process intelligence for quality assurance, faster changeovers and enhanced R&D capabilities. Future development should focus on building quantitative models (PLS), extending monitoring deeper in the barrel for reactive processes, and integrating Raman outputs into closed‑loop extrusion control schemes.

Reference

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2. Bolskis E, Adomavičiūtė E, Griškonis E. Formation and investigation of mechanical, thermal, optical and wetting properties of melt‑spun multifilament poly(lactic acid) yarns with added rosins. Polymers. 2022;14:379.