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

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

Significance of the topic:
Real-time, inline chemical monitoring during polymer extrusion enables tighter process control, faster material changeovers, reduced scrap, and enhanced product consistency. By detecting structural transformations (e.g., crystalline-to-amorphous transitions), contamination, and resin identity while material is being processed, vibrational spectroscopy such as Raman can directly support QC, process optimization and R&D efforts in plastics, pharmaceutical formulations and battery electrode manufacturing.

Objectives and overview of the study:
The study demonstrates integration of a MarqMetrix All‑In‑One Process Raman Analyzer with a Thermo Scientific Process 11 parallel twin‑screw extruder to: monitor polymer structural changes in real time; distinguish between LDPE and PLA during extrusion; and track resin changeover dynamics using chemometric analysis (PCA). The experiments used virgin LDPE and PLA pellets, characterized both prior to and during extrusion, and evaluated process condition variations and the timing of resin replacement at the die using inline measurements.

Methodology and workflow:
- Sample materials: low‑density polyethylene (LDPE) and polylactic acid (PLA).
- Integration: a MarqMetrix threaded Process BallProbe (sapphire ball tip, Hastelloy body) mounted in‑line in the extruder barrel provided direct contact spectra collection.
- Instrumental parameters: laser power 450 mW; integration time 1 s; 3 averages per scan; automatic background; total cycle ≈6 s per measurement.
- Extruder parameters: Process 11 extruder with eight independently controlled barrel zones (5 D/L segments each). Typical set temperatures ranged from ≈130–180 °C along the barrel depending on material/condition; screw speeds tested 200–300 RPM; feed rates 4–5% and torques 15–22% were evaluated.
- Data analysis: spectral interpretation of characteristic Raman bands, and principal component analysis (PCA) to classify and track resin identity during changeover. The study also discusses potential for PLS regression for quantitative composition modeling in future work.

Used instrumentation:
- Thermo Scientific Process 11 Parallel Twin‑Screw Extruder (eight barrel segments with independent temperature control).
- Thermo Scientific MarqMetrix All‑In‑One Process Raman Analyzer.
- MarqMetrix Threaded Process BallProbe (Hastelloy construction, sapphire ball optic, threaded screw design) rated >300 °C, designed for direct inline contact with extrudate.

Main results and discussion:
- Virgin pellet spectra: LDPE and PLA exhibit distinct Raman fingerprints enabling clear spectral discrimination prior to extrusion.
- LDPE structural changes during extrusion: characteristic crystalline methylene bending (δ(CH2)) band near 1,416 cm⁻¹ present in pellets disappeared in extruded material, consistent with melting of crystalline domains. Peaks linked to amorphous LDPE (≈1,080 and 1,303 cm⁻¹) became more prominent after extrusion, while symmetric/asymmetric C–C stretches (≈1,063 and 1,123 cm⁻¹) associated with all‑trans chain periodicity diminished, indicating loss of 1D translational order. Significant changes were also observed in C–H stretching region (≈2,800–3,000 cm⁻¹). Changes were evident under baseline processing conditions, and raising temperature/screw torque produced no major additional change, indicating the polymer was already fully molten in the baseline condition.
- PLA behavior: Raman spectra of PLA showed characteristic peaks that were stable in the extruded state; during the LDPE→PLA changeover PLA features gradually emerged in the inline spectra.
- Changeover monitoring and PCA: during a controlled resin switch, inline spectra collected every ≈6 s showed the PLA spectral features beginning to appear between ~30–60 s after introduction. PCA separated LDPE and PLA clusters clearly: PC1 explained the vast majority of variance (≈98.7%), with a visible transition path from LDPE to PLA. The system reached a stable PLA spectral cluster by approximately 84 s, with the full monitoring run extended to 114 s to visualize stabilization.

Benefits and practical applications of the method:
- Real‑time detection of melting and structural state (crystalline vs amorphous) to confirm process effectiveness.
- Rapid, inline material identification to prevent cross‑contamination during resin changeovers and speed material transitions.
- Process optimization capability (temperature, screw speed, feed rate) based on chemical/structural feedback rather than indirect physical proxies.
- Reduced off‑spec production, material waste and downtime through immediate corrective action enabled by continuous spectral feedback.
- Enhanced R&D throughput by directly linking process settings to molecular/structural outcomes for formulation and process development.

Limitations and considerations:
- The study is a proof‑of‑concept focusing on two well‑characterized polymers (LDPE and PLA); extension to mixed composites, additives or filled systems will require additional calibration and validation.
- Quantitative composition measurement would require development of PLS models and careful calibration against standards and process variability.
- Probe placement and flow/contact dynamics will influence response time; measurements reflect the local composition at the probe location (end‑of‑die monitoring here).

Future trends and potential applications:
- Development and validation of multivariate PLS models for inline quantification of polymer blends, filler loadings and additive concentrations.
- Integration of Raman analytics with process control loops (closed‑loop adjustments of temperature, screw speed, feed rate) to enable autonomous process optimization and quality assurance.
- Mid‑barrel probe placement for real‑time monitoring of reactive extrusion, compounding with chemical reactions, or devolatilization processes.
- Broader application across pharmaceutical hot‑melt extrusion, battery electrode mixing, and other continuous manufacturing processes where chemical identity and molecular structure are critical.
- Combining Raman with complementary sensors (e.g., NIR, melt pressure/torque) and digital twins for advanced process understanding and predictive maintenance.

Conclusion:
Inline Raman spectroscopy using a robust threaded ball‑probe integrated into a twin‑screw extruder can provide actionable, real‑time chemical and structural information during polymer extrusion. The study demonstrated clear detection of LDPE crystalline melting and established reliable polymer identification and changeover tracking from LDPE to PLA using PCA. These capabilities support improved quality control, reduced waste, accelerated changeovers, and enhanced R&D insight. Future work should expand quantification (PLS), probe positioning strategies, and closed‑loop process control applications to fully exploit inline vibrational spectroscopy in continuous polymer processing.

References:
1. Lu R, Gan W, Wu B. H., Zhang Z., Guo Y., Wang H. F. C‑H stretching vibrations of methyl, methylene and methine groups at the vapor/alcohol (N = 1–8) interfaces. J Phys Chem B. 2005 Jul 28;109(29):14118–14129. doi:10.1021/jp051565q.
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.