In situ density determination of polyethylene in multilayer polymer films using Raman microscopy

Importance of the Topic

Polyethylene is among the most widely produced plastics with density directly influencing crystallinity mechanical strength and barrier properties. Accurate determination of PE density at the layer level in multilayer films is essential for product design performance and quality control in packaging and industrial applications.

Objectives and Overview

This work presents the development of a quantitative Raman microscopy method combined with partial least squares regression to measure polyethylene density in single and multilayer films in situ without complex sample preparation.

Methodology and Instrumentation

The study utilized a Thermo Scientific DXR2 Raman Microscope equipped with a 532 nm laser confocal objectives and OMNIC software for data acquisition. TQ Analyst software handled spectral normalization and PLS model building. A total of 25 reference samples (12 pellets and 13 films) spanning HDPE LDPE and LLDPE densities were analyzed. Spectra were collected at multiple points baseline corrected normalized to the CH₂ bending band (1440 cm⁻¹) and focused on the 1400–1500 cm⁻¹ range for quantitative analysis.

Results and Discussion

Distinct CH₂ stretching (2848/2882 cm⁻¹) and bending (1416/1440 cm⁻¹) features reflected differences in crystallinity and density among PE types. A three-factor PLS model achieved a correlation coefficient above 0.99 and RMSE below 0.005 g/cm³ for calibration and validation samples. All 25 samples were predicted within ±0.81 % of actual densities. Confocal depth profiling of a multilayer nicotine patch resolved two PE layers with densities characteristic of LDPE/LLDPE in the backing layer and HDPE in the rate-controlling membrane.

Benefits and Practical Applications

• Non-destructive in situ density measurement within complex multilayer films
• Avoids laborious layer separation and microtoming
• Applicable to both pellet and film samples across the full PE density range
• Supports materials selection quality assurance reverse engineering and failure analysis

Future Trends and Applications

Expanding this approach to other polymer systems and integrating real-time monitoring could further optimize production control. Advances in automated Raman mapping and deeper confocal profiling may enable detailed multilayer characterization and online inspection of packaging films.

Conclusion

Raman microscopy combined with PLS regression offers a fast accurate and non-destructive method for quantifying polyethylene density in situ within multilayer films. The method correlates strongly with traditional techniques and streamlines analysis by eliminating extensive sample preparation.

Reference

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