Distinction of Polyethylene and Polypropylene by Infrared Spectrum

Significance of the Topic

The differentiation of polyethylene (PE) and polypropylene (PP) by FTIR offers a rapid, non-destructive method for polymer identification in quality control, recycling and material authentication.
It also enables the detection of subtle structural variations within the same polymer class, such as low-density and high-density polyethylene, which is critical for industrial processing and research.

Objectives and Overview

This study demonstrates how a Fourier Transform Infrared spectrometer with an ATR accessory can resolve characteristic spectral differences between PE and PP, and further distinguish between LDPE and HDPE.
Key absorption bands corresponding to C–H stretching and bending vibrations are correlated with polymer backbone and side-chain structures.

Methodology and Instrumentation

Measurements were performed with an IRSpirit-TX FTIR spectrophotometer equipped with a QATR-S diamond ATR accessory under the following conditions:

• Resolution: 4 cm⁻¹
• Accumulations: 40 scans
• Apodization function: Happ-Genzel
• Detector: DLATGS
• Wavenumber range: 4000–400 cm⁻¹

Main Results and Discussion

In the C–H stretching region (3200–2600 cm⁻¹):

• Both PE and PP display CH₂ asymmetric (~2930 cm⁻¹) and symmetric (~2850 cm⁻¹) stretching bands.
• PP shows additional CH₃ asymmetric (~2960 cm⁻¹) and symmetric (~2870 cm⁻¹) stretching peaks due to its methyl side groups.

In the bending region (1600–1200 cm⁻¹):

• PE exhibits a sharp CH₂ scissoring band near 1460 cm⁻¹.
• PP presents a broader feature at 1460 cm⁻¹ with an overlapping CH₃ symmetric bending band at ~1380 cm⁻¹.

In the fingerprint region (800–600 cm⁻¹):

• PE shows a distinct CH₂ rocking band at ~720 cm⁻¹ (accordion vibration), absent in PP.

Comparison of LDPE vs. HDPE revealed:

• Stronger CH₃ stretching (~2960 cm⁻¹) and bending (~1380 cm⁻¹) in LDPE, reflecting higher side-chain content.
• Enhanced CH₂ scissoring and rocking bands in HDPE, indicating greater crystallinity.

Benefits and Practical Applications

This ATR-FTIR method provides fast, reliable identification of hydrocarbon resins in routine QA/QC, counterfeit detection, and recycling operations without extensive sample prep.
It also supports material characterization and verification in research and industrial laboratories.

Future Trends and Potential Applications

Integration with chemometric algorithms can automate polymer classification and quantification.
Real-time, inline ATR-FTIR monitoring in manufacturing processes can improve process control.
Advances in detector technologies and ATR interfaces will extend applicability to copolymers, blends and composite materials.

Conclusion

ATR-FTIR spectroscopy using the IRSpirit-TX with QATR-S ATR accessory effectively distinguishes PE from PP and resolves density-related structural differences in polyethylene.
This approach offers a practical, non-destructive tool for polymer analysis across research and industrial environments.

References

• FTIR TALK LETTER Vol. 41