Distinction of Polyethylene and Polypropylene by Infrared Spectrum

Importance of the Topic

Accurate identification and differentiation of common polyolefin resins such as polyethylene and polypropylene are vital in quality control, recycling operations and material verification across chemical, packaging and automotive industries. Infrared spectroscopy provides a non-destructive, rapid approach for distinguishing these hydrocarbon polymers based on characteristic vibrational fingerprints of their molecular structures.

Objectives and Overview of the Study

This study demonstrates an ATR-FTIR application using the IRSpirit-TX instrument to differentiate polyethylene (PE) from polypropylene (PP) and to resolve structural variants of PE (low density vs. high density). The goal is to highlight spectral regions where functional group vibrations yield clear markers for polymer type and crystallinity.

Used Instrumentation

• IRSpirit-TX Fourier Transform Infrared Spectrophotometer
• QATR™-S Single-Reflection Diamond ATR Accessory
• DLATGS Detector

Methodology

Samples of PE, PP, LDPE and HDPE were analyzed by placing them directly on the diamond ATR crystal. Spectra were recorded over 4000–400 cm⁻¹ at 4 cm⁻¹ resolution, 40 scans accumulation and Happ–Genzel apodization. A background scan was acquired prior to sample measurement.

Main Results and Discussion

The 3200–2600 cm⁻¹ region shows CH2 asymmetric and symmetric stretching peaks near 2930 cm⁻¹ and 2850 cm⁻¹ common to both PE and PP. Unique to PP are stronger CH3 stretching absorptions at ~2960 cm⁻¹ and 2870 cm⁻¹. In the 1600–1200 cm⁻¹ range, PE displays a sharp CH2 scissoring band at ~1460 cm⁻¹, whereas PP exhibits a broader feature due to overlapping CH3 symmetric bending around 1375 cm⁻¹ and CH2 scissoring near 1460 cm⁻¹. The fingerprint region (800–600 cm⁻¹) reveals a distinct CH2 rocking vibration at ~720 cm⁻¹ in PE but is absent in PP. Comparing LDPE with HDPE, LDPE shows a pronounced CH3 asymmetrical stretch at ~2960 cm⁻¹ reflecting abundant side chains. HDPE, with higher crystallinity, exhibits relatively stronger intensity on the high-wavenumber side of the scissoring and rocking bands.

Benefits and Practical Applications

• Rapid, non-destructive resin identification in manufacturing and recycling.
• Quality assurance in polymer production lines to confirm material purity.
• Screening of mixed plastic waste streams for efficient sorting.
• In-field polymer verification using portable ATR-FTIR systems.

Future Trends and Applications

Advances may include integration of chemometrics and machine learning for automated spectral classification, expanded spectral libraries for diverse polymer blends, online process monitoring in extrusion and molding, and miniaturized FTIR modules for point-of-use polymer analysis.

Conclusion

This application study confirms that ATR-FTIR spectroscopy using the IRSpirit-TX platform enables clear differentiation of polyethylene and polypropylene and can further distinguish polyethylene density variants. Characteristic absorption bands linked to CH2 and CH3 vibrations serve as reliable markers for polymer type and crystallinity, supporting a wide range of analytical and industrial applications.