Determination of percent polyethylene in polyethylene/polypropylene blends using cast film FTIR techniques

Importance of the Topic

The accurate determination of polyethylene (PE) content in PE/polypropylene (PP) blends is essential for optimizing mechanical performance, ensuring consistent product quality, and supporting efficient recycling of polyolefin waste.

Objectives and Overview of the Study

This study presents rapid infrared spectroscopy methods to quantify the PE:PP ratio in cast films. It compares a conventional salt-plate transmission approach with a novel DialPath transmission accessory technique using the Agilent Cary 630 FTIR platform.

Methodology and Instrumentation

Sample Preparation:

• Calibration blends prepared at 35–85 wt % PE by dissolving PE and PP in hot tetrachloroethylene (3 % polymer solution).
• Cast films formed on KBr plates (0.3 mL solution) or PTFE coupons (20–50 µm films peeled off after drying at 70–80 °C).

Instrumentation:

• Agilent Cary 630 FTIR spectrometer (benchtop and portable 4500/5500 models) with patented interferometer technology.
• Transmission sampling accessories: traditional salt-plate compartment and DialPath/TumblIR linear-path cell (100 µm pathlength).

Data Acquisition:

• Infrared spectra recorded as 74 co-added interferograms at 4 cm⁻¹ resolution, total scan time 30 s.
• Quantification based on the absorbance ratio of the PP methyl band at 1376 cm⁻¹ to the PE/PP bend band at 1462 cm⁻¹ to correct for film thickness variations.

Key Results and Discussion

Both transmission approaches yielded identical linear calibrations with R² = 1.000 across the 35–85 wt % PE range. The DialPath accessory offered greater convenience by accommodating larger, self-supporting films and minimizing spectral fringing through its convex/concave window design. Fringing, a common interference from internal reflections in smooth films, was effectively suppressed in both methods.

Benefits and Practical Applications

• Rapid, reliable quantification of PE content without complex sample preparation.
• On-site and near-line analysis capability with portable FTIR models (4500/5500) using DialPath.
• Integration of the calibration method into the Cary 630 software library for automated sample analysis and reporting.
• Support for quality control in polymer blending, recycling streams, and copolymer verification.

Future Trends and Possibilities

• Extension of DialPath-based methods to other polymer blends and copolymers.
• Development of automated film-casting robots for high-throughput FTIR analysis.
• Integration with chemometric models for multi-component polyolefin systems.
• Field deployment of handheld FTIR for in-situ monitoring of recycled plastics streams.

Conclusion

The study demonstrates that both salt-plate and DialPath cast-film FTIR methods on the Agilent Cary 630 deliver precise, linear calibration for PE/PP blend analysis. The DialPath approach enhances throughput and ease of use, making PE content determination faster and more versatile.

Reference

• ASTM D3900-05a: Rubber—Determination of Ethylene Units in Ethylene-Propylene Copolymers by Infrared Spectrometry.
• Higgins F., Agilent Technologies Application Note 5990-9785EN, January 19, 2012.