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

Importance of the Topic

Polyethylene (PE) and polypropylene (PP) are widely used thermoplastics with complementary properties. Blending PE with PP enhances low-temperature impact resistance and adjusts mechanical performance. Accurate quantification of PE content in PE/PP blends is critical for quality control, product performance optimization and efficient recycling of polyolefin waste.

Objectives and Study Overview

The primary goal was to develop and validate rapid FTIR methods for determining weight percent PE in PE/PP blends over a 35–85% PE range. Two cast-film transmission approaches were compared: a traditional salt plate technique and a novel self-supporting film method using a portable FTIR accessory.

Methodology and Instrumentation

Calibration standards were prepared by dissolving PE and PP in hot tetrachloroethylene to produce ~3% polymer solutions. Films were cast on KBr plates or PTFE sheets and dried at 70–80 °C. Spectra were recorded on the Agilent Cary 630 FTIR spectrometer at 4 cm⁻¹ resolution with 74 co-added interferograms (30 s total acquisition). The analytical metric was the peak height ratio of the PP-specific 1376 cm⁻¹ band to the combined methyl/methylene 1462 cm⁻¹ band. Film thickness was controlled to maintain absorbance between 0.3 and 1.0 AU in the 1500–1200 cm⁻¹ region.

Used Instrumentation

• Agilent Cary 630 FTIR spectrometer
• Transmission sampling compartment with salt plates
• DialPath (TumblIR) accessory for self-supporting films
• Portable Agilent 4500 and benchtop 5500 FTIR options with DialPath technology

Key Results and Discussion

Both the salt plate and DialPath methods yielded identical linear calibration plots with R² = 1.000 for PE content determination. The novel PTFE film approach allowed rapid repositioning and minimized interference fringes due to the concave/convex window design of the DialPath accessory. Compliance with ASTM D3900-05a was confirmed through use of the same absorbance ratio correction for film thickness.

Benefits and Practical Applications

• Instantaneous calculation and display of PE:PP ratios via the Cary 630 software library
• Elimination of extensive sample preparation and cell handling with the DialPath method
• Enhanced field and near-line testing capabilities using portable FTIR instruments
• Support for polymer manufacturing QA/QC and recycling stream characterization

Future Trends and Opportunities

Advances in FTIR accessory design may further simplify blend analysis by integrating automated film thickness control and real-time data analytics. Expanding the methodology to other polymer blends and copolymers or incorporating machine learning for spectral interpretation could broaden applicability in industrial and recycling environments.

Conclusion

The Agilent Cary 630 FTIR spectrometer, combined with cast-film techniques, provides a robust, accurate and rapid solution for quantifying PE in PE/PP blends. The traditional salt plate and innovative DialPath methods deliver equivalent performance, enabling reliable blend analysis for quality control and recycling processes.

References

• Agilent Technologies. Determination of percent polyethylene in polyethylene/polypropylene blends using cast film FTIR techniques. Application Note 5990-9785EN, January 2012.