Determination of percent ethylene in ethylene-propylene statistical copolymers

Significance of the Topic

Accurate determination of ethylene content in ethylene–propylene statistical copolymers is critical for quality control, product performance and regulatory compliance in polymer manufacturing. The statistical distribution of ethylene units influences mechanical, thermal and barrier properties of the material. A rapid, reliable analytical approach enables producers and researchers to monitor copolymer composition without extensive sample destruction or lengthy sample preparation.

Objectives and Overview

This study presents a validated infrared spectroscopic method for quantifying low levels (0.3–3.5 wt%) of randomly distributed ethylene in ethylene–propylene copolymers. The method targets a characteristic absorption band at 733 cm⁻¹ for statistical ethylene and uses a reference band at 1044 cm⁻¹ from the polypropylene matrix. A Beer’s Law–type calibration correlates the normalized absorbance ratio to ethylene concentration, using C13 NMR as the primary standard.

Methodology and Used Instrumentation

Sample Preparation:

• Representative resin samples are melt-pressed into 0.5–0.7 mm films using a hydraulic press (200 °C, up to 25 000 lb force) and optional chase molds on aluminum sheets.
• Films with surface defects or out-of-range thickness are discarded; three acceptable replicates are required.

Spectral Acquisition:

• FTIR analysis is performed on an Agilent Cary 630 or equivalent FTIR spectrometer (e.g., Agilent 4500/5500) equipped with DialPath or TumblIR transmission accessories (1000 µm path length).
• Resolution is set to 4 cm⁻¹ or better, with a minimum of 74 scans per film.

Data Processing:

• Peak height at 733 cm⁻¹ is measured relative to a baseline between 759 and 703 cm⁻¹, and at 1044 cm⁻¹ relative to a baseline between 1068 and 949 cm⁻¹.
• The MicroLab PC FTIR software automates baseline, peak measurement and ratio calculation across replicates.

Calibration:

• Standards covering the target range are quantified by C13 NMR.
• Normalized absorbance ratios (A733/A1044) are plotted versus known ethylene content.
• Linear regression yields calibration constants M (slope) and N (intercept) according to % C₂(stat.) = M × (A733/A1044) + N.

Main Results and Discussion

The method demonstrated excellent linearity over 0.3–3.5 wt% statistical ethylene, with correlation coefficients consistently above 0.995. Repeatability across triplicate films yielded relative standard deviations below 2%. The DialPath and TumblIR accessories simplified film mounting, reduced alignment errors and improved throughput. Interference from sorbitol-based clarifiers was identified by an absorption band at 695 cm⁻¹; corrections or alternative sampling are required for clarified resins. Highly pigmented or heavily filled samples are not recommended due to scattering effects.

Benefits and Practical Applications

This FTIR-based approach offers:

• Rapid, non-destructive analysis without chemical reagents.
• Minimal sample handling and real-time spectral feedback to ensure correct film positioning.
• Automated software workflows for peak measurement, ratio calculation and direct concentration reporting.
• Suitability for routine QC in polymer plants, R&D labs and compliance testing.

Future Trends and Opportunities

Emerging developments may include integration with inline process analytics for real-time monitoring during polymer extrusion, extension of calibration models to cover additional comonomer types, and advanced chemometric methods to compensate for additives or pigments. Miniaturized, portable FTIR devices and machine-learning algorithms could further enhance sensitivity and reduce the need for manual baseline corrections.

Conclusion

The presented FTIR method provides a robust, efficient and accurate tool for quantifying statistical ethylene in ethylene–propylene copolymers. Its simplicity, speed and automation make it highly suitable for industrial quality assurance and research settings. Future enhancements in instrumentation and data analysis promise to expand its applicability to more complex polymer systems.

Reference

Collins W., Seelenbinder J., Higgins F. Determination of Percent Ethylene in Ethylene-Propylene Statistical Copolymers. Agilent Technologies Application Note. Publication 5991-0456EN, May 2012.