Determination of the vinyl content of polyethylene resins

Significance of the topic

Polyethylene resins are ubiquitous in packaging, automotive and medical industries. The concentration of vinyl end groups reflects polymerization catalysts' performance and affects downstream properties such as stability and reactivity. A reliable quantification of vinyl content is hence essential for quality control and material development.

Objectives and Study Overview

This work presents an infrared spectroscopic procedure to quantify vinyl (C=C) end groups in chromium-catalyzed polyethylene. It aims to correlate the absorption intensity at 908 cm-1 with vinyl concentration per 1000 carbon atoms via a calibration based on NMR reference standards.

Methodology

Polyethylene samples (powder, pellets or cut coupons) are compression molded into 0.4–0.5 mm films at temperatures below 250 °C. Infrared spectra are recorded at 2 cm-1 resolution using a transmission accessory. The vinyl CH wag absorption at 908 cm-1 and a polyethylene reference band at 2019 cm-1 are baseline corrected and integrated. The ratio of these peak areas is applied to a linear regression calibration to yield the number of vinyl groups per 1000 carbon atoms.

Used Instrumentation

• Agilent Cary 630 FTIR spectrometer with DialPath or TumblIR interface (1000 μm path length)
• Hydraulic press with heated platens (200 °C, up to 25 000 lbf force)
• Chase mold and aluminum sheets for controlled film thickness

Key Results and Discussion

Calibration against NMR-derived standards produced a highly linear response (R2 = 0.999, 95% CI ±0.1 vinyl per 1000 C). Triplicate analyses demonstrated precision and accuracy across the analytical range. Chromium-based resins exhibited vinyl contents above 0.5 vinyl per 1000 C, while titanium-based catalysts yielded values below this threshold.

Benefits and Practical Applications

This FTIR method enables rapid, non-destructive screening of vinyl content in polyethylene, supporting polymer quality assurance and catalyst performance evaluation. The simple film mounting and software-guided workflow reduce user variability and instrument downtime.

Future Trends and Potential Applications

Future developments may include adapting the technique for pigmented or filled systems, integrating chemometric analysis, and implementing in-line monitoring to enable real-time process control in polymer production.

Conclusion

The described infrared spectroscopy method provides a fast and reliable measure of vinyl end groups in polyethylene films, with strong correlation to NMR standards. Its ease of use and high accuracy make it a valuable tool for both research laboratories and industrial quality control.

Reference

• Collins W., Seelenbinder J., Higgins F. Determination of the vinyl content of polyethylene resins. Agilent Technologies Application Note, 5991-0458EN, May 11, 2012.