Determination of the vinyl content of polyethylene resins

Importance of the topic

The determination of vinyl end‐groups in polyethylene resins is critical for controlling polymer properties such as crystallinity, mechanical strength and chemical reactivity. Vinyl content reflects catalyst performance and polymer chain termination mechanisms, making its accurate measurement essential for quality control in polymer manufacturing, R&D and performance specification.

Objectives and Overview of the Study

This work describes an FTIR‐based method to quantify the number of vinyl (C=C) sites per 1000 carbon atoms in polyethylene samples. The method uses infrared absorption at 908 cm⁻¹ for the vinyl group and a reference band at 2019 cm⁻¹, calibrated against standards characterized by C-13 NMR. It applies to powders, pellets and molded films of unfilled, unpigmented resins produced with chromium‐based catalysts.

Methodology and Instrumentation

Sample Preparation:

• Statistically representative resin sampling (cone and quarter, rotary splitter).
• Molding into 0.4–0.5 mm films using a heated hydraulic press at 200 °C (minimal pre‐force 1–2 min, then 25 000 lb for 30 s).
• Inspection and selection of defect‐free films within thickness tolerance; at least three replicates per sample.

Instrumental Setup:

• Agilent Cary 630 FTIR (or equivalent 5500/4500 Series) with DialPath or TumblIR transmission cell (1000 µm path).
• Resolution: 2 cm⁻¹ or better; scan time: ≥30 s (37 scans).

Data Processing:

• Baseline‐corrected peak area measurement at 908 cm⁻¹ (dual baseline at 926–898 cm⁻¹).
• Reference band area at 2019 cm⁻¹ (baseline 2097–1987 cm⁻¹).
• Calculation of A908/A2019 ratio and application of linear regression equation: number C=C per 1000 C = 2.751×(A908/A2019) – 0.111.

Main Results and Discussion

Calibration across three standards yielded a highly linear response (R² = 0.999, 95 % CI ±0.1 vinyl/1000 C). The method consistently distinguished high‐vinyl chromium‐catalyzed samples (>0.5 vinyl/1000 C) from low‐vinyl titanium‐catalyzed resins (<0.5 vinyl/1000 C). Replicate analyses showed excellent precision and ease of real‐time sample positioning with the DialPath and TumblIR cells.

Benefits and Practical Applications

• Rapid, non‐destructive quantification of vinyl end‐groups without extensive sample preparation.
• Reproducible results enabled by user‐friendly transmission cells and method‐driven software.
• Applicable to diverse polyethylene formats for QA/QC, catalyst evaluation and product development.

Future Trends and Potential Applications

Advances may include automated film thickness monitoring, expanded spectral libraries for other functional end‐groups, and integration with process analytical technology (PAT) platforms. Combined FTIR‐NMR calibration sets and machine learning could further enhance sensitivity and predictive capability for emerging polymer chemistries.

Conclusion

The described FTIR method using Agilent Cary 630 with DialPath/TumblIR offers a straightforward, accurate and reproducible approach to quantify vinyl groups in polyethylene. Its high linearity, minimal sample handling and adaptable software streamline polymer quality assessment in research and production environments.