Determination of percent glycerol monostearate in polypropylene by infrared spectroscopy

Importance of the Topic

Glycerol monostearate (GMS) and related antistatic additives play a critical role in modifying surface properties of polypropylene for industrial applications. Accurate quantification of GMS in polymer matrices ensures consistent product performance, quality control of additive dosing, and compliance with regulatory and process standards. Infrared spectroscopy offers a rapid, non-destructive method for routine monitoring.

Objectives and Study Overview

This study develops and validates an FTIR-based analytical method to determine weight percent of GMS in polypropylene. The method targets an ester carbonyl absorption band specific to GMS at 1739 cm−1 and uses the polypropylene reference band at 1044 cm−1 for normalization. Calibration covers 0.05–0.8 wt% GMS and addresses potential interferences from other ester-bearing additives.

Methodology and Instrumentation Used

Sample Preparation:

• Polypropylene powder or pellets blended with known amounts of GMS under nitrogen.
• Resin molded into 0.4–0.7 mm films using a hydraulic press (200 °C platen temperature, 25 000 lb force) with optional chase molds and aluminum sheets.
• Films cleaned with lint-free wipe to remove surface residues.

Instrumentation:

• Agilent Cary 630 FTIR with DialPath or TumblIR transmission accessory (1 000 µm path length).
• Equivalent portable FTIR systems (Agilent 4500/5500 series) acceptable.
• Hydraulic press and optional chase mold setup for film thickness control.

Data Acquisition and Calibration:

• Spectral resolution ≥ 4 cm−1, 74 scans (30 s) per film.
• Peak height measured at 1739 cm−1 (GMS ester) and at 1044 cm−1 (polypropylene reference) against defined baselines.
• Normalized absorbance ratio (A1739/A1044) inserted into linear regression: Wt% GMS = 1.761 × (A1739/A1044) + 0.145.

Key Results and Discussion

The calibration curve exhibited high linearity across the validated range. Triplicate film measurements demonstrated reproducible results with absorbance values below 1.6 AU. Interference by ester-based antioxidants (e.g., Irganox 1010/3114) was identified by a band at 1745 cm−1, requiring spectral correction factors when present.

Benefits and Practical Applications

This method enables fast, reliable process control of GMS dosing in polypropylene manufacturing. It eliminates laborious wet-chemical analysis, reduces sample preparation time, and permits on-line or at-line quality checks. The turnkey MicroLab PC FTIR software streamlines data handling by automating peak ratio calculations and reporting final wt% values.

Future Trends and Potential Applications

Advancements may include expanding spectral libraries for diverse additive packages, integrating ATR-FTIR for filled or pigmented resins, and coupling with chemometric models to correct for matrix variations. Portable FTIR devices could facilitate in-field polymer monitoring and broader implementation in recycling quality assessment.

Conclusion

The described FTIR method offers a straightforward, accurate approach to determine GMS content in polypropylene. Its validated performance, minimal interference, and user-friendly software make it suitable for routine QC and process monitoring in polymer production facilities.

Reference

Collins W, Seelenbinder J, Higgins F. Determination of percent glycerol monostearate in polypropylene by infrared spectroscopy. Agilent Technologies Application Note, 2012.