Analysis of Diethylene Glycol (DEG) in Polyethylene Terephthalate (PET)

Significance of the Topic

Residual diethylene glycol (DEG) in polyethylene terephthalate (PET) influences mechanical and thermal properties of the polymer. Accurate quantification of DEG is critical for quality control in fiber manufacturing, food packaging, and industrial resin production. Traditional methods are complex and time intensive, prompting the need for faster, more efficient analytical techniques.

Objectives and Study Overview

The application note compares a rapid reactive pyrolysis method using tetramethyl ammonium hydroxide (TMAH) to conventional hydrazinolysis for determining DEG levels in PET. The goals are to:

• Demonstrate equivalence in accuracy between the two methods.
• Highlight the speed and simplicity of reactive pyrolysis.

Methodology and Used Instrumentation

Samples of PET were directly subjected to reactive pyrolysis with TMAH, eliminating the need for chemical pretreatment. Key instrumentation and conditions include:

• Pyrolyzer: PY-2020D (Frontier Laboratories Ltd.)
• Pyrolysis temperature: 400 °C
• Reagent: Tetramethyl ammonium hydroxide (TMAH, (CH₃)₄NOH)
• Detection: Pyrolysis–Gas Chromatography (Py–GC)

Hydrazinolysis was performed according to established protocols for comparison, involving overnight derivatization and subsequent GC analysis.

Main Results and Discussion

Reactive pyrolysis generated a pyrogram featuring major peaks from ethylene glycol (EG) and terephthalate monomers, with smaller, distinct peaks corresponding to DEG. The DEG/EG molar ratios obtained by reactive pyrolysis matched closely with those from hydrazinolysis (Figure 2), demonstrating quantitative agreement within experimental error. The direct pyrolysis approach reduced sample preparation time from hours to minutes.

Benefits and Practical Applications

The reactive pyrolysis technique offers:

1. Rapid turnaround: Direct sample introduction bypasses lengthy pretreatment.
2. Operational simplicity: Minimal handling and reagent use.
3. High reproducibility: Consistent DEG quantification comparable to hydrazinolysis.

These advantages support enhanced throughput in industrial QC labs and research environments, improving decision-making in process management and polymer formulation.

Future Trends and Potential Applications

Advancements may include:

• Integration with automated pyrolysis–GC systems for high-throughput screening.
• Extension to other residual oligomers in polyester and co-polymer matrices.
• Coupling with mass spectrometry for structural confirmation of degradation products.

Such developments will further streamline polymer analytics and expand reactive pyrolysis into broader quality assurance applications.

Conclusion

Reactive pyrolysis with TMAH represents a robust, efficient alternative to hydrazinolysis for DEG determination in PET. By offering comparable accuracy with significantly reduced analysis time and complexity, it stands to enhance polymer quality control workflows.

Reference

Kiura S., Wakabayashi M. Some Recent Applications of Py-GC. 2nd Pyrolysis GC Seminar, Frontier Laboratories Ltd., 1998.