Determination of Phenolic Antioxidant DBPC and DBP Levels in Electrical Insulating Oil

Importance of the topic

The durability and performance of electrical insulating oils in transformers and mineral-oil lubricants rely on phenolic antioxidants such as 2,6-ditertiary-butyl paracresol (DBPC, also known as BHT) and 2,6-ditertiary-butyl phenol (DBP). These inhibitors prevent oxidative degradation, extending oil service life and ensuring reliable cooling and insulation in high-value assets. Regular monitoring of inhibitor depletion is critical to maintain oil quality and prevent equipment failure.

Study objectives and overview

This work presents infrared spectroscopy methods aligned with ASTM D2668 and IEC 60666 standards to quantify DBPC and DBP in new and used mineral oils. The methods extend the measurable range up to 1.0 wt.% DBPC and 0.8 wt.% DBP, exceeding the original standard limits. They are designed for both laboratory and on-site mobile FTIR analysis.

Methodology and instrumentation

Calibration standards were prepared by dissolving known amounts of DBPC in phenolic-free base oils (Base 20 and Base 76).

• ASTM D2668 method: standards from 0–1.0 wt.% DBPC, measured with 100 µm pathlength cells at 8 cm⁻¹ resolution, 128 scans—measurement time ~30 s.
• IEC 60666 method: standards from 0–0.8 wt.% DBPC, measured at 200, 500, and 1000 µm pathlengths at 4 cm⁻¹ resolution, 64 scans.

Used Instrumentation

• Agilent 5500 FTIR spectrometer
• Agilent 4500 portable FTIR spectrometer
• Agilent Cary 630 FTIR spectrometer
• TumblIR and DialPath transmission accessory cells

Main results and discussion

The phenolic OH stretching band near 3650 cm⁻¹ served as the analytical marker. Calibration curves demonstrated excellent linearity (R² ≥ 0.999) across all pathlengths. For ASTM D2668, variance between actual and predicted DBPC concentrations remained below 0.02 wt.% up to 1.0 wt.%. IEC 60666 validation at 1000 µm pathlength showed repeatability and reproducibility well within specified limits, with prediction errors under 0.03 wt.%. The MicroLab software allows users to set threshold limits and provides color-coded results with actionable recommendations to streamline decision making.

Benefits and practical application

• Rapid, on-site quantification of phenolic antioxidants without sample transport delays.
• Extended measurement range beyond standard methods for both new and aged oils.
• Intuitive software with threshold alerts and color coding reduces operator dependence.
• Supports preventive maintenance and manufacturing quality control in power and industrial sectors.

Future trends and opportunities

Advances may include real-time monitoring of antioxidant depletion via inline FTIR probes, expansion to other inhibitor chemistries, cloud-based data management for predictive maintenance, and integration with digital twin platforms for asset health diagnostics.

Conclusion

The developed FTIR methods on Agilent 5500, 4500, and Cary 630 systems provide reliable, sensitive, and rapid measurements of DBPC and DBP levels in electrical insulating oils. By exceeding the concentration ranges of existing standards and featuring user-friendly software, these approaches enhance maintenance strategies for critical energy and industrial equipment.

References

• F. Higgins, Onsite additive depletion monitoring in turbine oils by FTIR spectroscopy, Agilent Technologies, publication 5990-7801EN (2011).