Turbine Oil Antioxidant Additives: Formulation History, Updates, and Impact on Varnishing

Introduction

The formulation of turbine oils has undergone significant changes over the decades, driven by advancements in base oil refining technologies and the evolution of additive chemistries. These changes, while improving the performance and longevity of turbine oils, have also introduced challenges such as an increased propensity for varnishing, especially when used with API Group II and III base oils. This article explores the history of antioxidant additive formulations in turbine oils, their updates, and their relationship with varnishing.

Historical Perspective: Group I Base Oils and Antioxidant Additives

Characteristics of Group I Base Oils

Historically, turbine oils were formulated primarily with Group I base oils. These base oils are produced using solvent refining methods and have:

• Higher levels of sulfur and aromatic compounds (polar molecules).
• Lower levels of saturates compared to modern base oils.
• Good solvency for oxidation byproducts due to their polar nature.

Antioxidant Additives in Group I Formulations

In the past, antioxidant systems in turbine oils were simpler, leveraging:

1. Phenolic Antioxidants: These additives scavenge free radicals, interrupting oxidation chain reactions.
2. Aminic Antioxidants: Often used in combination with phenolics, aminic antioxidants offer superior thermal stability and work synergistically to extend oil life.

Group I base oils’ natural solvency helped manage varnish precursors by keeping them dissolved, reducing deposits in the system.

Transition to Group II and III Base Oils

Advancements in Base Oil Refining

The development of hydrocracking and hydroisomerization processes led to the introduction of Group II and III base oils, characterized by:

• Higher saturates content (>90%) and negligible aromatic content.
• Lower sulfur levels.
• Superior oxidation and thermal stability.
• Reduced polarity compared to Group I base oils.

Benefits and Challenges

While these base oils brought about improved oxidation resistance and longer oil life, their low solvency created challenges, particularly:

• Poor dissolution of oxidation byproducts.
• Increased risk of varnishing as insoluble degradation products precipitate out.

Modern Antioxidant Additives: Updates and Formulation Strategies

Enhanced Antioxidant Systems

To address the solvency challenges of Group II and III base oils, modern turbine oils incorporate advanced antioxidant systems:

1. High-Performance Aminic Antioxidants: Designed to provide long-lasting protection against high-temperature oxidation.
2. Synergistic Blends: Combining phenolic and aminic antioxidants for balanced performance across a range of operating conditions.
3. Secondary Antioxidants: Including sulfur-containing compounds or metal deactivators to prevent catalytic oxidation from trace metals.

Varnishing Phenomenon in Modern Oils

Causes of Varnish Formation

The propensity for varnishing has increased with the use of Group II and III base oils due to:

• Reduced polarity leading to the precipitation of insoluble byproducts.
• Extended oil life allowing for the accumulation of degradation products over time.
• High operating temperatures accelerating oxidation rates.

Addressing Varnish Issues

Strategies to minimize varnish formation include:

1. Improved Oil Monitoring: Frequent analysis of varnish potential using techniques such as MPC (Membrane Patch Colorimetry).
2. Filtration Technologies: Employing Patented Ion Exchange Resin Based filtration solutions of EPT Clean Oil to remove varnish precursors.
3. Customized Additive Packages: Tailoring antioxidant and dispersant chemistries for specific applications.

Conclusion

The shift from Group I to Group II and III base oils has revolutionized turbine oil formulations, enhancing performance while introducing new challenges, such as varnishing. Antioxidant additive technologies have evolved to address these challenges, with synergistic systems and deposit control additives playing pivotal roles. To ensure optimal turbine performance and oil longevity, it is critical to select oils with formulations tailored to the specific requirements of modern equipment and to employ robust maintenance practices, including regular oil analysis and varnish mitigation strategies.

Great Resouces to know more:

Do Group II Oils Deserve Their Bad Reputation?

Ksp & Thermodynamic Considerations To Remove Varnish