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| Research on the Aging Behavior and Air-Tightness Failure of Nitrile Rubber for Transformers under the Combined Action of Thermal-Mechanical Stress and Oil Erosion |
| Lin Yuandi1, Wang Tonglei1, Liang Jiabi1, Li Xuan1, Gao Sihang2 |
1. Electric Power Research Institute of State Grid Jiangsu Electric Power Co. Ltd Nanjing 211103 China;
2. School of Automation Chongqing University of Posts and Telecommunications Chongqing 400065 China |
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Abstract Nitrile butadiene rubber (NBR) is extensively employed for transformer sealing owing to its superior oil resistance and mechanical properties. However, the aging kinetics and the fundamental mechanism leading to air-tightness failure in NBR under the synergistic influence of thermal, mechanical stress, and oil immersion conditions remain inadequately understood. This knowledge gap significantly constrains the accuracy of sealing state assessment and the reliability of long-term service life predictions. Particularly in practical transformer operations, sealing rings are subjected to complex multi-stress coupling effects, yet current research predominantly focuses on single-factor aging, failing to adequately reflect actual service conditions.
Consequently, accelerated aging tests were performed under four distinct conditions: thermal-air, thermal-stress-air, thermal-oil, and thermal-stress-oil combination, at 100℃, 110℃, and 120℃. This experimental design enables the isolation and analysis of individual factor effects while capturing their synergistic interactions. The evolution of key mechanical properties, including hardness, tensile strength, elongation at break, and compression set, was monitored throughout the aging process. The measurement intervals were arranged to capture the complete degradation trajectory, with higher frequency sampling during initial rapid degradation phases. A nitrogen pressure-holding test was employed for samples undergoing thermal-stress-oil aging to determine the end-point of functional air-tightness failure. This functional failure criterion represents a more engineering-relevant assessment compared to conventional mechanical property thresholds. To elucidate the underlying physicochemical mechanisms, the aged sealing rings were subjected to Fourier transform infrared (FTIR) spectroscopy and cross-sectional scanning electron microscopy (SEM) analysis.
The results demonstrate that the mechanical properties of NBR undergo significant degradation over aging time. The combined application of mechanical stress and oil erosion exacerbates the deterioration, accelerating the decrease in hardness and tensile strength while increasing the compression set rate and elongation at break. The air-tightness functionality failed after 124, 85, and 56 days of continuous thermal-stress-oil aging at 100℃, 110℃, and 120℃, respectively. Air-tightness failure consistently occurred when compression set reached about 65%, establishing a critical threshold for functional failure. Microscopic analysis reveals that the breakage, cross-linking, and oxidation of molecular chains within the rubber matrix lead to severe microstructural damage, manifesting as microcracks, voids, and particle aggregation. This microstructural deterioration causes the collaborative degradation of macroscopic mechanical properties and the ultimate loss of sealing function. In general, NBR exhibited a milder aging reaction, a later appearance of the mechanical property inflection point, and superior overall air-tightness durability. Finally, using the air-tightness failure data as the functional end-point, a service life prediction model for NBR was established based on the Arrhenius equation. The model shows correlation with experimental data. The model forecasts a service life ranging from approximately 9.59 years at 30℃ to 1.07 years at 80℃ for typical transformer operating conditions. This study provides critical experimental data and theoretical support for the accurate assessment of the sealing state and the prediction of the remaining service life of NBR seals in power transformers, thereby contributing to enhanced operational reliability.
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Received: 22 September 2025
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2026, Vol. 41 
