Aging Mechanisms and Lifetime Prediction Methods for Silicone Rubber Composite Insulators in High-Temperature and High-Humidity Regions

1. Introduction

Silicone rubber (SiR) has become the material of choice for composite insulator sheathing due to its excellent electrical strength, light weight, and superior anti-fouling properties. To date, over 10 million composite insulators are in service worldwide, with some exceeding 20 years of operational life. However, in high-temperature and high-humidity regions—particularly the developed coastal areas of southern China where temperatures and humidity peak from July to September each year—long-term exposure to such environments causes surface cracking, flashover, and sheath breakage. Understanding the aging behavior and degradation mechanisms of silicone rubber under these conditions is essential for ensuring the safe and reliable operation of power transmission systems.

2. Aging Mechanisms in High-Temperature and High-Humidity Environments

The aging of silicone rubber composite insulators in HTHH environments is a multi-faceted process involving physical, chemical, and electrical degradation mechanisms that operate synergistically.

2.1 Moisture Diffusion and Hydrolysis

Moisture ingress is one of the primary drivers of silicone rubber degradation in humid environments. The absorption of water vapor by HTV silicone rubber follows the Fick diffusion model, which is based on random thermal motion. As hygrothermal aging time increases, the surface morphology of HTV silicone rubber progressively deteriorates, with the appearance of pores and defects that further elevate both the moisture absorption rate and the diffusion coefficient. Elevated environmental temperatures accelerate the moisture absorption process, while water molecules that penetrate the material promote hydrolysis of the siloxane (Si-O-Si) backbone, leading to chain scission and reduced molecular weight.

2.2 Thermal Oxidation and PDMS Depolymerization

Thermal oxidation constitutes another critical degradation pathway. The lifespan of sampled insulators in HTHH regions is predominantly determined by the thermal oxidation process of silicone rubber. Under combined thermal and humid stress, polydimethylsiloxane (PDMS) molecular chains undergo continuous depolymerization. Research has demonstrated that the degree of PDMS degradation in samples subjected to 26 days of hygrothermal aging is comparable to that of shed samples that have been in service for 10 years. The degradation of the PDMS backbone results in a reduction of both Si-O-Si (main chain) and Si-CH₃/Si(CH₃)₂ (side chain) infrared absorption peak heights, which serve as reliable aging characteristic parameters.

2.3 Filler Migration and Surface Degradation

The migration of alumina trihydrate (ATH) filler—a flame-retardant additive commonly incorporated into silicone rubber—is another hallmark of aging in HTHH environments. Damage to the silicone rubber backbone caused by sunlight exposure and electric field stress facilitates ATH migration, which increases material porosity and diminishes corrosion resistance. Concurrently, the surface hydrophobicity of the material progressively deteriorates as the content of hydrophobic functional groups decreases. The decline in hydrophobicity, combined with increased surface roughness and porosity, significantly impairs the insulator's ability to resist contamination flashover.

2.4 Dielectric Degradation and Abnormal Heating

The degradation of silicone rubber materials and their enhanced moisture absorption capacity lead to an increase in the dielectric loss tangent of the saturated moisture-absorbing medium. This elevation in dielectric loss is a primary contributor to abnormal heating at the end of composite insulators—a defect frequently observed in HTHH regions. High-humidity conditions exacerbate this heating phenomenon, creating a positive feedback loop wherein elevated temperatures further accelerate material degradation.

3. Aging Characterization Parameters

To quantitatively assess the aging state of silicone rubber composite insulators, researchers have identified multiple characteristic parameters that exhibit strong correlation with service. Systematic studies of 371 to 391 composite insulators with operating durations ranging from 3 to 22 years in HTHH regions have led to the proposal of 18 to 19 aging characteristic indices.

Key aging characterization parameters include:

· Chemical parameters: Infrared absorption peak heights of Si-O-Si and Si-CH₃ groups, thermogravimetric residual mass, and activation energy calculated from thermogravimetric analysis.

· Mechanical parameters: Tensile strength, elongation at break, and hardness.

· Electrical parameters: Dielectric loss tangent, dielectric constant, and trap charge characteristics.

· Surface parameters: Static contact angle (hydrophobicity), surface morphology, and porosity.

4. Lifetime Prediction Methods

4.1 Physics-Based Models

Physics-based models correlate material degradation with fundamental physicochemical processes. The generalized-Eyring model has been successfully applied to establish the lifetime of silicone rubber under the combined effects of humidity and heat. This model predicts a lifetime of 19.72 years for silicone rubber under conditions of 20 °C and 70% relative humidity, with validation using actual insulators confirming that all predicted values fall within the 99% confidence interval of test results.

The Arrhenius equation, derived from activation energy calculations via thermogravimetric analysis, provides another physics-based approach for lifetime estimation. The thermal-oxygen aging model has also been employed to equivalently simulate the decline process of mechanical properties.

4.2 Statistical and Data-Driven Models

Statistical models leverage operational data to predict remaining lifespan. Research has established that the electrical lifespan of composite insulators in HTHH regions is approximately 18.9 years, while the silicone rubber material itself has a lifespan of about 14.6 years. Based on mechanical load and Fourier transform infrared spectroscopy (FTIR) test results, remaining lifespan can be predicted for insulators in HTHH regions. Validation using 20 insulators that had been in service for 14 years yielded a forecast error of only 1 year.

Grey models, such as GM(1,N), have been applied to predict remaining life using static contact angle and hardness test results. Comprehensive studies have further revealed that composite insulators in HTHH regions face cracking risk after 15.8 years of service and should be progressively replaced after 19.2 years.

4.3 Machine Learning Approaches

Recent advances have introduced machine learning techniques for enhanced prediction accuracy. The GA-SVM (genetic algorithm–support vector machine) model, combined with thermogravimetric analysis, considers not only thermal weight loss rate and temperature but also specific weight loss components. This approach achieves prediction accuracy improvements of 6% to 14% compared to traditional thermal life equations and BP neural networks, which are prone to converging to local optimal solutions.

4.4 Multi-Stress Accelerated Aging Approaches

Given the complexity of actual service conditions, multi-stress accelerated aging test platforms have been developed to simulate the coupled effects of humidity, heat, electricity, and salt spray. These platforms enable the study of silicone rubber aging under conditions that more closely replicate real-world HTHH environments, providing a foundation for more accurate lifetime predictions.

5. Conclusion

The aging of silicone rubber composite insulators in high-temperature and high-humidity regions is driven by synergistic mechanisms including moisture diffusion, thermal oxidation, PDMS depolymerization, filler migration, and dielectric degradation. These processes collectively result in the deterioration of mechanical properties (tensile strength decreasing to 48.5% of initial values), electrical properties (flashover voltage gradient decreasing to 83.3% of initial values), and surface hydrophobicity.

Lifetime prediction methodologies have evolved from traditional physics-based models (Arrhenius, generalized-Eyring) to sophisticated statistical approaches and machine learning techniques (GA-SVM, grey models), with multi-stress accelerated aging tests providing increasingly realistic simulation capabilities. Current research indicates that silicone rubber composite insulators in HTHH regions typically have an operational lifespan of approximately 15 to 20 years, with replacement recommended after 19.2 years of service.

Future research directions should focus on developing non-destructive evaluation techniques for in-service insulators, establishing unified aging characterization standards, and integrating multi-factor coupling effects into more comprehensive prediction models to enhance the reliability of power transmission systems in challenging climatic environments.