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Aging Mechanism of Silicone Fluid Under Heat and Humidity

As electrical and electronic systems continue to demand higher reliability, especially in tropical, coastal, and high-humidity regions, the long-term stability of insulation materials becomes crucial. Silicone fluid (also known as silicone oil) primarily composed of polydimethylsiloxane (PDMS), is widely valued for its excellent thermal stability, dielectric strength, and chemical inertness.

However, under prolonged high-temperature and high-humidity exposure, silicone fluids experience gradual degradation, leading to insulation performance decay. This article systematically explores the aging mechanisms, the chemical and physical transformations involved, and practical engineering measures for enhancing performance in demanding environments.

1. Overview of Silicone Oils in Insulation Systems

1.1 Composition and Key Properties

Silicone oils, mainly based on PDMS, consist of a Si–O–Si backbone with methyl or phenyl side groups. Their flexible molecular structure contributes to:

Broad operating temperature range (–50 °C to > 200 °C)
High dielectric strength and low loss factor
Excellent resistance to oxidation, UV radiation, and weathering

Despite these advantages, PDMS can degrade under oxidative, humid, or high-temperature stress, resulting in viscosity changes, structural instability, and altered electrical properties.

1.2 Typical Industrial Applications

Silicone fluids are integral in transformers, composite insulators, cable joints, power electronics encapsulation, and thermal management systems. They act as impregnation fluids, insulating fillers, and interface modifiers, ensuring stable electric field distribution and enhanced hydrophobicity.

2. Environmental Stress Factors: Heat and Humidity

2.1 Thermal Stress Mechanisms

Elevated temperature accelerates chain scission, oxidation, and volatilization. At sustained high temperatures:

Si–CH₃ side groups oxidize and detach
Si–O–Si backbones break down
Fillers or stabilizers may migrate or decompose

Thermal aging also induces crosslinking or embrittlement, reducing mechanical flexibility and increasing internal stress.

2.2 Moisture and Hydrothermal Effects

Moisture diffusion and condensation within silicone systems lead to:

Absorption and swelling of the polymer matrix
Hydrolytic cleavage of siloxane bonds
Increased dielectric loss due to water polarization
Studies have shown that moisture-saturated silicone rubbers exhibit up to 40 % higher dielectric loss tangent (tan δ) compared to dry samples.

2.3 Combined Environmental Impacts

In real-world conditions, heat, humidity, and electric field often act simultaneously. This combination accelerates surface erosion, microcrack propagation, and localized discharge, particularly in coastal or polluted atmospheres.

3. Chemical and Physical Aging Mechanisms

3.1 Chain Scission and Thermal Oxidation

The Si–O–Si backbone undergoes oxidative cleavage, forming silanol (Si–OH) and silica-like residues. Low-molecular-weight volatiles may evaporate, lowering viscosity and altering dielectric behavior.

3.2 Moisture Absorption and Interface Deterioration

Water molecules infiltrate through microdefects, reacting with siloxane bonds and promoting hydrolysis. These processes create voids and interfacial delamination, which amplify electrical stress concentration.

3.3 Filler Interaction and Additive Depletion

Common fillers such as silica and alumina improve heat resistance but can adsorb water, acting as aging accelerators if surface treatment fails. Additive depletion (antioxidants, crosslinking agents) also contributes to early deterioration.

3.4 Microstructural Evolution

Microscopic analysis (SEM/FTIR) reveals surface roughening, cracks, and filler migration during aging. These changes serve as pathways for moisture and ion ingress, reducing the material’s insulating integrity.

4. Pathways of Insulation Performance Decay

4.1 Dielectric Constant and Loss Variation

As molecular degradation and moisture absorption proceed, the dielectric constant increases, and the loss tangent rises sharply. This leads to higher energy dissipation and localized heating under AC operation.

4.2 Breakdown Strength Reduction

Thermal and hydrothermal stress introduce defects and voids, reducing breakdown voltage. PDMS-based materials show a consistent drop in dielectric strength after prolonged 150 °C aging.

4.3 Thermal Stability and Mechanical Fatigue

Crosslinking and oxidative reactions stiffen the material, lowering elongation at break and resilience. This mechanical fatigue further exposes internal defects.

4.4 Moisture–Polarization–Heating Cycle

Moisture increases dielectric loss → generates localized heat → accelerates further degradation — forming a self-reinforcing aging loop.

5. Accelerated Aging Studies and Modeling

5.1 Experimental Approaches and Case Studies

Accelerated testing combines temperature, humidity, and electric field cycling to simulate decades of field exposure. Such studies provide data for life modeling and reliability forecasting.

5.2 Typical Results and Key Indicators

Water uptake and diffusion coefficients rise over time.
Dielectric loss tangent (tan δ) correlates linearly with exposure duration.
Thermogravimetric analysis (TGA) reveals reduced onset temperatures for thermal decomposition.

5.3 Life Prediction Models

Empirical models suggest that under 20 °C and 70 % RH, silicone rubber systems can retain functionality for up to ~20 years, while harsher environments significantly shorten lifespan.

6. Engineering Strategies and Material Solutions

6.1 Advanced Silicone Formulations

Use heat- and oxidation-resistant silicone oils (e.g., phenyl-modified PDMS).
Incorporate nano-fillers (TiO₂, SiO₂, Al₂O₃) to improve dielectric and mechanical strength.
Employ surface-treated fillers to reduce moisture adsorption.

6.2 Design and Environmental Protection Measures

Seal interfaces to prevent moisture ingress.
Apply hydrophobic coatings or barriers in coastal and tropical climates.
Allow mechanical tolerance for thermal expansion and contraction.

6.3 Monitoring and Predictive Maintenance

Regular infrared thermography for hot-spot detection.
Dielectric loss monitoring for early warning of water ingress.
Scheduled maintenance in high-risk zones (e.g., high humidity substations).

6.4 Application Insights for Silico® Products

At Silico®, our silicone-based insulating oils are engineered to withstand thermal oxidation and moisture intrusion. By integrating advanced molecular stabilization and precision filler treatment, Silico® products deliver long-term dielectric stability and superior performance in extreme environments.

We also provide custom aging simulations and reliability assessments for clients operating in tropical, marine, or industrial sectors.

7. Conclusion

Silicone fluids experience gradual but significant degradation under high-temperature and high-humidity conditions, primarily through thermal oxidation, hydrolysis, and structural fatigue. These processes collectively lower dielectric strength, increase energy loss, and reduce system reliability.

Implementing optimized formulations, protective design strategies, and predictive maintenance can effectively extend the lifespan of silicone-based insulation systems. Silico® remains committed to advancing this field through continuous research and sustainable innovation.