Material Compatibility And Long-Term Aging Performance of Silicone Rubber Interfaces in High-Voltage Plug-In Connectors

1. Material Compatibility of Silicone Rubber Interfaces

Silicone rubber is valued for its hydrophobicity, flexibility, thermal stability, and dielectric strength. However, interface compatibility with adjacent materials is critical.

1.1 Conductor Interface (Copper/Aluminum)

Silicone rubber does not chemically bond to metals. While this allows for removable connections, poor interface adhesion can trap air gaps, leading to partial discharge. Compatibility can be improved by applying a suitable primer or using a silicone rubber formulation containing adhesion promoters. Additionally, metal inserts should be free of sharp edges to avoid stress concentration and interface delamination.

1.2 Plastic Housing Interface (PBT, PA, PC)

Thermoplastics like PBT or polyamide are common housing materials. Silicone rubber typically exhibits low adhesion to these plastics. Over time, plasticizers from thermoplastics may migrate into silicone rubber, causing swelling, softening, or loss of mechanical strength. Conversely, low-molecular-weight siloxanes from silicone rubber can diffuse into plastic surfaces, leading to stress cracking. Compatibility testing per ISO 10993 or ASTM D2000 is recommended to evaluate weight change, hardness shift, and surface cracking.

1.3 Grease and Coating Interfaces

In high-voltage plug connectors, silicone grease is often applied to the rubber interface to reduce friction, prevent tracking, and exclude moisture. While silicone grease is generally compatible with silicone rubber (same base polymer), hydrocarbon-based greases must be avoided as they cause severe swelling. Fluorosilicone greases offer better chemical resistance in harsh environments.

2. Long-Term Aging Performance

Aging phenomena at silicone rubber interfaces can lead to electrical failure, loss of sealing, or mechanical detachment. Key aging factors include temperature, electric field, humidity, and mechanical cycling.

2.1 Thermal Aging

Silicone rubber is rated for continuous service from –50 °C to 200 °C, depending on the formulation. However, the interface ages faster than the bulk material. At elevated temperatures (e.g., 150–200 °C), crosslink density increases due to post-curing, resulting in increased hardness and reduced elongation. More critically, thermal cycling induces differential expansion between silicone rubber (CTE ~ 300 ppm/K) and metal/plastic (CTE ~ 20–80 ppm/K). Cyclic stress can cause interfacial de-bonding, creating voids that become partial discharge sites.

2.2 Electrical Aging (Partial Discharge and Tracking)

Under AC or DC high voltage, the interface between silicone rubber and a conductive layer (e.g., a stress control tube) experiences electrical stress concentration. If the interface contains microscopic gaps, partial discharge (PD) erodes the silicone rubber surface, converting it into a brittle silica-like residue. This material has lower permittivity and reduced elasticity, further amplifying PD. Over time, this can lead to electrical treeing and eventual flashover. Standard tests such as IEC 60587 (tracking resistance) are used to evaluate performance.

2.3 Environmental Aging (Humidity, Salt Spray, UV)

Silicone rubber is hydrophobic, but the interface can absorb moisture if the seal is imperfect. Hydrolytic aging at high temperature (e.g., 85 °C/85% RH) may cause chain scission in certain filler systems, reducing tensile strength. Salt spray (coastal or de-icing environments) accelerates corrosion of metal inserts, and corrosion byproducts (e.g., copper oxides) can migrate into the silicone rubber, increasing conductivity and causing local dielectric breakdown. UV exposure primarily affects exposed surfaces, but not enclosed interfaces.

2.4 Mechanical Aging (Compression Set and Repeated Mating)

Plug-in connectors are subjected to insertion/withdrawal cycles. Silicone rubber exhibits excellent elasticity, but permanent compression set occurs over time, especially at high temperatures. A compression set exceeding 30% reduces contact force, compromising both electrical continuity and sealing. The interface region—where the rubber contacts a rigid surface—suffers the highest strain. Testing per ASTM D395 (compression set) is essential for design validation.

3. Failure Modes at the Interface

Based on field experience and accelerated life tests, common failure modes include:

· Debonding: Loss of adhesion between silicone rubber and metal/plastic, often preceded by moisture ingress.

· Swelling or dissolution: Caused by incompatible greases or plasticizer migration.

· Silica formation: From prolonged PD or corona exposure at the interface.

· Cracking: Resulting from thermal shock or excessive compression set.

4. Best Practices for Improving Interface Reliability

· Selectively match materials: Use adhesion-promoted silicone grades for metal interfaces and avoid incompatible thermoplastics.

· Control interfacial pressure: Design with adequate interference fit (typically 10–20% compression) to eliminate air gaps.

· Use filler-treated silicone rubber: Alumina trihydrate (ATH) or silica fillers improve tracking resistance.

· Perform combined aging tests: Simultaneously apply thermal, electrical, and mechanical stresses (e.g., CENELEC prEN 50696 or IEC 62271-209).

· Avoid hydrocarbon greases: Always use fluorosilicone or pure silicone greases designed for HV applications.

Conclusion

The long-term reliability of high-voltage plug-in connectors depends critically on the material compatibility and aging behavior of silicone rubber interfaces. While silicone rubber offers excellent intrinsic properties, interface failures often arise from thermal cycling, partial discharge, plasticizer migration, or incompatible lubricants. A systematic approach—combining proper material pairing, controlled interface geometry, and multi-stress accelerated aging tests—is essential for achieving service lifetimes exceeding 30 years in grid and EV infrastructure applications.