Silicone Rubber for Sealing Applications
Silicone rubber covers a service range of −60 °C to +200 °C for standard HTV compounds, extending to −60 °C to +230 °C with phenyl-modified grades — a continuous operating window no elastomer except PTFE can match. Compression set after 70 h at 175 °C (ASTM D395 Method B) is <25% for well-formulated HTV and <10% for platinum-cured LSR, making silicone the standard choice wherever long-term sealing force retention is required. Hardness can be tailored from Shore 20A (very soft, low-stress static seals) to Shore 80A (high-pressure dynamic applications) within the same silicone chemistry.
Compound Selection by Environment
| Environment | Recommended Grade | Key Property |
|---|---|---|
| Air, steam, water | Standard HTV (VMQ) | Cost, temp range |
| Fuel, oil, hydraulic fluid | Fluorosilicone (FVMQ) | Chemical resistance |
| Food contact / medical | FDA 21 CFR / USP Class VI | Compliance |
| Automotive coolant | Silicone + EPDM blend | Glycol resistance |
| Dynamic seals (rotating shaft) | LSR 40–60A | Low friction, precise |
Grade selection begins with the service fluid. Standard VMQ is cost-effective for hot air, steam autoclave seals, and clean water. Fluorosilicone (FVMQ) is mandatory wherever hydrocarbon fuels or oils are present. FDA 21 CFR 177.2600 compliance and USP Class VI biocompatibility testing are required for food-contact and implantable medical seals. Silicone-EPDM blends address the glycol swelling that limits pure silicone in automotive coolant circuits.
Fluorosilicone for Fuel Systems
FVMQ (fluorosilicone rubber) is based on 3,3,3-trifluoropropyl methyl siloxane — the trifluoropropyl pendant group imparts resistance to petrol, diesel, jet fuel, and MTBE that is impossible in standard polydimethylsiloxane. Volume swell of FVMQ in ASTM Reference Fuel C is <5%, compared with >50% for standard VMQ — a factor-of-ten difference that eliminates seal extrusion and bypass leakage. The usable temperature range of −54 °C to +175 °C positions FVMQ between nitrile rubber (limited to −40 °C cold-start) and Viton (FKM), which is significantly more expensive and offers less flexibility at low temperatures.
Key applications are automotive fuel injector O-rings, fuel pump bowl seals, and aerospace fuel system seals where the combination of fuel resistance, low-temperature flexibility, and thermal stability is non-negotiable.
Compression Set and Seal Life
Compression set measures the permanent deformation remaining after a seal is held at defined strain and temperature — lower values translate directly to longer seal life and sustained sealing force as the component relaxes in its groove. The path to low compression set begins with compound formulation: appropriate crosslink density, absence of processing aids that bleed into the crosslink network, and correct post-cure.
Post-cure at 200 °C × 4 h in a forced-air oven dramatically reduces compression set by removing residual peroxide fragments and completing condensation of Si-OH groups left over from peroxide cure. Platinum-cured LSR achieves compression set <10% directly from mould (vs 15–25% for peroxide HTV before post-cure), because addition cure leaves no byproducts and reaches full network conversion in the mould. This advantage makes LSR preferred for critical medical, food-contact, and precision automotive seals.
Related Reading
HTV Silicone Rubber · LSR Liquid Silicone Rubber · Silicone Rubber for Automotive Seals