Outdoor seals die from weather, not pressure. The failure sequence is predictable: UV and ozone attack the polymer chain, the surface micro-cracks, compression set accelerates, and the seal weeps at -20°C after the first winter.
Two compound families survive this: silicone and EPDM.
Silicone: best-in-class UV and ozone immunity (essentially inert), widest temperature range (-70~232°C — important for enclosures that freeze at night and cook at noon), low compression set, available in FDA grades. Trade-offs: poor resistance to mineral oils, higher material cost, lower tear strength — handle with gland design.
EPDM: excellent ozone and UV resistance too, superb in water and steam, flexible at -50°C, and significantly cheaper. Trade-offs: top temperature ~150°C, swollen by oils and hydrocarbons — avoid anywhere near fuel or grease.
Selection pattern by application: solar junction boxes and tracker joints — silicone; EV charging gun seals — silicone (temperature plus cleanliness); telecom cabinets and HVAC — EPDM; cable entry grommets — silicone for high-temperature panels, EPDM elsewhere.
A design detail that outlives material choice: compression ratio. For outdoor flat gaskets keep 20-30% compression at assembly — enough to absorb thermal movement, not so much that stress relaxation wins.