I pulled this description off the web:
Garlock 1303 FEP
GraphLock Valve Packing
The Style 1303 Filament Encapsulated Packing process combines GRPAH-LOCK flexible graphite with a thermally conductive, non-scoring inconel filament. It is self-lubricating, non-hardening, dimensionally stable and impervious to gases and fluids.
Style 1303 FEP has been engineered to deliver compliance to the most stringent VOC and VHAP emissions regulations while offering the security of fire safety. It handles easily, retains integrity when cut, packed, or formed, and reduces inventories and downtime by offering the convenience of a spool stock.
Garlock Style 1303 FEP Specifications
Temperature: -328°F to +1200°F in steam
+850°F in atmosphere
pH Range: 0-14 (except strong oxidizers)
Pressure: 4500 psi / (320 bar)
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Looks expensive to me. I'd have to have a pretty special reason to use it instead of something a bit more acessible/affordable.