PEEK (polyetheretherketone) is the preferred thermoplastic for backup rings and seal support elements in HPHT. Unfilled and carbon-filled grades resist 260°C continuously and H2S attack. PEEK exhibits virtually no volume swell and prevents elastomer extrusion at 250 MPa. Scarf-cut and endless designs accommodate thermal expansion differences. https://globalsealstech.com/oil-gas-seals/