Full definition
The ability of a seal or gasket material to resist degradation when in sustained contact with the fluid it is intended to seal — the single most important factor in seal material selection. Incompatibility manifests as: excessive volume swell (>25% — seal becomes oversized, extrudable), hardening/embrittlement (cross-link scission or further cross-linking — seal cracks under compression), softening (plasticizer extraction or polymer chain scission — seal loses sealing force), surface cracking (chemical attack on surface), and complete dissolution (in extreme cases). Evaluation method: immerse specimen in the fluid at service temperature for 70-168 hours per ASTM D471, then measure volume change, hardness change, tensile/elongation change. General compatibility guide: NBR — oils/fuels/greases (not ketones, esters); EPDM — water/steam/acids/bases (not mineral oils); FKM — oils/fuels/acids/most chemicals (not amines, ketones, hot water); silicone — wide temperature extremes (not steam, hydrocarbon solvents); FFKM — virtually everything (cost $50-500/seal). Always verify compatibility with the specific fluid at the actual service temperature — generic compatibility charts are starting points only. Manufacturer databases: Parker O-Ring Handbook, Trelleborg, Freudenberg chemical resistance guides.