HERCULES URETHANE BACKUP Ring 90A DUROMETER

90A puts it on the firmer end of the urethane range, which is actually what you want for backup ring duty. Softer materials tend to extrude under pressure, and a backup ring's whole job is to keep the primary seal from getting pushed into the hardware gap. Urethane at 90A holds its shape well under load, so it's a solid choice for hydraulic cylinder applications where extrusion resistance matters.