Description Description Freudenberg Sealing Technologies (formerly Tobul) backup ring is a ring of hard, extrusion resistant material such as high nitrile, nylon, Hytrel, or PTFE. A back-up ring is designed to fit between the O-ring and the extrusion gap and prevent the extrusion of the O-ring. State of California Proposition 65 WARNINGCancer and Reproductive Harm - www.P65Warnings.ca.gov. Literature & Docs Accessories