O-rings are commonly made out of four different materials. Silicone rubber, neoprene, aflas and polyurethene are most commonly used materials for o-rings.
Custom-made FFKM o-rings - Ridderflex
What are O-rings? - Quora
Which O-ring material is right for my application? - Parker Distributor
An Introduction to Grommets and How They Work
O-rings made of metal? Never seen plugs like this before! They
The Importance of O-Rings
O-Ring Material Options
What Are O-Rings Made Of? 5 Common Materials for Industrial O-Rings, sealing rings
What are O-Rings made of?, O-Ring Material Guide
O-Ring Seals A Versatile Solution for Reliable Sealing by
Seal the Deal: Your Guide to O-Ring Types Article
What is an O ring: materials and operation
O Ring - Precision Brand