FKM Rubber, short for fluoroelastomer, is a synthetic rubber originally developed for aerospace and industrial sealing, where heat, chemicals and ultraviolet light see off lesser compounds. Once it is moulded into a watch strap, it is vulcanised under high heat and pressure, cross-linking the polymer chains and locking in elasticity, structural integrity and chemical resistance. Because the material is non-porous, it does not absorb water or sweat, and because it carries no static charge, it does not attract dust and lint the way silicone does. On the wrist it feels firmer and denser than silicone, which reads as premium rather than floppy.
The easiest place to see what FKM offers is in fitted straps cut for a specific case. Crafter Blue makes curved-end vulcanised FKM straps such as the TD01 for the 41mm Tudor Black Bay and the TD03 for the 39mm Black Bay 58, usually selling around £50 to £60. Rubber B hand-finishes its vulcanised rubber in Switzerland and fits it to every Rolex Submariner generation, from the 40mm non-ceramic on an Oyster clasp to the current 41mm ceramic Glidelock, typically running a couple of hundred pounds. Zealande offers fitted FKM options for Rolex, Omega, Tudor, TAG Heuer, Panerai and Sinn, while universal straight-end FKM straps from various makers start much lower.
When you buy, confirm two things: that the strap is genuinely FKM and not silicone or NBR sold loosely as rubber, and whether it is fitted to your case or a universal straight-end design. A model-specific curved end closes the gap between case and strap and makes the watch look integrated. FKM is the sensible pick for divers, anyone in a hot climate, and collectors who want a strap that survives sunscreen, chlorine, salt water and DEET insect repellent without fading. The trade-offs are real: it costs more than silicone and feels stiffer at first, a firmness that does not soften much but rarely bothers anyone.
Do not confuse FKM with cheaper rubbers. Bonetto Cinturini's well-loved 300L, for instance, is vulcanised NBR rather than FKM, and most budget straps are silicone. FKM's edge is chemical resistance and colour stability, which is why it now equips serious dive and tool watches. Given that the first 1960s Submariner and Fifty Fathoms shipped on perforated Tropic-style rubber, FKM carries that idea forward with better material science, and a modern brand like Panerai, which leans heavily on rubber straps, takes to it naturally.