FKM is a class of fluoroelastomer synthetic rubbers; the most common trade name is Viton (DuPont). The materials were developed in the 1950s for industrial sealing applications (chemical-plant gaskets, automotive fuel-system seals) where chemical resistance and temperature stability were critical. The properties translate directly to watch-strap requirements: a strap that survives chlorine pool water, salt water, sunscreen, DEET insect repellent, vehicle fuels and oils, and high temperatures without degradation.
In watch-strap applications, the alternative materials are silicone (cheaper, less chemically resistant, yellows in sunlight, cracks over years) and standard rubber (NBR, EPDM, similar limitations). FKM's UV stability and chemical inertness mean an FKM strap can spend years in tropical / saltwater / pool conditions without visible degradation; silicone straps in the same conditions yellow and crack within 1-3 years.
"Silicone is a strap. FKM is a watch component. The price difference is the difference between buying twice and buying once."- Dive-watch retailer on rubber-strap selection
The defining modern FKM strap is Rolex Oysterflex: introduced 2015 on the Yacht-Master 116655, the strap is FKM rubber over a flexible titanium-nickel core, providing the comfort of rubber with the security of bracelet-style construction. Omega's integrated rubber straps on the Seamaster Diver 300M ceramic and Planet Ocean references are FKM. Hublot uses FKM extensively on Big Bang and Spirit of Big Bang references; AP Royal Oak Offshore Diver rubber strap is FKM.
The aftermarket FKM market exploded through the 2010s. Crafter Blue, RubberB, and Erika's Originals are dominant aftermarket FKM-strap suppliers; specialist makers offer brand-and-model-specific FKM straps for vintage and modern Rolex, Omega, Tudor, Panerai, IWC, and microbrand divers. Prices typically USD 80-200 retail; the cost reflects the material expense plus the precision-fit tooling required for individual watch references.
For buyers, the practical guide: FKM is the right strap for dive / sport / outdoor wear. The premium over silicone (typically 3-5×) is justified for owners who actually use their watches in pool / salt water / outdoor conditions; for desk-bound or occasional-wear pieces, silicone is acceptable. FKM straps do not stretch with age, do not discolour from sweat or sunscreen, and last essentially indefinitely. The disadvantage: FKM is heavier than silicone (the material has higher density), and the rubber feel is slightly stiffer.
