Rubber

In watchmaking, the term rubber covers a family of elastomers that are highly diverse, and confusing them is an error you pay for over time. The fundamental distinction is between natural rubber (vulcanized latex), first-generation synthetic rubbers like silicone, and high-performance technical elastomers — HNBR and FKM leading the way — which dominate professional production today.

Vulcanized latex is the historical material: you find it in the so-called tropic straps of the 1960s and 1970s, associated with models like the Speedmaster Professional in tropical and underwater settings. Vulcanization hardens and stabilizes natural latex, making it less prone to hydrolysis; manufacturer Bonetto had made it a specialty with the ladder pattern construction, which increases transverse flexibility and aeration. The drawback of latex is that it ages: it hardens, develops cracks, loses elasticity under prolonged UV exposure.

HNBR (Hydrogenated Nitrile Butadiene Rubber) solves part of this problem: it is a hydrogenated nitrile rubber, more thermally and chemically stable than latex, which maintains elasticity even at low temperatures and resists chlorine and salt water — critical conditions for serious diving, not just beach sports.

FKM, commercially known as Viton®, is the fluoroelastomer: the acronym stands for fluorine-kautschuk material, fluorocarbon rubber. It is the material you find today in the most demanding contexts, from bracelets built on internal metal blades to hybrid aviation straps with textile inserts. The practical advantages are concrete: it requires no break-in (unlike natural rubber, which is stiff the first few days), is hypoallergenic, dries quickly, doesn't absorb sweat and doesn't retain odors. It resists UV and thermal shocks without losing dimension or color. Working with it requires precision molds: this is why manufacturers using FKM with engraved textures — sailcloth patterns, Saffiano motifs, cordura surfaces — must invest in expensive tooling, and you can see it in the surface quality.

Mirum deserves separate mention: it is a material derived from natural rubber, oils and plant waxes, without plastics or PVC, proposed as an alternative to synthetic elastomers for those wanting to reduce the petrochemical component. It is still a niche, but the direction is interesting and the mechanical performance in the tests I've seen is acceptable for daily use.

From the bench: rubber should not be stored completely dry. The main enemy — after UV — is ozone, present in urban areas. A latex strap kept in a closed drawer next to an electric motor (which produces ozone) will crack in a few months. FKM is more tolerant, but not invulnerable.