In the early days, watches did not have straps, and pocket watches were considered symbols of identity and status. It wasn't until World War I that watch straps appeared, with leather and steel becoming the predominant materials.

As watch designs diversified, so did the materials used for watch straps. These now include metal, ceramic, leather, canvas, nylon, plastic, silicone rubber, and more.

The use of rubber straps in traditional high-end wristwatches showcases the dynamic personality of trendsetters, while enhancing the creativity of the watch's appearance.

Among the various rubber types, silicone rubber is the most familiar elastic material. Yet, do you know there’s another elastic material that surpasses silicone rubber in terms of heat resistance, oil resistance, and corrosion resistance?

That is fluoroelastomer.

1. Development Overview of Rubber Strap

Rubber is a highly elastic polymer material capable of reversible deformation. It exhibits elasticity at room temperature and can undergo significant deformation with minimal external force, returning to its original state upon removal of the force.

Early as twenty years ago, rubber was first incorporated into high-end wristwatches. Today, in the era of high technology, this material has become synonymous with "fashion". Their outstanding features, including resistance to sweat and moisture, being non-allergenic, and comfort of wear, have led to widespread use in sports watch series. With the development of smartwatches, the exploration and research of new rubber straps have become a major focus.

Since the first generation in 2015, the Apple Watch has utilized "high-performance fluoroelastomer," also known as "fluoro rubber," in its sports bands. This material boasts exceptional durability while maintaining remarkable softness.

2. Classification of Fluoroelastomers

Fluoroelastomer, FKM, FPM, and Viton® are all different names used to refer to the same family of synthetic rubber materials. There are even more names for this material than we have listed here, these are simply the most common names. Why are there so many different names for the same material?

Here is what each of those names means:

• Fluoroelastomers are a family of fluoropolymer rubbers.
• FKM is the American standard (ASTM) short form name for Fluoroelastomers or fluoro rubber material. F stands for Fluoro; the K is an abbreviation of the German word Kohlenstoff, meaning Carbon; and the M is the designation of saturated backbone rubber from ASTM.
• FPM is the international ISO 9000 and ISO/TS 16949 standard registration for Fluoroelastomers.
• Viton® is a registered trademark of DuPont Performance Elastomers LLC. Back in 1958,DuPont developed this compound to meet the stringent requirements of the aerospace industry for a higher-performing elastomer. The use of the material quickly spread to other industries such as the automotive, chemical, and fluid power industries.

Fluoroelastomers can be divided into various categories based on the amount of fluorine present. The three primary types include:

• FPM/FKM (Fluorocarbon Elastomers): This is the most common type with a fluorine content usually between 66% and 70%. This class includes the well-known brand of Viton®.
• FFKM (Perfluoroelastomers): These are the most chemically resistant elastomers available. They are fully fluorinated, containing more than 70% fluorine.
• FEPM (Tetrafluoroethylene Propylene Elastomers): Also known as Aflas®, these have a fluorine content of around 54% and offer unique resistance to certain chemicals.

3. Structure and Properties of Fluoroelastomers

Fluoroelastomers refer to synthetic high molecular weight elastomers containing fluorine atoms on the main chain or side chains.

The introduction of fluorine atoms imparts excellent resistance to heat, oxygen, oil, corrosion, and weather to rubber. It finds extensive applications in aerospace, aviation, automotive, petroleum, and household appliance industries, making it an indispensable material in cutting-edge defense industries.

Fluorine is the most electronegative element among known chemical elements. In fluoroelastomer, the C-F chemical bonds formed by fluorine atoms and carbon atoms have high bond energy, indicating its resistance to breakage and chemical stability.

The small radius of the fluorine atom, equivalent to half the length of a C-C bond, allows fluorine atoms to tightly surround carbon atoms, forming a protective barrier around the C-C bonds. This imparts chemical inertness to the C-C bonds in fluorinated high molecular weight elastomers. The fluorine atoms surround the carbon chain and C-F bonds, making them less susceptible to attack by other chemicals, molecules, ions, or atoms.

As the number of fluorine atoms attached to carbon increases, the strength of the C-F bonds also increases. The addition of fluorine to carbon atoms also increases the bond energy with other atoms, thereby improving the resistance to heat and corrosion of fluoroelastomers.

Additionally, fluorine atoms enhance the rigidity of fluoroelastomers, resulting in a decrease in flexibility and low-temperature resistance. Fluoroelastomers with higher fluorine content have increased fluid resistance as the fluorine levels go up.

Thanks to this unique molecular structure, fluoroelastomers exhibit excellent resistance to heat, chemicals, solvents, fluorination, vacuum, oil, and aging, among other properties. Fluoroelastomers were initially used in the aerospace industry, but their most widespread application is in the automotive industry, accounting for 60% to 70% of total usage.

(To be continued)