Evolution of Seal Materials
The evolution of seal materials evolved as seals faced more rugged demands. Early needs could be met using packed hemp or leather, but as the demands became more and more rigorous new materials were sought. This led to the introduction of natural rubber seals, which evolved into synthetic elastomers, and finally engineering polymers like teflon PTFE (Polytetrafluoroethylene).
Four of the most common modern material options for rotary shaft seal lips are nitrile rubber, polyacrylate, FKM, and PTFE. Of these materials, three are elastomers – nitrile rubber, polyacrylate rubber and FKM – and only one is polymer – PTFE. Let’s see how these materials stack up.
Material 1: Nitrile Rubber
Nitrile rubber goes by quite a few names, including acrylonitrile butadiene rubber, Buna-N, and NBR. Basically it is a synthetic rubber elastomer that is highly resistant to some key chemicals like oils, lubricants, and fuels. Compared to other elastomers, it does an outstanding job of resisting degradation and exposure to the sun and weather. It has it limits, though.
Material 2: Polyacrylate Rubber
Polyacrylate refers to polyacrylate rubber and is sometimes referred to as ACM. It provides better heat resistance and is compatible with higher shaft speeds than nitrile rubber. It’s also quite good in some specialty applications such as lubricants that include sulfur. Its limited strength and water resistance are its major limiting characteristics.
Material 3: FKM (Fluoroelastomers)
Here’s an interesting fact: people often get confused about the difference between FKM, FPM, and Viton. They are all referring to the same base material. The name FKM finds its roots in ASTM classifications of flouroelastomers, while FPM is the DIN/ISO abbreviation. Viton is its trade name, owned by DuPont. The properties of FKM are far superior to that of either nitrile rubber or polyacrylate rubber, both in terms of temperature and shaft speed, but also chemical resistance. It’s also the most expensive of the three elastomers discussed so far.
Material 4: PTFE (Polytetrafluoroethylene)
PTFE is best known by its trade name, Teflon (also owned by DuPont). teflon far exceeds the performance of nitrile rubber, polyacrylate rubber, and FKM in terms of shaft speed, temperature, and chemical resistance. In fact, is has the best chemical resistance of any polymer or elastomer as well as the lowest coefficient of friction.
The chart below shows how these four common seal materials stack up to each other in terms of their shaft speed limitations. PTFE outshines the rest, even at shaft speeds in excess of 30,000 rpm. When you are selecting a seal that needs to survive a corrosive and challenging environment at elevated speeds, look no further than PTFE.