When it comes to seal materials, Buna Rubber and Viton ® (FKM Fluoroelastomer) gaskets are two of the heavyweight contenders. These two rubber formulations are in a constant battle for the top spot, so what are the important differences between the two?
Both Buna Rubber and Viton ® (FKM Fluoroelastomer) are used extensively in pumping applications, so we will start by using that as our comparison point.
Buna Rubber (Butadiene Acrylonitrile) and Viton ® (FKM Fluoroelastomer) (A Dupont Synthetic rubber and Fluoropolymer Elastomer) are both superior over natural rubber seals in resisting heat aging.
Buna has a moderate temperature resistance with a -30°F to 250°F range of operation. This still gives it a much higher range than natural rubber, but does not allow it to be used in high temperature situations.
Viton ® (FKM Fluoroelastomer) has a higher temperature range, but falls short of Buna on lower temperatures. Viton ® (FKM Fluoroelastomer) can resist temperatures of 5°F to 400°F and can even withstand temperatures up to 600°F for up to 48 hours. If you are pumping high temperature fluids, or operating in high temperatures, Viton ® (FKM Fluoroelastomer) wins the battle.
Buna wins the battle if you are working in cold environments. The -30°F allows it to be used in outdoor pumps in frigid environments where Viton ® (FKM Fluoroelastomer) would become to inflexible to maintain a secure seal.
Will your seals be subject to sunlight? This is another area where Viton ® (FKM Fluoroelastomer) rubber steps to the forefront. Its resistance to UV rays makes it ideal for outdoor applications.
One of the key things to consider is resistance to chemicals the seals may come in contact with. Buna provides excellent oil resistance and good resistance to many solvents. Viton ® (FKM Fluoroelastomer) rubber provides excellent resistance to oil, fuels, solvents and many other chemicals.
How can you tell which seal is currently being used in your situation? Buna Rubber is normally black in color, while Viton ® (FKM Fluoroelastomer) rubber tends to be brown. You can also check the manufacturers specifications and parts lists to determine which type of seal they recommend.
What are some of the other physical differences between the two rubbers?
Buna rubber has more elasticity than Viton ® (FKM Fluoroelastomer) rubber. Buna rubber can be stretched up to a 600% elongation. This is double the amount of elongation of Viton ® (FKM Fluoroelastomer) rubber at 300%. You need to stop and consider if this is a benefit in your situation or not. Many seals are never stretched during application and usually only in small amounts when they are.
After seeing the elongation comparison, you probably will not be surprised to learn that Buna rubber is softer with a Shore A Durometer range of 20 to 95, while Viton ® (FKM Fluoroelastomer) rubber ranges from 60 to 90.
Another difference can be found in the tensile strength of the two rubbers. Buna rubber is rated with a 200-3000 psi tensile range, while Viton ® (FKM Fluoroelastomer) rubber is rated with a 500-2000 psi tensile range.
Viton ® (FKM Fluoroelastomer) rubber separates itself from Buna rubber with its added resistance to oxidation, UV rays, weather, fungus and mold growth.
While both seals can be used in a wide variety of applications, Buna is normally not used in either the food or pharmaceutical industries. Viton ® (FKM Fluoroelastomer) is used in industry, food, pharmaceuticals and a wide range of other industries.
How are you supposed to decide between the two materials?
You always need to start with your personal application. What chemicals will the seals encounter? What are the temperature ranges you expect? Will the seals be used outdoors? Is elongation important in your situation?
Both Buna and Viton ® (FKM Fluoroelastomer) make excellent seals, but you must determine what is ideal for your situation. As an example, while both seal materials work great at 200 degrees, which one works better? At that temperature Viton ® (FKM Fluoroelastomer) rubber wins the battle. It can be used indefinitely at that temperature. What happens if you drop a few degrees to around 170? Then Buna rubber becomes slightly superior, but not by much.
Start off by making sure the seal will work with any chemicals you are using. Then verify your temperature ranges. Once you know those two factors you can make a good decision between Buna Rubber or Viton ® (FKM Fluoroelastomer) Rubber. Viton ® (FKM Fluoroelastomer) is usually considered the more flexible option, but only you can decide what is right for you. Evaluate the difference, compare the costs and then you can order with confidence.