Nitrile “NBR” (buna N) with 50 to 60 duro (hardness) is generally the material that should be chosen for most transformer applications.
Do not confuse this material with butyl rubber. Butyl is not a satisfactory material for transformer gaskets. The terms butyl and buna are easily confused, and care must be taken to make sure nitrile (buna N) is always used and never butyl.
Replace all cork neoprene gaskets with nitrile if the joint has recesses or expansion limiting grooves. Be careful to protect nitrile from sunlight; it is not sunlight resistant and will deteriorate, even if only the edges are exposed. It should not be greased when it is used in a nonmovable (static) seal. When joints have to slide during installation or are used as a moveable seal (such as bushing caps, oil cooler isolation valves, and tap changer drive shafts), the gasket or O-ring should be lubricated with a thin coating of DOW No. 111, No. 714, or equivalent grease. These are very thin and provide a good seal.
Nitrile performs better than cork-neoprene; when exposed to higher temperatures, it will perform well up to 65 ºC (150 ºF).
Viton should be used only for gaskets and O-rings in temperatures higher than 65 ºC or for applications requiring motion (shaft seals, etc.). Viton is very tough and wear resistant; however, it is very expensive ($1,000+ per sheet). Therefore, it should not be used unless it is needed for high wear or high temperature applications. Viton should only be used with compression limiter grooves and recesses.
Store nitrile and viton separately, or order them in different colors; the materials look alike and can be easily confused, causing a much more expensive gasket to be installed unnecessarily.
Compression and fill requirements for Viton are the same as those for nitrile, as outlined
above and shown in table 3.