Chemical Compatibilities of Various O-Ring Materials
O-rings are typically manufactured from a variety of elastomers that differ greatly in their compatibility with different types of fluids and gases. Selecting an elastomeric material for a sealing application is dependent on a variety of factors ranging from material hardness to application temperature, lubrication, and sealing pressure. Apart from this, one of the main factors to be taken into consideration is the chemical compatibility of the material with the fluid that it comes in contact with during operation.
Some of the most common materials used to manufacture O-rings are:
- Viton <R>
- Teflon <R>
Buna N, also known as NBR or Nitrile rubber, is a synthetic rubber polymer that is commonly used for manufacturing O-rings. Offering a low cost sealing option, this material is a co-polymer of Acylonitrile and Butadiene, where the Acylonitrile component provides the O-ring with its chemical resistance properties and the Butadiene component provides physical properties. Buna N offers excellent resistance to oils and other petroleum products, making it ideal for petroleum applications. However, there are several limitations to its chemical compatibilities as well, including its inability to resist ketones, esters, and aromatic hydrocarbons. It is also non-resistant to ozone, making it imperative that Buna N O-rings are not stored adjacent to operational motors that typically produce ozone.
Ethylene Propylene Rubber or EPR is a type of rubber also used to manufacture O-rings. This material is particularly useful for sealing applications that involve exposure to ozone and sunlight. EPR offers good resistance to ketones and alcohols, but not ideal for exposure to aromatic hydrocarbons and food based applications. It also has very good resistance to water. Together, these chemical compatibility properties make it suitable for brake fluids and hydraulic fluids that are non-flammable up to 300°F. Another important application of Ethylene-propylene is its usage in steam sterilizers.
Silicone, an elastomeric polymer made from a combination of oxygen, silicone, hydrogen, and carbon, is a commonly used material for O-rings used in static applications. This rubber is moderately resistant to solvents, but is not recommended for concentrated solvents and acids. Apart from these chemicals, silicone is also not compatible with most oils. It is however quite resistant to the actions of ozone.
This rubber is a combination of silicone with fluorinate, which creates an elastomeric material that offers the low-temperature capabilities of silicone along with good chemical resistance. Fluorosilicone is chemically compatible with military and aerospace fuels as well as resistant to ozone. However, it is not recommended for applications that involve contact with polar solvents and hydrocarbon fluids. O-rings made from fluorosilicone are also particularly susceptible to hydrolysis by alkalis and acids.
Viton or FKM is another commonly used synthetic rubber that is used for making O-rings. This material has excellent compatibility with solvents and oils, especially with all aromatic, halogenated, aliphatic hydrocarbons, as well as acids, animal oils, and vegetable oils. This excellent compatibility has made it a popularly used material for the manufacture of O-ring seals. Viton is also compatible with most different hydrocarbons. However, it is not resistant to ketones, compounds containing nitro, and esters having low molecular weight. O-rings manufactured from Viton are found in safety glasses used for scuba driving.
O-rings made from neoprene offer fair, but not good, resistance to several solvents and oils. Neoprene offers good resistance to animal and vegetable oils, as well as fat, grease, and moderate chemicals. This material is however not ideal for use in applications that involve oxidizing acids, ketones, esters, and nitro as well as chlorinated aromatic hydrocarbons. Neoprene also has excellent resistance towards ozone and sunlight. One particular property of neoprene is its resistance to refrigerants such as Freon, which has made it ideal for sealing applications in various refrigeration operations.
PTFE, or Teflon as it is generally known, is a low frictional co-efficient fluoropolymer commonly used to make O-rings. This material is one of the most chemically inert materials and extremely compatible with solvents, acids, alkalis, oils, and several other chemicals.
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