O-rings are one of the oldest and most classic components used in the engineering industry. It is a doughnut- shaped ring that acts as a seal between two parts to prevent gases or liquids from escaping. There are so many types of O-rings available in the market today ranging fromrubber O-rings to Stainless Steel O-rings. O-rings materials today are tailored to thousands of applications and there are dedicated materials just for certain applications like Viton ® for example.
So, with the overwhelming choices for O-ring materials, it can be tough to choose the correct material for the application you want.
Factors to consider when choosing rubber O-ring material
The most important factor to consider when choosing a material for O-rings is the application and environment it will be used in. This will determine the operating temperature, pressure and chemicals it may come into contact with. Obviously, high-performance O-rings are also quite costly, so you need to make a well thought out compromise between cost and performance.
Let’s look at some of the operating conditions O-rings could be used in and how their materials will need to be tailored accordingly.
1.High-temperature applications
O-rings will need to withstand the extremely high temperatures with minimum deformation. The most common applications are in gas turbines, compressors, aerospace applications, oil refineries, etc.
It is interesting to note that despite being such a small, simple component, it can prove extremely critical in some applications. The most famous case of O-ring failure was in NASA’s Challenger space shuttle. A simple error in calculating O-ring failure at a certain temperature led to one of the biggest cautionary tales in aerospace history. So, it is extremely important to evaluate your application and its operating conditions thoroughly before selecting an O-ring.
For rubber O-rings, the best materials for high-temperature applications include Viton, nitrile and silicone rubber.
2.High-pressure applications
It is very important that O-rings should be able to withstand high internal pressures exerted on it by gases, fluids or mechanical components. If not, it will cause O-ring extrusion and won’t seal properly.
Examples of high-pressure scenarios include O-rings used in reciprocating pistons in engines and O-rings used in heavy-duty hydraulic or pneumatic applications.
3.Static vs Dynamic applications
In Static applications, O-rings are subject to more uniform forces. In Dynamic applications, however, the O-rings are secured either in a static or a moving component. This means that the O-ring will constantly be rubbing against other components (e.g. O-rings secured in reciprocating Piston). So, in dynamic applications, the O-ring material should be able to withstand high friction and abrasion. Abrasion is a common cause of O-ring failure.
The O-ring material will also need to withstand high cyclical forces and endure many lifecycles. So, durability and abrasion are key factors for dynamic applications.
4.Chemical Resistance
In applications where the O-rings can come into contact with corrosive liquids or even just a wide range of chemicals such as in the food and beverage industry, oil refineries, etc. The O-ring material will need to possess high chemical resistance to a wide range of chemicals.
5.Food-grade applications
In the Food and Beverage industry, O-rings should be made from non-toxic materials that are resistant to bacteria. There best food-grade rubber O-rings are made from materials such as EPDM, Neoprene, Nitrile, Silicone, and Flurocarbon. These materials have all been approved by the FDA.
6.Cost
As with any component, you need to factor in the cost-effectiveness of the material. You need to evaluate how to compromise between cost and performance for your application. In general, Nitrile Butadiene Rubber (NBR) is the cheapest and most popular for general applications while Hydrogenated Nitrile Butadiene Rubber (HNBR) is the most expensive.
Other factors include things like conformability, which is how well the O-ring material can take up the shape of the groove it is sitting in. Low conformability can lead to leakier seals.
Most common materials for Rubber O-rings
Let’s look in more detail, the most commonly used materials and their properties:
1.Viton
Viton O-rings are rubber O-rings made from fluorocarbon rubber. Viton ® is a high-performance elastomer and has the best abrasion resistance, oil resistance, and a relatively high-temperature resistance compared to materials such as nitrile rubber. The temperature range of Viton® is between -20 to 200 and a hardness rating of 75-90.
As Viton ® has excellent chemical resistance, it is ideal for harsh chemical environments and ideal for chemical processing and acid solvent applications in industries such as paper mills. It is pretty much resistant to all chemicals except Skydrol and certain esters.
Viton ® is ideal for applications requiring extreme chemical or heat resistance and is commonly used in hydraulic or pneumatic seals.
2.Nitrile Rubber (NBR)
NBR is the most common material for rubber O-rings. NBR has good abrasion resistance, good oil resistance, very high tear resistance and is quite cheap compared to other rubber materials. However, it does have poor temperature resistance. NBR’s properties make it ideal for low-pressure applications as hydraulic or pneumatic seals where cost is an important factor.
The temperature range of NBR is -30-100 and has a hardness rating of 85.
3.Highly Saturated Nitrile Rubber, Hydrogenated Nitrile Rubber (HNBR)
HNBR is a slightly better performing version of NBR. It can withstand a higher temperature range, has better wear resistance and has better chemical resistance than regular NBR. Its temperature range is from -25-150.
4.EPDM Rubber
EPDM is a rubber-like elastomer with good temperature resistance, excellent tear resistance, excellent surface speed limitation (great for dynamic applications) and a relatively cheap price point. It does, however, have poor oil and chemical resistance and decent abrasion resistance.
Its properties make it ideal for high-temperature water and steam applications but not good for applications dealing with mineral oil or grease.
Its temperature range is from -45-100.
5.Neoprene ®
Neoprene has good surface speed limitation, good abrasion resistance, excellent tear resistance, and a good price point. It does however, have a poor temperature resistance. It is a durable material that can resist petroleum oils, ozone, and sunlight. It is commonly used in refrigeration and air conditioner seals.
Its temperature range is from -54 -121.
6.Silicone Rubber
Silicone rubber has the highest temperature resistance from all rubber O-rings and have a medium price point. The material does, however, have poor oil resistance, low surface speed limitation, poor abrasion, and tear resistance. Hence it is not ideal for use in dynamic applications or environments exposed to mineral oils such as in reciprocating pistons. It is commonly used in static, extreme-temperature applications. The temperature range is from -60-200.
7.Flurosilicone
This material is a mix of Viton and silicone. Flurosilicone exhibits great temperature resistance, oil resistance, decent abrasion resistance, excellent oil and chemical resistance. It has poor abrasion resistance and low surface speed limitation so is not ideal for dynamic applications. It is also quite expensive.
It is most commonly used in aerospace applications such as aircraft fuel systems and ideal for environments full of jet fuel, petroleum oils or synthetic jet oil.
The temperature range is from -62-177.