2025 Ultimate Guide to O-Rings: Materials, Types, and Sizing

Rubber O-rings are ingenious yet simple devices that seal two surfaces together. These ring- or doughnut-shaped mechanical seals prevent the escape of fluids (like water, fuel, or gas) or the entry of contaminants.

As an O-ring manufacturer, we know these devices inside and out. Like most sealing solutions, O-rings come in a wide variety of materials, types, and sizes — each designed for a different application. Choosing the right O-ring requires careful analysis of the different options and understanding of what they’re each used for.

What is a Rubber O-Ring?

A rubber O-ring is a mechanical seal shaped like a ring or doughnut (sometimes called a torus). It sits between two surfaces, e.g., pipelines, and prevents the leakage of liquids. They’re often vital for containing hazardous substances like petroleum or chemicals.

O-rings are commonly used in most relevant industries, from food machinery to aviation and chemical works. This versatility makes them a popular choice and has led to dozens of different types.

They work by creating friction between the two surfaces, either in a static or dynamic application. As they move past each other (or the liquid tries to escape), this friction forms a seal preventing anything from escaping.

But, due to their hardness, abrasion resistance, heat resistance, pressure tolerance, and other factors, they’re suitable for even the toughest situations.

What Materials are Used for Rubber O-Rings?

Perhaps the biggest difference between rubber O-ring seals is the material they’re made from. Whether they’re used for fluid or gas leakage, food processing, plumbing, electronics manufacturing, or some other industry, the material must be suitable.

For example, if used in food, the material must be non-toxic and not likely to break into different pieces. Whereas, in an engine, it must be heat-resistant or tolerate high pressures. Each material has its pros and cons and is used in a different application.

Nitrile Rubber

Nitrile rubber is the most common form of rubber O-ring. Composed of acrylonitrile (AC) and butadiene, this synthetic rubber is preferred due to its affordability, performance, and chemical resistance.

Different nitrile O-rings have different ACN-to-butadiene ratios relevant to their tasks. A higher ACN content increases solvent resistance, while a lower ACN content performs better at lower temperatures.

Nitrile O-rings are perfect for the automotive industry, hydraulics, and industrial machinery. They form excellent seals against hydraulic fluids, lubricants, and petroleum-based oils, withstanding temperatures from –40°C to 120°C.

Viton® (FKM)

Viton® is the tradename for FKM synthetic rubber. These rings cost more than nitrile, but their extreme chemical resistance makes them ideal for industrial sealing where harsh chemicals, solvents, and fuels are involved.

They’re also more heat-resistant than conventional nitrile, operating in environments from –20°C to 210°C. You’ll find Viton rings work against:

• Oils
• Acids
• Hydraulic fluids
• Aromatic hydrocarbons
• Silicone fluids

That’s made them the preferred option in aerospace fuel transfer, vehicle engines, and chemical processing lines. Other beneficial traits include resistance to UV light and mould/fungus.

Neoprene (CR) Elastomers

Neoprene is most commonly known for its use in wetsuits. There, its thermal insulation and buoyance properties keep the diver warm.

In industrial applications, neoprene (or chloroprene rubber) is similarly chosen because of its resistance to environmental factors. Neoprene has low flammability, weather resistance, and is stable against UV light, ozone, ageing, and oxidation.

Such a unique blend of characteristics is ideal for outdoor or marine applications. For example, neoprene O-rings are used to seal refrigerants, ammonias, coolants, silicone fluids, and petroleum-based lubricants (operating where other materials struggle).

Silicone

Silicone is the most well-known material on the list. It’s a polymer made from a combination of silicon, oxygen, hydrogen, and carbon in a cross-linked matrix. It offers remarkable resistance to weathering and UV degradation. However, its non-toxicity makes it suitable for food and medical applications.

Indeed, while its temperature range is broad (–60°C to 225°C), the lower tensile strength and reduced wear resistance mean it’s rarely used in high-demand environments. Instead, its anti-mould and low corrosion properties make it the mainstay of food and beverage processing and medical device manufacturing.

Latex

Latex is a natural rubber harvested from the sap of rubber trees. The raw sap is processed via prevulcanisation to create an elastomer known for its incredible flexibility and strength. While it works well at low temperatures, the material is sensitive to oxygen, sunlight, and heat.

Useful in industries where biocompatibility and flexibility are preferred, such as sports equipment or the medical and pharmaceutical sectors. It’s not recommended for a chemical environment due to its rapid deterioration.

Polyurethane

Polyurethane O-rings are known for their high tensile strength, elasticity, and abrasion resistance. You’ll find them in hydraulic cylinders, mining equipment, automotive parts, and pneumatic actuators, where mechanical stress and repetitive movement are the primary issues.

Types of Rubber O-Rings

Most rubber seal O-rings are only differentiated by their material. However, there are a few other types:

• Back-up O-rings, or anti-extrusion rings, are exclusively used to seal pipes within high-pressure or high-temperature environments. They protect against extrusion failure in hydraulic or pneumatic systems.
• Coated rubber O-rings have a specialised surface coating that enhances their properties. Coatings include PTFE (Teflon®), parylene, or silicone. These coatings can reduce chemical reactions, increase friction resistance, or simply provide colour identification.
• Encapsulated rubber O-rings contain a rubber core (usually silicone or Viton) within a casing of high-performance plastic. They offer greater resistance to corrosive chemicals or extreme temperatures.
• Hollow O-rings reduce the overall weight of the ring, perfect for low-pressure, static, or vacuum-sealing environments. They often provide a tighter seal.
• Square O-rings have a square cross-section. Typically found in plumbing, water-filtration routes, or other applications where a lack of rotation is preferred.

Selecting the Correct Sized Rubber O-Ring

Rubber O-rings come in a vast array of sizes for almost any calibre of pipework or tubing. Here at Forever Seals, for example, we have more than 5,000 moulds of O-rings. Standard options include AS 568, BS 4518, GB/T 3452.1, JIS B2401 P/G/S/V, SMS 1586, METRIC, and JASO F404.

But for non-conventional applications, we can even make a mould specific to your criteria. That’s because the ring must be relatively precise to provide the necessary seal.

Browse our complete selection of O-rings. We have all the materials, shapes, and sizes, perfect for almost any application.

Frequently Asked Questions (FAQs)

How do I choose the right O-ring materials for my application?

Choosing the right O-ring materials is all about balancing factors. Usually, it is a limiting factor that determines the preferred material.

For example, in medical or food and beverage applications, silicone is by far the best option due to its low toxicity and mould resistance. In contrast, Viton might be better for aerospace applications where chemical resistance is crucial.

What is the difference between static and dynamic O-ring seals?

It’s all down to friction. Static environments allow the O-ring to not move, and so lower abrasion resistance is acceptable. In dynamic settings, the O-ring must provide a continual seal even as the component moves.

How do I measure O-ring size correctly?

The most important factor is the depth and the width of the groove where the O-ring will sit. Generally, the diameter of the O-ring is at least 15% smaller than the groove ring. Too small, and the O-ring can become damaged; too big, and it will not fit correctly.